Cation Group II (Copper Arsenic group ) - Weeblyhishamezzat.weebly.com/uploads/9/0/6/0/9060375/chapter_3cation.pdfCation Group II (Copper Arsenic group ) Group IIA: Hg2+, ... This

Group II cation Page 64

Cation Group II (Copper Arsenic

group )

Group IIA: Hg2+, Pb2+, Bi3+, Cu2+, Cd2+

Group IIB: Sn2+, Sb3+, As3+

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 ll 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 + H2O H2S + CH3COONH4

Thioacetamide

H2S 2H+ + S2-


The real significance of the [H+] as a control of [S2-] can be seen

by examining the ionization of H2S.

Ka=

H+ 2[S2−]

[H2S]= 1.3 x 10−20

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

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

Ka=

H+ 2[S2−]

0.1

Or [H+]2+ [S2-] = constant

H2S 2 H+ + S2-

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.

On the other hand, the addition of strong base shifls this

equilibrium to the fight with a resultant increase the [S2-].


Msl [M2+]+ [s2-]

Ksp = [M2+][S2-]

The metal will precipitate when the product of [M2+] and [S2-]

exceeds the solubility product. This means that the limiting factor

in precipitation is the sulphide ion concentration which is

controlled by the pH (the hydrogen ion conc.).

Therefore, to precipitate cations of group ll (having lower solubility

product than those of group llIB), we need relatively small amount

of sulphide ion.

If the [H+] is controlled to become 0.3 M. the [S2-] will be 1 x 10-19

M. This is sufficient to precipitate the sulphides of the elements of

cation group ll.

Adjustment of the acidity

An increase in [S2-] by decreasing the acidity (< 6.3 M HCl)

results in:

a) Precipitation of sulphides of group lll.

b) Dissolution of sulphides of group IIa as thio-anions (see

group Ila).


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

results in :

a) Prevention of the precipitation of CdS, PbS. SnS2;

(have higher solubility products).

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

[SnCl6]2-. [SbCl4]

-. so they cannot precipitate by addition

of H2S.

Subdivision of cations group II intoIIA and IIB.

Cation group ll 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 (arsenic group). The

subdivision is based on the differences in the electronegativity

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

electronegativily 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).


NB. : The eleclronegativity and the acidity increases by increasing

the oxidation number.

The thio and oxyanions of group IIB are re-precipitated on

acidification acetic acid is used; since if strong HCl is used,

antimony and tin sulphides will dissolve forming soluble

complexes.


Copper Subgroup

1. Boil with H2O2 and expel the excess 2. Adjustment the acidity

3. Add H2S

NaOH or Na2S

Flowchart for separation of cations group II

to subgroup IIA and IIB

N.B.:

(1) Unless Hg2+ is an element of group IIA, but it appears in

bothIIA and II B subgroups because its precipitate HgS in group

Cation group II

Residue

Sulphides of Group II

Residue Centrifuge

Soluble complexes of

subgroups IIB

Ppt of sulphides of

subgroup IIA


IIA is partially soluble in alkali sulphide (Na2S) and pass with

group IIB

HgS + Na2S Hg(SNa)2

soluble salt

(2) 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).

Sn2+ + H2O2 𝑯𝑪𝒍 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

2 H2O2 𝑺𝒕𝒓𝒐𝒏𝒈 𝒃𝒐𝒊𝒍𝒍𝒐𝒊𝒏𝒈 2 H2O + O2

H2O2 + H2S So + 2 H2O

colloidal sulphur


Adjustment the acidity

Add H2S

Heat with 1:1 HNO3/H2SO4

Ethanol/ H2SO4

Conc. NH4OH

Flowchart for analysis of cation group IIA

Copper subgroup (Cation group II A)

Hg2+, Pb2+, Bi3+, Cu2+, Cd2+

HgS,

PbS, Black ppt

CuS,

Bi2S3, Brown ppt

CdS Yellow

Residue Centrifuge

HgS, HgO

Dissolve in aqua regia Test for Hg2+

1) SnCl2 2) NH4OH 3) KI

Pb2+, Bi3+, Cu2+, Cd2+

Residue Centrifuge

PbSO4

White ppt Bi3+, Cu2+, Cd2+

Dissolve in ammonium acetate Test for Pb2+

1- Acetic acid +K2CrO4

2- KI

Residue Centrifuge

Bi(OH)3

white gelatinous ppt dissolve in HCl

Test for Bi3+ 1- Xss H2O 2- Sodium stannite

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

Test of Cu2+ in presence of Cd2+

1) Acetic acid + ferrocyanide 2) KI

Test for Cd2+ In presence of Cu2+ KCN + H2S


Mercuric (ll) Ion (Hg2+

)

