﻿524 BOTANICAL GAZETTE [june 



where P and Pi represent the concentrations of hydrogen ions in 

 solution. 1 If the concentration of hydrogen ions in one solution is 

 known, the concentration of the second may be calculated from the 

 observed potential. In practice it is customary to substitute a 

 standard half-cell (for example, a calomel electrode) for one of the 

 hydrogen electrodes. 



In a similar manner, when an oxygen electrode dips into a solu- 

 tion, the oxygen on the electrode tends to give up to the solution 

 negatively charged oxygen ions until equilibrium is reached. The 

 electrode, therefore, becomes positively electrified. If there be 

 added to the solution an oxidizing agent (which, by definition, gives 

 up positive charges), it will increase the positive charge on the 

 electrode. In the case of potassium permanganate in acid solu- 

 tions, the source of the positive charge may be represented by the 

 equation 



MnO>+4H+=Mn+++ 4 OH-+5+, 

 and the potential may be calculated from the formula 2 



Experimental complications are met with in using platinum as 

 an electrode, since it appears that oxygen combines with platinum 

 to form oxides. 3 These complications are of theoretical interest, 

 but for practical purposes they may be neglected, since the platinum 

 electrode, when charged with oxygen, gives satisfactory comparative 

 values of the oxidizing and reducing ability of different compounds. 

 This is true even when the manner in which the compound ionizes 



The measurements recorded in this paper, and in investigations 

 to be reported on later, were made with the apparatus illustrated 

 by fig. i. The reaction cell A consisted of a 200 cc. beaker con- 

 taining the solution to be tested, into which the two electrodes 

 dipped. The electrode B was made from a platinum crucible of 



'LeBlanc, M., Electro-chemistry; translation by Whitney and Gowan. New 

 York. 1907 (P- i95). 



'Crotogino, F., Zeit. Anorg. Chem. 24:225. 1902. 

 'Schoch, E. P., Jour. Phys. Chem. 14:665. 1910. 



