A comparative study of the stability of hexachloroiridate and hexacyanoferrate in electrochemical mass transfer measurements

Hexacyanoferrate(II)/hexacyanoferrate(III) is a commonly used redox couple for measuring the mass transfer performance of electrochemical reactors. However, this redox couple can irreversibly poison the electrode, potentially leading to unreliable results. There are alternatives such as the hexachloroiridate(IV)/hexachloroiridate(III), but relatively little is known about their stability. Therefore, we have investigated the stability of the hexachloroiridate redox couple and compared this to the stability of the hexacyanoferrate couple. Voltammetry confirms that the hexachloroiridate couple is reversible, with a clear limiting current density plateau for the reduction on both platinum and nickel. Diffusion coefficients for hexachloroiridate(IV) and hexachloroiridate(III) were determined to be respectively 8.38*10⁻¹⁰ m²/s and 6.10*10⁻¹⁰ m²/s. Chronoamperometric limiting current density experiments of 16 h at pH 4 show that the electrode is not poisoned. However, as evidenced by inline UV-VIS measurements the bulk concentration of hexachloroiridate(IV) decreases, which is probably due to a spontaneous reduction or hydroxylation reaction. UV-VIS measurements of iridate solutions show that in more acidic solutions (pH 2–3) the degradation occurs more slowly, whereas at pH > 6 rapid hydroxylation of the complex occurs, making the couple unsuitable for mass transfer experiments at high pH. Experiments with hexayanoferrate show that that couple is more stable in solution, but that irreversible electrode poisoning cannot be avoided. Therefore, the hexachloroiridate couple seems especially suitable for testing electrode materials that are difficult to clean, such as three dimensional structures.