Basic Inorganic Chemistry Lectures (Group I & II)

1

Basic Inorganic Chemistry

PHR 125

Prof. Dr. Mona Bedair

2

Analysis for anions (acid radical) and for cations (basic radical),

the two parts of inorganic qualitative analysis, are carried out separately.

Either part may be attacked first.

Cations are positively charged fragments or ions of salt or

compound. They are frequently referred to as the metals or basic

radicals.

In studying an unknown, a visual examination should come first.

If the unknown is a solution, what color is it ? Color is important

because some inorganic ions reveal their identities through color alone.

If the unknown is a solid, the color is also important, but it does

not necessarily indicate the colors of the individual ions. Thus, Pb2+

and

I- are both colorless, but they combine to give the bright yellow PbI2.

Identification of cations can not be performed by tests on the

original substances (or salts), therefore we have to do some schemes for

the systematic separation of cations.

Flowcharts are used to represent the steps in qualitative analysis,

particularly for the analysis of cations, which is more systematic than the

analysis of anions. A flowchart is a schematic outline that begins with

the ions in question, indicated the reagents and the conditions for each

step, and gives the formulas for the chemical products that result.

In considering qualitative analysis and in carrying out the

procedures in the laboratory, keeping track of the analytical steps is

essential. We recommend that you refer to the flowcharts frequently.

Flowcharts can be written in various ways, in which precipitates or

undissolved solids are carried to the left and ions in solution are carried

to the right.

3

ANALYTICAL SEPARATION

OF CATIONS

4

Common cations (basic radical) may be divided, for purposes of

qualitative analysis, into a number of groups, the members of any group

are precipitated by a particular group reagent. Thus by the addition of a

slight excess of dilute hydrochloric acid to a solution containing all the

common carions, a precipitate is obtained consisting of the chlorides of

silver, lead and mercurous, similarly by the use of the appropriate group

reagents, the remaining cations are separated into different groups.

Generally, analytical separation of cations depends on the varying

solubilities of their chlorides, sulphides, hydroxides and carbonates. The

various groups are summarised in the following Table.

Group Group

Reagent

ions Formula of

Precipitate

I

(Silver group)

Dilute HCl Ag+, Pb

++, Hg2

++ AgCl, PbCl2,

Hg2Cl2

II

(Copper and

arsenic groups)

H2S in presence

of dilute HCl

Hg++

, Pb++

,

Bi+++

, Cu++

,

Cd++

, Sn++

As+++

, Sb+++

HgS, PbS, Bi2S3,

CuS, CdS, SnS,

As2S3, Sb2S3

IIIA

(Iron group)

Aq. NH3 in

presence of

NH4Cl

Al+++

,

Cr+++

,

Fe+++

,

Mn3+

Al(OH)3,

Cr(OH)3,

Fe(OH)3

Mn(OH)3

IIIB

(Zinc group)

H2S in presence

of aq. NH3 and

NH4Cl

Ni++

,

Co++

,

Mn++

,

Zn++

NiS,

CoS,

MnS,

ZnS

IV

(Calcium group)

(NH4)2CO3 in

presence of aq.

NH3 and NH4Cl

Ba++

,

Sr++

,

Ca++

BaCO3,

SrCO3,

CaCO3

V

(Alkali group)

No particular

reagent

Mg++

, Na+, K

+,

NH4+

different ppted.

