Sulphonated hydrocarbons

Sulphonated hydrocarbons are not hydrolysed by either acids or alkalis. 

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Alkyl and alkylether sulphates and all types of sulphonated hydrocarbons always contain a small proportion of unsulphated or unsulphonated starting material. 

Nevertheless, there are many other mixtures of surfactants that can be analysed by classical methods without prior separation. Mixtures of surfactants of one type, e.g. the anionic, cationic and nonionic fractions recovered from an ion-exchange column, can often be so treated, the most notable exception being mixtures of different kinds of sulphonated hydrocarbons, which require high-tech methods. Mixtures of nonionics and of cationics are, however, not often encountered in practice, but a fair proportion of mixtures of surfactants of different classes can be analysed without recourse to high technology, and this is of considerable practical use. 

In all cases the first action is to determine total activity by titration with benzethonium chloride (BEC), and this is to be taken as mandatory in each of the following cases. After acid hydrolysis a second BEC titration to determine the sulphonated hydrocarbon is probably all that is required in the majority of cases, but the following are alternatives. In the case of a-sulphonated fatty esters, the second BEC titration measures the total a-sulphonated fatty acid plus the sulphonated hydrocarbon (see below). 

General. Acid-hydrolyse by the appropriate method and determine the surviving titratable surfactant (sulphonated hydrocarbon) or one of the hydrolysis products. 

Combinations of species other than sulphonated hydrocarbons... 

The presence of sulphonated hydrocarbons does not really make much difference. In the following examples it is assumed that a sulphonated hydrocarbon is present. 

Any sulphate with a-sulphonated fatty ester. BEC titration in alkaline solution after acid hydrolysis gives twice the a-sulphonated ester (approximately—see above) plus any sulphonated hydrocarbon, less the sulphate. Determine the sulphate separately by measuring any one of its decomposition products. 

The results then give total active (yl), other ester or amide (B) and twice the a-sulphonated fatty ester plus any sulphonated hydrocarbon (C). (C — A- -B) then gives (twice) the a-sulphonated ester, and any sulphon-ate hydrocarbon is found by difference. 

BEC titration after acid hydrolysis gives sulphonated hydrocarbon, if... 

BEC titration in alkaline solution gives the sulphonated hydrocarbon plus twice the a-sulphonated fatty ester plus soap from the isethionate and acyl methyltaurate. 

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