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Materials Each student will have 1 to 2 spotting plates

*Knowns (all approximately 0.5 M) *Test Reagents

NaCl Na2SO4 0.5 M Mg(NO3)2 0.5 M Cu(NO3)2

NaOH Na2CO3 0.5 M Ca(NO3)2 0.1 M AgNO3

NaNO3 Na3PO4 6 M HNO3 litmus paper (red or neutral)

*Several sets of knowns and test reagents, provided in dropper bottles, will be distributed throughout the laboratory. For the knowns, potassium salts (e.g., KI) may be substituted for sodium salts.

Procedure

You will use a variety of acids and bases (including nitric acid and sodium hydroxide), and all of these are corrosive. As usual, wear safety goggles at all times.

Getting Started You will work with a laboratory partner on this experiment. The knowns and test reagents will be organized on a labeled, plastic-covered sheet. It will be to your own benefit to return bottles to the sheet after each use and to generally keep your work area organized and clean.

Set yourself up with a large collection beaker for used solutions. You should also have a wash bottle of deionized water, and stirring rods.

Types of Tests There will be several types of tests that you can carry out. For litmus tests, you will transfer a drop of sample to a piece of litmus paper, and note any color change. For spot tests, you will simply mix a drop of sample (known or unknown) with a drop of test reagent and then observe what happens – formation of a precipitate, gas evolution, etc. For these tests, you will use a spot plate, a ceramic, glass, or plastic plate with several wells (depressions) in it for mixing the solutions. In some cases, it is possible to carry out secondary tests in the spot plate. For example, if a precipitate forms when a test reagent and an anion solution (known or unknown) are combined, you could add acid directly to the precipitate on the spot plate to see whether or not the precipitate dissolves and whether or not gas is evolved upon dissolution.

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Recording Data for “Knowns” The key requirement for collecting data for the knowns is that the data be organized, clearly labeled, and accurate. A table format such as Table 1 is often very helpful. However, other formats are acceptable as long as they satisfy the requirements.

Testing Knowns

Litmus Tests 1. To carry out a litmus test, place a piece of red or neutral (violet) litmus paper on a clean,

dry paper towel. 2. Add a drop of the known solution. Never dip litmus paper directly into a solution – this

may lead to contamination. 3. Observe the paper's color. A distinctly basic solution will cause either type of litmus to

turn to a purple-blue color. You should only use this test to check for solutions that are distinctly basic and turn the litmus blue – do not rely on any litmus test that produces a subtle color change.

Precipitation/Dissolution Tests 4. Start by spot testing the knowns with the four metal ion test reagents (Mg2+, Ca2+, Cu2+,

and Ag+). Most of the spotting plates have nine wells in a 3×3 pattern. 5. Carry out the spot test by putting one drop of the first

known into each well in a row, the second known into the next row, and a third known into the final row. It may help to place a piece of paper under the spot plate and label it to make sure you can keep track of what is in each column and row. Use a plastic dropper to dispense the knowns, taking care to:

a. Use small drops similar in size to those dispensed by the dropper bottles. To obtain results for unknowns comparable to those for the knowns, it is important to keep the ratio of sample to test solution about the same.

b. Rinse the dropper when switching between different samples. A simple way to do this is to have a small beaker of deionized water ready.

i. Squeeze the dropper bulb, wiping away any solution that comes out, and ii. With the bulb still squeezed, dip the dropper tip into the beaker and draw

up some water. iii. Expel this into a collection beaker, and repeat at least once.

6. Once sample has been distributed to the wells, put one drop of the first test solution into the first well of each row, the second test solution into the second well of each row, and so on. It is important to stir each spot with stirring rod to make sure the solutions mix well.

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a. Use a clean glass stirring rod to stir. Make sure the end of the stirring rod is blunt (no sharp edges) to prevent scratching up the spotting plate.

b. Some precipitates, particularly CaSO4(s), form relatively slowly, and may not form a noticeable precipitate at all if the mixture is not stirred. Failure to stir mixture is a common source of error in this experiment.

c. Observe carefully. Some precipitates (particularly those of Mg2+) are gelatinous and somewhat translucent. It is sometimes helpful to inspect the wells with both a light-colored background (e.g., white paper) and a dark background (lab bench).

d. If (and only if) a precipitate does form, add 1-2 drops of nitric acid and stir it to see if the precipitate dissolves and whether gas bubbles are formed upon dissolution. There is no reason to add acid if no precipitate formed!

e. Each time a precipitate is formed, note (= record on your data sheet) not just that a precipitate formed, but also its color and appearance, whether or not it dissolved in acid, and whether or not gas was formed upon dissolution. It is essential that you record your observations in an organized manner, perhaps in the format similar to Table 1.

7. When you are done with one set of known-test reagent combinations: a. Use a wash bottle to rinse the spots into your collection beaker. b. If some precipitates are sticking to the spotting plate, you can use cotton swabs to

wipe them. Do not use a brush with a metal wire center (e.g., a test tube brush) to clean the spotting plates (the metal can scratch the spotting plates).

c. If your collection beaker gets full, transfer the used solutions to the designated spent chemical container at the fume hood.