forms

5

Ag+, Pb

2+, Hg2

2+, Hg

2+, Cu

2+, Bi

3+, Cd

2+, Sb

3+, Sn

2+, As

3+, Al

3+, Cr

3+, Fe

3+, Mn

2+,

Co2+

, Ni2+

, Zn2+

, Ca2+

, Ba2+

, Sr2+

, Mg2+

, Na+, K

+, NH4

+

Cold, dil HCl

Residue Centrifugate

Hg2Cl2, PbCl2, AgCl Hg2+

, Cu2+

, Bi3+

, Cd2+

, Sb3+

, Sn2+

, As3+

, Al3+

, Cr3+

, Fe3+

, Mn2+

,

Co2-

, Ni2+

, Zn2+

, Ca2+

, Ba2+

, Sr2+

, Mg2+

, Na+, K

+, NH4

+

Group I H2S/0.3 M HCl

Residue

Centrifugate

Bi2S3, HgS, PbS, CuS, CdS, Sb2S3 Al3+

, Cr3+

, Mn2+

, Fe2+

, Co2+

,

Ni2+

, Zn2+

, Ca2+

, Ba2+

, Sr2+

, Mg2+

, Na+, K

+, NH4

+

As2S3, SnS NH4Cl/NH4OH

Group II

Residue Centrifugate Al(OH)3, Fe(OH)3, Cr(OH)3, Mn(OH)3 Co

2-, Ni

2+, Zn

2+,

Ca2+

, Ba2+

, Sr2+

, Mg2+

, Na+, K

+, NH4

+

H2S/NH4Cl/NH4OH

Group IIIA

Residue

Centrifugate

CoS, NiS, ZnS, MnS

(NH4)2CO3/NH4Cl/NH4OH

Group IIIB

Residue

Centrifugate

CaCO3, BaCO3, SrCO3

Group IV GroupV Mg2,

K+ + Na

+, NH4

+

Flowchart for separation of cations into

groups

7

Cation Group I

Silver Group

Mercurous Hg22+

Lead Pb2+

Silver Ag+

Cations Group I brings together three metal ions that form

chlorides which are insoluble in acidic solution. The group reagent is

cold, dilute hydrochloric acid, and this group is sometimes known as

the hydrochloric acid group, the chloride group, or the silver group. The

chlorides of all the other cations in our cation analysis scheme are

soluble in acidic solution.

Condition of precipitation

(1) Cold hydrochloride acid is used to prevent the solubility of

PbCl2 in hot solution.

(2) 6M cold hyrochloric acid

Use of a moderate excess of hydrochloric acid to precipitate the

group serves two purposes :

(1) The excess chloride ion encourages the precipitation of group I

(Le Chatelier’s principle) by the common ion effect of the Cl

i.e: (decrease the solubility of the chloride precipitates of group I)

Example : AgCl AgCl Ag+ + Cl

-

soluble

HCl H+

+ Cl

moderate excess c.i.e.

8

(2) Also, if the unknown contains Bi3+

or Sb3+

(from the subsequent

group) the hydrogen ion prevents precipitation of them as

oxychlorides.

Bi3+

+ Cl- + H2O BiOCl + 2H

+

Sb3+

+ Cl- + H2O SbOCl + 2H

+

HCl Cl- + H

+

moderate excess c.i.e.

i.e. Solubility of BiOCl or SbOCl if these cations are present with

group I cations (Bi3+

, Sb3+

) by the common ion effect of the H+.

Too large excess of the acid is avoided :

(1) Dissolves the silver and lead chlorides by complex formation.

Mercury (I) chloride does not dissolve because the chloro

complexes of mercury (I) are very unstable.

Example :

AgCl + Cl- [AgCl2]

-

Large excess Silver dichloride

soluble complex

PbCl2 + Cl- [PbCl3]

- or [PbCl4]

2-

Large excess Lead trichloride Lead tetrachloride

soluble complexes

(2) Also, if the unknown contains Cd2+

and Sn4+

(from the subsequent

groups), the high chloride concentration forms stable soluble

complexes with them and inhibits the precipitation of CdS and

SnS2 by H2S/H+.

Cd2+

+ Cl- [CdCl4]

2-

or Sn4+

+ Cl- [SnCl6]

2

9

Separation of Cation group I

Cold 6M HCl (Procedure 1)


Hg2Cl2, PbCl2, AgCl (If only Cation Group I present,

all white ppt discard. Otherwise save for Cation

H2O, boil (Procedure 2) Group II)


Hg2Cl2, AgCl PbCl2

Conc. NH4OH Divide into 3 portions

(procedure 4) (Procedure 3)

Residue Centrifugate H2SO4 Cool K2CrO4

Hg + HgNH2Cl [Ag(NH3)2]++Cl

-

black white PbSO4 PbCl2 PbCrO4

white white yellow

Dissolve in aqua regia HNO3 until acidic Pb2+

present

Evaporate, test with (Procedure 5)

1- SnCl2 AgCl

2- KI white

Hg22+

present Ag+ present

Flowchart for Analysis of Cation Group I

Hg22+, Pb2+, Ag+

10

Procedure 1:

The solubility product of the precipitates of group I cations are

PbCl2 Ksp 1.6 x 10-5

AgCl Ksp 1.1 x 10-10

Hg2Cl2 Ksp 3.0 x 10-18

Solubility product constants indicate that lead (II) chloride is much

more soluble than the chlorides of mercury (I) or silver (I).

