For solubility reasons, many of the organic polarographic reductions reported in the literature have been studied in mixtures of water with ethanol or other water miscible organic solvent. As pointed out by Kolthoff and LinganeI2 the effect of such solvents on the measured halfwave potentials and diffusion currents is often overlooked, with resulting confusion in the interpretation of organic polarographic data. When reductions a t the dropping mercury electrode are carried out in the approved manner, i. e., in well buffered solutions, the over-all effect of added organic solvent on the potential will include: (a) the change resulting from any alteration in the hydrogen ion activity of the aqueous buffers; (b) any change in potential of the quiet mercury anode (or change due to the introduction of a liquid junction if an outside reference anode is used) ; and (c) any effect on the inherent reduction potential of the reducible compound. The effect on the diffusion currents will reflect the influence of the solvent on one or more of the terms in the Ilkovic equation (diffusion coefficient or capillary characteristics).Investigations of the effect of organic solvents on metal ion reductions3 indicate that diffusion currents are depressed but potentials only slightly affected. In the organic field a few experiments on the effect of varying solvent composition4 have been reported, but these are limited in scope and incidental to the investigator's chief purpose.The present report describes a complete quantitative study of the effect of ethanol, the most commonly used polarographic solvent, on (a) the pH values as calculated from hydrogen electrode potentials, for a series of commonly used buffers covering a wide pH range and (b) the usual polarographic values measured in the reduction of p-nitroaniline in these buffers. The appreciable solubility of this compound in water makes possible a direct comparison of its polarographic be-(1) Present address: E. I. Jessop, ibid., 158 (1946); Brasher and Jones, Tvans. Faraday (4) Shikata and Tachi, Mcm. Coll. Agr., Kyoto Imp. Univ., 8, 31 (1930); Tnchi, ibid.. 40, 17 (1937): Kolthoff and Lehmicke, THIS TOVRNAL, 70, 1879 (1948): Pasternak and Halban. Hrlu. Chim. Acfa. SOC., 4a, 775 (1946). as. I P O ( l w e i havior in the presence of varying amounts of ethanol with that in water.
ExperimentalUsing the polarographic cell-aqueous calomel electrode combination described below, polarographic and g1as.i electrode potential measurements were made on 5.4 X 10-4 M p-nitroaniline solutions in nine buffer series. each series comprising a commonly used buffer system made up in water and in six ethanol-water mixtures. All solutions contained 0.02y0 gelatin as maximum suppressor and the total concentrations of buffer constituent5 in each series were held constant a t those calculated to yield an ionic strength of 0.1 in water.6Using the same cell combination, hydrogen and glass electrode potential measurements were made on 5 of the above series (compositions being identical except for the a...
