Epton method

The first two-phase titration method to come into widespread use was that of Epton [10], who used methylene blue as indicator. Methylene blue is cationic, and in the Epton method all the dye passes into the chloroform as its salt with the anionic at the beginning of the titration. In the region of the end-point it returns to the aqueous layer. Provided that the ratio of the volumes of aqueous and chloroform layers is 3 1 at the end-point, equal colour intensity in the two layers indicates equivalence between anionic present and cationic added. This is quite empirical and does not correspond with the completion of any chemical process or any clearly defined event. The more the volume ratio differs from 3 1, the greater the deviation of the observed result from true equivalence. Another difficulty is that the hues of the two layers are different, and matching them is very operator-dependent. Nevertheless, the method was the standard for many years and performed invaluable service to the surfactants industry. It is still in use in some laboratories and ASTM Method D 1681-83 [11] includes it, but ISO 2271 [7] and some of the methods described below are to be preferred. 

Volumetric methods are the traditional methods employed in routine controL specifically two-phase titration for anionic and cationic surfactant determination, and are characteristically highly sensitive. They are based on the reaction between an anionic surfactant and cationic surfactant (one of which is a sample surfactant and the other a titrant surfactant) in a two-phase system (chloroform-water). The endpoint is detected by a transfer-phase indicator, the most common of which is a mixture of dimidium bromide and disulfine blue although methylene blue (Epton method), which is the first indicator chronologically, is also applied. Two commonly used titrants are sodium lauryl sulfate for cationic surfactants and benzethonium chloride, currently named Hyamine 1622, for anionic surfactants. 

The method developed by Epton [212,213] became the universally accepted method for the analysis of active matter of anionic and cationic surfactants. Epton s method, also known as the two-phase titration, is based on the titration of the anionic surfactant with cetylpyridinium bromide, a cationic surfactant, in the presence of methylene blue as indicator. A solution of the anionic surfactant is mixed with the indicator dissolved in dilute sulfuric acid, followed by further addition of chloroform, and then it is titrated with the cationic surfactant. Methylene blue forms a complex with the anionic salt that is soluble in chloroform, giving the layer a blue color. As the titration proceeds there is a slow transference of color to the water layer until the equivalence point. At the equivalence point colors of the chloroform and water layers are visually the same. On successive additions of titrant the chloroform layer lightens in shade and finally becomes colorless. 

Epton, S.R. 1948. New method for the rapid titration analysis of sodium alkyl sulphates and related compounds. Trans. Faraday Soc. 44 226-230. 

Adsorption Studies Appropriate amounts of cleaned latex and surfactants were mixed, equilibrated for 24 hours and the excess surfactant in serum analyzed after separation by centrifugation. Igepal CO-630 and Alipal surfactants were analyzed by colorimetric titration with cetyltrimethylammonium chloride in the presence of methylene blue and a chloroform layer similar to the method of Epton (9). Some non-ionic surfactants and solubil-... 

There are a number of books [168-173] and reviews [68,174,175] that provide good starting points. Refs. [168,176-178] discuss methods for the determination of anionic surfactants, which are probably the most commonly encountered in industry. Most of these latter methods are applicable only to the determination of sulfate-and sulfonate-functional surfactants. Probably the most common analysis method for anionic surfactants is Epton s two-phase titration method [179,180] or one of its variations [171,172,181-183]. Additional references are provided elsewhere [163], Aqueous surfactant micellar systems have also been utilized successfully in virtually every area of analytical chemistry (for example, [184—186]). 

Epton R, Goddard P, Ivin KJ, Gel phase (13)C NMR spectroscopy as an analytical method in solid (gel) phase peptide synthesis, Polymer, 21 1367-1371, 1980. 

Epton, R., Hydrophobic, Ion Exchange and Affinity Methods, in Chromatography of Synthetic and Biological Polymers, Vol. 2, E. Horwood, Chichester, 1978, 1-9. 

Epton, S. R. (1947) A rapid method of analysis for certain surface-active agents. Nature 160, 795-796. 

Concerning the synthesis of iV-aminopyrroles (52) Epton [276] has reported that the condensations of acetonylacetone and 2,5-diethoxytetrahydrofuran (succindialdehyde source) with A -aminophthalimide are superior to earlier methods where the partly-protected benzyloxycarbonyl- [277] and toluene-sulphony 1-hydrazines [278] were employed. Apparently this method fails with other 1,4-diketones, possibly due to steric effects [279]. 

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