771 
as in practice is always the case, the current density is given with 
great approximation by: 
D, 
d — Lt 1 7 5 (C1 ee Cia) . or as Cla pmen ni 
€ 
De. 
a Pals, ; (l — n)C,. 
C 
If the total quantity of halogen is J/ gramme equivalents, the 
diminution of this per time unit is equal to the current strength, 
divided by 96500, bence: 
dM Od + f , ; 
ee if O is the surface of the anode. 
dt 96500 : 
If further the volume of the solution is WV, then M— VC, and 
consequently : 
dC, Od OD, 
er ha = ae > TA (l—x) C; 
dt 96500 V 86400 dV 
from which follows: 
86400 DV C1 
ROAN MP Soe ORO BENEN 
OD, (1 ae n) Cie 
It appears from this equation that for a given value of Ci, and 
Cie the analysis will proceed the more rapidly as d is smaller, 
consequently as the stirring is more vigorous, the volume is smaller, 
n is smaller, and O and DP, are greater. D, may be increased by 
rise of temperature. A large area of the anode is particularly de- 
sirable here because the deposited silver chloride covers the metal, 
and the cffective area becomes, therefore, smaller during the analysis. 
The electro-analytical determination of the halogens has been fully 
examined by Sith‘) and his collaborators, and the recorded results 
are satisfactory. The method for chlorides has appeared to give the 
best results when the solution contains a sufficient quantity of 
OH-ions, to form AgOH with the silver, when the chlorine ions are 
almost consumed. In this way the loss of silver of the anode is 
prevented. In consequence of the precipitated AgOH the anode 
weighs too heavy, but the silver hydroxide can be easily decomposed 
by heating, so that the increase of weight now actually gives the 
deposited chlorine. There always remains a deficit, however, as not 
all the chlorine from the solution can be deposited. For the 
other halogens the error will be smaller than for chlorine, when 
the same method of working is followed. 
The difference between the three halogens appears clearly from a 
research by Respy*), who found a deficit of 0.2 °/, for a diluted acid 
1) SmitH, Electroanalysis. 
2) Amer. ‘ourn. of Science. (4) 40 (1915) 281. 400. 
