Relation Between the SHE and the Internal Ag/AgCl Reference Electrode at High Temperatures

This article reviews the electropotential relation between the internal Ag/AgCl reference electrode used in nuclear light water reactor (LWR) applications and the standard hydrogen electrode (SHE). The focus is on the formation of AgCl(n)(1-n) complexes and how these might affect the potential of the Ag/AgCl electrode. In LWR applications the internal Ag/AgCl reference electrode is subjected to temperatures up to above 300 degrees C, depending oil reactor design. At temperatures of > 150 degrees C, the solubility products governing the concentration of AgCl(n)(1-n) complexes and thus the concentration of free Cl(-) in solution are different from what is known at room-temperature conditions. The potential of the Ag/AgCl electrode is in turn dependent on the concentration of free Cl(-). Therefore, it is likely that also the potential of the reference electrode is altered, compared to the value derived by ignoring the formation of AgCl(n)(1-n) complexes. The review is based partially oil data from literature and partially oil thermodynamic calculations. Thermodynamical calculations based on the revised Helgeson-Kirkharn-Kirkham-Flowers model were used to derive the equilibrium concentration of free Cl(-). The revised electropotential relation between the internal Ag/AgCl reference electrode and the SHE assigns the electrode a higher potential compared to currently used models. (c) 2009 The Electrochemical Society. [DOI: 10.1149/1.3056125] All rights reserved.