Question

What does the test reveal about an unknown halide ion? Suggest another test which would confirm that identity.

Addition of acidic 0.1M NaNO2, 0.5M HNO3 and CH2Cl2 to a unknown halide ion solution resulted in a colorless CH2Cl2 layer

Answer 1

(NO₂)^- + 6H⁺ + 4e       N₂O + 3H₂O; E° = 1.297 volts

It is enough to oxidize either bromide or iodide, to produce bromine or iodine respectively. Brome turns CH₂Cl₂ brownish and iodine turns CH₂Cl₂ violet. But it is not enough to oxidize chloride into chlorine. No color in the organic layer implies no Br- or I- present in solution. It is likely to be chloride present in the solution.

An additional test to confirm the identification is to add a solution of HNO₃ acidified AgNO₃ solution. Chloride produces white precipitate of AgCl which is soluble in concentrated ammonia by forming argento ammonium chloride. The precipitate returns when re-acidified with HNO₃.