Planning At this point, you must review your data to organize it to make sure it is complete, then develop a flowchart. Look to see if your plan will provide a way to identify each anion. You must write out a detailed flowchart similar to the one shown earlier that allows you (or any other person using it) to correctly identify any of the six possible anions.

Below is a very idealized flowchart. The idea is to set up a plan in which you will carry out the first test, Test 1, on your unknown. If the test is negative (e.g., no precipitate forms), the ion must be one of the Group A ions, but the ions of Group B are eliminated. If Test 1 is negative, you go on to Test 2 with the unknown – there is no reason to carry out any of the tests on the right-side branch of the flowchart if Test 1 is negative!

Depending on whether Test 2 gives a positive or negative result, you will know whether the unknown ion is from Group C or D, and then perform either (but not both) Test 3 or Test 4, depending on the result of Test 2. You would continue working down the flowchart, carrying out tests on the unknown, until only one ion is possible.

Once your first unknown anion has been identified, you will then analyze your second and third anions in the same manner. Each will take a different path through the flowchart, e.g., your 2nd

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unknown may give a positive result for Test 1, in which case you would follow the right-side branch of the flowchart and carry out Test 3 next. Each branch of the flowchart will continue until there is a separate “group” for each individual ion.

A few comments:

• The “branches” in the flowchart need not be symmetrical, e.g., your first test might split the possibilities into groups of two and four anions instead of three and three.

• A test may simply be the color of the precipitate in the preceding step, e.g., Test 2 may produce precipitates for two or more ions (Group D), and Test 4 may ask what the color of the precipitate is, e.g., it might branch based on “white/light-colored” versus “brown/dark-colored” instead of “negative – positive.” A precipitation and color test may actually be integrated into one step of a flowchart as indicated on the right.

• You might consider using the litmus test first. For the litmus test, the two results should be “distinctly basic (litmus turns blue)” and “not distinctly basic (litmus does not turn distinctly blue.”

• This analysis is actually “overdetermined”, i.e., there is way more than enough information for its completion, and many different approaches that will work. You probably will not need to use all of the different test reagents. Some flowchart schemes are more efficient than others, but that should not be your primary concern here. Your main concern should be to come up with a logical and workable flowchart based on your own observations.

• You will be required to show your flowchart to your instructor before receiving your unknown sample. Your instructor will check to see that you have set up a flowchart in some reasonable manner, but will not check it in detail to see if it should work properly.

A few words of caution. When devising your flowchart, recognize that there can be variability in what you will observe for any given mixture of chemicals.

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Color Variations You are directed to combine “one drop” each of test and sample solution, but drops are not always the same size, and differences in the ratio of the two solutions may cause the color to shift. Also, the appearance of precipitates may change over time as the particles “age.” This is particularly true of silver precipitates, which are light sensitive and which will start to appear violet to gray upon standing.

Do not base decisions in the flowchart on subtle color differences, e.g., shades of yellow or brown. Generally, precipitation tests and pH tests are more reliable than color distinctions. Use color differences as a last resort, and only when the differences are very pronounced, e.g., a white precipitate versus a brown precipitate.

Slight Precipitation Contamination of knowns and sample solutions by carbon dioxide can also cause some problems. Strongly basic solutions may be contaminated by exposure to carbon dioxide. For example, NaOH solutions exposed to air often contain some carbonate due to the reaction:

2OH% +CO,(𝑔) ⟶ CO$,% +H,O

Carbonate contamination sometimes leads to traces of precipitation when none is expected. CO2 contamination may also occur after the test and sample solutions have been mixed.

Do not rely on reactions that produced slight precipitation (just a few grains of precipitate). Base your flowchart design on reactions that produce distinct, heavy precipitates.

Analyzing Unknowns When you have completed your flowchart, you will receive a set of three unknowns in labeled tubes (ABC), with a sample number. Be sure to record the sample number in the unknown data section of the lab report.

Your instructor will check to see that you have devised a flowchart before giving you a set of samples, but will not check it to verify that it will work properly.

Each unknown will contain a solution of one salt. Each of your three unknowns will contain a different anion.

Proceed with your plan for analysis, recording your observations on your data sheet.

When you are satisfied with your results, turn in your report sheet. Note: While it makes sense to set up a table for recording data for the knowns, a table format does not work as well for the

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unknowns. For the unknowns, simply recording results from each successive test is probably best, e.g.,

Sample # 2187

Sample A 1st test: Added Ca2+ ® no ppt 2nd test: Added Ag+ ® white ppt Added H+ – ppt did not dissolve

3rd test: ....

Cleanup Checklist � Excess unknown solutions should be returned to your instructor. These will be collected in

a rack or box labeled “Used Unknowns”. (Do not pour out the excess!) � Place all chemicals in their proper place on the organizing sheet. � Empty your collection beaker into the spent chemical container in the fume hood. � Clean all glassware (beakers, stirring rods, etc.) and spotting plates used during the lab. � Leave the spotting plates (cleaned!) on the lab bench (not in a drawer). � Leave the collection beaker (cleaned!) on the lab bench next to the chemicals (not in a

drawer). � Wipe any spills from your work area. � Your work area should be left in good condition so that the next group of students can work

properly.