QUINHYDROXE ELECTRODE 



241 



scale or +.4250 volts when measured against the saturated 

 calomel cell at 25° C. The temperature coefficient is — .77 

 millivolts per degree, K x and K 2 are the primary and secondary 

 acidic (phenolic) ionization constants of hydroquinone. The 

 symbol "a" refers to the activity or thermodynamic concen- 

 tration of the respective substances. 



Subject to the restrictions given below this general equation 

 reduces to the simplest form 



k o — observed E.M.F. 



which differs from the usual hydrogen electrode formula only 

 in the magnitude of tt 0 ; namely, by .6990 volts. 



From the values for K x and K 2 , determined by Sheppard 8 ; 

 namely, 1.8 X 10" 10 and 4 X 10~ 12 , it can readily be seen that 

 the term involving these constants, which is the correction for 

 factor (A), becomes negligible (less than 0.2 mv.) when the 

 reaction is more acid than P H 8.0, but that it rapidly increases 

 in magnitude as the solution is made more alkaline. 



With regard to factor (B) our experiments have shown 

 that the total salt concentrations present in physiological solu- 

 tions, except in certain special cases, are seldom sufficient to 

 affect the activities of the quinone substances appreciably, but 

 in any given case the magnitude of this effect may be evaluated 

 and taken into account by determining the activity coefficients 

 from the solubilities of the quinone and of the hydroquinone in 

 pure water and in the salt solution under examination, accord- 

 ing to the equations : 



activity (a) — activity coefficient (/) X molar concentration (/) 



solubilitv in salt solution 



in salt solution = 1 



solubility m pure water 



In contrast to (A) and (B), factor (C) is important for 

 solutions that are exposed to oxygen owing to the conversion 

 of hydroquinone to quinone which alters the equimolecular 

 ratio between these two substances. The as yet unpublished 

 results of kinetic experiments by LaMer and Rideal on the 

 mechanism of the autoxidation of hydroquinone have shown 



6 Trans. Amcr. Elect roch cm. Soc, 1921, xxxix, 428. 



