Principle: It gives the most accurate results and is the recommended procedure for sulphate concentrations above 10 mg/mL. The sulphate ions in the sample are precipitated by the addition of barium chloride solution to water sample acidified with hydrochloric acid and kept near the boiling point.
SO42- + Ba2+ ? BaSO4.
In highly alkaline water maintained near the boiling temperature, BaCO3 may get precipitated and eliminate this, the sample is acidified. To precipitate sulphate ions as completely as possible, excess barium chloride is used. The precipitate of BaSO4 is highly insoluble and hence there is considerable tendency for most of the precipitate to form in colloidal condition which cannot be removed by ordinary filtration procedures. To facilitate the conversion of colloidal form to crystalline form, the samples at temperatures near the boiling point for a few hours are digested.

Procedure: About 200 mL of water sample is transferred into a beaker. Two drops of methyl red indicator is added to it. Conc.HCl is added to it drop by drop till the colour changes to pink. Two drops of conc.HCl are added in excess. It is heated to nearly boiling and reduce the volume to 50mL. Hot barium chloride solution is added to it, with stirring until the formation of white precipitate is complete. Two drops of hot BaCl2 are added in excess. The precipitate (BaSO4) is digested for about 2 hours (or until the precipitate becomes settles down). It is filtered using Whatman filter paper No. 42 (ash less filter paper), quantitatively. The precipitate is washed several times with distilled water until the washings are free from chloride ions. After it completely drains out, the filter paper along with the precipitate is transferred carefully into previously weighed Gooch crucible and ignited at 800-9000C till all traces of filter paper are burnt. The crucible is cooled in a desiccator and finally weighed. Let the amount of BaSO4 precipitated be W g.

Calculations: We know that, 233.4 g of BaSO4 contains 96.06 g of sulphate.
Therefore, W g of BaSO4 contains (96.06 x W)/233.4 g of sulphate.
That is, 200 mL of water sample contains (96.06 x W)/233.4 g of sulphate.
Therefore 1000 mL of water sample contains(96.06 x W x 1000)/ (233.4 x 200) g of sulphate