» 
Chemistry. —- ‘On some Particular Cases of Current Potential Lines’. 
Il. By Dr. A. H. W. Arex. (Communicated by Prof. A. F. 
HorLEMAN). 
(Comraunicated in the meeting of October 28, 1916.) 
In the preceding paper > an equation was derived which represents 
the ratio between the quantity of silver chloride which is deposited 
on the anode in case of anodic polarisation of a silver electrode in 
a, solution of a chloride, and that which is deposited in the liquid. 
This equation now enables us to indicate in what way the electro- 
lytical determination of halogens as silver halides takes place with 
the smallest error possible. 
In these determinations the conditions must be satisfied that the 
halogen is deposited as completely as possible on the anode as silver 
halide and that as little silver as possible goes into solution from 
the anode. 
During the analysis the halogen content of the liquid diminishes 
continually. The course of the process can therefore not be indicated 
by a single current potential line, but only by a number of current 
potential lines for all concentrations which the halogen ion passes 
through from the beginning to the end of the analysis. If e.g. we elec- 
trolyse a 0.01 » solution of NaCl, the course is in the beginning 
represented by line 1 in Fig. 2 of the preceding paper. If in course 
of time the solution has become 0.001 7, line 2 is applicable, later 
on, when the concentration has become 0.0001, line 3 applies, ete. 
As was demonstrated on p. 668 the points on the leftside of AB 
give current densities which chiefly yield AgCl on the anode. If 
C,=0.01 we can, therefore, work with a comparatively large 
current density, without Ag appreciably going into solution. For 
C, = 0.001 this current density is ten times smaller, for C, = 0.0001 
a hundred times smaller, etc. 
If we now wanted to execute the electrolysis with a constant 
current density, till C, bad become = 0.0001, we should have to 
work with a very small current density from the very beginning, 
for which with (C,—0.000!1 no appreciable quantity of Ag goes 
into solution. In consequence of this the analysis would last very 
long. It is more advisable to work with a greater current density, 
as long as (’, is still great, and diminish it according as C, descends. 
This may be done by measuring the anode potential during the 
analysis, and by regulating the intensity of the current so that the 
1) These Proc, Vol. XIX, p. 653. 
