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+ + S2Group II cation
Page 64
The real significance of the [H+] as a control of [S2-] can be seen
by examining the ionization of H2S.
K a=
H + 2 [S2− ]
= 1.3 x 10−20
[H2 S]
A saturated solution of H2S is approximately 0.1 M; therefore, for
a saturated solution of H2S, the expression may be simplified to,
K a=
H+ 2 [S2− ]
0.1
Or [H+]2+ [S2-] = constant
H2S  2 H+ + S2HCl 
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.
On the other hand, the addition of strong base shifls this
equilibrium to the fight with a resultant increase the [S 2-].
Group II cation
Page 65
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 [S 2-] 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).
Group II cation
Page 66
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).
Group II cation
Page 67
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.
Group II cation
Page 68
Copper Subgroup
Cation group II
1. Boil with H2O2 and expel the excess
2. Adjustment the acidity
3. Add H2S
Residue
Sulphides of Group II
NaOH or Na2S
Residue
Centrifuge
Ppt of sulphides of
Soluble
complexes
subgroup IIA
subgroups IIB
of
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
Group II cation
Page 69
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
Group II cation
Page 70
Copper subgroup (Cation group II A)
Hg2+, Pb2+, Bi3+, Cu2+, Cd2+
HgS,
PbS,
Black ppt
CuS,
Bi2S3, Brown ppt
CdS Yellow
Adjustment the acidity
Add H2S
Heat with 1:1 HNO3/H2SO4
Residue
Centrifuge
Pb2+, Bi3+, Cu2+, Cd2+
HgS, HgO
Ethanol/ H2SO4
Dissolve in aqua regia
Test for Hg2+
1) SnCl2
2) NH4OH
3) KI
Dissolve in ammonium
acetate
Test for Pb2+
1- Acetic acid +K2CrO4
2- KI
Test for Bi3+
1- Xss H2O
2- Sodium stannite
Test of Cu2+ in presence of Cd2+
1) Acetic acid + ferrocyanide
2) KI
Centrifuge
Residue
Bi3+, Cu2+, Cd2+
PbSO4
White ppt
Conc. NH4OH
Residue
Bi(OH)3
white gelatinous ppt
dissolve in HCl
Centrifuge
[Cd(NH3)4 ]2+ colourless
[Cu(NH3)4]2+ blue
Test for Cd2+ In presence of
Cu2+
KCN + H2S
Flowchart for analysis of cation group IIA
Group II cation
Page 71
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
Bi
3+
+ 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
Group II cation
Page 72
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]Stannite reagent
Group II cation

