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Acidify and add barium chloride
To test a fresh portion of a solution for sulfate ions, add dilute hydrochloric acid followed by barium chloride solution. A positive result is a white precipitate of barium sulfate. The correct reagent pair matters: the halide test uses dilute nitric acid and silver nitrate, whereas this sulfate test uses dilute hydrochloric acid and barium chloride. Keep the two methods and their observations distinct.
