Highly oriented pyrolytic graphite (HOPG; ZYA quality, mosaic spread 0.4 ± 0.1 o supplied by Scanwel, UK) served as the working electrode (WE). Electrical connection was made by stripping an enamel copper wire (RS components, UK) for about 0.5 cm at each end, attaching one side to the edge plane of the HOPG with conductive silver epoxy resin (RS component, UK). Following overnight curing, the epoxy was covered by an insulating resin and left to dry for a few hours. In this way the direct contact of the silver epoxy and stripped copper wire end with the electrolyte (and hence the introduction of ionic impurities in the latter) was eliminated.