Chemical Ion Test

7/24/2019 Chemical Ion Test

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CHEMICAL TESTS FOR SMALL SPECIMENSBy Jesse Crawford

INTRODUCTION

This is a work in progress. The objective is to collect together the

chemical tests that are useful for identifying minerals and that are alsowithin the reach of the typical hobbyist with the typical hobbyists

budget. If anyone would like to suggest a test for the collection,

please email me ([email protected]) with the details. Put Mineral test in

the title so the message can get through my spam filter. Tests should be

easy to perform, and should use materials that are reasonably easy to

obtain.

The tests described here are intended for small samples. For most tests,

a piece that's 1 or 2 millimeters across is adequate for at least 3 or 4

tests. That's about 5 to 20 milligrams.

Scientists have directed a lot of effort toward developing ways to make

chemical tests on tiny samples using a microscope to interpret theresults. Most of these tests have become obsolete in recent years, but

they still offer useful and fairly low cost methods that amateur

scientists can use to test minerals.

What follows is a description of some chemical tests that work reasonably

well when scaled down to a size appropriate for testing tiny samples. The

author has tried most, but not all, of the tests included. Not much

detail is included about what positive or negative test results look like

because it is assumed that the tests will be performed on both the

unknown sample, and a sample that is known to contain the element being

tested for. It's also a good idea to run the test on a blank sample known

to not contain the element being tested for.

No test is perfect. Most of these tests offer a level of confidence

probably no more than the 80 to 90 percent level, which is pretty good in

a world as uncertain as this one. As I see it, it's the uncertainty that

keeps things interesting. Remember, if youre not having fun, then youre

not doing it right.

Chemists (at least old chemists) form the habit early in their careers of

treating all chemicals as if theyre dangerous.

BE SAFE! Respect the fact that chemicals can be hazardous. Scientists

dont know all the ways that chemicals can injure people. You dont want

to be the first to discover a new one. Don't let chemicals stay on your

skin, and don't breathe them. If you can smell them, then you probably

need to improve the ventilation.


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SAMPLE PREPARATION

After we have a sample to test, the next thing to do is dissolve it, or

at least dissolve enough of it to test. Ideally, the objective is to

dissolve as much as possible of the sample so that the result is a drop

of clear liquid about 10 to 50 microliters in volume containing all of

the ions of interest. (in other words, a small drop). Most of the time,it's not necessary to go for complete dissolution. Often there will be a

part of the sample that remains as pulverized fragments or sometimes a

gelatinous mass of silica. If the grinding of the specimen is done with a

mortar and pestle, the acid can be added while the grinding is being

done. Then touching the pestle to a slide makes a drop that's sufficient

for a test.

Almost all samples are prepared by dissolving them in some kind of acid.

The following is a list in the suggested order to use in trying to

dissolve the sample. If nothing in the list attacks the sample, then

that's a lot of information already. The list of minerals that are

impervious to all acids is a comparatively short one. A table of

solubilities of some minerals is included at the end of this paper. Acidsshould be full strength. When one is found that attacks the sample, the

solution can be diluted with a drop of water before beginning the tests.

Some of the tests need to be carried out in a neutral or basic

environment. Ammonia is handy for neutralizing acids.

THESE ACIDS ARE DANGEROUS! Handle them carefully in a well ventilated

environment. Don't breathe the fumes. Be especially careful with fluoride

minerals. Hydrofluoric acid and sometimes elemental fluorine is evolved

when fluorides are treated with some acids. Its very nasty stuff.

Water

Hydrochloric Acid

Nitric AcidSulfuric Acid

Aqua Regia (3 parts Hydrochloric 1 part Nitric) CAUTION! Chlorine

is evolved from aqua regia.

Hydrofluoric acid, if it were less dangerous, would certainly belong on

this list. It neatly solves the problem of dissolving silicate minerals

by converting silicon to a gas, silicon tetrafluoride. With that goal in

mind, there is an alternative to using a strong solution of hydrofluoric

acid. Small samples of silicate minerals can be digested in platinum or

teflon dishes with a mixture of sulfuric acid and calcium fluoride.

Hydrofluoric acid is thereby generated in situ and immediately reacts

with the silica in the mineral. The technique is not without dangers, butwith proper precautions can be used when necessary. The hydrogen fluoride

generated is still dangerous, and must be respected, but the risk is more

manageable.

There is a method that can be employed to dissolve even the minerals that

resist all the above acids. Heating the sample with a flux to a high

temperature until it is thoroughly fused alters the composition of most

minerals so that they can be dissolved in water or one of the above


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acids. The usual flux used is sodium or potassium carbonate, or for some

minerals sodium or potassium bisulfate.

Fusing the mineral sample at red heat with a flux can induce almost any

mineral to dissolve either in water or in one of the acids. These are

extreme measures, and because they involve a lot more handling of the

sample than simply treating it with acid it's usually good to start witha larger piece. 50 to 100 milligrams is good. Carbonate fusions can be

carried out in a platinum crucible or piece of platinum foil, but

bisulfate fusions should not be made on platinum, as the platinum will be

attacked. Fusions with carbonate can be done in a small ceramic crucible,

or on a block of charcoal, or a loop of platinum or nichrome wire using

pretty much any small torch.

To do a carbonate fusion, start by grinding the sample as fine as

possible. Add about twice the volume of dry sodium carbonate, and mix

them. If you have a platinum crucible, then put in the sample mixed with

flux, cover with a little more pure flux and support the crucible for

heating. Begin heating the side of the crucible and as the mass begins to

fuse, regulate the heat so as to avoid any loss of sample. The melt willevolve carbon dioxide and water vapor and possibly other gasses, and it

will probably do a lot of bubbling. After the bubbles stop, raise the

heat to redness and continue heating for 10 or 15 minutes, until its

thoroughly melted. Let everything cool down and add a few drops of nitric

acid and a little water, and let it sit for a while. The melt will loosen

and dissolve. Put the contents into a beaker, rinse the crucible with

water, and add the washings to the beaker. Then set the beaker on a low

source of heat so that the water and nitric acid can evaporate. It should

not boil at any time. A double boiler arrangement is desirable for this

phase of the operation. When the contents of the beaker are dry, add the

minimum amount of water necessary to dissolve the soluble part. There may

be an insoluble residue of silica. If the dried sample doesn't dissolve

in water, one of the acids may be necessary. The fusion can also be doneusing a wire loop. Start with a hot loop, pick up as much sample and flux

mixture as will stick to it, and fuse it. Then, touch the fused bead to

the sample mixture to pick up a little more, and continue. Repeat the

process until enough of the sample is fused.

The procedure for a bisulfate fusion is similar, but should be carried

out in a porcelain crucible. It's messier, and the fumes are more toxic,

so BE CAREFUL. During the bisulfate fusion, there's a lot of bubbling at

first. After the bubbling stops, there comes a point where the melt

solidifies, and a higher heat is needed to get it to fuse again. This is

the point at which the generation of sulfur trioxide and other corrosive

sulfur oxides begins, which is the objective of the procedure. If the

melt is allowed to cool at this point, the process will not be complete.The heat should continue until the mass fuses again, and no further

changes are in evidence. At this point, cool the melt, add a drop of

concentrated sulfuric acid (carefully) and resume heating. This is

repeated two times. Then the melt is cooled and removed from the crucible

as above using a little sulfuric acid and water. Sulfuric acid gives off

dense clouds of white fumes when it is heated to dryness. DON'T BREATHE

ANY OF IT. This procedure is not for the faint hearted. It's noisy and

hot and frightening and suitable only for a well ventilated garage or


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lab. Have a fire extinguisher close by and an escape route cleared in

case of emergency. Other than that, it's kind of fun.

Vycor labware works well for fusions.

Sodium peroxide also makes a good flux. One author asserts that any

mineral can be brought into solution by sodium peroxide fusion. Peroxidefusions are ordinarily carried out in a zirconium crucible.

DECIDING WHAT TESTS TO PERFORM

In deciding what tests to make, it's sometimes handy to remember that it

can be just as valuable to know what isn't present in a sample as what

is.

Once we know what will dissolve the sample, tables of the solubility of

minerals can be consulted to help in selecting which further tests to

undertake. Try to find a test that will split the list of possibilities

in half. This has been called the half-split technique.

Whenever we read about a test, it usually starts out with a list of

needed equipment and reagents, then a description of the procedure, and

somewhere near the end will be a list of ions that interfere with the

test. That's always the catch. There are very few tests that respond only

to one element. Usually there's a list of them.

A lot of the difficulty with interfering ions can be sidestepped by

careful selection of the sample. Picking a well formed crystal of the

mineral of interest improves the chances that there won't be a lot of

interfering ions. Naturally, those are always the prettiest crystals.

MATERIALS for SPOT TESTS

A small mortar and pestle for grinding samples.

A box of microscope slides.

A box of cover slips.

A glass or plastic ring about 15 or 20 millimeters in diameter ( it

must be smaller than the cover slips) and about 2 to 3 millimeters thick

A glass rod 1 to 2 millimeters in diameter.

