Abstract
Current methods of estimating "free HC1" in acid mixtures are examined and reasons adduced for preferring either the Gunzberg method or a pH 2 titration. Both methods are superseded in accuracy and simplicity by a titration in 90% acetone solution using [rho]-benzenesulphonic acid-azo-a-naphthylamine (described as naphthylamine orange) as indicator. The exact end-point is controlled at pH 3.4 or 4.0 (pH in acetone) according to the nature of the acids of the mixture. A comparison of the methods on acid solutions of known composition is carried out.

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