Basic_Inorganic_Chemistry_

Basic Inorganic Chemistry PHR 125

Prof. Dr. Mona Bedair

1

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, Pb 2+ 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.

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ANALYTICAL SEPARATION OF CATIONS

3

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 I (Silver group) II (Copper and arsenic groups)

Group Reagent Dilute HCl

ions Ag+, Pb++, Hg2++

H2S in presence of dilute HCl

Formula of Precipitate AgCl, PbCl2, Hg2Cl2

Hg++, Pb++, Bi+++, Cu++, Cd++, Sn++ As+++ , Sb+++ Al+++, Cr+++, Fe+++, Mn3+

HgS, PbS, Bi2S3, CuS, CdS, SnS, As2S3, Sb2S3

IIIA (Iron group)

Aq. NH3 in presence of NH4Cl

IIIB (Zinc group)

H2S in presence of aq. NH3 and NH4Cl

Ni++, Co++, Mn++, Zn++

NiS, CoS, MnS, ZnS

IV (NH4)2CO3 in (Calcium group) presence of aq. NH3 and NH4Cl

Ba++, Sr++, Ca++

BaCO3, SrCO3, CaCO3

Mg++, Na+, K+, NH4+

different ppted. forms

V (Alkali group)

No particular reagent

4

Al(OH)3, Cr(OH)3, Fe(OH)3 Mn(OH)3

Ag+, Pb2+, Hg22+, Hg2+, Cu2+, Bi3+, Cd2+, Sb3+, Sn2+, As3+, Al3+, Cr3+, Fe3+, Mn2+, Co2+, Ni2+, Zn2+, Ca2+, Ba2+, Sr2+, Mg2+, Na+, K+, NH4+

Cold, dil HCl Residue

Centrifugate 2+

Hg , Cu , Bi , Cd , Sb , Sn , As3+, Al3+, Cr3+, Fe3+, Mn2+, Co , Ni , Zn , Ca , Ba , Sr2+, Mg2+, Na+, K+, NH4+

Hg2Cl2, PbCl2, AgCl 2-

2+

2+

2+

2+

3+

2+

3+

2+

2+

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 Ca2+, Ba2+, Sr2+, Mg2+, Na+, K+, NH4+

Co2-, Ni2+, Zn2+,

H2S/NH4Cl/NH4OH

Group IIIA Residue Centrifugate CoS, NiS, ZnS, MnS (NH4)2CO3/NH4Cl/NH4OH Group IIIB Residue Centrifugate CaCO3, BaCO3, SrCO3 Group IV + K + Na+, NH4+

GroupV

Flowchart for separation of cations into groups

5

Mg2,

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 soluble HCl moderate excess

7

Ag+ + Cl

-

H+ + Cl c.i.e.

(2) Also, if the unknown contains Bi 3+ 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+ Cl-

HCl moderate excess

+

H+ 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 + ClLarge excess

[AgCl2]Silver dichloride soluble complex

 PbCl2 + ClLarge excess

[PbCl3]or [PbCl4]2Lead trichloride Lead tetrachloride soluble complexes

(2) Also, if the unknown contains Cd 2+ 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+.

or

Cd2+ + Cl-

[CdCl4]2-

Sn4+ + Cl-

[SnCl6]2 8

Separation of Cation group I Hg22+, Pb2+, Ag+ Cold 6M HCl Residue

(Procedure 1)

Centrifugate

Hg2Cl2, PbCl2, AgCl all white ppt H2O, boil (Procedure 2)

(If only Cation Group I present, discard. Otherwise save for Cation Group II)

Residue

Centrifugate

Hg2Cl2, AgCl

PbCl2

Conc. NH4OH (procedure 4)

Residue Hg + HgNH2Cl black white

Centrifugate

H2SO4

[Ag(NH3)2]++Cl-

 Dissolve in aqua regia  Evaporate, test with 1- SnCl2 2- KI

 Hg22+ present

Divide into 3 portions (Procedure 3)