A small bulb type pipet (eyedropper).

REAGENTS for SPOT TESTS

Acetic Acid (Glacial)

Acetylsalicycilic Acid (Aspirin)

Ethyl AlcoholAluminon 0.1 percent solution

Ammonium Acetate Solution 3N

Ammonium Chloride

Ammonium Hydroxide

Ammonium Molybdate

Ammonium Oxalate

Ammonium Phosphate (Dibasic)

Aniline hydrochloride


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Barium Chloride

Cesium Chloride

Chloroplatinic Acid

Citric Acid

Curcumin

Dimethylglyoxime

Hydrogen PeroxideHydroquinone

Lead Acetate

Oxalic Acid or sodium or potassium oxalate

m-phenylenediamine hydrochloride or sulfate

Potassium Dichromate

Potassium Iodide

Potassium Mercuric Thiocyanate (This reagent is made by combining

mercuric nitrate with potassium thiocyanate in molar proportions of 1

part mercuric nitrate to 4 parts potassium thiocyanate. Tabular and

needle-like crystals separate easily from acidic aqueous solution).

Potassium Nitrite

Potassium Phosphate (Dibasic)

Potassium or Sodium SulfiteRubidium Chloride

Silica sand

Sodium Acetate

Sodium Chloride

Sodium Fluoride

Sodium Phosphate (Dibasic)

Silver Nitrate

Starch

Tartaric Acid

Thiourea

Uranyl Acetate

The following are spot tests that are carried out on microscope slidesand viewed through the microscope. Some authors recommend coating the

microscope slides with wax or some other hydrophobic material to make it

easier to control the drops. Some manufacturers make microscope slides

with small wells that prevent solutions from running off the slide or to

use with the "hanging drop" method (to be described below). They're all

good ideas, yet just a plain microscope slide works fine for most tests.

It's also a good idea to have a piece of black paper and a piece of white

paper handy to put under the slide for contrast when viewing crystalline

precipitates.

TECHNIQUES

The most general method for carrying out tests is to place a drop of thesolution of the sample on a slide and put a drop of a reagent solution

near it. Then a thin glass rod is used to bring the two drops together.

The entire process is observed under the microscope.

Another important technique that's used is the hanging drop method. It's

used to trap gaseous reaction products that are evolved from the sample

as it reacts with a test reagent. For this technique a glass or plastic

ring supports a cover glass with a drop of reagent or water hanging from


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the underside. The hanging drop is positioned over the sample, so it's

close but not touching.

It is occasionally desirable to separate a drop of a solution from solid

material, such as a precipitate or the fragments that remain after

grinding the sample. It's often possible to precipitate an interfering

ion and then move the clear sample solution to another slide for furthertests. To remove the iron, for example, from a drop of solution, the pH

of the sample solution can be raised by adding a drop of ammonia. At high

values of pH, iron forms a dark gelatinous precipitate. To separate the

sample from the iron precipitate a small piece of filter paper, an eighth

of an inch or so in diameter, is placed on the slide near the sample.

Then a dropper tube with an opening a little smaller than the diameter of

the paper is pressed against the paper. The bulb of the dropper should be

squeezed so that a small amount of suction will be supplied when the bulb

is released. The tip of the tube with the filter paper is slid across

into the sample drop, and the pressure on the bulb is released. If the

dropper tube is not pressing too hard on the filter paper, the fluid will

be drawn up into it through the filter paper, and it can be picked up and

moved to another slide. As described, this procedure for separating ironis not selective, and would also leave behind other elements that

precipitate at high values of pH, notably aluminum. Something else is

needed to separate iron and aluminum (See "Aluminon Test" below). It

takes a little practice to get this technique just right, but it opens a

lot of possibilities when mixtures of ions interfere with one another. It

helps to roughen the end of the tip of the dropper tube with fine

sandpaper to prevent it from slipping off the filter paper when sliding

it along the glass.

It is sometimes necessary to protect a glass slide or cover slip from the

action of hydrogen fluoride. Plastic slides can often be used in these

situations, or the glass can be coated with a hydrophobic material.Smearing grease on the glass works, but its difficult to get a uniform

thickness, and the irregularity of the coating can interfere with

visibility. It works well to keep on hand a thin solution of microscope

grease dissolved in xylene for this purpose. A drop is spread easily over

the slide, and the xylene evaporates quickly, leaving a thin film of

grease that prevents the hydrogen fluoride from attacking the glass.


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TESTS for SPECIFIC IONS

In the descriptions of the tests to follow, references in parentheses

following the name of each test are to the sources listed in the

bibliography.

CATIONS

Aluminum

Aluminon Test: (Welcher) (Lange) If the sample is not already

acidic, acidify it with dilute hydrochloric acid (1 drop). Some authors

also recommend adding a drop of ammonium acetate buffer (3N). Place a

drop of 0.1 percent Aluminon nearby and combine the two drops using a

glass rod. A Red precipitate develops in the presence of aluminum and a

number of other ions. If the precipitate persists after adding a drop of

ammonium hydroxide, aluminum is indicated. This is the simplest form of

the test and unless the material being tested is reasonably free of other

ions, it is likely to produce a false positive. Aluminon is a veryversatile reagent that can be used to detect very small amounts of

aluminum, but in order to be confident of the results it must be

recognized that it forms colored precipitates with a number of other

ions. It forms a purple precipitate with iron, and red to brown

precipitates with aluminum, actinium, barium, beryllium, calcium, cerium,

chromium, europium, gadolinium, hafnium, indium, lanthanum, magnesium and

neodymium, and white precipitates with antimony, bismuth, lead, mercury,

and titanium. In order to interpret the result of this test it's

necessary to separate the aluminum from at least some of these other

ions, particularly iron.

Iron may be separated from aluminum by adding tartaric acid or citric

acid to the sample solution. These will bind with the iron in such a wayas to make it soluble in alkaline solution from which aluminum can be

separated as a gelatinous hydrous oxide. Add a little of the tartaric or

citric acid, and stir the drop until it dissolves, then add a small drop

of ammonia to form the aluminum precipitate. It's important to realize

that if the pH will go too high, the precipitate of aluminum will re-

dissolve. This does not ordinarily happen with ammonium hydroxide, but if

it does, warm the slide a little to drive off some of the ammonia. When

the ph of the drop is in the right range, the aluminum will be in the

form of a white translucent gelatinous precipitate and the iron will

still be in solution. The aluminum precipitate will be translucent white.

If it takes on a dark color then it probably means that the iron (or

something else) is also precipitating. Add more tartaric acid and adjust

the amounts until it looks right. Then use the eyedropper to filter offthe liquid phase, or decant it carefully. Add a drop of water to wash the

precipitate and draw that off through the filter too. Repeat the wash a

couple of times. Then dissolve the white precipitate containing the

aluminum and test it with the aluminon reagent as described above. It's

good to practice on some fake "unknowns" until you have a feel for the

amounts needed to make it go right.


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Aluminon works better as a reagent for use with the enhanced spot tests

described later.

Ammonium Molybdate Test: (Chamot and Mason) This test should be

carried out at a fairly neutral pH. To the drop of sample, if it is not

already neutral, add a small drop of saturated sodium acetate buffer.

Then add a small pinch of ammonium molybdate. Watch for the formation oftransparent four sided plates indicating the presence of aluminum. At

first the crystals look square, but on closer examination they prove to

have a more interesting shape. Too much buffer tends to inhibit the

formation of the crystals. These crystals show symmetrical extinction

when viewed between crossed polarizers. The presence of some ions can

inhibit their formation. Warming the slide and/or adding more water to

the drop sometimes helps. These should always be tried before deciding

that the test is negative for aluminum. Nickel and iron both form similar

crystals. Mercury forms six sided slightly elongated crystals under these

conditions.

Barium

Sodium Bicarbonate Test: (Chamot and Mason) See the sodium

bicarbonate test under Calcium below. Barium carbonate crystals form

more slowly than calcium or strontium carbonates, and the crystal are

larger and more well formed.

Beryllium

Aluminon Test: (Welcher) See the test for aluminum. A red

precipitate develops in the presence of aluminum or beryllium. The red

color from the beryllium looks much like the color from aluminum, but

when ammonia is added, the red precipitate dissolves if it's beryllium.

This is not a very good test for beryllium, because several of the otherions that form red precipitates with aluminon behave the same way.

Aluminum is the only one that does not dissolve when the ammonia is

added.

Potassium Oxalate Test:

(Chamot and Mason) This test

produces characteristic crystals

of a double salt of potassium

and beryllium oxalate. A large

drop of potassium oxalate

solution is placed near the

sample and the two drops are

joined using a glass rod. As thewater evaporates, rhombs and

prisms will become evident if

beryllium is present. It is easy

to mistake crystals of potassium

oxalate for the double salt, so

care should be exercised in

interpreting the result of this test. The double salt is strongly

birefringent, and exhibits an extinction angle of 39 degrees. Prisms are


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sometimes formed with difficulty, but if the solution is heated so the

crystals re-dissolve and a tiny drop of a solution containing mercuric

ions is added, the prisms will have more of a tendency to form as the

solution cools.