Silver (I) ion (Ag+)

Reactions Important in the Separation and Identification of Ag+

Group Precipitation

Ag+ + Cl

- AgCl

white

Dissolution by Complex Ion Formation

1- AgCl + 2NH3 (aq) [Ag(NH3)2]+ + Cl

-

silver diammine

2- AgCl + 2KCN [Ag(CN)2]- + K

+

silver dicyanide

3- AgCl + xss HCl [AgCl2]- + H

+

silver dichloride

11

Reaction with NaOH

Produces black precipitate insoluble in excess NaOH.

Ag+ + NaOH Ag2O

black

Ag2O + excess NaOH insoluble

(non amphoteric)

Lead(II) ion (Pb2+

)

Its oxide is amphoteric (dissolves in both acid and alkali)

Reactions Important in the Separation and Identification of

Lead (Pb2+)

group Precipitation

cold

Pb2+

+ 2Cl- PbCl2 in group I

Pb2+

+ S2-

/H+ PbS in group IIA

Dissolution

By oxidation of sulphide

PbS + HNO3 Pb2+

+ So + NO + H2O

Reaction with sodium hydroxide

Pb+2

+ NaOH Pb(OH)2

Pb(OH)2 + excess NaOH [HPbO2]- or [Pb(OH)3]

-

Reaction with ammonium hydroxide

Produces white basic salts (ppt) which is insoluble in excess

NH4OH

12

PbCl2 + NH4OH Pb(OH) Cl

Lead hydroxychloride

white basic salt

Confirmatory Tests

1- Pb2+

+ SO42-

PbSO4

white

CH3COOH

2- Pb2+

+ CrO42-

PbCrO4

yellow

The reaction of Pb2+

with chromate gives yellow precipitate in

weakly acidic medium (dil acetic acid).

As if we use strong acidic medium (mineral acids) as dil HNO3, it will

dissolve the yellow precipitate of PbCrO4 and transform it to soluble

dichromate salt.

PbCrO4 + HNO3 PbCr2O7 + H2O

lead dichromate (soluble)

Also if we use strong alkaline medium, the yellow precipitate of

PbCrO4 will dissolve (amphoteric character)

3- Pb2+

+ KI PbI2

yellow

lead iodide

PbI2 + excess KI (PbI4)2-

soluble complex lead tetraiodide

13

Mercurous (I) ion, Hg22+

Mercuric (II) ion, Hg2+

The mercurous ion is precipitated as chloride in the silver group;

the mercuric ion is precipitated as sulphide in the copper-arsenic group.

Each mercury atom has two (6s). The loss of two electrons by a mercury

atom produces the mercuric ion (Hg2+

). The loss of one electron by a

mercury atom gives a mercurous ion (Hg22+

) which is unique among

ions in that it has a single (6s) electron in its outermost orbit. Two of

these ions combine to form the ion (+Hg:Hg

+) or simply Hg2

2+, in which

the two mercury atoms are bonded together by a pair of shared electrons

(a covalent metal-metal bond).

The Hg22+

ion is found in a number of solid compounds. In

aqueous solutions, its existence is limited to the pH region below about 3

to 4. At higher pH values, a disproportionation reaction with water or

hydroxide ion takes place:

H2O

Hg22+

Hgo + Hg

2+

mercury mercuric ion

metal

This also may called self oxidation-reduction reaction.

Therefore precipitation reactions involving mercurous ion are

complicated by disproportionation reactions yielding elemental mercury

and mercuric compounds.

Example :

Hg2Cl2 + 2NH3 HgNH2Cl + Hgo + Cl

-

white black

mercuric mercury

amino chloride metal

14


Mercurous (Hg22+)

Group Precipitation

Hg22+

+ 2Cl- Hg2Cl2

white

mercurous chloride

Complex formation

Hg22+

+ 4CN- Hg

o + [Hg(CN)4]

2-

mercuric

tetracyanide

Hg22+

+ 4I- Hg

o + [HgI4]

2-

mercuric

tetraiodide

Reaction With NaOH (non amphoteric)

Hg22+

+ 2OH- Hg

o + HgO + H2O

mercury yellow

metal mercuric oxide

Confirmatory Test

Hg2Cl2 + 2NH4OH Hgo + Hg(NH2)Cl + NH4

+ Cl

-

black white

mercury mercuric

metal amino chloride


Mercuric (Hg2+)

15

Mercury(II) sulfide (group II) is the least soluble sulfide. It is not

soluble in either dilute nitric acid or hydrochloric acid, but dissolves in

hot aqua regia with the formation of a chloro complex.

