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The diphenylbenzidine reaction of Letts and Rea has been examined and is recommended for use in sea-water; 2·5 ml. of sea-water is mixed with 6·0 ml. of the purest strong sulphuric acid and then allowed to cool. Subsequently 1·5 ml. of a sulphuric acid solution of diphenylbenzidine is added, the concentration being 20 mg. per 100 ml. of acid. The colour should be compared, after 20-24 hours, with a standard solution made up by adding a definite quantity of nitrate to the sea-water. A blank correction should be used. Diphenylbenzidine used must be recrystallised from boiling toluene. 2. If the sulphuric acid used is found to give a blue colour with diphenylbeniddine, this may be removed, if not too intense, by warming the acid for a few minutes. With a more intense blue the nitric acid-present may be eliminated by previous cautious treatment with hydrogen sulphide. Phosphoric acid may be used instead of sulphuric acid with fresh water, but not with sea-water.
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W. R. G. Atkins (1932) studied this question.
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