The Reduction of Coumarin in the Electrodeposition of Nickel

SummaryThe behaviour of coumarin in the electrodeposition of nickel has been studied using a rotating disc electrode to give known rates of transport of coumarin to the electrode surface. The kinetics of coumarin consumption have been studied at constant potential for a range of pH from 5·1 to 1·5. At pH 4 and 1·5, the nickel current density is determined by the total rate of consumption. The incorporation into nickel deposits of carbon from coumarin has been measured. The consumption of coumarin represented by carbon incorporation is small compared with consumption by reduction reactions. There are two products of coumarin reduction, melilotic acid being the major product at pH 4. The second product has been isolated and partially identified. Estimates of the relative rates of reduction to second product and to melilotic acid have been obtained. At low pH and coumarin concentrations, the rates of the two reactions are similar. The second product has been found in used production plating solutions. Rates of reduction to second product are, generally, an order of magnitude greater than rates of incorporation. The results suggest that the second product reaction may play an important part in the inhibition, by coumarin, of nickel deposition.