140 



hydi'ogen charges which uii electrode ac(|iiires for dilFerent |)otential8 

 when the concentration of tiie hj drogen ions in the solution is 1. 



The same thing is given hy the line CD for a hydrogen ion 

 concentration 0.01 etc. These lines are nothing but the graphical 

 representation of the potential of the hydrogen electrode. 



Let us now assume that the potential of a chromium electrode 

 in a solution of a given chromium concentration is determined by 

 the density of the hydrogen charge, and can be represented by the 

 line PQ in figure 1. There is little to be said about the course of 

 this line; only on increasing hydrogen charge this line must draw 

 near to a limiting value, which represents tiie real potential of 

 equilibrium of chromium. 



This latter sui)|)Osition, that the electrode presents a chromium 

 potential, which c.p. is only dependent on the density of the hydrogen 

 charge, goes slightly further than the usual hydrogen hypothesis, 

 according to which the hydrogen would only accelerate the setting 

 in of the equilibrium. According to our conception a certain chro- 

 mium potential corresponds to a given hydrogen charge. This suppo- 

 sition may seem improbable considered in itself, without it an 

 explanation by the aid of the hydrogen hypothesis is not possible 

 in my opinion. In what way the chromium potential comes about 

 here, will not be discussed. 



When we now take a chromium electrode with a gas charge 1, 

 then the potential of this is R, according to figure i. If this electrode 

 is placed in a solution, which is 0.01 n. of acid, it can generate 

 from it exactly hydrogen of one atmospliere. In consequence of the 

 overvoltage no hydrogen will be generated, but the chromium will 

 not lose its gas charge either. Hence the potential remains unchanged. 

 When the same chromium is brought in an acid in which the con- 

 centration of the hydrogen ions is = 1, the chromium at the 

 potential R can develop hydrogen in this acid to a pressure of 10^ 

 atm. In consequence of the increase of the hydrogen yharge the 

 potential now descends below R, this causes the hydrogen charge 

 to increase, and thus the potential will continue to descend. The 

 lowest value that can be attained, is 'S. This value need not be 

 reached, however. It can only exist for a hydrogen charge of 10'" 

 atm., and this is only possible when there is a very great over- 

 voltage for hydrogen generation at chromium. 



It appears therefore from this, that chromium with a hydrogen 

 charge 1 will spontaneously activate, when placed in an acid which 

 is stronger than 0,01 n. If the acid is weaker, the chromium does 

 not spontaneously become active. In 10-* n. acid a hydrogen charge 



