The Evaluation of the Purity of Various Organic Products by the Dichromate Method.

THE writer has previously described [1914] a method for the volumetric estimation of carbon, the essence of which is oxidation by a mixture of phosphoric acid and potassium dichromate and the measurement of the 002 produced. It was shown that where methyl groups are present the substance is not oxidised to carbon dioxide alone but to a mixture of carbon dioxide and acetic acid; for example, lactic acid, alanine and ethyl alcohol each yield one molecule of acetic acid per molecule of substance taken; that is to say each methyl group gives rise to one molecule of acetic acid. The method proved of service in estimating the amount of such substances as mannitol and glycerol or, in the case of a mixture, of the residual oxidisable material after a fermentation. It is possible with a few cc. of solution to estimate how much material remains after fermentation, and so on. At the time of publishing this method the writer was unaware of Martin's method [1904] for the estimation of alcohol. Martin's method is now probably fairly generally employed in industry; certainly in France. It is exceedingly accurate and is applicable to very dilute solutions of alcohol. Martin distils the alcoholic solution into a mixture of equal parts of N potassium dichromate and sulphuric acid; the steam keeps the mixture hot and the oxidation is immediate, the alcohol being oxidised to acetic acid (1 cc. N dichromate