382 Proceedings of the Royal Society of Edinburgh. [Sess. 
XXIV. — The Electromotive Force of Iodine Concentration Cells in 
Water and Alcohol. By A. P. Laurie, M.A., D.Sc., Principal of 
the Heriot-Watt College, Edinburgh. 
(Read May 4, 1908. MS. received May 21, 1908.) 
In a recent paper ( Zeitschrift fur JElectrochemie, 1906, page 265) Mr 
Maitland has redetermined with great care the electromotive force of an 
iodine solution with a platinum electrode against a mercury - calomel 
electrode. As, however, no observations seem to have been published of 
the electromotive force of iodine against iodine of different concentrations, 
it seemed advisable to determine those values before proceeding to the 
determination of the values for alcohol solutions. 
If a cell be made up consisting of a platinum wire inserted in a strong 
solution of iodine in potassium iodide — potassium iodide — platinum wire 
in a weak solution of iodine in potassium iodide, the potassium iodide 
being of the same strength throughout the cell, an electromotive force can 
be obtained between the platinum wires, the strong solution of iodine forming 
the positive pole and the weak solution of iodine forming the negative pole. 
The reactions in the cell are due to the transference of iodine from the 
strong to the weak solution, through the potassium iodide, the iodine ions 
travelling from the strong to the weak solution, and the potassium ions 
moving the opposite way. The transference of one atom of iodine from 
the strong to the weak solution involves the formation of one molecule of 
potassium iodide in the strong solution, and the decomposition of one 
molecule of potassium iodide in the dilute solution. 
The method of measuring the E.M.F. adopted in this paper was as 
follows : — The electromotive force was measured by means of a Dolezalek 
electrometer, which was arranged to have a sensitiveness which enabled 
charges of -0003 volts to be determined. An accumulator was connected 
through a resistance box, and a portion of the resistance used to measure 
against the E.M.F. of the cell to be measured. The accumulator was kept 
always connected to the resistance box when not in use, through some 
5000 ohms, and in the course of several weeks slowly lost about 01 volt. 
The arrangement was as shown in fig. 1. 
By this arrangement no current is drawn from the cell being measured. 
The accumulator was standardised by replacing C by a standard cadmium 
cell, the E.M.F. of which was 1*0196 volts at 17° C., according to the report 
of the National Physical Laboratory. 
