The Effect of Some Additives on Volumes of Activation in the Electrodeposition of Cobalt, Nickel, and Silver

Abstract

A study was made on the use of volumes of activation in investigations of the mechanisms of action of simple anionic additives in the electrodeposition of cobalt and silver, and of sulfonates in the deposition of nickel. The cobalt was chosen because of its similarity to nickel, while the silver served as an example of a metal ion that was different in charge and coordination number. Cobalt deposition from the aquo complex has a volume of activation, , of 12.3 cm3 mol−1, which corresponds to the loss of two water molecules (12.2 cm3 mol−1). It was concluded that one of the water molecules lost was complexed with the ion and one was adsorbed on the electrode. Addition of chloride caused to drop to 6.0 cm3/mol−1, indicating that the chloride acts as an inner‐sphere electron bridge so that only one water molecule needs to be lost for electron transfer. Addition of thiocyanate caused to drop to zero, indicating that the thiocyanate, with its π electron clouds, acts as an outer‐sphere electron bridge, so that no water needs to be lost before electron transfer occurs. for silver deposition was 9.5 cm3 mol−1 for deposition from the aquated ion, 20.6 cm3 mol−1 for deposition from the ammonia complex, and zero for deposition from the cyanide complex, indicating that the cyanide ion, with its π clouds, serves as an outer‐sphere electron bridge. Propane sulfonate ions did not alter for nickel deposition from the value indicating the loss of two water molecules (14.0 cm3 mol−1). However, the unsaturated 1,5 naphthalene disulfonate lowered to 7.2 cm3mol−1, indicating the loss of one water molecule and the formation of an inner‐sphere bridge. Propene sulfonate decreased to 2.9 cm3 mol−1, and 2‐naphthalene sulfonate decreased it to 5.1 cm3 mol−1, indicating in both cases a mixed mechanism involving some inner‐sphere bridging with the loss of one water molecule and some outer‐sphere bridging with the loss of no water.