Previously discussed with cation group I

Bismuth (Ill) ion (Bi3+

)

Reactions Important in the Separation and Identification

of Bismuth (Bi3+)

1. Group reagent

Bi3+ + H2S 𝟎.𝟑 𝑴 [𝑯+] Bi2S3 + H+

dark brown

2. Dissolution

Oxidation of sulphides

BiS3 + HNO3 Bi3+ + So + NO + H2O

Complex formation reaction

BiCl3 + conc. HCl [BiCl4]-

bismuth te-trachioride soluble complex

3. 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 [BiCl4]

- white gelatinous

We dissolve this precipitate in HCI

Bi(OH)3 + HCl BiCl3 + H2O

Two tests for bismuth by:

Oxysalt formation

Addition of large excess water will give white turbidity soluble in

excess acid by common ion effect of the H+

BiCl3 + H2O BiOCl + 2 H+

soluble large white turbidity

excess bismuth oxyehioride

BiOCl + HCI BiCl3 + 2H+

or HNO3 soluble

Sodium Stannite Test

Bi3+ + [HSnO2]- Bi° + [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 + 2 NaOH 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.

[Sn(OH)4]2

𝒃𝒚 𝒕𝒊𝒎𝒆 𝒂𝒖𝒕𝒐𝒙𝒊𝒅𝒂𝒕𝒊𝒐𝒏 Sno + [Sn(OH)6]

2-

black

4. Reaction with NaOH

the bismuth hydroxide is formed which is insoluble in excess

NaOH (not amphoteric)


Bi3+ + NaOH Bi(OH)3

xss NaOH

Insoluble

Copper (II) ion (Cu2+

)

Copper (I) and copper (II) are known in solid compounds.

Copper(ll) is the only common species in aqueous solution.

The Cu2+ ion in solution is blue in colour, while the anhydrous

cupric salts are white.

In acidic solution, Cu+ and Cu2+ are related by a redox

disproportionation equilibrium like that of the mercury.

2Cu+ Cu (s) + Cu2+

𝐊 = [𝐂𝐮+]𝟐

[𝐂𝐮𝟐+]=

At equilibrium the concentration of Cu2+ is always (1.4 x 106)

[Cu+]2 .

The volatile compounds of some elements give characteristic

colors when the compound or its solution is exposed to a flame.


This usually is done by dipping a clean platinum wire into the

compound or its solution and holding we wire ine the oxidizing

part of a Bunsen burner flame. Copper (II) nitrate gives a bright

blue flame color; copper (ll) chloride gives a green flame color.

Reaction Important in the Separation and Identification

of Copper (Cu2+):

1. Group Precipitation

Cu2+ + H2S 𝟎.𝟑 𝑴 [𝑯+] CuS + 2H+

black

2. Dissolution :

Oxidation of sulpide reaction

CuS(s) + NO3-(aq) Cu2+

(aq) + SO42-

(aq) + NO(g)

Complex formation reaction

o With NH4OH

Cu2+ + 4 NH4OH [Cu(NH3)4]2+ + 4 H2O

blue:

copper tetramine (soluble complex)


o With KCN

Cu2+ + KCN [Cu(CN)4]3-

cuprocyanide complex

(tetracyanocuprous complex)

stable complex.

KCN is called masking reagent

3. Confirmatory Tests:

Cu2+ + 4 NH4OH [Cu(NH3)4]2+ + 4 H2O

deep - blue colour

Cu2+ + 2 SCN Cu(SCN)2

thiocyanate black

Cu2+ + KI Cu2I2 + I2

white

cuprous iodide brown solution

Cu2+ + [Fe(CN)6]4-

𝑯𝑨𝑪 Cu2

[Fe(CN)6]

chocolate


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(NH3)4]2+ + dil. 4 CH3COOH Cu2+ + CH3COONH4

Reaction with NaOH

xss NaOH insoluble (not amphoteric)

Cu2+ + NaOH 𝑯𝑨𝑪 Cu2

(OH)2 blue

CuO

Black


Cadmium II ion,Cd2+

Reaction Important in the Separation and identification

of cadium (Cd2+)

Group precipitation and Confirmatory Test

Cd2+ + H2S 𝟎.𝟑 𝑴 [𝑯+] CdS + 2H+

Cadmium sulphide Canary yellow Dissolution :

By Oxidation of sulphide

CdS + dil. HNO3 Cd2+ + NO + H2O + So

Complex Formation

With NH4OH

Cd2+ + 4 NH4OH Cd(NH3)4]2+ + NO + H2O + So

cadium tetramine complex


with conc. HCI

Cd+ + conc. HCl [CdCl4]2-

with KCN

Cd2+ + 4 KCN [Cd(CN)4]2-

cadium tetracyanide unstable complex

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


Arsenic subgroup

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

As3+, As5+ , Sb3+ , Sb5+ , Sn2+

Airsenious, Arsenic , Antimonous ,Antimonie ,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 , As2S5 ], [Sb2S3 , Sb2S5 ], SnS2

Yellow ppt orangeppt brown ppt

Because of their high electroneganvities, 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).


