A novel electrolytic titrator design - Coating platinum cathodes with an electron sink using Nafion® and hexaammineruthenium (III)

•A novel approach to devising an electrolytic microliter titrator•Production of OH − ions inhibited at cathode•A Nafion and Ru(NH3)63+ ion membrane depolarizer acts as a cathodic electron sink.•A controlled constant H + ion flux from the anode titrates the surrounding medium.

Water electrolysis produces a constant flux of H + and OH − ions at the anode and cathode respectively. If the distance between the electrodes is reduced significantly, the pH gradients produced at the electrode surfaces become steep, and recombination of species is near instantaneous. In this experimental work, we show how microfabricated platinum cathodes coated with Nafion®, and further loaded with hexaammineruthenium (III) (Ru(NH3)63+) ions, can inhibit the production of OH − ions. Electrons from the cathode reduce Ru(NH3)63+ to Ru(NH3)62+ instead of water. Simultaneously, a constant flux of H + ions from the un-modified anode, titrates the sample. The Nafion® film's coating, redox couple film loading, stability and pH related changes at the electrode surfaces, have all been studied. Crucial to the development of the electrolytic titrator, this research work shows the importance of crystalline ion-exchange membranes and an optimised method of loading them with redox couples.

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