401 



Mg, hydrogen diid elecliolj'te. Between iliese limits tlie observable 

 oases lie. 



One liinit is indicated in fig. 2. Here tlie hydrogen is in inner 

 equilibrium but that of the Mg is displaced to such an extent that 

 the potential line of this metal has the position a,c,. 



At the other limit the Mg remains in inner equilibrium but the 

 liberating hydrogen is in a state of formation so that its potential 

 line has the position b^c, in tig. 3. 



In the latter case the observed potential of the three phase equi- 

 librium nc,e will practically correspond with the equilibrium poten- 

 tial of Mg'). The ob.served cases lie between these limits. 



The above remarks concerning Mg with a potential of — 1.86 V. 

 also apply to Mg with a potential of — 1.3 Volt. In this case 



I =10^^ -''•' and then ^^ = bll = w' "" '\ so that ,f (Mg-) 

 ^ (Hl)' Lh,^ 



= l,(H-) = 10-22 ^^. (OH') = 10^ 



Consequently when Mg of a potential of — 1.3 V. was liberating 

 hydrogen in inner equilibrium from a solution of a Mg salt in 

 which (Mgj*) = J, then OH' in the surrounding liquid layer would 

 be 10^. This is practically also an impossibility. 



From the above it follows that the hydrogen which coexists with 

 magnesium and the surrounding liquid must be in such a condition 

 that the value of 1j|j is much greater than that corresponding to 

 the inner .equilibrium. 



This statement arouses a suspicion lo the precipitations of Mg- 

 hydi'oxide in the surrounding liquid layei', but if this occurred the 

 coexisting hydroged would be formed in a stronger state of formation 

 tlian even in the case that the surrounding liquid is no longer 

 saturated with respect to Mg(OH),. 



The solubility product of MgtOH)^ is about 10 ' since the value 

 we choose for Mg is immaterial we will assume (Mg'*) ==: 1 . In 



this case (OH'j^) — 10~^•^^ 



If the Mg-electrode has the value LMg^JO' " we have already 



calculated that (Hjj = 10 "' or (OHl) = 10" which is quite im- 

 possible for the solubility product of Mg(OH\ requires here (OH') = 



= 10~"^ 



1) Here it must be remarked, that if hydrogen is being liberated the composi- 

 tion of the bounding liquid layer will always lie more to the lef than that of 

 the liquid outside. 