Group Precipitation

H+

Hg2+

+ H2S HgS + 2H+

black

mercuric sulphide

Dissolution by :

1- Aqua regia (1:3) Conc. HNO3 : Conc. HCl

Oxidation of S2-

and complex ion formation with mercuric

boil

3HgS + 8H+ + 12Cl

- + 2NO3

- 3[HgCl4]

2- + 4H2O + 3S

o + 2NO

___________________

aqua regia slightly ionized

2- Hypochlorite reagent

HgS + NaOCl [HgCl4]2-

+ SO2

Confirmatory Tests

1- With ammonium hydroxide NH4OH

HgCl2 + 2NH4OH HgNH2Cl + NH4+ + Cl

-

white

2- With stannous chloride reagent SnCl2

HgCl2 + SnCl2 Hg2Cl2 + [SnCl6]2-

white

mercurous stannic hexachloride

chloride

Hg2Cl2 + SnCl2 Hgo

+ [SnCl6]2-

xss. black stannic hexachloride

16

mercury

metal

It is an oxidation-reduction reaction.

3- With potassium iodide

HgCl2 + KI HgI2

scarlet red

HgI2 + KI [HgI4]2-

xss mercuric tetraiodide

soluble complex

Differentiate between mercuric and mercurous.

How can you separate and identify mixture of Hg22+

/ Hg2+

??

17

CATION GROUP II

COPPER-ARSENIC GROUP

Group IIA :

Mercury Hg2+

Bismuth Bi3+

Copper Cu2+

Cadmium Cd2+

Lead Pb2+

Group IIB :

Arsenic As3+ , As5+

Antimony Sb3+ , Sb5+

Tin Sn2+ , Sn4+

18

This analytical group of cations is composed of eight cations

which are subdivided into the copper group consisting of mercuric,

bismuth, cadmium, copper and lead; and the arsenic group consisting of

arsenic, antimony and tin.

The group reagent (precipitating agent) of cation group II is

hydrogen sulphide in acid medium (0.3 M HCl). Thioacetamide may

also be used; it is hydrolysed in acid solution to give hydrogen sulphide

CH3CSNH2 + 2H2O H2S + CH3COONH4

thioacetamide.

H2S 2H+ + S

2-

The real significance of the [H+] as a control of [S

2-] can be seen

by examining the ionization of H2S.

[H+]

2 [S

2-]

Ka = _____________________

= 1.3 x 10-20

[H2S]

A saturated solution of H2S is approximately 0.1 M; therefore, for

a saturated solution of H2S, the expression may be simplified to,

[H+]

2 [S

2-]

Ka = _____________________

0.1

or [H+]

2+ [S

2-] = constant

H2S 2H+ + S

2-

HCl H

+ + Cl

-

c.i.e.

According to the theory of common ion effect, increasing the

concentration of H+ ions in solution of H2S in water, shift the equilibrium

to the left, decreases the dissociation of H2S in H2O and hence decreases

the sulphide ion concentration.

19

Adjustment of the acidity

An increase in [S2-] by decreasing the acidity (< 0.3 M HCl)

results in :

a) Precipitation of sulphides of group IIIB.

b) Dissolution of sulphides of group IIB as thioanions (see group IIB).

A decrease in [S2-] by increasing the acidity (> 0.3 M HCl)

results in :

a) Prevention of the precipitation of CdS, PbS, SnS2 (have higher

solubility products).

b) Formation of stable soluble complexes as [CdCl4]2-

, [SnCl62-

], [SbCl4]-,

so they can not precipitate by addition of H2S.

20

Subdivision Of Cation Group II

into IIA and IIB

Cation group II contains eight elements; this large number is too

difficult to analyse satisfactorily unless it is subdivided into two

subgroups: group IIA (copper group) and IIB (aresnic group). The

subdivision is based on the differences in the electronegativity values.

Thus, the members of the copper group have low electronegativity values

and are, therefore, insoluble in alkali sulphides or alkali hydroxides. On

the other hand, the members of the arsenic group are sufficiently

electronegative to dissolve in alkali sulphides giving thioanions, and in

alkali hydroxides giving both thioanions and oxyanions (amphoteric

sulphides).

