Potentiometric titration with sodium dodecyl sulphate

2 Potentiometric titration with sodium dodecyl sulphate 

Quats may be titrated potentiometrically with sodium dodecyl sulphate. The procedure is straightforward, being the converse of that described for anionics in section 5.2.2, and end-points are mostly sharp. Quat solutions and titrant may be up to 0.04 M, solubility permitting. Any of the electrode types described in section 3.6.2 is suitable, as are many others, with calomel or silver-silver chloride as reference. 

The best known two-phase method is that of Cross [4]. By titrating at three different pHs, it permits a degree of classification of the analyte. It requires two buffer solutions and two anionic indicators. 

Buffer solution, pH3.0 mix equal volumes of 0.5 M citric acid and 

Buffer solution, pH 10.0 mix 100 ml 0.2 M disodium hydrogen phosphate with 6 ml 0.25 M trisodium phosphate. 

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Total active is determined by two-phase or potentiometric titration with sodium dodecyl sulphate (SDS), or with NaTPB at pH 3. This is the mandatory first step in all cases. 

A. Titration with sodium dodecyl sulphate in acid solution. The solution must be at least 0.1 M in hydrogen ion, but otherwise the procedure is straightforward. Potentiometric titration is preferred. 

This section includes methods for determining the total, primary, secondary and tertiary amine contents of fatty amines which are nominally primary, secondary or tertiary or mixtures. For determination, another approach is to titrate with sodium dodecyl sulphate. Either two-phase or potentiometric titration may be used. The solution must be distinctly acid, pH 3 or lower. Two-phase or potentiometric titration with NaTPB is also applicable. Procedures for all of these are given in section 7.1. The difficulty with these approaches is in deciding what value to use for the molecular weight. 

The determination of amine oxides is straightforward they can be titrated in acid solution with sodium dodecyl sulphate or NaTPB, either poten-tiometrically or in a two-phase system, for which purposes the solution must be at least 0.1 M in hydrogen ion, and they can be titrated with acid. However, for quality-control purposes it is necessary to determine the free tertiary amine content. This can be done by potentiometric titration, either in a solvent which permits discrimination on the basis of base strength, or before and after removal of the amine oxide. Methods are described for both of these. 

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