Cool

K2CrO4

PbSO4 PbCl2 PbCrO4  white white yellow

HNO3 until acidic (Procedure 5) AgCl white

 Pb2+ present

 Ag+ present

Flowchart for Analysis of Cation Group I 9

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 AgCl 

Ag+ + Cl-

white

Dissolution by Complex Ion Formation 1- AgCl  + 2NH3 (aq)

[Ag(NH3)2]+ + Clsilver diammine

2- AgCl  + 2KCN

[Ag(CN)2]- + K+ silver dicyanide

3- AgCl  + xss HCl

[AgCl2]- + H+ silver dichloride

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Reaction with NaOH  Produces black precipitate insoluble in excess NaOH. Ag2O 

Ag+ + NaOH

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

Pb

2+

+ 2Cl

PbCl2

-

Pb2+ + S2-/H+

PbS

in group I in group IIA

Dissolution By oxidation of sulphide Pb2+ + So + NO + H2O

PbS + HNO3

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 11

PbCl2 + NH4OH

Pb(OH) Cl Lead hydroxychloride white basic salt

Confirmatory Tests PbSO4 

1- Pb2+ + SO42-

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 HNO 3, 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 + excess KI

PbI2 yellow lead iodide

(PbI4)2soluble complex lead tetraiodide

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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 Hg22+, 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  + Hg2+ 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 + Clwhite black mercuric mercury amino chloride metal 13

Reactions Important in the Separation and Identification of Mercurous (Hg22+) Group Precipitation Hg22+ + 2Cl-

Hg2Cl2  white mercurous chloride

Complex formation Hg22+ + 4CN-

Hg22+ + 4I-

Hgo + [Hg(CN)4]2mercuric tetracyanide Hgo + [HgI4]2mercuric tetraiodide

Reaction With NaOH (non amphoteric) Hg22+ + 2OH-

Hgo + HgO  + H2O mercury yellow metal mercuric oxide

Confirmatory Test Hg2Cl2 + 2NH4OH

Hgo  + Hg(NH2)Cl  + NH4+ Clblack white mercury mercuric metal amino chloride

Reactions Important in the Separation and Identification of Mercuric (Hg2+)

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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 + 3So + 2NO ___________________ aqua regia slightly ionized +

-

-

2- Hypochlorite reagent  HgS  + NaOCl

[HgCl4]2- + SO2

Confirmatory Tests 1- With ammonium hydroxide NH4OH HgCl2 + 2NH4OH

HgNH2Cl  + NH4+ + Clwhite

2- With stannous chloride reagent SnCl2 HgCl2 + SnCl2  Hg2Cl2  + [SnCl6]2white mercurous stannic hexachloride chloride Hg2Cl2 + SnCl2  Hgo + [SnCl6]2xss. black stannic hexachloride 15

mercury metal It is an oxidation-reduction reaction.

3- With potassium iodide HgCl2 + KI

HgI2  scarlet red

HgI2 + KI xss

[HgI4]2mercuric tetraiodide soluble complex

 Differentiate between mercuric and mercurous.  How can you separate and identify mixture of Hg22+ / Hg2+ ??

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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+

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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. 2H+ + S2-

H2S

The real significance of the [H+] as a control of [S2-] can be seen by examining the ionization of H2S. [H+]2 [S2-] 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 [S2-] Ka =

_____________________

0.1 or [H+]2+ [S2-] = constant H2S

2H+ + S2-

HCl

H+ + Clc.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 H 2S in H2O and hence decreases the sulphide ion concentration. 18

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, SnS 2 (have higher solubility products). b) Formation of stable soluble complexes as [CdCl4]2-, [SnCl62-], [SbCl4]-, so they can not precipitate by addition of H2S.

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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.

