Electrolyte-resistance change due to an insulating sphere in contact with a disk electrode

This work is aimed at improving the quantitative analysis of the electrolyte-resistance (ER) fluctuations generated by two-phase systems with dispersed gaseous, liquid, or solid insulating entities in a conductive electrolyte. The primary potential distribution around a disk electrode in contact with a small insulating sphere, which simulates a spherical particle, drop, or gas bubble sitting on a disk electrode used as a sensor, was calculated with a collocation method in order to derive the increment in ER caused by the sphere. For a sphere of size equal to one-tenth of the electrode size, the values of the ER increments were found to be very low and to depend on the sphere position: 0.3% close to the edge of the electrode and 0.05% at its centre. Despite the influence of variations in the electrolyte temperature and of the approximate horizontality of the electrode, these low values could be measured experimentally by scanning insulating spheres of 1 and 2 mm in diameter above or in contact with a stainless steel electrode of 10 mm in diameter, using the motorized translation stage of a scanning electrochemical microscope and a home-made electronic device measuring low-amplitude ER fluctuations.