A Comparison of the Immersion and Open-Circuit Scrape Methods for Determining the Potential of Zero Charge of Metal Electrodes

Abstract

Open-circuit potentials were measured for freshly immersed metal surfaces and for metal surfaces renewed abrasively (scraped-off) in situ in aque-ous electrolytes. Electrode materials included Au, Ag, Cu, Pt, Ga, In, Bi, and Sn and the electrolytes included alkali halides and alkali sulfates in acid, neutral, and alkaline solutions. The fresh-surface potentials for Au, Ga, In, Bi, and Sn were approximately the same following either immersion or scraping in solution and yielded the potential of zero charge at certain pH conditions. For Ag, Cu, and Pt the immersion potentials were several hundred millivolts more positive than the scrape potentials. This is attributed to the presence of adsorbed oxygen on these metals at the instant of immersing them in solution. For Ag, Cu, and Pt the scrape potentials are interpreted as the potential of zero charge for the bare metal while the immersion potentials reflect a superimposed dipolar effect or faradaic reduction of the oxygen. Values for the potential of zero charge (pzc) of most solid metals have suffered from irreproducibility among various methods and workers (1, 2). Much of this variance is attributed to differences of impurities or other "foreign" species (e.g., H or O) on the surface, or to the crystallographic state of the metal surface resulting from different electrode pretreatments. In some cases it is difficult to verify that faradaic reactions do not interfere. In the present work the immersion and open-circuit scrape potentials are compared for a number of metals in aqueous solutions. Although both of these methods are based on essentially the same principle, there is some disagreement on published values obtained with the two methods (cf. Table I). There are other methods used to measure the pzc of solid electrodes which depend on various properties of the metal such as mechanical properties or capacity of the metal-solution interphase. These methods have a different basis than the kinetic methods being compared in this work. Inas-much as they are reviewed elsewhere (1) we need not discuss them further here. In the dip or immersion method a fresh metal test electrode, connected to a reference electrode through Key words: electrical double layer, zero charge, fresh metal surfaces .