Both of the photos show the

results of a positive test forberyllium, made on a known

sample of phenakite. The well

developed crystals in the upper

one appeared only after

recrystallization. These are

crystals of the double salt of

potassium and beryllium

oxalate. The extinction angle

is 39 degrees from the

direction of elongation of the

prisms. The colors are due tothe birefringence of the crystals viewed between crossed polarizers. The

lower photo includes crystals of other compounds as well, and is more

difficult to interpret.

Curcumin Test: (Chamot and Mason) (Smith) See Curcumin Test for

borate under Anions below.

Bismuth

Thiourea Test: (Chamot and Mason) (Lange) Thiourea added to a

solution containing bismuth in nitric acid makes a strong yellow colored

solution.

Dimethylglyoxime Test: (Budavari) A sample containing bismuth forms

a bright yellow color and precipitate with this reagent.

Boron

See borate under Anions below.

Cadmium

Potassium Mercuric Thiocyanate Test: (Chamot and Mason) (Schaeffer)

Put a small drop of potassium mercuric thiocyanate solution near the

sample drop. Combine the two drops with a thin glass rod and watch forthe characteristic crystals that indicate cadmium. This test is also used

for other ions. Crystal shapes are distinctive for each type of ion.

Oxalic Acid Test: (Chamot and Mason) Cadmium oxalate crystallizes

as long prisms with oblique ends, or as Xs or radiating groups. From

concentrated solutions it forms octahedrons. Calcium, zinc and strontium

interfere with this test.


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Calcium, Barium, Strontium

These three elements are difficult to separate because they have very

similar chemistries. Getting a good identification is possible by using two

of the following tests in combination. The difference in solubilities of the

sulfates makes a good way to tell the difference between them.

Ammonium Oxalate Test: (Chamot and Mason) Add a small drop of sodium

acetate buffer to bring the pH to neutral. Place a drop of the reagent near

the test sample and join the two drops with a glass rod. A white precipitate

indicates calcium or strontium or barium. The test can also be made using a

crystal of oxalic acid. The crystals typical of calcium are quite small,

squarish tablets. Strontium oxalate looks much the same. The crystals are

larger, and some are elongated but differentiating the two types of crystals

in a mixture of both is not practicable. Barium makes distinctive crystals

with oxalic acid. They assume the form of branching tree-like structures.

The presence of calcium or strontium will suppress the formation of the

barium crystals. Barium oxalate is very soluble in acids. If just a trace of

nitric acid is present, the crystals will not form.

Sodium Bicarbonate Test: (Chamot and Mason) To the sample drop add a

small drop of saturated sodium acetate to adjust the pH to a value near

neutral. Then add a pinch of sodium bicarbonate. If calcium is present,

small crystals of calcium carbonate will begin to separate out, floating on

the surface and adhering to the slide. After some of the water has

evaporated, larger crystals of the double salt of sodium and calcium

carbonate will form beginning at the edge of the test drop. Its not always

obvious which is the calcium carbonate and which is the double salt, however

the double salt is more soluble in water. Calcium carbonate, once it has

precipitated from neutral solution, will not redissolve on the addition of

water. The double salt of calcium and sodium carbonate can be redissolved by

adding more water to the drop. This is a good test for calcium. Strontium

and barium, which also precipitate as insoluble carbonates, do not form a

double salt under these conditions.

Sulfuric Acid Test: (Chamot and Mason) Add a small drop of sodium

acetate buffer to make the pH neutral and join the sample drop with a drop

of dilute sulfuric acid. In the presence of calcium, prisms of calcium

sulfate separate gradually from the solution. The ends of the prisms are

terminated at an angle of 66 degrees, which serves to confirm their

identity. Twinning is common. The precipitation with barium and strontium

is too finely divided to recognize crystal forms. There are several other

elements that react to form insoluble sulfates, so its best to do a

preliminary separation with oxalic acid or sodium bicarbonate, so that the

other elements will not interfere. The oxalates and carbonates of calcium,

barium and strontium tend to adhere to the surface of the microscope slide,

so the precipitate containing these can be carefully washed and redissolved

prior to making the sulfate test. The precipitate containing strontium can

be recrystallized from hydrochloric acid to produce recognizable crystals,

but they look a lot like the calcium oxalate crystals, so its not really

worth the effort. The solubility of the sulfates of calcium, barium, and

strontium differ widely. Calcium sulfate dissolves readily in hydrochloric

acid. Strontium sulfate dissolves slightly, while barium sulfate is

virtually insoluble.


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Cobalt

Potassium Mercuric Thiocyanate

Test: (Chamot and Mason) Adding

potassium mercuric thiocyanate

solution to a sample containing

cobalt results in deep blue crystalslike the ones shown in the photo.

These crystals are somewhat more

soluble than the ones that develop

in the presence of other ions, and

sometimes do not appear until the

drop has been allowed to sit for a

while so that some of the water has

evaporated.

Quinoline - Ammonium Thiocyanate Test: This test responds to Co,

Fe, Mo, Ti, U, V, and Zr. For this test, the sample should be dissolved

in strong hydrochloric acid. A small drop of quinoline mixed with an

equal volume of 6N hydrochloric acid is mixed with the test drop. It isthen connected in the usual way with a drop of saturated ammonium

thiocyanate. An oil phase separates out quickly and over time crystals

develop from the droplets of oil. After a half hour or so, the drop

becomes filled with ammonium chloride crystals from the reaction between

the hydrochloric acid and the ammonium thiocyanate. Antimony and bismuth

must be absent for this test to work, as these cause an immediate

precipitation when the quinoline reagent is added to the sample. In the

case of Mn, Cd, Sn, and Hg (and possibly others), crystals may be formed

immediately before the addition of the ammonium thiocyanate solution.

Cobalt develops light blue dendrites and blue crystalline blades from

blue oil droplets. In the case of titanium, the oil is yellow to orange

and the crystals, if they appear, are small thin yellow discs, scales andelongated hexagons or prisms. Zirconium yields a colorless oil and thin

scales, plates, and rosettes from yellow to orange in color. Vanadium

causes a colorless oil, and crystals are difficult to form. Uranium

causes a yellow oil with rectangular plates and prisms of a light yellow

color. Molybdenum produces a reddish oil but seldom produces crystals.

Nickel and copper both yield dark colored oils but rarely produce

crystals.

Quinaldine Ammonium Thiocyanate Test: Quinaldine is a compound

almost identical to quinoline. Its comprised of the same heterocyclic

ring system with a single methyl substituent. Its chemistry is much the

same as quinoline, and when substuted for quinoline in the above test

protocol, the results are similar. The crystal shapes and colors producedare a little different however, presumably because of steric effects due

to the methyl group. If the test solution is not already in hydrochloric

acid, it should be evaporated to dryness and dissolved in a drop of

hydrochloric acid before adding the quinaldine. If crystals do not

develop readily, sometimes it helps to also add a drop of water to the

sample. In the case of some ions (Co, Cu, Ni, U) crystals can take up to

two or three hours to develop. The slide must be covered in those cases


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12

with an inverted petri dish or watch glass in order to retard

evaporation.

The presence of cobalt in the test drop is indicated by a blue oil

separating out. Crystals do not form immediately. If conditions are not

perfect, they do not form at all. If the crystals dont make their

appearance before the slide becomes covered with a mixture of ammoniumchloride and unreacted quinaldine hydrochloride crystals, then its too

late. These metastable states are common with a number of ions, making

this test a little frustrating at times. Im still working on improving

it because crystals, when they can be coaxed to appear, can be quite

distinctive.

Copper

Ammonia Test: (Chamot and Mason) A dilute nitric acid solution of

copper ions will turn a strong characteristic blue color with the

addition of a drop of ammonia. This is not a precipitate, tetraamine

copper ions are soluble but strongly colored.

Triple Nitrite Test: (Schaeffer)(Chamot and Mason) Evaporate the

sample to dryness and then just cover the residue with a small drop of 30

percent acetic acid. Add a small crystal of sodium acetate. Wait for the

crystal to dissolve and add a small crystal of lead acetate. When that

has dissolved, add a crystal of potassium nitrite. Characteristic

crystals will form if copper is present. The triple nitrite test has

several variations and is used to test for a number of ions. It requires

some practice, and even then the results can be confusing. Theres a good

discussion in Handbook of Chemical Microscopy by Chamot and Mason.

Quinaldine Ammonium

Thiocyanate Test: See the notes

for this test under Cobaltabove. The coordination compound

made by copper and quinaldine

produces clusters of long

slender crystals arranged in

branching structures, dark

reddish brown in color. These

crystals only appear when

conditions are perfect, and they

take a long time to form.

Ordinarily they dont put in an

appearance before patience runs

out. The appearance of the oil is similar to the oil that separates when

the sample contains nickel.


