Some batches of phosphoric acid contain impurities that affect colorimetric determination of urea in soil extracts by the method of Douglas and Bremner, which involves measurement of the red color formed when an aliquot of extract is heated with diacetyl monoxime and thiosemicarbazide in the presence of phosphoric and sulfuric acid. A modification of this colorimetric procedure is described that permits use of phosphoric acid containing these impurities and allows accurate and precise determination of urea in soil extracts.
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Mulvaney et al. (1979) studied this question.
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