Volt Equivalent diagrams as a means of displaying the electrochemical thermodynamics of the sulfur–water system

•Thermodynamics of S–H2O system was studied.•The Volt Equivalent for redox species is defined as: reduction potential × average oxidation state.•Volt-Equivalent diagrams derived for the expected repository conditions in Sweden.•The Volt-Equivalent diagrams are very useful in interpreting and predicting the redox chemistry.

In this paper, we describe the use of Volt-Equivalent Diagrams (VEDs) to display the electrochemical thermodynamics of one of the most complicated chemical systems in nature; the S–H2O system, as it relates to the use of copper for the isolation of high level nuclear waste (HLNW) in granitic repositories in Sweden. The complexity stems from this element having oxidation states that vary from −2 to +8 including a multitude of fractional oxidation states. The volt equivalent describes the reactivity of a species and foretells the various chemical transformations that may occur between different sulfur-containing species in the system.