N.B. : The electronegativity and the acidity increases by increasing the

oxidation number.

21

Cation group II

1) Biol with H2O2 and expel the excess

2) Adjustment of the acidity

3) Add H2S or thioacetamide

Residue

Suphides ppt of group II

NaOH

or Na2S


ppt of sulphides soluble complexes

of subgroup IIA of subgroup IIB

(continue as scheme for

subgroup IIB separation)

Flowchart for separation of cation group II

to subgroup IIA and II B

22

N.B. :

Stannous sulphide is not amphoteric so, it is incompletely soluble in

either NaOH or Na2S. Therefore, to separate stannous with the

subgroup IIB cation sulphides, we have to oxidize stannous to

stannic (Sn4+

) by boiling with hydrogen peroxide. The stannic (Sn4+

)

has higher oxidation number and higher electronegativity).

HCl

Sn2+

+ H2O2 Sn4+

+ H2O

The excess H2O2 must be decomposed by boiling, otherwise it will

oxidize the H2S reagent to colloidal sulphur which is difficult to separate.

docompose

2H2O2 2H2O + O2

xss strong boiling

H2O2 + H2S So + 2H2O

colloidal

sulphur

23

Copper Subgroup (Cation Group IIA)

Hg2+

, Bi3+

, Cu2+

, Cd2+

, Pb2+

1) adjustment of the acidity

at 0.3 M [H+]

2) add H2S

HgS , CuS

, PbS, Bi2S3 , CdS

black ppt brown ppt yellow ppt

heat with 1:1 HNO3


HgS , HgO Bi3+

, Cu2+

, Cd2+

, Pb2+

dissolve in ethanol, H2SO4

aqua regia

or hypochlorite Residue Centrifugate

PbSO4 Bi3+

, Cd2+

, Cu2+

Test for Hg2+

white

1) SnCl2 dissolve in

2) NH4OH ammonium Conc. NH4OH

3) KI acetate

Test for Pb2+

1- acetic acid + K2CrO4

2- KI


Bi(OH)3 [Cd(NH3)4]2+

colourless

white gelatinous [Cu(NH3)4]2+

blue

dissolve in HCl

Test for Bi3+

Test for Cu2+

Test fo Cd2+

1- xss H2O in presence of Cd2+

in presence of Cu2+

2- Sodium stannite 1) Acetic acid + KCN + H2S

ferrocyanide

2) KI

Flowchart for analysis of cation group IIA

24

Mercuric (II) ion (Hg2+

)

Previously discussed with cation group I

Bismuth (III) ion (Bi3+

)


Bismuth (Bi3+)

Group reagent

0.3 M

Bi3+

+ H2S Bi2S3 + H+

[H+] dark brown

Dissolution

By oxidation of sulphide

BiS3 + HNO3 Bi3+

+ So + NO + H2O

Confirmatory Test

1- Reaction with conc. NH4OH

Bismuth is separated from Cu2+

and Cd2+

by precipitation with

Conc. NH4OH. The precipitate is insoluble in excess of the reagent (no

ammine complex).

Bi3+

+ Conc. NH4OH Bi(OH)3 + NH4

white gelatinous

We dissolve this precipitate in HCl

Bi (OH)3 + HCl BiCl3 + H2O

Test for bismuth by two tests :

2- Oxysalt formation

Addition of large excess water will give white turbidity soluble in

excess acid by common ion effect of the H+.

25

BiCl3 + H2O BiOCl + 2H+

soluble large white turbidity

excess bismuth oxychloride

BiOCl + HCl BiCl3 + 2H+

or HNO3 soluble

3- Sodium Stannite Test

Bi3+

+ [HSnO2]- Bi

o + [Sn(OH)6]

2-

stannite reagent black stannic hexahydroxide

bismuth metal soluble complex

This is an oxidation-reduction reaction where the bismuth ion is

reduced to metallic bismuth (black precipitate) and the stannous ion is

oxidized to stannic.

N.B. : Sodium stannite reagent must be freshly prepared.