Bi°  + [Sn(OH)6]2-
black
bismuth metal
stannic hexahydroxide
soluble complex
Page 73
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]2stannite
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)
Group II cation
Page 74
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+
[𝐂𝐮+ ]𝟐
𝐊=
=
[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.
Group II cation
Page 75
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
Cu
2+
+ 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)
Group II cation
Page 76
o With KCN
Cu2+ + KCN  [Cu(CN)4]3cuprocyanide 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 
white
cuprous iodide
Cu2+ + [Fe(CN)6]4-
𝑯𝑨𝑪
+
I2
brown solution
Cu2 [Fe(CN)6] 
chocolate
Group II cation
Page 77
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
Group II cation
Page 78
Cadmium II ion,Cd2+
Reaction Important in the Separation and identification
of cadium (Cd2+)
Group precipitation and Confirmatory Test
Cd
2+
+ 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
Group II cation
Page 79
with conc. HCI
Cd+ + conc. HCl  [CdCl4]2with KCN
Cd2+ + 4 KCN  [Cd(CN)4]2cadium tetracyanide
unstable complex
N.B. How can you separate and identify a mixture of CdS, CuS ?
Group II cation
Page 80
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 ,
Yellow ppt
Sb2S5 ],
orangeppt
SnS2
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).
Group II cation
Page 81
As2S3
+ NaOH  AsS2-
+ AsO2-
thioarsenite
As2S5
+ NaOH  AsS3-
oxyarsenite
+ AsO3-
thioarsenatc
Sb2S3
+ NaOH  SbS2thioantimonite
Sb2S5
+ NaOH  SbS3-
oxyarsenate
+ SbO2oxyantimonite
+ SbO3-
thioantimonate
oxyantimonate
 SnS + NaOH  sparingly soluble
 SnS2 + NaOH  [SnOS22-]2Thio 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
Group II cation
Page 82
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]2soluble complex
Group II cation
Page 83
Centifugate of Cation Group II
acidify with acetic acid
add H2S
Residue
As2S3 , Yellow ppt
As2S5 ,
Sb2S3 orange ppt
Sb2S5 ,
SnS2 brown ppt
Centrifuge
Discard
Heat with 1:1 HCl
Just before boiling
Residue
Centrifuge
As2S3 ,
SbCl3 ,
SnCl4 ,
As2S5 ,
Heat to dissolve
alkalinization
in conc. HNO3
Test for arsenic
1. Ammonium
molybdate reagent
2. Magnesia mixture
reagent
3. Battendroff reagent
with NH4OH
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
Flowchart for analysis of cation group IIB
Group II cation
Page 84
Arsenious (Ill) ion and Arsenic (V) ion, As3+, As5+
Reactions
Important
in
the
Separation
and
Identification of As3+, As5+
1. Group precipitation
2As
3+
2As
5+
+ 3 H2S
+ 5 H2S
𝑯+
𝑯+
As2S5  + 6H+
As2S5  + 10 H+
yellow
2. Dissolution by:
A- Complex formation:
(1)
Alkali sulphide (Na2S)
 As2S3 + S2-  AsS2 As2S5 + S2-  AsS3(2)
Alkali hydroxide (NaOH)

As2S3 + NaOH

AsS2- + AsO2-

As2S5 + NaOH

AsS3- + AsO3-
Group II cation
Page 85
(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.
Group II cation
Page 86
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
Group II cation
Page 87
reducing agent) which can reduce the arsenic and not the
antimony.
2- Ammonium molybdate Test
∆ 𝒄𝒐𝒏𝒄.𝑯𝑵𝑶𝟑
 (NH4)3AsO4.12MoO3
H3AsO4 + (NH4)8MoO4
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
Group II cation
Page 88
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 Sb
3+
2 Sb
5+
+ 3 H2S
+ 3 H2S
𝑯+
𝑯+
 Sb2S3 + 6 H+
 Sb2S5 + 10 H+
orange
Dissolution by complex formation
1- Alkali sulphide (Na2S)
 Sb2S3 + S2-  SbS2 Sb2S5 + S2-  SbS3-
Group II cation
Page 89
2- Alkali hydroxide (NaOH)
 Sb2S3 + NaOH  SbS2- + SbO2 Sb2S5 + NaOH  SbS3- + SbO33- 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
Group II cation
white turbidity
antimony oxychlotidc
basic salt
Page 90
The reaction is reversed by adding HCl
SbOCl  + HCl

or HNO3
SbCl3 + H2O
soluble
(common ion effect of H+)
2- Sodium thiosulphate Test
Sb3+ + S2O32- + H+  SbOS2
solid
orange
antimony oxysulphide
3- Iron wire Test
Sb
3+
o
+ 3 Fe
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.
Group II cation
Page 91
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
Sn
4+
+ H2S
𝑯+
 SnS2
brown
Group II cation
Page 92
Dissolution by complex formation
1- Alkali sulphide (Na2S)
 SnS + S2- 
sparingly soluble
 SnS2 + S2-  SnS322- Alkali hydroxide (NaOH)
 SnS + NaOH

 SnS2 + NaOH 
sparingly soluble
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-
Group II cation
Page 93
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]2white
 Hg2Cl2 + xxs HgCl2   Hgo + [SnCl6]2Black
4- Test for stannic in presence of Sb3+
We have to boil with iron wire (Fe°) in acid medium for two
reasons;
Sb
3+
o
+ 3 Fe
𝑯+
 Sbo + 3 Fe2+
removed by fiitration
Sn
Group II cation
4+
o
+ 3 Fe
𝑯+
 Sn2+ + 2 Fe2+
Page 94