Determination of Sulphates.—A soil extract is made with slightly acidulated water (HCl 1 per cent). In the filtrate the sulphate is precipitated with barium chloride, BaCl2. The barium sulphate, BaSO4, is washed carefully and dried. Its weight, multiplied by 0.4114, gives the amount of sulphuric acid: (1g.BaSO4 = 0.4115g.SO4) (see also Wiegner, 1926, p. 39). (Braun-Blanquet 1932:203)
BRAUN-BLANQUET, Josias. 1932. Plant sociology: the study of plant communities. (Trans.: George D. Fuller; Henry S. Conard) New York: McGraw-Hill Book Company.