Page 76 - Lab Manual & Project class 12

LABORATORY MANUAL CHEMISTRY
which is soluble in excess of sodium hydroxide solution due to the formation
of sodium meta aluminate.
AlCl + 3NaOH Al(OH) + 3NaCl
3 3
Al(OH) + NaOH NaAlO + 2H O
2
3
2
White gelatinous Sodium
precipitate meta aluminate
(b) In the second test when blue litmus is added to the solution, a red
colouration is obtained due to the acidic nature of the solution. On addition
of NH OH solution drop by drop the solution becomes alkaline and
4
aluminium hydroxide is precipitated. Aluminium hydroxide adsorbs blue
colour from the solution and forms insoluble adsorption complex named
‘lake’. Thus a blue mass floating in the colourless solution is obtained. The
Maxbrain Chemistry
test is therefore called lake test.

3+
2. Test for ferric ions (Fe )
Reddish brown precipitate of ferric hydroxide dissolves in hydrochloric acid and
ferric chloride is formed.
Fe(OH) + 3HCl FeCl + 3H O
3 3 2
(a) When the solution containing ferric chloride is treated with potassium
ferrocyanide solution a blue precipitate/colouration is obtained. The colour
of the precipitate is Prussian blue. It is ferric ferro-cyanide. The reaction
takes place as follows:

4FeCl + 3K [Fe(CN) ] Fe [Fe(CN) ] + 12KCl
3 4 6 4 6 3
Potassium Prussian blue
ferrocyanide precipitate
If potassium hexacyanoferrate (II) (i.e. potassium ferrocyanide) is added in
excess then a product of composition KFe[Fe(CN) ] is formed. This tends to
6
form a colloidal solution (‘soluble Prussian blue’) and cannot be filtered.
FeCl + K [Fe(CN) ] KFe[Fe(CN) ] + 3KCl
3 4 6 6
(Soluble prussian blue)
(b) To the second part of the solution, add potassium thiocyanate (potassium
sulphocyanide) solution. The appearance of a blood red colouration
3+
confirms the presence of Fe ions.
3+
Fe + SCN – [Fe(SCN)] 2+
Blood red colour

(V) Analysis of group-IV cations

If group-III is absent, pass H S gas in the solution of group-III for a few
2
minutes. If a precipitate appears (white, black or flesh coloured), this indicates

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24-04-2018