1. Analytical Chemistry PHCMp 101
Lab 4
Identification of
Group IV Cations
Group IV Cations

2. І ΙΙ ΙΙΙ ΙV V VІ Group reagent Ions Formulae of precipitates
Grp
Group reagent
Ions
Formulae of precipitates
Distinguishing features І
Cold dil HCl
Ag+, Pb++,Hg2++
AgCl, PbCl2, Hg2Cl2
Chlorides insoluble in cold dil HCl ΙΙ
H2S in presence of HCl ( M)
Subgroup IIA Cu++,Cd++,Hg++,Bi++,
Pb++
Subgroup IIB
Sn++, Sn4+, Sb3+, As3+, As5+
CuS, CdS, PbS, HgS, Bi2S3, SnS, SnS2, Sb2S3,
Sb2S5,
Precipitated as Sulfides in acidic medium ( M HCl)
ΙΙΙ
NH4OH in Presence of NH4Cl
Al3+, Cr3+, Fe3+
Al(OH)3, Cr(OH)3,
Fe(OH)3
Precipitated as Hydroxides by NH4OH in presence of excess NH4Cl ΙV
H2S in Presence of NH4OH
Ni++, Co++, Mn++, Zn++
NiS, CoS, MnS, ZnS
Precipitated as Sulfides by H2S from ammoniacal solution in presence of NH4Cl V
(NH)2CO3 in presence of NH4OH &NH4Cl
Ba++, Ca++, Sr++
BaCO3, SrCO3,
CaCO3
Precipitated as Carbonates in alkaline medium by NH4OH in presence of NH4Cl VІ
No particular reagent
Na+, K+, NH+4 , Mg++
Different precipitated forms
Ions not precipitated in previous groups

3. Group IV cations are:
Zn2+ / Mn2+ / Co2+ / Ni2+ Zinc/Manganese/Cobalt/Nickel They are precipitated as sulphides:
Zn2+ + H2S ZnS (s) White ppt.
Mn2+ + H2S MnS (s) Buff ppt.
Co2+ + H2S CoS (s) Black ppt.
Ni2+ + H2S NiS (s) Black ppt.
Group IV Cations

4. In the presence of NH4Cl & NH4OH
Group Reagent is: H2S
In the presence of NH4Cl & NH4OH
forming insoluble sulphide salts
Group IV Cations

5. The concentration of the [S-2] should be kept high!
WHY?
The solubility products of group IV sulphides are higher than those of group II sulphides
i.e. group IV sulphides require higher conc. of S-2 for their precipitation.
Group IV Cations

6. HOW? H2S 2H+ + S2- NH4OH OH- + NH4+
NH4OH is added to enhance the ionization of H2S as follows:
H2S H+ + S2-
NH4OH OH- + NH4+
NH4Cl is added to decrease the [OH-] by common ion effect to prevent the precipitation of the Mg(OH)2­ (group VI).
NH4OH OH- + NH4+
NH4Cl Cl- + NH4+
Group IV Cations

7. NH4Cl + xss conc NH4OH + K3Fe(CN)6 If Not, take a new portion
sample + xss NH4Cl + xss NH4OH + xss H2S
If black ppt. appears CoS or NiS
New portion of the sample
If White ppt. appears ZnS
New portion of the sample
White ppt: Zn2+
If Buff ppt. appears MnS
New portion of the sample
NH4Cl + xss conc NH4OH + K3Fe(CN)6
xss NaOH +
3 drops of H2O2 +
Boil
CH3COOH + K4Fe(CN)6
If Not, take a new portion
Reddish brown ppt : Co2+
Brown ppt: Mn2+
NH4Cl +
Conc NH4OH +
Drops of DMG
Pink ppt : Ni2+
Group IV Cations