SnCl2 + 2NaOH Sn(OH)2

Stannous hydroxide

Sn(OH)2 + xss NaOH [HSnO2]- or [Sn(OH)4]

2-

stannite

On standing (by time), the sodium stannite reagent under goes self

oxidation-reduction and black precipitate of tin is formed

by time

2[Sn(OH)4]2-

Sno + [Sn(OH)6]

2-

autoxidation

black

Reaction with NaOH, the bismuth hydroxide is formed which is

insoluble in excess NaOH (not amphoteric)

Bi3+

+ NaOH Bi(OH)3

xss NaOH

insoluble

26

Copper (II) ion (Cu2+

)

Reaction Important in the Separation and Identification of

Copper (Cu2+)

Group Precipitation

0.3 M

Cu2+

+ H2S CuS + 2H+

[H+] black

Dissolution :

By oxidation of sulpide reaction

CuS + dil HNO3 Cu2+

+ 2NO + H2O + So

Complex formation reaction

* with NH4OH

Cu2+

+ 4NH4OH [Cu(NH3)4]2+

+ 4H2O

blue

copper tetramine (soluble complex)

* with KCN

Cu2+

+ KCN [Cu(CN)4]3-

cuprocyanide complex

(tetracyanocuprous complex)

stable complex.

KCN is called masking reagent

Confirmatory Tests

1- Cu2+

+ 4NH4OH [Cu(NH3)4]2+

deep - blue colour

2- Cu2+

+ KI Cu2 I2 + I2

white

cuprous iodide brown solution

27

HAC

3- Cu2+

+ [Fe(CN)6]4-

Cu2 [Fe(CN)6]

choclate

N.B. :

To carry out the test with ferrocyanide on the soluble copper

ammine complex we have to acidify first with dil acetic acid (until the

blue colour disappear) to decompose the copper complex, and liberate

the free Copper ion in the medium

[Cu(NH)42+

] + dil 4CH3COOH Cu2+

+ CH3COONH4

Reaction with NaOH

xss NaOH

insoluble (not amphoteric)

Cu2+

+ NaOH Cu(OH)2 blue CuO black

Cadmium II, ion, Cd2+


Cadium (Cd2+)

Group precipitation and Confirmatory Test

0.3 M

Cd2+

+ H2S CdS

[H+] cadmium sulphide

yellow

Dissolution :

By Oxidation of sulphide

CdS + dil HNO3 Cd2+

+ NO + H2O + So

28

Complex Formation

* with NH4OH

Cd2+

+ 4NH4OH [Cd(NH3)4]2+

cadium tetramine

complex

* with Conc. HCl

Cd2+

+ Conc. HCl [CdCl4]2-

* with KCN (masking agent)

Cd2+

+ 4KCN [Cd(CN)4]2-

cadium tetracyanide

unstable complex

N.B. :

How can you separate and identify a mixture of CdS, CuS ?

29

Arsenic Subgroup

Cation Group IIB

The elements of this group are arsenic, antimony, and tin

As3+

, As5+

, Sb3+

, Sb5+

, Sn2+

Arsenious, Arsenic , Antimonous , Antimonic , Stannous

When group II is boiled with H2O2, the acidity is adjusted at 0.3 M

HCl and H2S or thioacetamide is added, the following amphoteric

sulphide precipitates of subgroup IIB are formed.

AS2S3

yellow ppt

AS2S5

Sb2S3

orange ppt

Sb2S5

SnS2 brown ppt

Because of their high electronegativities, their sulphides (except SnS) are

amphoteric.

These precipitates are soluble in NaOH (separation of subgroups

IIA & IIB) (or Na2S) producing a mixture of thio- and oxysalts

(centrifugate in the separation of subgroup cation precipitate of IIA &

IIB).

30

Group IIB Cations

As+3

, Sb+3

, Sn+4

1- acidification with acetic acid ... (1)

2- add H2S or thioacetamide

Residue Contrifugal

As2S3 , Sb2S3 , SnS2 discard

As2S5 Sb2S5

yellow orange brown

ppt ppt ppt

heat with 1:1 HCl (12 M)

Just before boiling ..... (2)

Residue Contrifugate

As2S3 SbCl3, SnCl4

As2S5

heat to Alkalinize with

dissolve in NH4OH

Conc. HNO3 ... (3)

Test for arsenic

1- Ammonum molybdate reagent.

2- Magnesia mixture reagent.