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Potassium Mercuric Thiocyanate Test: (Chamot and Mason) Copper

causes long green needle shaped crystals to form when combined with

potassium mercuric thiocyanate reagent. These crystals, like the ones

that develop with cobalt, are fairly soluble. Allowing the slide to sit

undisturbed for a period of time while the water evaporates produces

crystals like the ones in the photos above.

Gold

Potassium Mercuric Thiocyanate Test:

(Chamot and Mason) Adding a drop of

potassium mercuric thiocyanate reagent and

joining it to a drop of sample solution

containing gold causes the immediate

separation of a densely branched structure

of finely divided crystals. The crystals

have a reddish hue and are unmistakable,

making this an easy test for the presence

of gold.

Potassium mercuric thiocyanate is a

versatile reagent, but it must be realized that with mixtures of ions,

the results can be variable and confusing. It works best when one ion

dominates the sample mixture. Chamot and Mason give an extensive

discussion of the behavior of this reagent under various conditions in

Handbook of Chemical Microscopy.

Iridium

Thiourea Test: (Chamot and Mason) In a solution of the sample in

concentrated hydrochloric acid, a few crystals of thiourea are added. If

iridium is present, the reddish color of the sample drop will decolorize,

and become water clear. Iridium does not cause the formation of crystals.


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Iron

A quick assessment of iron can be made by adding a little sodium

hydroxide solution, or ammonia to the unknown. Iron will cause a

precipitate immediately of a dirty green color if its ferrous, or brown

if its ferric. This is a quick test for iron, and can easily lead to

false conclusions unless followed up with more specific tests, becausethere are several elements that yield gelatinous precipitates with bases.

Specifically, the following ions all yield gelatinous hydrous oxides

under these conditions: aluminum, chromium, tin, titanium, zirconium,

hafnium, thorium, bismuth, and uranium. There are probably others.

Iron is a common element in the earths crust. In minerals, it assumes

one of two oxidation states, either the +2 or ferrous state, or +3,

ferric. Its sometimes important to be able to determine which of the two

states are present. Often both are, and if both, then its good to get

some idea of the ratio. This ratio is destroyed if the sample is

dissolved in nitric acid, since nitric acid is a strongly oxidizing acid,

all iron in nitric acid solution is in the ferric state, even if it was

originally ferrous. This difficulty does not apply if the solution ismade in hydrochloric acid. Ferric iron can be changed into ferrous iron

by the addition of a reducing agent, such as sodium sulfite. Ferrous iron

can be changed back to the ferric state by adding an oxidizing agent such

as hydrogen peroxide. Because some of the reagents respond only to iron

in one state and not the other, advantage can be taken of these facts to

design a sequence of operations that will give a pretty good idea of the

proportions of each in an unknown sample. Thiocyanate produces a red

color in the presence of ferric iron, but remains colorless if only

ferrous iron is present. If a solution in hydrochloric acid is first

treated with ammonium or potassium thiocyanate (potassium works best)the

red color, if there is any, gives an estimation of the ferric iron

present. If the color increases significantly after adding hydrogen

peroxide, then the change in the color gives an idea of how much ferrousiron was in the sample.

Quinoline Test: (Chamot and Mason) See Quinoline Test under

Cobalt above.

Quinaldine Test: See Quinaldine Test under Cobalt above.Iron

causes the immediate separation of a dark red oily phase and the prompt

separation of dark red, almost black rectangular tabular and skeletal


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crystals, some elongated, as well as elongated blades and rhombohedral

forms. Dendritic clusters predominate. A concentrated solution of iron

turn opaque very quickly. Because of the strong color, this is a very

sensitive test for iron.

Iron (Ferric)

Potassium Ferrocyanide Test: (Chamot and Mason) Potassium

Ferrocyanide and ferric (Fe3+) iron produce Prussian blue.

Ammonium or Potassium Thiocyanate Test: (Chamot and Mason) Either

of these reagents react with a solution of ferric ions to produce a red

color.

The ferrocyanide and thiocyanate tests for iron may fail in the case of

minerals that contain phosphate, fluoride, or borate. Also cobalt,

chromium, nickel and copper interfere.

Iron (Ferrous)

Potassium Ferricyanide Test: (Chamot and Mason) Potassium

Ferricyanide and ferrous (Fe2+) iron produce Turnballs blue.

Orthophenanthroline is an excellent reagent for detecting ferrous

iron.

Lead

Thiourea Test: (Schaeffer)

This test must be carried out on

a solution of the sample innitric acid. Add a small drop of

nitric acid to the sample and a

small lump of thiourea.

Characteristic crystals will

slowly form in the presence of

small amounts of lead. It's

important to observe the form of

the crystals. Other ions may form

other kinds of crystals and if

large quantities of some impurities are present, the crystals may not

form at all. The form of the crystals varies depending on the acidity and

concentration of lead present. Thiourea makes distinctive crystals with

several elements, including gold, platinum, ruthenium, palladium,rhodium, and osmium. Unfortunately, most if not all of these elements can

produce crystals of several different habits, depending on the

concentration, pH, and the nature and amount of interfering ions.

Consequently, interpreting the results of a thiourea test is something of

an art. More so than with most tests, running standards and blanks in

parallel with the test sample is necessary in order to have much

confidence in the results. Its worthwhile to take some extra precautions


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to make sure the thiourea is pure. Thiourea can be purified by

recrystallization from alcohol.

Hydrogen Chloride Ammonia Test: (Chamot and Mason) This test for lead is

also a test for silver and for mercury. In a dilute nitric acid solution

of the sample, add a small drop of hydrochloric acid. A white precipitate

indicates lead, silver, or mercury. Adding a drop of ammonia willdissolve the precipitate if it's silver. If not, then lead or mercury is

indicated. Remove the liquid by touching the edge of the drop with a

piece of filter paper. Then add two drops of water and heat, but not to

boiling. Put a small drop of potassium dichromate solution near the hot

solution and combine the two drops with a glass rod. A yellow precipitate

indicates lead. These tests are sometimes ambiguous because silver and

lead often occur together and sometimes all three can be present. Lead

and mercury chloride are more soluble than silver chloride. This fact can

be exploited by washing the precipitate several times with warm water to

separate the silver from the other two.

Potassium Iodide Test: (Chamot and Mason) Drop a few small crystals

of potassium iodide into the sample solution. Lead will cause a yellowprecipitate.

Magnesium

Sodium Phosphate (dibasic) Test: (Chamot and Mason) To the sample

drop add a few crystals of ammonium chloride, stir and add a few crystals

of citric acid. Warm and stir until dissolved. Add a crystal of disodium

phosphate, warm gently and stir. Put a drop of strong ammonium hydroxide

near the sample and cause the two drops to join using a glass rod.

Ammonium magnesium phosphate slowly develops as dendritic forms, featherystars, and Xs turning into plates and tabular forms. The precipitate can

be recrystallized by decanting, then dissolving the crystals in dilute

hydrochloric acid and precipitating with ammonium hydroxide. This should

be done in order to reduce the chance of false results from interfering

ions. Similar double ammonium phosphates are formed with Fe2+, Mn2+,

Co2+, and Ni2+. Of these only Mn2+ precipitates (partly) in the presence

of citric acid, and then only if the Mn is in high concentration. If in

doubt, decant the crystals, wash with distilled water, and add hydrogen

peroxide. If manganese is present the crystals will turn brown.

In ammoniacal citrate solution, disodium phosphate will completely

precipitate Mg, Ca, Sc, Pb, Au, and the rare earth elements. In addition,

Be, Sr, Ba, Hg, In, U, Zr, and Mn are partially precipitated.

Manganese

Sodium Phosphate (dibasic) Test: (Chamot and Mason) See sodium

phosphate test under magnesium.

Sodium Bismuthate Test: Manganese dissolved in dilute nitric acid

gives a purple color with sodium bismuthate.


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Acetylsalicylic acid test: (Welcher) The reagent must be freshly

prepared by dissolving a 15 grain aspirin tablet in 1 ml of 10 percent

ammonia. Add 0.5 ml of hydrogen peroxide solution. The color developed is

red to reddish brown in the presence of manganese. Iron also produces a

strong color which may mask the results. The color produced with iron is

dark brown at high concentrations and brown to yellow at lowconcentrations.

Ammonium Molybdate Test: (Chamot and Mason) Evaporate the sample

drop to dry without overheating. Place a very small crystal of ammonium

molybdate on the spot and put a drop of water on it. Set aside for a half

hour. The orange crystals produced in the presence of manganese are

markedly dichroic, going from red-orange to a pale yellow color as the

polarization of the light is rotated. Its important not to use too much

ammonium molybdate. If too much is used, the result will be a white

crystalline mass that covers the entire spot, making any red-orange

crystals difficult to see. This is a good test, but not very sensitive.

Manganese concentration needs to be at least two parts per thousand. The

presence of significant amounts of copper, chromium, strontium, titaniumor tungsten can prevent the crystals from developing.

