Classification of surfactants using indicators

ISO 2271 (1989) [3] is a method for the determination of anionics by titration with a cationic surfactant. It uses a mixed indicator system in a chloroform-water solvent system. One indicator reacts with excess anionic to colour the chloroform layer pink, and the other reacts with excess cationic titrant to colour the chloroform layer blue. It is possible to use this mixed indicator to classify surfactants, by taking advantage of the following differences. 

Sulphates and sulphonates are anionic at all pHs. Carboxylates are anionic only at high pH. 

Quaternary ammonium salts are cationic at all pHs. Weak bases are cationic only at low pH. 

SW sulphobetaines (with weakly basic nitrogen) are zwitterionic at low pH and anionic at high pH. 

The classification is done by adding 1 ml of the mixed indicator (disulphine blue V and dimidium bromide) to two small portions of a solution of the surfactant, one acid and the other alkaline, adding a few 

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