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The importance of minimizing the amount of impurities in solution in hydrogen overpotential measurements has been stressed repeatedly (1). Following the example of Frumkin and his co-workers (2), pre-electrolysis is used extensively to this end. In purified solutions, hydrogen overpotential on mercury follows the Tafel equation with closely reproducible exchange current and Tafel slope (3). Results with solid electrodes vary widely, however. Deviations from Tafel behavior and time dependence of the overpotential at any given current density have been reported frequently (4, 5).We have measured hydrogen overpotential on nickel ("spectroscopic" purity) by the direct method. Unlike previous work with this system (6), results were time independent and coincident curves were obtained with increasing and decreasing current densities. This study will be reported in detail later. The method used to prepare electrodes, which we believe applicable to similar studies with other metals, is presented here.Nickel electrodes were cleaned by immersion in a chromic-sulfuric acid solution ("cleaning solution"), followed by washing with boiling conductivity water. Upon immersion in alkaline (0.1N NaOH) test solutions, electrodes exhibited passive potentials (>0.200 v vs. hydrogen in the same solution). They were activated by cathodic polarization at 2 ma/cm ~ for about 5 min. Rest potentials were within 2-3 mv of the reversible hydrogen potential. After a run, which generally required about 100 min, the electrode was passivated by anodic polarization at about 40 t~a/cm ~ and left in this condition until the next run whereupon the electrode was again activated by cathodic polarization as described 1 Present address:

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To numerically classify solution adsorption isotherms, a difference or deviation measure, DS(mc) (the relative difference between two sums of the adsorption maximum's characteristics of selectivity isotherms x 100), is derived. The measure is applicable to completely miscible binary solutions on solids. This quantity evaluates the difference between an adsorption system and the ideal adsorption system (ideal adsorbed and bulk phases, homogeneous surface, and equal molar area solution components) at the point of maximum adsorption. For model systems, DS(mc)s are calculated at several levels of surface heterogeneity (Gaussian distribution of surface energy) and for different signs of phase nonideality (regular solution phases) on a homogeneous surface and on a simple two-site-type heterogeneous surface. All heterogeneous surfaces have negative DS(mc) values, but nonideal phases have DS(mc)s with signs opposite to the sign of deviation from Raoult's law. DS(mc)s from both U- and S-shape isotherms are reported for 16 experimental systems consisting of hydrocarbon mixtures and both alcohol + hydrocarbon and alcohol + water solutions or acetone + carbon tetrachloride on several silica gels and a variety of carbons.