1495 
Us 
FS Pes Do . . “7 - 
The difference a - is, namely, the potential difference, which 
M5 
occurs when two metals come in contact, and which is always 
small in comparison with the potential difference metal-solution. 
Though equation (34) is little suitable to indicate the potential 
difference between a metal and a solution, it gives a very simple 
relation for the potential difference between two metals, e.g. Cu 
and Zn, which are each imimerged in a solution of their ions. 
In this case we get, namely, 
RT L RT My 
Ne aa) eee: 4 G5) 
vk Lu, vk Mr, 
Wo, Hy. 
because and En are practically equal for the two metals. In 
this it has been assumed that the valency rv is the same for the two 
metals. 
When now the concentration of both metal ions in solution is 1, 
we get: 
Jd Lu. 0.058 D : 
A A= Ih ze log ee „ (36) 
rf Ear, PY “(La 
It follows from this that the difference between the normal potentials 
of two metals is equal to 0.058.*°/og of the ratio of the solubility 
products. 
If the valency of the metal ions is different, then 
A meenen L Ri 
1 Er Ms) — 
1 
In (Lu,) Seer (87) 
in general for (My) = 1. 
If we take the hydrogen electrode as zero point, through which 
A, becomes = 0, then: 
058 
son 
10707 (Lu) 2 Sees) 
1 
A, = 0,058 101, (La,) — 
The normal potential of a metal with respect to H, =O is there- 
fore, exclusively determined by the solubility product of the metal 
and that of the hydrogen. By this latter we then understand the 
value which the product (H ) (4) has in a solution which is saturate 
with respect to hydrogen of one atmosphere. 
When the solubility preduct of a metal was known, then by the 
aid of the known normal potentials we should be able to calculate 
the solubility products of all other metals. This is, however, not 
the case. We may, however. say that the most negative metal, Lz, 
