Question

Write a note on the chromyl chloride test.

Answer 1

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.