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Scientific Proceedings (129) 



tential 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 sys- 

 tem and references to the literature see 4 . Using this method, 

 Biilmann 3 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 dis- 

 tinct advantage that it may be used where the platinum black 

 hydrogen electrode is inapplicable, e.g., in the presence of cer- 

 tain 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 or- 

 der 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. 

 (B) Changes in the activities of the dissolved quinone or hydro- 

 quinone molecules due to the presence of salts. (C) The pres- 

 ence of other oxidizing or reducing substances which react with 

 the quinone substances with measurable velocity. 



In order to show the relative importance of these factors 

 we have derived the following general form of the expression 

 for the oxidation-reduction potential (electron pressure) at an 

 inert electrode which will take into account the factors (A) and 

 (B) at varying hydrogen ion and salt concentrations: 



Observed E.M.F. = w<? + 0.059 log + 

 0.059 / K a KjKjA 



hydroquinone 



where tt 0 is the normal potential of the quinhydrone system 

 and has the value +.6990 volts on the hydrogen electrode 



4 LaMer and Baker, Jour. Amer. Chem. Soc, 1922, xliv, 1954, 

 *Ann. Chim., 9th Ser., 1921, xv. 119. 