3- Battendroff reagent

Test for Sn4+ in Test for Sb

3+ in

presence of Sb3+

presence Sn4+

- boil with Conc. HCl 1- oxalic acid and H2S ... (5)

and iron wire ....... (4) 2- acetic acid and solid sodium

add HgCl2 thiosulphate

3- excess H2O

Flowchart for analysis of cation group IIB

31

Arsenious (III) ion and Arsenic (V) ion, As3+

, As5+


As3+, As5+

Group precipitation

H+

2As3+

+ 3H2S As2S5 + 6H+

H+

2As5+

+ 5H2S As2S5 + 10H+

yellow

Dissolution oxidation by Conc. HNO3

As2S3 + Conc. HNO3

H3AsO4

As2S5 + Conc. HNO3

arsenic acid

Separation and Confirmatory Tests

1- Gutziet Test

The arsenic sulphide is dissolved by Conc. HNO3 and treated with

zinc dust in acid medium (strong reducing agent).

A filter paper moistend with AgNO3 is exposed to the arsine gas

produced from the previous reaction, Blackening to the filter paper

occurs.

H3AsO4 + Zno / H

+ AsH3 + Zn

2+

arsine gas

AsH3 + AgNO3 Ago + H3AsO3

black

}

32

2- Magnesia mixture Test

Mixture of (NH4OH, NH4Cl, MgCl2)

H3AsO4 + NH4+ + Mg

2+ MgNH4AsO4

white crystalline ppt

magnesium ammonium arsenate

3- Battendroff Test

The stannous chloride in conc. HCl, reduces arsenic salts to

metallic arsenic

H3AsO4 + SnCl2 + C. HCl Aso + SnCl6

2- + H2O

It is an oxidation-reduciton reaction.

Antimonous (III) ion and Antimonic (V) ion (Sb3+, Sb5+)


Sb3+, Sb5+

Group precipitation

H+

2Sb3+

+ 3H2S Sb2S3 + 6H+

H+

2Sb5+

+ 3H2S Sb2S5 + 10H+

orange

Separation and Confirmatory Test

1- Oxysalt formation

SbCl3 + H2O SbOCl + H+

large excess white turbidity

antimony oxychloride

basic salt

This reaction is reversed by adding HCl

33

SbOCl + HCl SbCl3 + H2O

or HNO3 soluble

(common ion effect of H+)

2- Iron wire Test

H+

Sb3+

+ 3Feo Sb

o + 3Fe

2+

black deposit

Oxidation-reduction reaction

3- Test for Sb3+

in presence of Sn4+

Here we test for Sb3+

by H2S to give orange ppt. But the Sn4+

interferes and give the brown ppt of SnS2. To get rid of this interference,

we add oxalic acid which forms two complexes with Sb3+

and Sn4+

. The

complex with Sn4+

is stable, while that with Sb3+

is unstable, so on

passing H2S, orange ppt of Sb2S5 is formed.

COO-

Sn4+

+ [Sn(C2O4)3]2-

COO-

stable

COO-

Sb3+

+ [Sb(C2O4)3]3-

COO-

oxalate unstable

+ H2S

Sb2S3

orange

Oxalic acid is called masking agent.

34

Stannous II ion, Stannic (IV) Ion (Sn2+,Sn4+)


Sn2+, Sn4+

Group precipitation

Sn2+

+ H2O2 + 2H+ + Sn

4+ + 2H2O

H+

Sn4+

+ H2S SnS2

brown

Separation and Confirmatory Test

1- Mercuric chloride test

We have to reduce the Sn4+

to Sn2+

using iron wire in acid medium.

Then HgCl2 oxidizes Sn2+

to Sn4+

and being reduced according to the

amount of HgCl2 used. The test may give white ppt (Hg2Cl2) or grey ppt

(Hg2Cl2 + Hgo) or finally black ppt (Hg

o) (oxidation-reduction reaction).

Sn4+

+ Feo + H

+ Sn

2+ + Fe

2+

(iron wire/H+) white

Sn2+

+ HgCl2 Hg2Cl2 + [SnCl6]2-

white

Hg2Cl2 + xss SnCl2 Hgo + [SnCl6]

2-

black

4- Test for stannic in presence of Sb3+

We have to boil with iron wire (Feo) in acid medium for two reasons:

H+

Sb3+

+ 3Feo + Sb

o 3Fe

2+

removed by filtration

H+

Sn4+

+ 2Feo Sn

2+ 2Fe

2+

Documents

Basic Inorganic Chemistry Lectures (Group I & II)