Amphoteric surfactants titration

Many amphoteric surfactants at acidic pH behave like cationic ones, so they can be determined by the standard two-phase titration method using dimidium bromide/disulfme blue VN mixed indicator. Precise analysis can, however, be difficult due to the formation of emulsions in the vicinity of the end-point. These problems can be minimized by reversing the titration, i.e., using amphoteric surfactant as a titrant, and by adding ethanol. 

Titrations of anionic, cationic and amphoteric surfactants with surfactants of opposite charge, using a surfactant-sensitive electrode. Titrations of cationic and amphoteric surfactants, and metal complexes of nonionic surfactants, with tetraphenylborate. 

The standard two-phase titration usually applied to cationics can also be used with amphoteric surfactants if the pH is adjusted. This is discussed in chapter 16. The ethanol concentration of the aqueous phase must be controlled within narrow limits for accurate results (9). 

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). 

Buschmann, N., H. Wille, Titration of amphoteric surfactants—comparison of methods, Comun. Torn. Com. Esp. Deterg., 1997,27, 65-70. 

Alkylbetaine amphoteric surfactants behave as cationics at pH 1, at which the standard two-phase titration of anionics is performed. Their presence thus leads to low results for anionics (1). 

FIG. 1 Potentiometric titration with sodium tetraphenylborate of a mixture of a cationic (cetylpy-ridinium chloride) and an amphoteric (cocoamidopropylbetaine) surfactant, followed with a specially made PVC-membrane electrode. The first inflection is due to titration of the cationic and the second to titration of the amphoteric surfactant. (Reprinted with permission from Ref. 81. Copyright 1992 by Karl Hanser Verlag.)... 

Quaternary ammonium salts Amphoteric Two-phase or surfactant-electrode monitored titration, or GC or HPLC... 

Vytfas and co-workers have reported successful titrations of large numbers of anionic, cationic, amphoteric and even nonionic surfactants using such electrodes. The paper cited [18] is a review with 38 references. The response appears to be sub-Nernstian, but when used as end-point indicators these electrodes exhibit potential jumps of up to 100 mV or more for very small increments of titrant. 

The titration must be done with alcoholic acid or alkali, and is best done potentiometrically with an autotitrator. Next best is manual potentiomet-ric titration, but it is satisfactory to use indicators, methyl orange or bromophenol blue for the low-pH end-point and phenolphthalein for the high-pH end-point. The information obtained is often a useful crosscheck on the mass balance in the analysis of an unknown, but it can also help in the analysis of WW amphoterics, which contain appreciable amounts of non-surfactant weak acids and bases. 

The possible procedures involving titration with SDS or BEC all depend on pH control, and are therefore not applicable to mixtures containing alkali-labile quats. It is better to assume that such mixtures will necessitate separation, and the guidelines given below assume that alkali-labile quats are absent, and that only one quat and one amphoteric are present, although the astute analyst will be able to work out schemes for some mixtures containing more than one surfactant of each class. 

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. 

Some amphoterics can be titrated at high pH as anionic surfactants, using benzetho-nium chloride titrant. Betaines do not lose their cationic nature at high pH and are not measured. 

Moody and coworkers and Buschmarm and coworkers were leaders in constructing efficient surfactant sensors. ISEs containing PVC membranes with ortho-nitrophenyloctyl ether (p-NPOE) as a plasticizer were suitable for the determination of amphoteric (ZS) and catirmic (CS) surfactants. At pH < 1.5, the ZS reacts as a CS and can be titrated by a bulky counter ion such as tetraphenylborate. Complexation of the ethoxylated part of nonirmic surfactants with a bivalent cation such as Ba " allows potentiometric titratirMi with tetraphenyl borate. Jones et al. " report a detection limit of 1 x 10 mol for Antarox 880. 

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