1907-8.] Electromotive Force of Iodine Concentration Cells. 383 
The method used was a null one, the resistance ratio being altered till 
no deflection was obtained on the electrometer. 
The standard cell and the cells being measured were kept in a thermostat, 
the temperature of which, with a corrected thermometer, was 20*4° C. for 
the measurements with the water cells, but afterwards raised to 25° C. for 
the alcohol measurements, as this is the temperature usually selected. 
The room in which the experiments were made is fairly constant in 
temperature, and the thermostat was not found to alter as much as ^th of 
a degree. A special form of cell was devised for the experiments. A glass 
tube about J centimetre in diameter had a stopper ground into one end 
and the other end was drawn out, and a platinum wire fused into it, so as 
to form a little vessel. Two of these vessels were used, the iodine solu- 
tions being introduced into the vessels and stoppered up, and the vessels 
C E 
C - Cell being measured 
E - E /ectrome ter 
A - Accumulator 
Fig. 1. 
then introduced upside down into a solution of potassium iodide. The 
complete arrangement is shown in fig. 2. 
If the stoppers are free from grease, the layer of moisture round them 
forms an efficient conductor for a quadrant electrometer. To prepare fresh 
platinum wires for use, they were heated red hot and then steeped for some 
hours in strong iodine solution. No difference could be detected on reversing 
the relation of the wires in the cells, the readings obtained from platinum 
wires in even very dilute solutions of iodine being evidently quite reliable. 
The wires were never touched or handled in any way, being simply washed 
or left in contact with the iodine solutions, after having been once pre- 
pared in the way described above. 
The solutions were prepared in freshly boiled redistilled water from 
Merck’s guaranteed pure iodine and pure potassium iodide. The potassium 
iodide solution showed no signs of liberating iodine from an aqueous solu- 
tion kept for some weeks, and the addition of a minute quantity of iodine 
tinted the solution, a tint which persisted for weeks. 
The freshly made strong solutions were titrated against sodium thio- 
sulphate solution which agreed exactly with a decinormal iodine solution, 
which had been standardised against pure recrystallised barium thiosulphate. 
