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प्रश्न
Write a note on the chromyl chloride test.
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उत्तर
Heating solid potassium dichromate with concentrated sulphuric acid and a soluble metal chloride (e.g. NaCl, KCl) produces orange-red vapours of volatile oily chromyl chloride (CrO2Cl2).
\[\ce{K2Ct2O7 + 4NaCl + 6H2SO4 ->[Heat] 2KHSO4 + 4NaHSO4 + \underset{(Orange red vapour)}{\underset{Chromyl chloride}{2CrO2Cl2}} + 3H2O}\]
Chromyl chloride vapours react with water to produce a yellow solution containing chromic acid.
\[\ce{CrO2Cl2 + 2H2O -> 2HCl + \underset{(Yellow solution)}{\underset{Chromic acid}{H2CrO4}}}\]
Treating the solution with lead acetate and acetic acid results in a yellow precipitate of lead chromate.
\[\ce{H2CrO4 + \underset{Lead acetate}{(CH3COO)2Pb} -> \underset{(Yellow ppt)}{\underset{Lead chromate}{PbCrO4}} + 2CH3COOH}\]
The above test is known as the Chromyl chloride test and is used for the detection of chloride ions in qualitative analysis.
