Sulphate ion- confirmatory test
The method consists
of slowly adding a dilute solution of barium chloride to a hot solution of the
sulphate slightly acidified with hydrochloric acid

It is  customary to carry out the precipitation in weakly acid solution in order
to prevent the possible formation of the barium salts of such anions as chromate,
carbonate, and phosphate, which are insoluble in neutral solutions; moreover,
the precipitate so obtained consists of large crystals, and is therefore more easily filtered

Reference: Vogel's Textbook of 'Quantitative Chemical
Analysis 11.72 Sulphate: Determination of sulphate as barium sulphate