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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

or

Residue

NaOH Na2S

Centrifugate

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

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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 (Sn 4+) 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

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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, black ppt

Bi2S3 , CdS brown ppt yellow ppt heat with 1:1 HNO3

Residue

Centrifugate Bi3+ , Cu2+, Cd2+ , Pb2+

HgS , HgO dissolve in aqua regia or hypochlorite Test for Hg2+ 1) SnCl2 2) NH4OH 3) KI

ethanol, H2SO4 Residue

Centrifugate

PbSO4  Bi3+ , Cd2+ , Cu2+ white dissolve in ammonium Conc. NH4OH acetate Test for Pb2+ 1- acetic acid + K2CrO4 2- KI Residue

Centrifugate

 Bi(OH)3 white gelatinous dissolve in HCl Test for Bi3+ 1- xss H2O 2- Sodium stannite

[Cd(NH3)4]2+ colourless [Cu(NH3)4]2+ blue

Test for Cu2+ in presence of Cd2+ 1) Acetic acid + ferrocyanide 2) KI

Test fo Cd2+ in presence of Cu2+ KCN + H2S

Flowchart for analysis of cation group IIA 23

Mercuric (II) ion (Hg2+) Previously discussed with cation group I

Bismuth (III) ion (Bi3+) Reactions Important in the Separation and Identification of Bismuth (Bi3+)  Group reagent 0.3 M Bi3+ + H2S

[H+]

Bi2S3  + 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). Bi(OH)3  + NH4

Bi3+ + Conc. NH4OH

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+.

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BiCl3 + H2O soluble large excess

BiOCl  + 2H+ white turbidity bismuth oxychloride

BiOCl  + HCl or HNO3

BiCl3  + 2H+ soluble

3- Sodium Stannite Test Bi3+ + [HSnO2]stannite reagent

Bio  + black bismuth metal

[Sn(OH)6]2stannic hexahydroxide 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

Sn(OH)2  Stannous hydroxide

+ 2NaOH

Sn(OH)2  + xss NaOH

[HSnO2]- or [Sn(OH)4]2stannite

On standing (by time), the sodium stannite reagent under goes self oxidation-reduction and black precipitate of tin is formed by time

Sno  + [Sn(OH)6]2autoxidation black  Reaction with NaOH, the bismuth hydroxide is formed which is 2-

2[Sn(OH)4]

insoluble in excess NaOH (not amphoteric) Bi3+ + NaOH

Bi(OH)3   xss NaOH insoluble

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Copper (II) ion (Cu2+)

Reaction Important in the Separation and Identification of Copper (Cu2+)  Group Precipitation 0.3 M Cu

2+

+ H2S

[H+]

CuS  + 2H+ 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]3cuprocyanide complex (tetracyanocuprous complex) stable complex. KCN is called masking reagent

Confirmatory Tests 1- Cu2+ + 4NH4OH 2- Cu2+ + KI

[Cu(NH3)4]2+ deep - blue colour  Cu2 I2 + I2 white cuprous iodide brown solution 26

HAC 3- Cu

2+

Cu2 [Fe(CN)6]  choclate

4-

+ [Fe(CN)6]

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) Cu(OH)2 blue   CuO  black

Cu2+ + NaOH

Cadmium II, ion, Cd2+ Reaction Important in the Separation and Identification of Cadium (Cd2+) Group precipitation and Confirmatory Test 0.3 M Cd2+ + H2S

+

[H ]

CdS  cadmium sulphide yellow

 Dissolution : By Oxidation of sulphide  CdS + dil HNO3

Cd2+ + NO + H2O + So 27

 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]2cadium tetracyanide unstable complex

N.B. : How can you separate and identify a mixture of CdS, CuS ?

28

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).

29

Group IIB Cations As+3, Sb+3, Sn+4 1- acidification with acetic acid ... (1) 2- add H2S or thioacetamide Residue As2S3 , As2S5 yellow ppt

Contrifugal

Sb2S3 , Sb2S5 orange ppt

SnS2

discard

brown ppt

heat with 1:1 HCl (12 M) Just before boiling ..... (2) Residue

Contrifugate

As2S3  As2S5 

SbCl3, SnCl4

heat to dissolve in Conc. HNO3 ... (3)

