cIvArk: reduction potentials 259 



as indigo can be obtained only in the presence of such substances 

 as tartrates. Perhaps there is a specific action here to be ac- 

 counted for, but the more probable explanation is that the proper 

 relations for a sharp-end point are determined by the hydrogen- 

 ion concentration, which mixtures such as the tartrates can con- 

 trol. 



In the next column of the tables is given the percentage reduc- 

 tion estimated from the total amount of reducing agent required 

 to completely reduce the dye. As was just mentioned, it was not 

 always possible to estimate with precision the total requirement 

 by determining the end point potentiometrically. In such cases 

 the disappearance of color or merely a judgment of the end point 

 from the form of the titration curve was used. In any case the 

 error was not large. 



In the third column are given the single electrode potentials 

 observed. These were determined from the potential of the 

 calomel electrode and the difference of potential between calomel 

 and gold electrodes. In each case the standard reference value 

 was used, so that the values given are in terms of the hypothetical 

 normal hydrogen electrode. The sign indicates the sign of the 

 electrode. 



Since both methylene blue and indigo are reduced in accord- 

 ance with reactions which may be written as 



Dye -f 2 electrons = reduced dye, 



the potential equation of Peters becomes at 30° C. 



„ , , 7- , 0.06 , [Oxidation product] 



E observed = £0 H log — 



2 [Reduction product] 



The ratios 



Concentration of oxidation product 



Concentration of reduced product 



multiplied by 0.03 are given in column four of the tables. In the 

 last column are given the values of Eo obtained from the applica- 

 tion of the equation written above. 



It will be seen that there is a substantial constancy in the values 



