The effect of platinum electrode surfaces on precise primary pH measurements

Abstract

For primary pH measurements, a platinum hydrogen electrode and a silver chloride electrode are immersed in the same solution in a cell without transference (Harned cell). The platinum electrode is covered with dispersed platinum (platinum black) to increase the surface area. To determine the influence of specific deposition conditions (current density, duration, and composition of the electrolyte) on the properties of platinum black, the surface area, and the electrode performance, platinized platinum electrodes were investigated systematically using scanning electron microscopy and optical microscopy. Confocal laser scanning microscopy was applied to obtain quantitative information about roughness parameters. After assessment of the surface structures, improved conditions for the fabrication of platinized platinum electrodes were derived. It was found that all investigated electrochemically coated platinum electrodes obtained comparable values of standard potential of Ag/AgCl-electrodes within the measurement uncertainty. The platinum layer obtained by sputtering was significantly less distinct and less homogenous than the layer of platinum obtained by electrochemical deposition. The examinatio of shiny (uncoated) platinum electrodes evidenced a significant difference between multiple-used and freshly prepared platinum electrodes. Thus, uncoated platinum electrodes are not suitable for the Harned cell measurement either. Thus, to obtain reliable and reproducible results in pH measurements using the primary method, an electrochemical coating of the platinum electrodes under defined conditions is required.