Thermometric titration technology can make surfactant analysis easier; a simple, long-lasting thermometric probe is the easy alternative to toxic reagents or expensive electrodes
The classical method for the analysis of anionic active matter in surfactants is the 'two-phase' titration with the cationic titrant hyamine 1622 (benzethonium chloride).
A mixed indicator of disulphine blue VN and dimidium bromide is used, and the titration is carried out in the presence of chloroform. Dimidium bromide is toxic to the liver and causes cancer.
The use of chloroform in many laboratories is severely circumscribed due to a range of toxic effects and suspected carcinogenicity.
This sounds like a technique that should be avoided at all costs! In recent years, the development of so-called 'surfactrodes' (electrodes sensitive to ionic surfactants) have enabled the potentiometric titration of anionic and cationic surfactants in aqueous media.
The sensing media are highly specialised, and for optimum results with a range of surfactants and formulations, two different types must be used. One type requires refilling with a special paste, and is not suitable for use with water-immiscible solvents.
Thermometric titrimetric techniques for analyzing ionic surfactants were developed nearly 40 years ago, and Multitrator decided to re-visit this technology to determine its applicability to modern thermometric titration instrumentation.
It used 1M cetylpyridinium chloride (a cationic surfactant) instead of hyamine 1622 as titrant, as it has been reported to give a more exothermic reaction with anionic surfactants.
It analysed two samples of sodium lauryl ether sulphate of different activities, and says it obtained excellent precision in both instances.
High purity sodium dodecylbenzene sulphonate was used to standardise the titrant.
The hydrophilic groups on the two surfactants are electrostatically neutralised, solvated water is rejected and a precipitate forms.
The enthalpy for the reaction between cetylpyridinium chloride and dodecylbenzene sulphonate has been reported to be -24.3kJ/mol.
While earlier workers reported rounded endpoints, Multitrator says its were unequivocal and sharp.