Mercury

Hydrogen Chloride - Ammonia Test: (Chamot and Mason) To a nitric

acid solution of the sample, add a small drop of hydrochloric acid. A

white precipitate forms if silver or mercury or lead are present. Adding

a drop of ammonium hydroxide will cause the precipitate to go back into

solution if it is silver. See notes for the lead test above.

Potassium Iodide Test: (Chamot and Mason) Add a tiny crystal of

copper sulfate to the sample drop. Put a drop of potassium iodide

solution nearby and bring the two drops together in the usual way. Redmercuric iodide indicates the presence of mercury.

Ammonium Molybdate Test: (Chamot and Mason) See the discussion of this

test under aluminum above.

Molybdenum

Dipotassium Phosphate Test: (Chamot and Mason) The test sample must

be strongly acidified with nitric acid, A solution of dipotassium

phosphate is combined in the usual way, and, if no precipitation occurs,

warm the slide gently. Then set the slide aside to cool. Examine at highmagnification. If molybdenum is present, small yellow isotropic

octahedral crystals are formed. If the principal element is tungsten the

crystals will be white. A negative test result does not mean that

molybdenum or tungsten are not present, only that they are not present in

the form of molybdate or tungstate ions. If diammonium phosphate is used

instead of the dipotassium salt, the test is more sensitive, but its

harder to read.


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Nickel

Quinaldine Ammonium Thiocyanate

Test: See the notes for this test above

under Cobalt. Crystals of the nickel

quinaldine coordination compound are

deep garnet red. They are rarely seen

however, because conditions must be

perfect, and even then theyre very slow

to form. Dark, almost black drops of oil

separate out on addition of the reagent.

The initial appearance is similar to the

oil droplets seen when the sample contains copper.

Dimethylglyoxime Test: (Schaeffer) (Lange) This reagent forms a

bright red precipitate in the presence of nickel. Make the sample

alkaline with a drop of ammonium hydroxide. Put a drop of saturated

dimethylglyoxime in water near the sample, and combine the two drops with

a glass rod. A deep pink or magenta precipitate indicates nickel. It

might be necessary to warm the sample.

Ammonium Molybdate Test: (Chamot and Mason) See the discussion of this

test under aluminum above.

Osmium

Thiourea Test: (Chamot and Mason) Add a few crystals of thiourea

to the sample dissolved in concentrated hydrochloric acid. In thepresence of osmium, a red color develops immediately and, over time,

red crystals form.

Palladium

Dimethylglyoxime test: (Smith) Dimethylglyoxime forms a yellow

precipitate with palladium under acid conditions which is soluble in a

solution made basic by ammonia.

Thiourea Test: (Chamot and Mason) Adding a few crystals of thiourea

to a drop of the sample in concentrated hydrochloric acid causes an

orange or yellow region to develop around the crystals, the outer edge of

which is crystalline. In concentrated solutions of palladium, the orange

crystals form closely around the reagent crystals and prevent them from

dissolving.


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Platinum

Potassium Chloride Test: (Schaeffer) Octahedral crystals of

potassium chloroplatinate form when a drop of potassium chloride solution

is joined to a drop of a sample solution containing chloroplatinic acid.

This is the form produced by the action of aqua regia on platinum.Rubidium chloride can also be used in place of the potassium chloride.

The rubidium chloride test is more strongly colored.

Thiourea Test: (Chamot and Mason) Adding a crystal of thiourea to

a solution containing chloroplatinic acid causes a yellow reaction

followed by reddish brown feathery dendrites.

Potassium

Uranyl Acetate Test: (Schaeffer) Characteristic crystals of

potassium uranyl acetate are formed in the presence of potassium.

Tartaric Acid Test: (Schaeffer) Tartaric acid causes crystals

typical of potassium acid tartarate to precipitate if potassium is

present. Ammonia must not be present for this test to work. Add a little

sodium hydroxide solution and warm the slide first to remove it. Then

make the test for potassium.

Chloroplatinic acid Test: (Schaeffer) To use this reagent, ammonium

must not be present. Add a little sodium hydroxide solution and warm the

slide first to remove it. Then place a drop of chloroplatinic acid

solution near the sample, and proceed in the usual way. In the presence

of potassium, characteristic crystals will form. The test can also beused in the same way to test for the presence of ammonium. If a mixture

of ammonium and potassium is suspected, use the hanging drop method to

trap the ammonia in a drop of water. Then test the water drop separately.

Ruthenium

Thiourea Test: (Chamot and Mason) This test works only on a

sample dissolved in concentrated hydrochloric acid. The sample

solution should not be too darkly colored, if it is, dilute it with

concentrated hydrochloric acid. Add several small crystals of

thiourea to the test drop. Warm the slide gently. Over time, a blue

color will develop in the presence of ruthenium.

Silver

Hydrogen Chloride Ammonium Hydroxide Test: (Schaeffer) To a

nitric acid solution of the sample, add a small drop of hydrochloric

acid. A white precipitate forms if silver, lead or mercury are present.

The silver precipitate will dissolve in ammonium hydroxide.


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Sodium

Uranyl Acetate Test: (Schaeffer) This test is conducted in the

usual way. Characteristic crystals of sodium uranyl acetate will form in

the presence of sodium.

Fluosilicic Acid Test: The initial step of this procedure is to create a

drop of water containing fluosilicic acid. Mix powdered silica sand with

powdered calcium fluoride and put it in a small lead dish. Add a drop of

concentrated sulfuric acid. Then put a hanging drop of water over it. It

works well to use a cover slip coated with a very thin film of something

hydrophobic such as stopcock grease. Silicon hexafluoride is evolved from

the acid mixture and is trapped in the drop of water where it breaks down

forming silicic acid which separates out, and fluosilicic acid which

remains dissolved in the water. After a few minutes, lift the cover slip

and touch the drop to a microscope slide. Then put a drop of the sample

solution nearby, and cause the two drops to join using a thin glass rod.

Set the slide aside for several minutes while the water evaporates. If

sodium is present, hexagonal crystals of sodium fluosilicate, some

looking like little flowers, will appear beginning near the edges of the

drop. These crystals have a very low index of refraction so they may be

difficult to see. If necessary, let the slide dry completely and examineit with a high powered objective. Often the crystals appear as small

prisms lying on their sides with irregular terminations.

Strontium

Sodium Bicarbonate Test: (Chamot and Mason) See the sodium

bicarbonate test under Calcium above. Strontium carbonate looks much

the same as calcium carbonate at first, but does not form the double

salt, and after standing for a while it forms small acicular tufts of

crystals attached to the slide near the test reagent. These are easy to

overlook.

Tin

Potassium Iodide Test: (Chamot and Mason) A yellow to reddish

orange precipitate is formed with the addition of a solution of potassium

iodide to a sample containing stannic tin (+4). If the sample contains

stannous ions (+2) the precipitate is a lighter yellowish white which

changes to orange in the presence of an excess of potassium iodide.


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Stannic Tin (Sn+4)

Cesium Chloride or Rubidium Chloride Test: (Schaeffer) (Chamot and

Mason) These tests are conducted in the usual way. Small crystals

characteristic of tin can be recognized. It is difficult to have much

confidence in this test, since the reagents form insoluble crystals witha number of other ions. If the sample is first treated with nitric acid

and evaporated to dryness on a double boiler several times before making

the test, all the tin will be converted to an insoluble hydrous oxide,

which can be washed several times again with dilute nitric acid. This

removes many of the interfering ions. Then the insoluble oxide can be

dissolved in hydrochloric acid and tested for tin as described above.

Stannous Tin (Sn+2)

Oxalic Acid or Alkali Oxalate Test. (Chamot and Mason) The addition

of oxalic acid or a solution of an alkali oxalate causes a precipitate of

irregularly shaped crystals. Prisms, if formed, exhibit either parallelextinction or, if twinned, an extinction angle of approximately 15

degrees to the direction of elongation.

Titanium


Cobalt above.

Tungsten

Dipotassium Phosphate Test: (Chamot and Mason) See Dipotassium

Phosphate Test under Molybdenum above.

Vanadium


Cobalt above.

Quinaldine Test: See Quinaldine Test under Cobalt above.

Uranium

Sodium Fluoride Bead Test. This test is incredibly sensitive,

however it is not very specific to uranium. A sodium fluoride bead is

made by heating a loop of platinum wire until it is red hot and

touching it to some sodium fluoride so that a little of it adheres to

the loop. This is re-heated until fused, and the bead is built up in

increments until it reaches the desired size. Then the hot bead is

used to pick up a bit of the pulverized sample and heated thoroughly

until its completely fused. Let the bead cool, and examine it under


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an ultraviolet light source. If uranium is present, the bead will

glow brightly with green fluorescence. Because there are other

elements that can also cause fluorescence, this test should be

followed up with a confirmatory test. There is a lot of old

literature devoted to bead tests. They are still some of the most

useful of field tests. With some practice its possible to glean agreat deal of information from them.


Cobalt above.

