Abstract
The work describes the influence of coumarin on the polarography of the nickel ion at the dropping mercury electrode under the influence of a direct current. It was found that the nickel polarograms were distorted by adsorption of the coumarin on the surface of the electrode. The extent of the adsorption was a function of the coumarin concentration in the bulk of the solution, and also of the cathode potential. As the cathode potential was gradually made more negative the coumarin was eventually desorbed. The potential at which the nickel was reduced was coincident with that at which the desorption occurred. Studies on the influence of coumarin and some of its derivatives on the mercury electrocapillary curve indicated that they were adsorbed with the plane of the molecule parallel to the mercury surface. Coumarin is also reducible at the dropping electrode, the reduction requiring one electron per molecule of coumarin. The reduction products have been reported in the literature as the meso and racemic forms of tetrahydrodi-4,4'-coumarinyl. In the presence of excess nickel the apparent height of the coumarin wave is reduced to one-half its value in nickel-free solutions. For this, an explanation is offerred which requires that the products of the reduction of four coumarin molecules react with each nickel ion, in the diffusion layer surrounding a mercury drop, in such a way that reduction of the nickel ion can no longer take place. Evidence is given in favour of the suggestion that, in unbuffered solutions, cathodic reduction of coumarin produces hydroxide ions, which consequently cause removal of nickel ions from the diffusion layer by precipitation of nickel hydroxide. Further experimental evidence indicates that the phenomena controlling the electrochemical reduction of the nickel ion are confined to the boundary region between an electrolyte solution and the electrode surface.