Determination of sulphonated surfactant species

Titration with benzethonium chloride in acid solution measures the sulphonated ester plus the sulphonated carboxylic acid, only the sulphon-ate group of the latter being titrated. In alkaline solution the titration measures the sulphonated ester plus twice the sulphonated carboxylate, both the sulphonate and the carboxylate group being titrated. Alpha-sulphonated esters give poor potentiometric titration curves, and two-phase titration is strongly preferred. 

If the titration in alkaline solution is done without pretreatment, the soap will be titrated as well. The following method extracts the fatty acid to eliminate this complication. 

Dissolve a sample containing 1.0-1.5g active in 50 ml water and transfer to a separating funnel using 50 ml ethanol. 

Just acidify with IM hydrochloric acid and extract with three 50 ml portions of petroleum ether. Collect the lower layer in a 11 volumetric flask. 

Combine the petroleum ether extracts and wash with 50 ml water. Collect the washings in the 11 flask. Discard the petroleum ether extracts, unless they are required for the determination of ester and fatty acid. 

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