Potassium Oxalate Test: This is not an especially sensitive test

for uranium, but because the crystals produced are dichroic, its fairly

definitive. A large drop of sample containing something on the order of a

half a milligram of uranium in dilute nitric acid is combined with a

similarly sized drop of saturated potassium oxalate solution. Crystals of

oxalic acid are immediately precipitated. Over time these re-dissolve

leaving small pale yellow rectangular prisms and tablets of the uraniumcompound. These are dichroic, going from pale yellow to colorless as the

polarization of the light is turned through 90 degrees. The crystals are

small and require an hour or two to develop. After three or four hours

they will be large enough to easily determine their dichroic character.

The best crystals seem to develop near the edges where the two drops come

together.

Zinc

Potassium Mercuric Thiocyanate

Test: (Schaeffer) (Chamot and Mason) Put

a small drop of potassium mercuric

thiocyanate solution near the sample

drop. Bring the two drops together with

a thin glass rod and watch for the

characteristic crystals that indicate

Zinc.

The photos show crystals of zinc mercuric

thiocyanate. These are typical of the

crystals that form after adding a

solution of potassium mercuric

thiocyanate to a sample containing zinc.

The graceful branching is typical of a

solution with a high concentration of

zinc.

Sodium Bicarbonate Test: (Schaeffer) Expose the test drop to

ammonia fumes long enough to make it alkaline, or add a small drop of

sodium hydroxide solution, then join with a drop of saturated sodium


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bicarbonate solution. Watch for the formation of characteristic crystals

that indicate zinc. The reaction begins as a slightly milky area where

the two drops join, and the crystals grow slowly. Avoid stirring the drop

when adding the baking soda. If it is agitated, the crystals that form

may be too small to be seen even at maximum magnification.

Zirconium

Quinoline Test: (Chamot and

Mason) See Quinoline Test under

Cobalt above.

Quinaldine Test: See

Quinaldine Test under Cobalt

above. Red crystals of the

quinaldine zirconium complex look

like little red footballs. They

tend to be a little slow in

forming and develop from a lightcolored oil that separates on

addition of the reagent. This is

another one thats a little

tempermental. Conditions must be just right for the crystals to develop,

and often they dont.


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ANIONS

Borate

Curcumin Test: (Smith) (Chamot and Mason) Use an alcoholic solution

of curcumin (0.5 percent). This test works well on paper. First put thespot of unknown on the paper and let it dry. Then put a spot of sodium

fluoride solution over the spot and let it dry. Then add a drop of dilute

hydrochloric acid. Let the spot evaporate until almost dry. Then add a

drop of alcoholic solution of curcumin (0.5 percent). A red color

indicates boron or beryllium. Then hold the test strip over a bottle of

ammonia so the fumes can reach it. If the spot turns blue, boron is

indicated. Titanium, columbium, molybdenum, tantalum, and zirconium

interfere.

Bromide

m-phenylenediamine or aniline Test: (Schaeffer) Either of these

reagents can be used to demonstrate the presence of bromide ions. Theprocedure involves the use of the hanging drop technique to trap the

volatile bromine as it is released from the sample. First, place a small

ring about 2 mm in thickness around the sample drop. Add several crystals

of potassium dichromate to the sample and warm it until it's dry. Then

add a drop of concentrated sulfuric acid. If bromide ions are present in

the sample, free elemental bromine will be evolved from the reaction.

This bromine must be trapped in a solution of the reagent by placing a

cover glass with a droplet of a solution of m-phenylenediamine (sulfate

or hydrochloride) in the middle over the reaction mixture, supported on

the ring. After a few minutes, small crystals characteristic of the

tribromo derivative of the test reagent will appear on the underside of

the cover glass. Aniline works the same way.

Chloride (in the absence of fluoride)

Chromyl Chloride Test: (Schaeffer) This is an indirect test for

chloride ions. Add potassium dichromate to the test solution and

evaporate to dryness. Then add a small drop of concentrated sulfuric Acid

to the sample. Set up a hanging drop of water to catch any gas evolved by

the reaction. Allow to stand for a few minutes, and retrieve the hanging

drop. Evaporate it to dryness and add a small drop of water to the dry

residue. Add a small crystal of silver nitrate. A red precipitate of

silver chromate establishes the presence of chloride ions in the sample.

For this test to work, fluoride ions must not be present. Bromide and

iodide ions do not interfere. The reason this test works to identify

chloride is that the mixture of sample containing chloride mixed withpotassium dichromate and treated with sulfuric acid produces chromyl

chloride which is trapped by the hanging drop, where it decomposes to

chromic acid and hydrochloric acid. Evaporation to dryness leaves only

the chromic acid which reacts with the silver nitrate to produce red

silver chromate. if there are no chloride ions in the original sample,

then there will be no chromic acid, and thus no silver chromate.


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Chromate or Dichromate

Silver Nitrate Test: (Chamot and Mason) (Schaeffer) Acidify the

sample with nitric acid. Place a drop of 2 percent silver nitrate

solution close by and combine the two drops. Dark red crystals of silver

chromate form if the sample contains chromate or dichromate ions.

Lead Acetate Test: (Smith) The same procedure using lead acetate

instead of silver nitrate produces a yellow precipitate in the presence

of chromate or dichromate.

Fluoride

Sulfuric acid and silica Test: (Schaeffer) This is another test

that involves the hanging drop technique to catch the reaction product in

a drop of water. The sample is mixed with some pulverized silica sand and

a drop of concentrated sulfuric acid is added. Silicon tetrafluoride gasis evolved if a fluoride is present. This is collected in a hanging drop

of water where the silicon tetrafluoride breaks down into silicic acid

and fluosilicic acid. The silicic acid is insoluble and forms a

precipitate, while the fluosilicic acid remains in solution. It can be

detected by converting it into its insoluble sodium salt by the addition

of a few crystals of sodium chloride. Compare the fluosilicic acid test

for sodium.

Halides (other than Fluoride)

Silver Nitrate Test: (Schaeffer) The presence of a precipitatewhen the sample is combined with a drop of silver nitrate solution

indicates chloride, bromide or iodide.

Iodide

Potassium nitrite and starch Test: (Schaeffer) Potassium (or

sodium) nitrite is an oxidizing agent that releases free iodine from a

mixture containing the iodide ion. Put a few crystals of potassium

nitrite in the test drop together with a few grains of starch. The starch

grains will turn blue if iodine is present. A drop of hydrogen peroxide

to which a little hydrochloric acid has been added can be used instead of

the potassium nitrite. Bleach also works well for this test. As

confirmation, adding potassium or sodium sulfite reduces the iodine back

to iodide, causing the blue color to disappear.

Phosphate or Arsenate

Ammonium Molybdate Test: (Smith) Add a drop of ammonium molybdate

reagent, and a 1 drop of concentrated nitric acid. Warm the slide. A

yellow precipitate indicates phosphate or arsenate.


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Selenium

Hydroquinone Test: (Chamot and Mason) Hydroquinone is a reducing

agent that works well to detect the presence of selenium and tellurium.

The sample, dissolved in nitric acid is placed on a slide and evaporated

to dryness without overheating. The spot is covered with sulfuric acid

and heated until dense fumes of sulfur trioxide begin to come off. Theslide is cooled and another drop of sulfuric acid is added, then separate

the clear solution from any insoluble material. This can be problematic.

Glass fiber microfilters work well, if you have them, otherwise decanting

the drop is about the best that can be done. Let the drop settle for a

long time and then decant very slowly. The clear drop is then heated

again until sulfur trioxide fumes begin to come off. Let the drop cool,

and then combine it in the usual way with a saturated drop of

hydroquinone dissolved in sulfuric acid. Warm the slide gently. Selenium

will separate as a brown or red precipitate. After a few minutes, decant

the clear liquid from the selenium precipitate, and put the drop on a

fresh slide. Heat the drop again until the dense fumes of sulfur trioxide

begin to come off. Tellurium, if present will precipitate as black

bundles and aggregates. Hot sulfuric acid is dangerous. Drops tend tospread out when hot and its a littledifficult to keep things together.

For this reason, this test would probably work better carried out on a

small watch glass, or something that has a shape that helps to keep the

drop in one place.

Silicate

The same reaction that is used to detect fluoride can be used to

test for silicon. The hanging drop setup is used. Concentrated sulfuric

acid is added to a mixture of the unknown material with calcium fluoride.

Any gas that is evolved is trapped in a hanging drop of water. After a

few minutes, if silicon is a major component of the sample, a precipitate

of silicic acid will be visible in the water drop. If the water issubsequently treated with a few crystals of sodium chloride, insoluble

hexagonal crystals of sodium fluosilicate will confirm the presence of

silicon. This test gives a false positive if carried out in the presence

of glass. A small lead dish works well. A glass cover slip can be coated

with a film of stopcock grease to prevent the hydrogen fluoride from

attacking it, or a plastic cover slip can be used.

Sulfate

Barium Chloride Test: (Chamot and Mason) Use the normal procedure.

A white precipitate indicates sulfate.

Tellurium

Hydroquinone Test: (Chamot and Mason) See Hydroquinone Test under

Selenium.


