( 190 ) 
If the formula of the compound is Me J/,’ , then owing to disso- 
ciation, ions M, and J, will occur along with ions Mt /," and 
between these an equilibrium will exist expressed by the equation: 
Pi pt = KP 
When the total concentration of the ions remains constant, /—p, 
may be substituted for p, and the equation becomes 
pPi* (k—p,)? = Kp 
12° 
The maximum value of p,, is reached when the first differential 
quotient with respect to p, = 0, that is, when 
ap, (k—p,)’ — b p,* (k—p,)’ = 0 
or atk =p:) = bp, 
ee Pi? P,.—: b. 
P,. therefore reaches a maximum and a a minimum where the 
ratio of the ions J/, and MZ, in the electrolyte 
Fig 5. 
is equal to that of the metals in the compound. 
a. If the compound can be in equilibrium 
with an electrolyte in which the ratio of the 
cathions is the same as that of the metals in 
the compound and if in addition to the com- 
pound only the metals in a pure condition 
are capable of existence, then the a-curve 
will have the form indicated in fig. 5. 
The points on the line AG give the compositions of electrolytes in 
equilibrium with pure J/, and the corresponding potential differences. 
With the electrolyte G both JM, and the compound are in equilibrium. 
So long as both metal phases are present, the potential difference remains 
constant. Should JM, have entirely disappeared, so that the electrode 
consists of the pure compound (composition = /), the electrolyte may 
vary from G to K while the potential difference first falls to Hand 
then again rises to A. In A there is again a non-variant equilibrium 
between the compound, pure J/, and the electrolyte A’ and so long 
as these phases exist the potential difference remains constant. But 
when the compound has disappeared, it falls to B, while the 
electrolyte changes from A to pure J/,Z. 
From an electrolyte having a composition situated between G and A” 
the compound .W,M, is precipitated by J/, and also by M,. 
