Question

why is it essential to boil off H2S gas before precipitation of radicals of group III ? ​

Answer 1

It is essential to boil off the $H_2S$ gas before precipitation of radicals of group III because the group cations are first treated with concentrated $HNO_3$ and if not boiled off the $H_2S$ gas can react with $HNO_3$ and will form a colloidal solution of sulphur which will disturb the further analysis of the chemical reaction.

It is also known to us that the solubility of group IV sulphides is much higher than the group IV radicals, therefore we require a greater concentration of sulphide ions to precipitate the radicals hence $H_2S$ is boiled off.

Answer 2

If H2S is not boiled off then it would react with HNO3 and would be oxidized to colloidal sulphur, which would interfere with further analysis.