148 



When we now anodicall}- polarize witli the same strength of current 

 as wh^en the liydrogen concentration was represented by i?i6\, the 

 hydrogen charge at the l)oiindary surface will now descend below 

 C'l, since the thickness of the diflfnsion layer is now so much greater 

 than immediately after polarization, and the diffusion of hydrogen 

 accordingly proceeds so much more slowly. In consequence of this 

 the chromium gets a more positive potential. In this way it is, 

 therefore, explicable that the chromium can bear a stronger current 

 when this is again put on immediately after the interruption than 

 when the current has remained broken for some time, though in 

 the latter case the j)olential is more negative in current-less condition. 

 It now also apj)ears that there is no immediate connection between 

 the potential of a cliromium electrode in current-less condition, and 

 the possibility of its becoming passive. The former is namely deter- 

 mined by the density of the hydrogen charge at the boundary surface, 

 whereas it depends on the gradient of concentration in the diffusion 

 layer whether the electi'ode can be made passive. The before described 

 phenomenon that not always the most negative electrode is most 

 difticult to j)assivate, is in agreement with this. 



So far the phenomena can, therefore, be explained by the aid of 

 the hydrogen theory. That the phenomena are caused by a parti- 

 cular state of the metal surface, and not of the liquid, appears from 

 this that they qualitatively remain the same, when the li(juid is 

 vigorously stirred, and also when the liquid is entirely renewed. 

 The potential of the electrode only becomes somewhat more positive 

 by stirring. This is probably owing to this, that in consequence of 

 the stirring the solution contains more oxygen, and the hydrogen is 

 more quickly withdrawn from the chromium surface. When the 

 stirrer is stopped, the potential falls again. 



In these experiments the chromium anode was placed in a saturate 

 solution of KCl, the cathode in a same solution in a porous vessel. 

 This latter liquid became alcalic during passage of the current. The 

 solution round the anode became on the other hand acid. To deter- 

 mine the degree of acidity a hydrogen electrode was placed in this 

 solution. It presented a potential — 0.58 V. with respect to the n. 

 calomel electrode, corresponding with a hydrogen ion concentration 

 of 10^^. A current of 4 mA had been led through this solution for 

 20 hours. 



That the solution became acid can be explained by a hydrolytic 

 splitting up of the formed CrCi^, or by the hydrogen present in the 

 chromium going anodically into solution as H. 



As the volume of the solution amounted to about 300 cm", and 



