Clxxxi. Dissociation Constants of Cys-

MATHEWS and WALKER [1909] and, in greater detail, Dixon and Tunnicliffe [1923] have demonstrated the great significance of the hydrogen ion activity in relation to the rates of oxidation and of autoxidation of a number of thioacids. To assist the analysis of these results and of certain observations of our own it became desirable to know the dissociation constants of cysteine, thioglycollic acid and thiolactic acid. It was thought worth while at the same time to determine the corresponding constants of cystine in order that a gap in the electrochemical data on the natural amino-acids might be filled. With respect to cysteine, thioglycollic acid and thiolactic acid no difficulty was encountered in obtaining reproducible hydrogen electrode titration curves from which the several constants could be deduced. The method was,' however, not available in the case of cystine as this substance suffers reduction at the hydrogen electrode surface with consequent depolarisation of the electrode. The quinhydrone electrode suggested itself as an alternative since its potential is positive to that which, on chemical and other grounds, may reasonably be attributed to solutions of cystine. This electrode, indeed, proved satisfactory within its limitations but was rejected because it does not permit the extension of a titration to the low concentrations of hydrogen ions at which stages in the dissociation of cystine occur. An oxidation-reduction electrode was sought, therefore, which would be available over the whole significant PH range and whose equilibrium would not be disturbed by the presence of cystine. The methylene blue-methylene white electrode fulfilled these requirements.