As2S3 + NaOH AsS2- + AsO2

-

thioarsenite oxyarsenite


-

thioarsenatc oxyarsenate

Sb2S3 + NaOH SbS2- + SbO2

-

thioantimonite oxyantimonite


-

thioantimonate oxyantimonate

SnS + NaOH sparingly soluble

SnS2 + NaOH [SnOS22-]2-

Thio oxystannate

N.B. :

Re-precipitation of the Arsenic Group Sulphides

The alkaline solution, containing the ions of the arsenic group in

the form of soluble complex thio- or oxy-anions, is acidified to


destroy these complex ions and to re-precipitate the sulphides of

the arsenic group.

3 AsS3- + AsO3

- + 4H+ 2 As2S3 + 2 H2O

5 AsS3- + AsO3

- + 6H+ 2 As2S5 + 3 H2O

3 SbS2- + SbO2

- + 4H+ 2 Sb2S3 + 2 H2O

5 SbS3- + SbO3

- + 6H+ 3 Sb2S5 + 3 H2O

SnOS22- + 2H+ SnS2 + H2O

In acidification of the previous soluble complex solutions, dilute

acetic acid is preferred to HCl. As HCl will form soluble

complexes with Sn4+ and Sb3+, so prevent their reprecipitation as

sulphide salt (are lost with the discarded centrifugate.)

Example

SbS2- + SbO2

- + conc. HCl [SbCl4]-

SnOS22- + conc.HCl [SnCl4]

2-

soluble complex


Centifugate of Cation Group II

acidify with acetic acid

add H2S

Discard

Heat with 1:1 HCl

Just before boiling

Heat to dissolve alkalinization

in conc. HNO3 with NH4OH

Flowchart for analysis of cation group IIB

Residue Centrifuge

As2S3 , Yellow ppt As2S5 , Sb2S3 orange ppt Sb2S5 , SnS2 brown ppt

Residue

Centrifuge

As2S3 ,

As2S5 ,

SbCl3 , SnCl4 ,

Test for arsenic

1. Ammonium

molybdate reagent

2. Magnesia mixture

reagent

3. Battendroff reagent

Test for stannic

in presence of

Sb3+ (boil with

conc, HCl and

iron wire

Test for Sb in presence of sn

1. Oxalic acid + H2S

2. Acetic acid and solid

sodium thiosulphate

3. Excee water


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

, As5+

Reactions Important in the Separation and

Identification of As3+, As5+

1. Group precipitation

2As3+ + 3 H2S 𝑯+

As2S5 + 6H+

2As5+ + 5 H2S 𝑯+

As2S5 + 10 H+

yellow

2. Dissolution by:

A- Complex formation:

(1) Alkali sulphide (Na2S)

As2S3 + S2- AsS2-

As2S5 + S2- AsS3-

(2) Alkali hydroxide (NaOH)


-


-


(3) Yellow ammonium sulphide

(Poly ammonium sulphide) (NH4)2Sx it is mixture of ammonium

sulphide and sulphur. The later has an oxidizing power. All the

products of the dissolution will give the soluble complex of the

higher oxidation state, (thio and oxy arsenate)

As2S3 + (NH4)2Sx

AsS3- + (NH4)2Sx-1

As2S5 + (NH4)2Sx

B- Oxidation by conc. HNO3

As2S3 + conc. HNO3

H3AsO4

As2S5 + conc. HNO3

4. Separation and Confirmatory Tests

The arsenious sulphide and arsenic sulphide are insoluble in l2 M

HCl (differ from sulphides of antimony and tin), but by boiling they

are partially soluble.


To confirm the presence of arsenic several tests can be done.

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 moistened with AgNO3 is exposed to the arsine gas

produced from the previous reaction, Blackening to the filter paper

occurs. .

H3AsO4 + Zno /H+ AsH3 + Zn2+

arsine gas

ASH3 + AgNO3 Ago + H3AsO3

black

If Sb3+ is present it will give the same reaction with the production

of stibine gas which also reduced AgNO3 to Ag°.

Flietmann Modification

To prevent the interference of Sb3+ in the test, we use zinc dust

in alkaline medium or aluminium in alkaline medium (mild


reducing agent) which can reduce the arsenic and not the

antimony.

