Page 78 - Lab Manual & Project class 12

LABORATORY MANUAL CHEMISTRY
(a) On addition of sodium hydroxide solution it gives a white precipitate of
zinc hydroxide, which is soluble in excess of NaOH solution on heating.
2+
This confirms the presence of Zn ions.
ZnCl + 2NaOH Zn(OH) + 2NaCl
2 2
Zn(OH) + 2NaOH Na ZnO + 2H O
2
2
2
2
Sodium zincate
(b) When potassium ferrocyanide K Fe(CN) solution is added to the solution
6
4
after neutralisation by NH OH solution, a white or a bluish white precipitate
4
of zinc ferrocyanide appears.
2 ZnCl + K [Fe(CN) ] Zn [Fe(CN) ] + 4 KCl
6
2
2
6
4
Zinc
ferrocyanide
Maxbrain Chemistry
2. Test for Manganese ion (Mn )
2+
Manganese sulphide precipitate dissolves in dil. HCl on boiling. On addition of
NaOH solution in excess, a white precipitate of manganese hydroxide is formed
which turns brown due to atmospheric oxidation into hydrated manganese dioxide.
MnS + 2HCl Mn Cl + H S
2 2
MnCl + 2NaOH Mn(OH) + 2NaCl
2 2
(White precipitate)
Mn (OH) + [O] MnO(OH) 2
2
Hydrated manganese dioxide
(Brown colour)

2+
3. Test for Nickel ion (Ni )
The black precipitate of nickel sulphide dissolves in aqua regia and the reaction
takes place as follows:

3NiS + 2HNO + 6HCI 3NiCl + 2NO + 3S + 4H O
3 2 2
After treatment with aqua regia nickel-chloride is obtained which is soluble
in water. When dimethyl glyoxime is added to the aqueous solution of nickel
chloride, made alkaline, by adding NH OH solution, a brilliant red precipitate is
4
obtained.

Complex of red colour
(Stable form of complex)
78

24-04-2018