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ENHANCED SPOT TESTS

CHEMICALS

70% Isopropyl Alcohol

8 Hydroxyquinoline

Alizarin

Aluminon

Curcumin

Dimethylaminobenzyledene Rhodanine

Dimethylglyoxime

Sodium Sulfide

Quercetin

Rhodamine B

You don't need all of these, just the Isopropyl alcohol (available at any

drug store) and one or two of the other reagents for making the spotsvisible. 8-hydroxyquinoline and alizarin are the best. They are both

very versatile reagents. The colors that develop are often unique for a

given ion, and some ions produce spots that show fluorescence under

ultraviolet light. Curcumen and quercetin are also pretty good and

they're a lot cheaper, since they're both available at health food

stores. They also show fluorescence with certain ions.

There are a number of reagents that react to a wide variety of ions with

colors that, in many cases, are diagnostic. They can be used as simple

spot test reagents, but the chance for success can be increased by using

the following method to spread the sample over a wider area and separate

the different ions somewhat using a technique borrowed from

chromatography.

Chromatography is a method that has evolved into one of the main

technologies both for detection and for the separation of compounds that

are mixed together. There are many variations on the method, but the one

that seems most useful for the basement scientist uses paper as the

support. There are a lot of possibilities for the mobile phase. The best

solvent system for a given application has been the subject of a lot of

research. Isopropyl alcohol is not optimum, but it has the virtue of

being easily available and seems to work reasonably well. It's also

fairly non-toxic, which is always an important consideration.

It may be stretching the definition a little to call this method

chromatography. It's really just a spot test, with a slight enhancementborrowed from chromatography. Before applying the reagent that develops

the color, the sample spot is caused to spread across a region of the

paper by allowing the isopropyl alcohol to climb the length of the paper

by capillarity. Since different ions in a mixture in the sample have

different solubilities in the isopropyl alcohol (and differing affinities

for the paper), they will move along the paper at different rates. A

particular ion then can be recognized by how far along it moves, and by

the color it develops with a sprayed on reagent. Spreading the test spot


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across a region of the paper overcomes many of the problems of

interfering ions by moving each ion to a different part of the paper

before adding the reagent to detect it. Resolving two ions that may be

eclipsing one another can sometimes be accomplished by using a longer

strip of paper, or by trying a different solvent. Arthur Ritchie's book

"Chromatography in Geology" contains a lot of helpful information.

You'll need a a stock of chromatography paper. It comes in various sizes.

Tests of the method were made with pieces 1x4 inches cut from larger

sheets. Precondition the strips to be used by soaking them in 70 percent

isopropyl alcohol (or whatever solvent youre using) for several hours

and then let them dry before use. Cleanliness is very important. Don't

let fingerprints get on the paper. About a half inch from one end of the

paper, place a spot of the sample solution made by dissolving a small

crystal (5 to 20 milligrams) of your unknown mineral. The spot should be

approximately a quarter inch in diameter. Let the spot dry. Prepare a

jar that is large enough for the paper to stand in without touching the

sides. Fabricate a way to hang the paper in it so that the bottom end of

the paper is about a half inch above the bottom of the jar, and the sides

of the paper do not touch anywhere else. Then put about a half inch of 70percent isopropyl alcohol in the jar and hang the paper that you prepared

with the spot down so that the end just dips into the alcohol. It must

not go deeply enough that the spot is beneath the surface of the alcohol,

or the test will not work. Cover the jar and watch as the alcohol rises

up the paper. The front of the solvent should rise fairly evenly up the

paper over a time of several minutes until it reaches somewhere near the

top. The time required will depend on the kind of paper used. Some papers

are very fast, others may take an hour or more. Don't let the solvent

front reach all the way to the top. When it's ready, remove the paper and

hang it up to dry. After it's dry, spray the paper with a developer made

from one of the reagents described below. Let it dry again. The

positions and colors that will be on the paper will depend on what ions

were present in the spot that you applied, and the type of spray reagentused. The possibilities are many, and this is both the power and the

weakness of the method. The interpretation of the result is strictly

empirical. To determine whether a sample contains, for example, gold,

compare it with a reference strip that was made with a sample known to

contain gold. Ideally, the reference strip should contain about the same

amount of gold that the unknown has, in order for them to look the same.

Even when they don't look exactly the same though, the colors and the

distance moved, expressed as a percentage of the distance moved by the

solvent front, will be the same, or nearly so. This can be an extremely

sensitive test. There is no right or wrong way to do it. The important

thing is to keep the paper clean, and do enough of them that you develop

a system that works for you.

There are a lot of chemicals that can work as developers. Below is a

list of several, and the ions that they are sensitive to. Reagents

can also be applied by dipping the paper in them, but spraying works

better. There are some really cute little chromatography sprayers

available on ebay from time to time.


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8 Hydroxyquinoline 0.5 percent in ethyl alcohol Al, Ag, Au, Ba, Be, Bi,

Ca, Cd, Co, Cr, Cs, Cu, Fe, Ga, Ge, Hf, Hg, In, K, La, Li, Mg, Mn, Mo,

Nb, Ni, Pb, Pd, Pt, Rb, Sb, Sc, Sn, Sr, Ta, Th, Ti, Tl, U, V, W, Zn, Zr

Alizarin: A saturated solution in ethyl alcohol Al, As, Bi, Ce, Cr, Cs,

Cu, Fe, Hg, In, Li, Mg, Mn, Pb, Sb, Ta, Th, Ti, V, W, Y, Zn, Zr

Aluminon: 0.1 percent in 1 percent ammonium acetate in water Ac, Ag, Al,

Ba, Be, Ca, Ce, Cr, Cu, Eu, Ga, Ha, In, La, Li, Mg, Mn, Nd, Ni, Ti

Curcumin: 0.1 percent in ethyl alcohol Ag, Al, Au, B, Be, Cr, Cu, Fe, Li,

Ni, Pt, Ta, Ti, V, W, Zr

Dimethylaminobenzyledene Rhodanine 1 percent in ethyl alcohol Al, Au, Ag,

Co, Cu, Fe, Hg, Li, Mn, Ni, Pb, Pd, Pt, Ta, Ti, V, W, Zn

Dimethylglyoxime 1 percent in ethyl alcohol Al, Cu, Fe, Co, Li, Ni, V, W,

Zn

Sodium Sulfide 0.5 percent in water Au, Cd, Co, Cu, Fe, Hg, Mn, Ni, Pb,

Pd, Pt, Zn

Quercetin 0.2 percent in ethyl alcohol Ag, Al, Bi, Ca, Cd, Co, Cr, Cu,

Fe, Hg, Mg, Mn, Ni, Pb, Sb, Sn, U, Zn

Rhodamine B 0.1 percent in ethyl alcohol Ag, Au, Cu, Fe, Ni, Pt, Sb, V, W

After development, exposing the paper to ammonia fumes will sometimes

enhance the picture and sometimes not. Also, viewing them under

ultraviolet light can reveal features that otherwise are not visible.


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BIBLIOGRAPHY

1. "Identification and Qualitative Chemical Analysis of Minerals" by

Orsino C. Smith 1953

2. "Microscopy for Chemists" by Harold F. Schaeffer 1953

3. "Chromatography in Geology" by Arthur S. Ritchie 1964

4.

Handbook of Chemical Microscopy Chamot and Mason 19405. Handbook of Chemistry (9th Edition) Norbert Adoplph Lange Ph.D.

1956

6. Organic Analytical Reagents (Volume 2) Frank J. Welcher Ph.D.

1947

7. The Merck Index (11thEdition) Susan Budavari Editor 1989

EXPERIMENTAL

SAMPLE: Approx 20 mg sample of heulandite. PROCEDURE: Pulverized sample

in a small mortar. Added 1 drop of concentrated nitric acid. Continued

grinding for a minute or two. Added 1 drop of water. Grind some more andtouch the pestle to a microscope slide, leaving a small drop with quite a

bit of undissolved material suspended in it. Placed a small drop of 0.1

percent aqueous aluminon beside it. Used a glass rod to cause the two

drops to touch. Over a period of several minutes the point where the two

drops meet developed a pink color that spread across to eventually cover

the entire reagent drop. The color persists after adding a drop of

ammonia. CONCLUSION: This test is positive for aluminum. DISCUSSION:

Heulandite is approximately 9 percent aluminum. This experiment

demonstrates the sensitivity of aluminon as a reagent for the detection

of aluminum.

SAMPLE: Approx 25 mg crystal of heulandite. PROCEDURE: Ground the sample

with about twice the volume of calcium fluoride. Placed in a small leaddish with a drop of sulfuric acid. Placed a cover slip with a hanging

drop of water over the sample, supported on a plastic ring about 2 mm

thick. The hanging drop spread out and ran under the plastic ring, but

did not run down into the acid solution because of the hydrophobic nature

of the plastic ring. Over several minutes gas bubbles evolved from the

sulfuric acid and a residue accumulated on the cover slip in a ring

around the inner edge of the plastic ring. It appears to be salicic acid.

