Amphoteric surfactants assay

The most recent patent references to amphoteric surfactants for the pharmaceutical industry involve the development of new diagnostic test methods. These include immunochemical assays and improved genetic diagnostic methods. In these applications, the selective use of amphoteric surfactants has been shown to enhance the sensitivity of detection (117-119). 

Potentiometric titration with HCl gives a characteristic value proportional to the content of amphoteric surfactant (1,10,11). In some cases, solvent systems have been optimized so that acid-base titration is suitable for assay of the product, as described below for characterization of alkylbetaines (12). 

Acid-base potentiometric titration is applicable to amphoterics. Typically, either an excess of acid or of base is added, and the titration is conducted with base or acid, respectively. The first inflection is due to the excess acid or base and subsequent inflections are due to end points associated with the surfactant. Hydrolysis and consequent blurring of the end point can be minimized by using a highly alcoholic solvent (114). Since other acidic or basic components will interfere, this approach is only applicable to concentrated surfactant solutions, not to formulations. In fact, impurities in the concentrated surfactant will interfere, which sometimes limits the usefulness of titration for assay. For determination of amphoterics in formulations, ion-pair titration is often used. 

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