2- Ammonium molybdate Test

H3AsO4 + (NH4)8MoO4

∆ 𝒄𝒐𝒏𝒄.𝑯𝑵𝑶𝟑 (NH4)3AsO4.12MoO3

canaly yellow

ammonium arsenomolybdate

3- Magnesia mixture Test

Mixture of (NH4OH, NH4Cl, MgCl2)

H3AsO4 + NH4+ + Mg2+ MgNH4AsO4

white crystalline ppt

magnesium ammonium arsenal

4- Battendroff test

The stannous chloride in conc. l-lCl, reduces arsenic salts to

metallic arsenic


H3AsO4 + SnCl3 + HCl Aso + SnCl62- + H2O

It is an oxidation-reduction reaction.

Antimonous (III) ion and Antiomnic (V) ion (Sb3+

,

Sb5+

)

Reaction Important in the separation and Identification

of Sb3+, Sb5+

Group precipitation

2 Sb3+ + 3 H2S 𝑯+

Sb2S3 + 6 H+

2 Sb5+ + 3 H2S 𝑯+

Sb2S5 + 10 H+

orange

Dissolution by complex formation

1- Alkali sulphide (Na2S)

Sb2S3 + S2- SbS2-

Sb2S5 + S2- SbS3-


2- Alkali hydroxide (NaOH)


-


-

3- Yellow ammonium sulphide (NH4)2Sx

Sb2S3 + (NH4)2Sx

SbS3- + (NH4)2Sx-1

Sb2S5 + (NH4)2Sx

4- Conc. HCl

Sb2S3 + conc. HCl SbCl4- + H2S

Sb2S5 + conc. HCl SbCl6- + H2S

Separation and Confirmatory test:

1- Oxysalt formation

SbCl3 + H2O SbOCl + H+

large excess white turbidity antimony oxychlotidc basic salt


The reaction is reversed by adding HCl

SbOCl + HCl SbCl3 + H2O

or HNO3 soluble

(common ion effect of H+)

2- Sodium thiosulphate Test

Sb3+ + S2O32- + H+ SbOS2

solid orange antimony oxysulphide

3- Iron wire Test

Sb3+ + 3 Feo 𝑯+

Sbo + 3 Fe2+

black deposit

Oxidation-reduction reaction

4- Test for Sb3+ in presence of Sn4+

Here we test for Sb3 by H2S to give orange ppt. But the Sn4+

interferes and gives 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.


Sn4+ + [Sn(C2O4)3]2-

stable

Sb3+ + [Sb(C2O4)3]3-

oxalate unstable

H2S

Sb2S3

orange

Oxalic acid is called masking agent.

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

,Sn4+

).

Reaction Important in the Separation and Identification

of Sn2+,Sn4+.

Group precipitation

Sn2+ + H2O2 + 2H+ Sn4+ + 2 H2O

Sn4+ + H2S 𝑯+

SnS2

brown


Dissolution by complex formation

1- Alkali sulphide (Na2S)

SnS + S2- sparingly soluble

SnS2 + S2- SnS32-

2- Alkali hydroxide (NaOH)

SnS + NaOH sparingly soluble

SnS2 + NaOH SnoS22-

Thio oxystannate complex

3- Yellow ammonium sulphide (NH4)2Sx

SnS + (NH4)2Sx

SnS32-

+ (NH4)2Sx-1

SnS2 + (NH4)2Sx

4- Conc. HCl

SnS + conc. HCl [SnCl4]2-

SnS2 + conc. HCl [SbCl6]2-


Separation and Confirmatory Test

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. (HgCl2) or gray ppt. (Hg2Cl2+ Hgo) or finally black ppt. (Hgo)

(oxidation-reduction reaction).

Sn4+ + Fe° + H+ Sn2+ + Fe2+

(iron wire/H+) white

Sn2+ + HgCl2 Hg2Cl2 + [SnCl6]2-

white

Hg2Cl2 + xxs HgCl2 Hgo + [SnCl6]2-

Black

4- Test for stannic in presence of Sb3+

We have to boil with iron wire (Fe°) in acid medium for two

reasons;

Sb3+ + 3 Feo 𝑯+

Sbo + 3 Fe2+

removed by fiitration

Sn4+ + 3 Feo 𝑯+

Sn2+ + 2 Fe2+

Cation Group II (Copper Arsenic group ) - Weeblyhishamezzat.weebly.com/uploads/9/0/6/0/9060375/chapter_3cation.pdfCation Group II (Copper Arsenic group ) Group IIA: Hg2+, ... This

Documents