After oxidising sulphides to sulphates with a mixture of sodium chlorate and hydrochloric acid, the sample is refluxed with a reducing mixture of sodium iodide, red phosphorus, hypophosphorous acid, orthophosphoric acid and propionic acid. The hydrogen sulphide generated is absorbed in potassium hydroxide solution and titrated against 2-(hydroxymercuri)benzoic acid solution with dithizone as indicator. The approximate range covered is 5 to 2000 mg kg^–1 of sulphur in the rock.