Page 61 - Lab Manual & Project class 12

SYSTEMATIC QUALITATIVE ANALYSIS
–
4. Test for Nitrate ion [NO ]
3
(a) If on heating the salt with conc. H SO light brown fumes are evolved
2
4
then heat a small quantity of the given salt with few copper turnings or
chips and conc. H SO . Evolution of excess of brown fumes indicates
2
4
the presence of nitrate ions. The solution turns blue due to the formation
of copper sulphate. Copper
sulphate
NaNO + H SO NaHSO + HNO
3 2 4 4 3
4HNO 4NO + O + 2H O
3 2 2 2
2NaNO + 4H SO + 3Cu 3 CuSO + Na SO + 4H O + 2NO
3 2 4 4 2 4 2 Nitric
Copper sulphate
(Blue) acid
2NO + O 2NO
4Maxbrain Chemistry
2 2
(Brown fumes)
(b) Take 1 mL of an aqueous solution of the salt and add 2 mL conc. H SO 4 Oxalates
2
slowly. Mix the solutions thoroughly and cool the test tube under the
tap. Now, add freshly prepared ferrous sulphate solution along the sides
of the test tube dropwise so that it forms a layer on the top of the liquid
already present in the test tube. A dark brown ring is formed at the
junction of the two solutions due to the formation of nitroso ferrous
sulphate (Fig. 7.2). Alternatively first ferrous sulphate is added and
then concentrated sulphuric acid is added.

Fig. 7.2 : Formation of brown ring

NaNO + H SO 4 Na HSO + HNO 3
4
3
2
6 FeSO + 3H SO + 2HNO 3Fe (SO ) + 4H O + 2NO
2 4 3 2 4 3 2
FeSO + NO [Fe(NO)]SO
4 4
Nitroso ferrous sulphate
(Brown)
2–
5. Test for Oxalate ion [C O ]
2 4
If carbon dioxide gas along with carbon monoxide gas is evolved in the
preliminary examination with concentrated sulphuric acid, this gives indication
about the presence of oxalate ion.

(COONa) + Conc. H SO 4 Na SO + H O + CO 2 + CO
2
2
2
4
2
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24-04-2018