260 



of the concentration ; on the assumption that the pressure (1 atm.), 

 temperature, and total ion-concentration (metal ions -|- hydrogen 

 ions) are constant. In the foregoing figure 1 hydrogen is taken for 

 one electrode, and palladium for the other, but instead of the 

 latter platinum might, of course, have been chosen just as well. 



Line bh indicates the potentials of the Series of electrolytes that 

 can coexist with different palladium phases. These phases of the 

 palladium are different, because palladium dissolves the hydrogen 

 in quantities wrhich increase with the hydrogen-ion concentration 

 of the electrolyte. 



Line bf indicates the potentials of the different palladium phases 

 containing hydrogen '), which coexist with the different electrolytes. 

 In our £',A-figure the potential of the metal-phase can be read on 

 the E-&x\s, but it is clear that on this axis also the potential oftiie 

 electrolyte can be read, when we reverse the sign. 



The line ag represents the potentials of the different electrolytes 

 coexisting with the gaseous hydrogen phases. These hydrogen phases 

 consist of pure hydrogen, and lie, iherefore, on the hydrogen axis. 

 Accordingly the portion ak of the hydrogen axis gives the potentials 

 of the hydrogen phases coexisting with the different electrolytes. 



The point of intersection c of the lines bh and ag represents the 

 electrolyte which can coexist at the same time with the palladium 

 phase [e) and with the hydrogen phase [d), so that it also shows 

 the potential of this three-phase equilibrium. The situation of this 

 point of intersection follows from the solubility products of hydrogen 

 and palladium : ') 



Lpd= (Pd- •) ((9)' = 1 0'' ^ - «". 

 At the three-phase equilibrium 



(6)H,={e)pd 

 from which follows: 



(Pd--) _Lpd_ JQ2X-U.2 



iffy Lh, 



If (H') is put =1, then {Pd") = 10^^'*-^. 



From this it is seen that the point e lies very much on one side, 

 and that when a palladium electrode was immersed in a 1-N sulphuric 



') This line indicates the gross hydrogen concentrations, and gives, therefore, 

 no information about the stale in which the hydrogen is 



•) Compare with regard to the smallness of these products the remarks in 

 "The Theory of Allotropy" in the chapter: "Small concentrations" p. 172. 



