Analytical Chemistry Trinidad & 

Tobago Lab Resources
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Group Classification of Cations



Add dilute hydrochloric acid to the cold original soln as long as any ppt. forms. Centrifuge.
Residue group 1
Silver gp.

AgCl - white
Hg2Cl2 - white
PbCl2 - white
Centrifugate gp 1. Heat to boiling and add 30% thioacetamide until pptn is complete. Centrifuge.
Residue group 2

(a) Copper gp

PbS - black
HgS = yellow,
going black
Bi2S3 - shinihg
black
CuS - black
CdS - yellow

(b) Arsenic gp.

AS2S3 - yellow
Sb2S3 - yellow
Sb2S5 - orange
Centrifugate gp 2. Evaporate the soln. to 1 ml.Add 1 drop conc. nitric acid and boil. Add xs. solid NH4Cl and then conc. ammonia until alkaline. Boil gently, cool under tap, and centrifuge.
Residue gp 3
Iron gp.

Fe(OH)3 - reddish
Cr(OH)3 - pale green
MnO2 - brown
Centrifugate gp 3. Add a few more drops of ammonia, then add thiocetamide and ammonium chloride. Centrifuge.
Residue gp 4
Zinc gp.

MnS - pink ZnS - white NiS - black CoS - black
Centrifugate gp 4. Evaporate to 1 ml, add ammonium carbonate soln. and ammonia, warm, stand awhile, then centrifuge.
Residue gp 5
Calcium gp.

BaCO3 - white
CaCO3 - white
SrCO3 - white
Centrifugate gp 5
Alkali gp.

Na; K; Mg;
Test original soln.
for NH4+ by
boiling with NaOH


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Dhanlal De Lloyd, Chem. Dept, The University of The West Indies, St. Augustine campus
The Republic of Trinidad and Tobago.
Last Update: February 2004.