CLARK: REDUCTION POTENTIALS 



261 



The potential of this cell will then be at 30° C. 



'R 



-Eh = 0.03 log 



(H2) 



where E^ is the potential of the reduction electrode, E'h that of 

 the hydrogen electrode and (H2) represents the pressure in atmos- 

 pheres of the hydrogen at the reduction electrode. 



As mentioned before, we cannot accurately measure the hydro- 

 gen-electrode potential of the mixtures now under consideration, 

 but the hydrogen-electrode potentials of the buffer mixtures 

 without the small proportion of the oxidizing agents were care- 

 fully measured. These are given in table 13 together with the 



reduction potentials. The values of log - — - for methylene blue and 



(H2) 



for indigo differ sufficiently to distinguish between the two sub- 

 stances, but it may appear that the agreement among the values 

 for either one of the substances is not satisfactory. However, 

 when these values are plotted against p^ they will be found to 

 fall upon a fairly smooth curve. 



We may just as well consider only the differences of potential 

 instead of calculating the hypothetical hydrogen pressure as was 



done above, but the values of log — furnish convenient numbers 



H2 



with which to characterize the intensity of oxidation-reduction 



actions. 



TABLE I 



Reduction of Indigo by Titanium at Ph i-55 



Buffer: M/10 KCl with HCl 



