Determination of total unesterified carboxylate

The combined sulphonated carboxylic acid and fatty acid or their salts are determined by potentiometric acid-base titration, after oxidation of sulphite/sulphur dioxide. 

Weigh about 7.5 g of the well-mixed sample in a 200 ml beaker or autotitration vessel. 

Add 20 ml ethanol and stir until dissolved. If the sample is the free sulphonic acid, add about 25 ml carbonate-free 1M ethanolic sodium hydroxide and stir. 

Add 1 M ethanolic hydrochloric acid or carbonate-free 1 M ethanolic sodium hydroxide to adjust the pH to between 1.5 and 2.0. 

Titrate potentiometrically with carbonate-free 0.2 M sodium hydroxide. There are end-points around pH 3 and 7.7. 

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