for their assistance in the operation respectively of the heating camera and the x-ray diffractometer, and Professor R. Ban for helping to establish the precision of the x-ray film data. References and Notes(1) 17465 Plaza Animado #144, San Diego, Calif. 92128. Reprint requests should be addressed to the University of Southern California.

1 2 3 4 5 E ? +POI cow (eq r') (IO) Publication costs assisted by the Air Force Office of Scientific Research Sir: In studies of the electrosorption of organic compounds on electrodes from aqueous solutions it is customary1 to

A gas Chromatographic method is described which permits the direct measurement of the activities of neutral organic compounds in aqueous electrolyte solutions. This method has a relative precision of about 0.5%; it is convenient and fairly rapid, and the apparatus required is relatively inexpensive. The significance of such activity measurements to electrosorption studies of organic compounds on electrodes is discussed. The method is equally applicable to organic compounds which are either liquid or solid in the pure state at the temperature of the electrosorption studies. It may also be used to determine the complete activity‐composition phase diagrams of two component mixtures of the organic compound and water.

Publication costs assisted bv the Air Force Office of Scienrifk ResearchA theory is proposed to explain noncongruent electrmorption of organic compounds at the metal-solution interface in the absence of specific ionic adsorption. Under this theory the inner (compact) part of the electrical double layer is treated as a two-component nonelectrolyte solution called the surface solution. By means of this theory it is possible to calculate the excess Gibbs free energy of mixing of the surface solution as well as the activities and activity coefficients of the adsorbed organic compound and of the adsorbed water in the inner layer. It is shown that in a certain range of excess charge densities on the metal the dilute surface solutions exhibit positive deviations frotn Raoult's law, and it is proposed that, like bulk aqueous solutions of aliphatic alcohols, the properties of the surface solution are under entropy control. For this reason, it is concluded that the so-called Flory-Huggins electrosorption isotherm is probably incorrect because the statistical mechanical theory on which that isotherm is based predicts that the excess electrochemical entropy of mixing of the surface solution will be positive. It is shown that in one extreme limiting case this theory leads to the Frumkin isotherm, but the interpretation of the parameter appearing in the exponential term of that isotherm would be quite different under this theory than under the theory of Fnimlrin. I. IntroductionIn part l3 the experimental measurements of the electrosorption of 2-butanol on mercury from aqueous sodium sulfate solutions and the thermodynamic analysis of the data were described. It was shown that the electrosorption is congruent neither with respect to the electrode potential nor with respect to the excess charge density on the metal surface. l n the present paper we develop a general theory of the adsorbed layer in the absence of specific ionic adsorption in terms of a model, the surface solution, in which the inner (compact) part of the electrical double layer is treated as a two-component nonelectrolyte solution. We then apply this theory to our data for 2-butanol in the horizontal orientation and calculate the electrochemical excess free energy of mixing of the surface solution and the activity and activity coefficients of the 2-butanol and water in the inner layer as functions of the surface mole fraction and the electrical state of the system. It will be shown that at constant composition in the surface solution the magnitude of the deviations from ideality (Raoult's law) depends on the electrode potential or excess charge density. The noncongruence of the electrosorption isotherm is a direct consequence of this dependence of the nonideality on the electrical state.

The apparatus and technique for making differential capacitance measurements on a dropping mercury electrode under computer control are described. Sample results for electrolyte solutions and for aliphatic alcohol solutions are presented. A comparison of interfacial tension and differential capacitance measurements, both under computer control, reveals a source of systematic error in differential capacitance measurements which has generally been ignored in the past. Theoretical and experimental results are used to argue that differential capacitance is not a suitable experimental route to electrosorption isotherms for organic compounds.