Other variants of two-phase titration

Many variations have been published on the basic idea of two-phase titration, with colour transfer from one layer to the other to mark the end-point, and in view of the importance of this general technique it is desirable for the analyst to have some appreciation of the subtle but significant differences between the variants, only a few of which are discussed below. 

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. 

Lew [12] used bromocresol green in alkaline solution, again taking the end-point as the point at which the two layers were of equal intensity. Bromocresol green is an anionic dye, and at this point most of the anionic and a substantial proportion of the indicator have been titrated, but it does not correspond with true stoichiometry (though that does not necessarily imply inaccuracy). Users report that the end-point is not very sharp and the solution is not clear at the end-point, which is no doubt due to the small amount of untitrated anionic remaining in the aqueous layer. 

Battaglini et al. [13] recommended phenol red in alkaline solution for titration of a-sulphonated fatty esters, taking the first appearance of a 

The present author [14] has also proposed the principle that the indicator should have the same charge as the species being titrated and that the end-point is the point at which the whole of the surfactant and the whole of the indicator have been titrated, i.e. when colour transfer to the chloroform layer is complete. Bromophenol blue is used for titration of anionics and methylene blue for titration of cationics. In both cases titration is continued until the top layer is clear and colourless. A measured 1 ml of indicator is used and a blank titration is done on the indicator alone, but the blank is appreciable only for bromophenol blue, because methylene blue is more intensely coloured and a much smaller quantity is used. 

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