Optimization of a novel two-phase potentiometric titration method of barbiturates with mercury (II). Part I: Theory.

A two-phase potentiometric titration procedure for barbiturates with mercury(II) was developed. In the new titration procedure the barbituric acid derivative is dissolved in an aqueous borate buffer. An organic phase consisting of chloroform and benzyl alcohol is added, and the vigourously stirred contents of the titration vessel are titrated by a mercury(II) nitrate solution. The theory of the conditional formation constant in a two-phase system is derived to provide a basis for the selection of optimal titration conditions. It is shown that the new conditional formation constant of the two-phase system, K, increases if the mercury-barbiturate complex is extracted into the organic layer. By extraction of the barbituric acid derivative into the organic layer K is diminished, but this effect can be minimized by a correct choice of the pH of the aqueous layer. The influence of these factors on the pM jump in the equivalence point is demonstrated by calculation of the corresponding theoretical titration curves.