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

Abstract

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 AgnormalCln1−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°C , depending on reactor design. At temperatures of 150°C , the solubility products governing the concentration of AgnormalCln1−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 AgnormalCln1−n complexes. The review is based partially on data from literature and partially on thermodynamic calculations. Thermodynamical calculations based on the revised Helgeson–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.

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