Determination of diffusivity and solubility of oxygen in phosphoric acid using a transit time on a rotating ring-disc electrode

A transit time method on a rotating ring-disc electrode was used to simultaneously determine the diffusivity and solubility of oxygen in phosphoric acid. The method consisted of measuring the limiting current for the diffusion of dissolved oxygen on the central disc electrode, and a transit time for the diffusion of oxygen from the disc to the ring electrode. The diffusivity of oxygen was calculated from the measured transit time, electrode rotational speed and kinematic viscosity of the electrolyte without the need of its bulk concentration and the number of electrons transferred in the electrode reaction. Once the diffusivity was known, the solubility of oxygen in the bulk electrolyte was determined from the limiting current on the disc using the Levich equation. This work has demonstrated that the transit method is a swift and reproducible technique for the diffusivity measurement. The diffusivity and solubility of oxygen in phosphoric acid of a concentration range of 0.79–14.7m were determined at 23° C and an oxygen partial pressure of 1 atm. A set of empirical equations are presented to correlate the Stokes-Einstein constant,Dµ/T, and solubility of oxygen to the phosphoric acid concentration.