Determination of transport properties of Ni(II) through a Nafion cation-exchange membrane in chromic acid solutions

The transport properties of a cation-exchange membrane in the presence of nickel sulphate solutions were investigated by means of chronopotentiometry. The interest of this study relies on the application to the treatment of chromium plating rinse baths. The presence of chromic acid in the solutions and the initial concentration of nickel sulphate affect significantly the shape of the chronopotentiograms. The different shape of chronopotentiograms as well as the values of different parameters is thereby justified by the calculation of the speciation diagrams of Ni(II). Parameters like the limiting current density, resistance of the ohmic region, plateau length, and nickel transport numbers through the cation-exchange membrane were calculated from the chronopotentiometric and the current–voltage curves. The obtained results show an important competence between Ni2+ and H+ ions, with increases of the nickel transport numbers when the [Ni2+]/[H+] ratio corresponding to the initial equilibrium exceeds the unity. The increase in the Ni2+ concentration brings about a facilitated arising of electroconvection, which promotes overlimiting currents and additional mixing at the membrane surface.