Arsenic deposition onto a gold substrate was studied as a function of pH, deposition potential, and time. Deposition of less than three monolayers was observed. The rate of deposition decreased as the thickness of the deposit increased. A set of partial reactions leading to arsenic deposition was proposed. The kinetics of deposition at the most cathodic potentials corresponds to a high-field deposition mechanism.Arsenic is a semimetal which has the ability to reduce the grain size in gold electrodeposits (1-3). However, the mechanism by which this occurs is not well understood. An examination of the deposition of arsenic itself upon a gold substrate may serve to help explain the process by which this occurs.Electrodeposited arsenic is usually amorphous (4-6) but becomes crystalline when heated to temperatures greater than 250~ (4, 5). Tammann and Warrentrup (5) claimed that electrodeposited arsenic has an electrical resistance 10 ~3 times greater than crystalline arsenic. Wranglen (7) has also found that electrodeposited arsenic films have a large electrical resistance. Piontelli and Poli (8), however, attributed the behavior of arsenic during electrodeposition to a large overvoltage at the surface. This large resistance, whether due to bulk resistance or overvoltage, prevents arsenic from plating indefinitely upon substrates. It reaches a limiting thickness in most cases. When plated potentiostatically, as has been done in this paper, the deposition current gradually drops. When plated galvanostatically, the current efficiency drops continuously. Kochegarov and Lomakina (4) have noted that after being annealed, arsenic films can be plated upon to form thicker arsenic layers. This indicates that the self-limiting nature of arsenic deposition is connected to the amorphous structure of the electrodeposit.The deposition of arsenic upon gold was studied by using cyclic voltammetry and potentiostatic plating. Cyclic voltammetry was done over a range of pH values in order to determine the potential ranges over which arsenic deposition occurred. The cyclic voltammetry also revealed that there were three cathodic reactions that were taking place before arsenic deposited on the gold substrate. The potentiostatic plating was performed as a function of potential and time in order to determine the amount of arsenic deposited and the type of kinetics that the process exhibited.

The design of a recessed rotating disk electrode is described. Its current distribution and mass‐transfer characteristics are compared to those of a rotating‐disk electrode without recess. The ohmic resistance of the recessed‐disk electrode was found to vary linearly with aspect ratio (recess depth/electrode radius). For aspect ratios greater than 0.5, the deviation of the current density at the edge of the electrode from the average current density was less than 10% under conditions of primary current distribution. The mass‐transfer‐limited current varied linearly with the square root of rotation rate up to an aspect ratio of 1.0, but the absolute values of the current decreased with increasing aspect ratio. The mass‐transfer‐limited thickness distribution on plated Cu samples was measured at aspect ratios of 0.1, 0.5, and 1.0. The thickness was uniform near the center of the electrode but decreased rapidly near the edge. The radius at which the thickness began to decrease approached the edge of the electrode as the aspect ratio decreased.

Experiments were conducted using plating baths containing gold (7 mM) and arsenic. The hardness of the electrodeposited gold was measured and its composition analyzed by electrochemical stripping. Arsenic was always found in the deposit when a hardening effect was shown. It was therefore concluded that arsenic must be incorporated into the deposit to show a hardening effect.

The Fe2+/Fe3+ redox reaction on platinum is investigated in liquid and frozen HC104. 5.5H20 over a temperature range from 130-300 K. The exchange current density follows an Arrhenius law in the liquid and in the solid state. Both the energy of activation and the preexponential factor are higher in the solid state; the exchange current density shows a discontinuous rise as one approaches the freezing point from higher temperatures. In the liquid state, mass transport to the electrode occurs through diffusion of the ions; in the solid state mass transfer is much slower, and is probably due to electron hopping rather than ion transport.

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