Alkalinize with NH4OH

Test for arsenic 1- Ammonum molybdate reagent. 2- Magnesia mixture reagent. 3- Battendroff reagent Test for Sn4+ in presence of Sb3+ - boil with Conc. HCl and iron wire ....... (4) add HgCl2

Test for Sb3+ in presence Sn4+ 1- oxalic acid and H2S ... (5) 2- acetic acid and solid sodium thiosulphate 3- excess H2O

Flowchart for analysis of cation group IIB

30

}

Arsenious (III) ion and Arsenic (V) ion, As3+, As5+ Reactions Important in the Separation and Identification of As3+, As5+ Group precipitation H+ 2As3+ + 3H2S

H+

2As5+ + 5H2S

As2S5  + 6H+ As2S5  + 10H+ yellow

Dissolution oxidation by Conc. HNO3  As2S3 + Conc. HNO3  As2S5 + Conc. HNO3

H3AsO4 arsenic acid

Separation and Confirmatory Tests 1- Gutziet Test The arsenic sulphide is dissolved by Conc. HNO 3 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. AsH3  + Zn2+ arsine gas

H3AsO4 + Zno / H+

AsH3 + AgNO3

Ago  + H3AsO3 black 31

2- Magnesia mixture Test Mixture of (NH4OH, NH4Cl, MgCl2) MgNH4AsO4  white crystalline ppt magnesium ammonium arsenate

H3AsO4 + NH4+ + Mg2+

3- Battendroff Test The stannous chloride in conc. HCl, reduces arsenic salts to metallic arsenic Aso  + SnCl62- + H2O

H3AsO4 + SnCl2 + C. HCl It is an oxidation-reduciton reaction.

Antimonous (III) ion and Antimonic (V) ion (Sb3+, Sb5+) Reaction Important in the Separation and Identification of Sb3+, Sb5+ Group precipitation H+ Sb2S3  + 6H+

2Sb3+ + 3H2S H+ 2Sb5+ + 3H2S

Sb2S5  + 10H+ orange

Separation and Confirmatory Test 1- Oxysalt formation SbCl3 + H2O large excess

SbOCl  + H+ white turbidity antimony oxychloride basic salt

This reaction is reversed by adding HCl 32

SbOCl  + HCl or HNO3

SbCl3 + H2O soluble

(common ion effect of H+)

2- Iron wire Test H+ Sbo  + 3Fe2+ black deposit Oxidation-reduction reaction Sb3+ + 3Feo

3- Test for Sb3+ in presence of Sn4+ Here we test for Sb3+ by H2S to give orange ppt. But the Sn 4+ interferes and give the brown ppt of SnS 2. To get rid of this interference, we add oxalic acid which forms two complexes with Sb 3+ and Sn4+. The complex with Sn4+ is stable, while that with Sb3+ is unstable, so on passing H2S, orange ppt of Sb2S5 is formed. COOSn4+ +  COO-

[Sn(C2O4)3]2stable

COOSb3+ +  COOoxalate

[Sb(C2O4)3]3unstable + H2S   Sb2S3 orange

Oxalic acid is called masking agent.

33

Stannous II ion, Stannic (IV) Ion (Sn2+,Sn4+) Reaction Important in the Separation and Identification of Sn2+, Sn4+ Group precipitation Sn4+  + 2H2O

Sn2+ + H2O2 + 2H+ + H+ SnS2  brown

Sn4+ + H2S

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 (Hg 2Cl2) or grey ppt (Hg2Cl2 + Hgo) or finally black ppt (Hgo) (oxidation-reduction reaction). Sn4+ + Feo + H+ (iron wire/H+) Sn2+ +

Sn2+ + Fe2+ white  Hg2Cl2 + [SnCl6]2white

HgCl2

Hg2Cl2 + xss SnCl2

Hgo + [SnCl6]2black

4- Test for stannic in presence of Sb3+ We have to boil with iron wire (Feo) in acid medium for two reasons: H+ Sbo  3Fe2+ removed by filtration

Sb3+ + 3Feo + H+ Sn4+ + 2Feo

Sn2+  2Fe2+ 34