Page 79 - Lab Manual & Project class 12

SYSTEMATIC QUALITATIVE ANALYSIS
2+
4. Test for Cobalt ion (Co )
Cobalt sulphide dissolves in aqua regia in the same manner as nickel sulphide.
When the aqueous solution of the residue obtained after treatment with aqua
regia is treated with a strong solution of potassium nitrite after neutralisation with
ammonium hydroxide and the solution is acidified with dil. acetic acid, a yellow
precipitate of the complex of cobalt named potassium hexanitritocobaltate (III)
is formed.
CoS + HNO + 3HCl CoCl + NOCl + S + 2H O
3 2 2
CoCl + 7KNO + 2CH COOH K [Co(NO ) ] + 2KCl + 2CH COOK + NO + H O
2 2 3 3 2 6 3 2
Potassium
hexanitritocobaltate(III)
(Yellow precipitate)
preserved precipitate. Maxbrain Chemistry
(VI) Analysis of Group–V cations

If group-IV is absent then take original solution and add a small amount of solid
NH Cl and an excess of NH OH solution followed by solid ammonium carbonate
4
4
(NH ) CO . If a white precipitate appears, this indicates the presence of group–V
3
4 2
cations.
Dissolve the white precipitate by boiling with dilute acetic acid and divide
2+
2+
2+
the solution into three parts one each for Ba , Sr and Ca ions. Preserve a
small amount of the precipitate for flame test. Summary of confirmatory
tests is given in Table 7.16.
Table 7.16 : Confirmatory test for Group–V cations
Dissolve the precipitate by boiling with dilute acetic acid and divide the
2+
2+
solution into three parts one each for Ba , Sr and Ca 2+ ions
2+
2+
2+
Ba ions Sr ions Ca ions
(a) To the first part add (a) If barium is absent, take (a) If both barium and
potassium chromate second part of the solution strontium are absent,
solution. A yellow and add ammonium take the third part of
precipitate appears. sulphate solution. Heat the solution. Add
and scratch the sides of the ammonium oxalate
(b) Perform the flame test tube with a glass rod solution and shake
test with the and cool. A white well. A white
precipitate is formed. precipitate of calcium
A grassy green flame oxalate is obtained.
is obtained. (b) Perform the flame test with
the preserved precipitate. (b) Perform the flame test
A crimson-red flame with the preserved
confirms the presence of precipitate. A brick red
Sr ions. flame, which looks
2+
greenish-yellow through
blue glass, confirms the
2+
presence of Ca ions.

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24-04-2018