69^ 



by the solution L and the solid phase S. This solid phase, therefore, 

 contains very nmcli of the less noble pseudo component a. The 

 metastable unary electromotive two-phase equilibrium is indicated 

 by the solution L' and the solid piiase S'. 



As was set forth before, on anodic polarisation tlie metal phase >S 

 will move down along Sri, hence become nobler, whereas the metal 

 surface will move upward along Sa on cathodic polarisation, hence 

 become less nol)le. 



The stable unary electromotive e(|iiilibrium requires an electrolyte 

 which contains only exceedingly few ferri-ions {^) by the side of 

 the ferro-ions («). When iron is immerged in a solution of ferri- 

 chloride, the s^^stem tends to assume unary electromotive equilibrium, 

 in which we may assume the metal phase to send ferro-ions into 

 solution, whereas ferri-ions are deposited from the solution on the 

 melal. 



As follows from the A, .r-figure, a solution containing many ferri- 

 ions could only be in pseudo-electromolive equilibrium with the iron 

 for much less negative potential of the iron. Hence there Avill be a 

 tendency to make the electrolyte richer in ferro-ions, and the metal 

 in ferri-ions, but until the unary equilibrium concentration has been 

 reached, the iron potential will possess a too small negative value, 

 as was also observed. 



It is further to be seen that the negative value of the iron poten- 

 tial will have to increase in a solution of ferro-sulphate, when 

 during the measurement the ferri-ions are precipitated as much as 

 possible. 



This follows, indeed, from the following table. 



Initial potential of Fe in Vio N. FeSOj-solution ') = — 0,538 V 



Potential „ idem „ with a little NH4CNS = — 0,578 „ 



„ idem „ „ „ H,P04 = — 0,569 „ 



„ „ idem „ „ „ NH4 oxalate = — 0,555 „ 



The removal of the ferri-ions makes the iron clearly baser. 



It is here the place to point out that in the just mentioned etching 

 experiments with chlorine, this substance has only indirectly caused 

 etching. It is namely very well possible that the action of the 

 chlorine has consisted in this that the ferro-ion emitted by the iron 

 is immediately converted into the ferri-ion, in consequence of which 

 the electrolyte remains as far as possible from the concentration 

 of the unary electromotive equilibrium, and that this gives to 

 the electrolyte its maximum etching action with respect to the iron 

 electrode. 



V This solution contained traces of ferri salt. 



