Pyrite-electrode potential measurements in reference solutions versus measured and computed redox potential: at the roots of an innovative generic pH electrode for nuclear waste repositories

Because, each electrode response depends upon its material nature as well as its surface properties, gold (Au) and glassy carbon (GC) electrodes were investigated as inert indicator electrodes (IIE) in addition to platinum (Pt). Indeed, platinum differs from gold as it has the higher exchange current density (10 mA/cm2 for Pt, against 0.3 mA/cm2 for Au). However, gold is known as a more inert material: Pt is susceptible to catalyze reactions, form oxides and adsorb H2; in contrast, gold does not. Gold has moreover a greater potential range (-0.8 to 1.8 V/NHE) towards the positive potentials (≈ 400 mV) than Pt. In comparison, glassy carbon possesses a larger potential range (-1.0 to 1.0 V/NHE) towards the negative potentials (≈200 mV) than Au and Pt. In addition to inert indicator electrodes pure pyrite (FeS2) was selected as alterable or alternative electrode. Its interest for potentiometric measurements in bio-leaching has already been demonstrated by Cabral and Ignatiadis (2001). It was even more interesting because pyrite occurs naturally in the Callovo-Oxfordian (COx) formation. It was thus of a great interest to integrate it to the bunch of electrodes and to identify the relevance of different redox reactions at the vicinity of the pyrite electrode surface.