INHIBITION OF CATHODE REACTIONS IN SODIUM-HYDROXIDE SOLUTION CONTAINING CHROMATE

The formation of a film on the cathode surface due to electroreduction of chromate and its effect on other cathode reactions have been studied by cyclic voltammetry using rotating disc electrodes of platinum and gold in 1 M NaOH solution. It was found that the film formed hinders very effectively the cathodic reduction of ClO-, Fe(CN)63- and O2(aq) as well as further reduction of chromate, while the hydrogen evolution reaction and the oxidation of the electrode surface can still proceed. The suppression of the cathodic reaction is achieved at a smaller amoun of deposit and is more efficient when adsorbed intermediates are involved [reduction of ClO- and O2(aq)] than for simple outer sphere reactions (reduction of Fe(CN)63-. The largely unhindered adsorption and desorption of hydrogen on platinum as well as the oxidation of the metal surface beneath the film show that it is permeable to hydroxide and/or hydrogen ions, whereas it is impermeable to the larger species used. This also explains the effect of the addition of chromate to the electrolyte in the chlorate process, where the film formed on the cathode effectively hinders the undesired hypochlorite reduction, while it still allows the desired hydrogen evolution reaction to occur.