ELECTRODE POTENTIALS 173 



but the pPI values are different, one of these being known and the 

 other unknown, the latter may be determined as in the ordinary Ho- 

 gas chain. For pH values > 8.5 the above described limitation 

 renders the method unusable. 



In practice the mixture of quinone-hydroquinone in a constant 

 proportion is replaced by the addition to the solution whose pH is to 

 be measured of a small amount of quinhydrone. This is a molecular 

 addition compound consisting of 1 mol hydroquinone + 1 mol 

 quinone, and which in solution splits into its components. 



For a more detailed account of the theory of oxidation-reduction 

 potentials and of the use of the quinhydrone electrode the reader is 

 referred to the original literature."* 



«E. Biilman and collaborators, Ann. de China. 15, 109 (1921); Ibid. 16, 321 

 (1921). Trans. Faraday Soc. 19, Part 3, (1924). Jour. Chem. Soc. 125, 1954: 

 (1924). 



S. P. L. S0rensen, M. S0rensen and K. Linderstr0m-Lang. Compt. rend, 

 trav. lab. Carlsberg, 14, No. 14 (1921); K. Linderstr0m-Lang. Ibid. 16, No. 3 

 (1925). 



W. M. Clark and collaborators. U. S. Public Health Reports, 38-39„ 

 (1923-4). 