CONCLUSION: This test demonstrates a positive test for a silicate

mineral.

SAMPLE: Approximately 50 mg piece of phosphate rock containing mostly

strengite with some rockbridgeite. PROCEDURE: Pulverized the sample with

a drop of concentrated nitric acid. After a minute or so, added a drop ofwater. Grind more, and allow to stand for a few minutes. Touch pestle to

a slide, leaving a small drop with a little solid material. Placed a drop

of 1 percent ammonium molybdate nearby and let the two drops flow

together. There was no reaction immediately. Placed the slide on a low

heat source (a small transformer). After a few minutes the drop, still

wet, shows a border of yellow material visible against a white paper

background. CONCLUSION: Test is positive for phosphate.


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SAMPLE: Approximately 50 mg piece of phosphate rock containing mostly

strengite with some rockbridgeite. PROCEDURE: Pulverized the sample with

a drop of concentrated nitric acid. After a minute or so, added a drop of

water. Grind more, and allow to stand for a few minutes. Sample stands

for 5 to 10 minutes. Touch pestle to a slide, leaving a small drop with a

little solid material. Placed a drop of 9 percent ammonium molybdate

nearby and let the two drops flow together. A yellow color developsimmediately at the interface between the two. Put the slide on a low heat

source (a small transformer). A yellow crystalline mass develops as the

drops proceed to dryness. CONCLUSION: Test is positive for phosphate.

DISCUSSION: The stronger ammonium molybdate reagent develops a stronger

yellow residue, as expected, however, it was not difficult to conclude

that the test was positive, even with the 1 percent reagent.

SAMPLE: Blank. PROCEDURE: Placed about 50 mg of calcium fluoride in a

small lead dish with a drop of concentrated sulfuric acid. A cover slip

with a hanging drop of water was placed over the sample. No bubbles were

observed coming from the sulfuric acid. The hanging drop spread out and

ran under the edges, but did not run down into the acid. Over time the

cover glass appeared to have a white film on the underside where thewater drop was. The cover slip was removed and inverted on a piece of

black paper. There appears to be a residue of salicic acid where the

water drop was. The amount is much less that it was in a similar

experiment in which the sample contained heulandite. CONCLUSION: This

test could be interpreted as a false positive for silicate in a sample.

It is significant that there was no visible evolution of gas bubbles from

the acid. When silicate is present, Silicon tetrafluoride gas bubbles are

distinctly visible, and probably should be part of the criterion for a

positive result. The salicic acid on the cover slip in this case was

probably from the hydrofluoric acid attacking the glass of the cover

slip. A plastic cover slip might work better for this test or a glass one

covered with a thin film of stopcock grease.

SAMPLE: Approximately 25 mg piece of aurorite. Procedure: The sample was

ground together with a small drop of concentrated nitric acid for several

minutes. The sample dissolved almost completely. The reagent solution was

prepared by dissolving an aspirin tablet in a milliliter of ammonia

solution and adding a half milliliter of strong hydrogen peroxide. A

small drop of the sample solution was transferred to a microscope slide

by touching the pestle to the slide. A reagent drop of similar size was

placed nearby using a small glass rod, and the drop was carefully moved

until it just barely touched the sample drop. A dense light gray

precipitate developed immediately at the interface between the two drops

and faint but clearly visible red streamers developed along the reagent

side of the precipitate. The streamers slowly spread and intensified over

time lending a pink cast to the solution. After a few minutes theprecipitate turned a dirty brown. CONCLUSION: Test is positive for

manganese. The red color was weak but unmistakable against a white paper

background at first, but faded to a light brown as it spread.

SAMPLE: About 100 mg piece of aurorite. PROCEDURE: Sample is ground with

a drop of nitric acid, and the solution is diluted with a drop of

distilled water. A small drop is placed on the slide near a drop of a

solution of oxalic acid, made by mixing a few crystals with a drop of


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water on the slide. The oxalic acid solution is then connected to the

sample solution using a glass rod. A white precipitate develops

immediately at the interface between the drops. CONCLUSION: The test is

positive for calcium. DISCUSSION: The results might be due to the

presence of strontium or barium. PROCEDURE: Another drop of sample

solution was placed on a fresh slide, and tested with sodium chloride

solution. The solution remained clear of any precipitate. CONCLUSION:Negative for silver, mercury and lead. DISCUSSION: Silver sometimes

occurs in aurorite, but is absent from this specimen.

PROCEDURE: Another drop of test solution was placed on a slide and caused

to join with a small drop of ammonium thiocyanate solution. No color was

observed. CONCLUSION: Negative for iron. DISCUSSION: At this point, it

has been established that the sample of aurorite probably contains

manganese and calcium, and that it does not contain significant amounts

of silver or iron.

SAMPLE: Approx 50 mg of picotite. PROCEDURE: The steps taken were similar

to the ones described above. The sample was macerated in a drop of nitric

acid and a drop placed on a microscope slide. A similar drop ofacetylsalicylic acid reagent was placed nearby and carefully encouraged

to join. There was a heavy white precipitate at the point where the two

drops joined, and a yellow hue developed in the reagent drop over the

following minute or so. CONCLUSION: Negative for manganese, positive for

iron. DISCUSSION The absence of a red color is consistent with the fact

that picotite does not contain manganese. This yellow color appears to be

due to iron in the sample.

SAMPLE: Approx 50 mg of picotite. PROCEDURE: The sample was ground

together with a drop of nitric acid for one minute. A small drop was

transferred to a microscope slide. Another small drop of aluminon reagent

was placed next to it, and the two drops caused to touch. A red streamer

reached immediately into the reagent drop. Color persists after adding adrop of ammonia. CONCLUSION: Test is positive for aluminum.

SAMPLE: Approx 50 mg of picotite. PROCEDURE: The sample was ground

together with a drop of nitric acid and allowed to stand for some time. A

small drop of the nitric acid solution is placed on a microscope slide

and a small lump of thiourea is placed into it. A reddish brown color

spreads out from the thiourea as it begins to dissolve. Over time, the

red color fades. No crystals are observed to form. CONCLUSION: Test is

negative for lead. DISCUSSION: This test can also be taken to imply that

thallium is absent, since it is also known to form crystals with

thiourea.

SAMPLE: A drop of solution known to contain zinc ions. PROCEDURE: A dropof sample solution was placed on a slide and joined to a nearby drop of

saturated potassium mercuric thiocyanate solution. Within a minute, small

crosses were beginning to become visible. These grew and developed a fine

branching structure, taking on a feathery appearance, eventually becoming

like round fluffy snowflakes. CONCLUSION: Test is positive for zinc.

DISCUSSION: The published description for this test is confirmed in this

experiment. This test is also sensitive to cadmium, and produces crystals


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of a different form in the presence of cadmium, or a cadmium and zinc

mixtures.

SAMPLE: A drop of solution made from dissolving a crystal of bertrandite

in dilute hydrochloric acid. PROCEDURE: A drop of potassium oxalate

solution was made on the slide by the following method. A small quantityof oxalic acid was placed on the slide and several small drops of

potassium hydroxide solution were mixed into it and stirred. After a few

minutes, most of the oxalic acid dissolved. The pH of the drop was

measured to be about 6 to 7. A drop of the solution was decanted from the

solid oxalic acid remaining and this was caused to join the drop of

sample. Then a glass rod was touched to a solution of mercuric nitrate,

and most of the adhering drop of mercury ions was transferred from the

glass rod to an unused part of the slide, so that very little of the

mercury solution remained on the glass rod. Then the glass rod was

lightly touched to the point where the sample drop joined the potassium

oxalate drop, and removed without stirring the drop. Over the next minute

or two, crystals separated from the solution, mostly rhombs, and some

prisms. The slide was placed under a polarizing microscope, and theextinction angle of several crystals was measured. Some of the crystals

measured around 44 degrees, and some measured 38 degrees. CONCLUSION: The

test is positive for beryllium, based on the presence of crystals with a

measured extinction angle of 38 degrees. DISCUSSION: It is apparent that

not all of the crystals are the double oxalate salt of potassium and

beryllium, but some of the observed crystals fit the criterion for this

salt, and therefore the conclusion that beryllium is present is

supported. This test is a little difficult, and the first attempt to

perform it failed. Making the potassium oxalate in situ in the manner

described is probably not the best way to perform the test, but was

necessitated by the fact that no potassium oxalate was available.


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SOLUBILITIES OF SOME COMMON MINERALS

Information for the following table was taken, for the most part,

from Orsino Smiths book Identification and Qualitative Analysis of

Minerals. In Smiths tables, minerals soluble in hydrochloric acid

were not tested for solubility in other acids, so subsequent entries

for that mineral under nitric and sulfuric acids will indicate No,

even though the mineral might in fact be soluble in those acids.

Likewise, minerals not soluble in hydrochloric acid, if soluble in

nitric acid, were not tested in sulfuric acid, and so may show an

erroneous No under sulfuric acid. These errors are regrettable, but

hopefully the table will nevertheless be useful if this caveat is

kept in mind. M

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