Page 74 - Lab Manual & Project class 12

LABORATORY MANUAL CHEMISTRY
2+
2. Test for Copper ion (Cu )
(a) Copper sulphide dissolves in nitric acid due to the formation of copper
nitrate.
3CuS + 8HNO 3Cu(NO ) + 2NO + 3S + 4H O
3 3 2 2
On heating the reaction mixture for long time, sulphur is oxidised to
sulphate and copper sulphate is formed and the solution turns blue. A
small amount of NH OH precipitates basic copper sulphate which is soluble
4
in excess of ammonium hydroxide due to the formation of
tetraamminecopper (II) complex.
S + 2HNO 3 H SO + 2NO
4
2
2
2+
2Cu + SO +2NH +2H O Cu(OH) . CuSO +2NH
Maxbrain Chemistry
4 3 2 2 4 4
–
Cu(OH) .CuSO + 8NH 2 [Cu(NH ) ]SO + 2OH + SO 2
2 4 3 3 4 4 4
Tetraamminecopper (II)
sulphate (Deep blue)
(b) The blue solution on acidification with acetic acid and then adding
potassium ferrocyanide [K Fe(CN) ] solution gives a chocolate colouration
6
4
due to the formation of copper ferrocyanide i.e.Cu [Fe(CN) ].
2 6
[Cu(NH ) ] SO + 4CH COOH CuSO + 4CH COONH
3 4 4 3 4 3 4
2CuSO + K [Fe(CN) ] Cu [Fe(CN) ] + 2K SO
4 4 6 2 6 2 4
Potassium Copper
hexacyanoferrate (II) hexacyanoferrate (II)
(Chocolate brown precipitate)

Group-II B (Arsenic Group)

If group- II precipitate dissolves in yellow ammonium sulphide and the colour of
3+
the solution is yellow, this indicates the presence of As ions. Ammonium
thioarsenide formed on dissolution of As S decomposes with dil. HCl, and a
2
3
yellow precipitate of arsenic (V) sulphide is formed which dissolves in concentrated
nitric acid on heating due to the formation of arsenic acid. On adding ammonium
molybdate solution to the reaction mixture and heating, a canary yellow
3+
precipitate is formed. This confirms the presence of As ions.
As S + 3 (NH ) S 2 (NH ) As S + S
2 3 4 2 2 4 3 4
Yellow ammonium
sulphide

2(NH ) AsS + 6HCl As S + 3H S + 6NH Cl
4 3 4 2 5 2 4
3As S + 10HNO + 4H O 6H AsO + 10NO + 15S
3
4
5
2
2
3
Arsenic acid
74

24-04-2018