26o CLARK: REDUCTION POTENTIALS 



of Eo . Since no correction had been made for the true concen- 

 tration of these products as influenced by the various factors of 

 the solution the agreement is remarkable in some instances. 



In the case of methylene blue it will be noted that only the 

 potentials taken during the early part of the titration are given 

 when the p^ of the solution is greater than 4.55. This is because 

 in the more alkaline solutions the methylene white base precipi- 

 tates. During the early part of the titration the solution remains 

 clear and the potentials follow a smooth curve which begins 

 normally. The cur\'e of potentials soon tends to flatten and there 

 comes a moment when the potential rises. It is at this moment 

 that the reduced compound separates from its now supersatu- 

 rated solution. Its removal leaves the oxidation product in 

 greater excess and the potential of the electrode becomes more 

 positive. Perhaps there is at hand in such phenomena the means 

 of accurately determining the solubility of such compounds. 



When there thus arose the impossibility of obtaining a com- 

 plete titration curve of methylene blue at the concentration used, 

 the part of the curve that was obtained was plotted and overlaid 

 with the theoretical form which applies to all the curves. The 

 probable position considered in conjunction with the calculated 

 values of Eo which were obtained, determined the "probable 

 values" of Eo which are given. 



It is sometimes customary to consider a reduction electrode as 

 a h} drogen electrode under diminished hydrogen pressure. 

 Although the hydrogen pressures so estimated become impossibly 

 small in some instances this may be a legitimate way of correlating 

 reduction potentials at different acidities. We may consider the 

 relation in the following form. 



Suppose our reduction electrode were a hydrogen electrode 

 under that pressure of molecular hydrogen which is in equilibrium 

 with the oxidation-reduction products of the solution. Let the 

 same solution be in contact with a hydrogen electrode under a 

 pressure of one atmosphere of hydrogen, and let the two elec- 

 trodes and the solution be connected as in a cell. 



