Electrometric acid-base titrations by means of the quinhydrone electrode and its application under physiological conditions. 1

Abstract

It has recently been shown that an acid solution containing equimolecular proportions of benzoquinone and hydroquinone, a condition which is most easily obtained by dissolving the crystalline addition product, benzoquinhydrone, in the medium under examination, promptly produces a stable and reproducible oxidation-reduction potential on a properly prepared inert electrode, preferably of gold. In acid solutions the observed potential has been shown to be a linear function of the P_H and may, therefore, serve as a very simple means of determining hydrogen ion concentrations. For measurements on this system and references to the literature see 4 . Using this method, Biilmann 5 has shown that the potential is stable in a phosphate buffer at P_H 6.81, but unstable in an alkaline borate solution at P_H 9.24, and further that the method possesses the distinct advantage that it may be used where the platinum black hydrogen electrode is inapplicable, e.g., in the presence of certain mild oxidizing agents as 0.1 M nitric acid or unsaturated compounds of the acryhic acid type.

We have made a study of the quinhydrone electrode in order to determine more definitely the range of its applicability under physiological conditions. The validity of the method depends upon maintaining the ratio of the concentrations or more correctly of the activities of the quinone and hydroquinone strictly equal to unity or to some other constant and known value.

The factors which may upset this relationship are: (A) Deviations from the simple oxidation-reduction equation of Peters due to hydroquinone acting as a weak dibasic acid. (13) Changes in the activities of the dissolved quinone or hydroquinone molecules due to the presence of salts. (C) The presence of other oxidizing or reducing substances which react with the quinone substances with measurable velocity.