Volt Equivalent Diagrams as a means of displaying the electrochemical thermodynamics of aqueous systems

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-H ₂O 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. There exist at least three homologous series of interest: Polythionates: S _xO ₆ ^2-; sulphur oxidation state (SoS) (z) = 10/x (z < 6), ranging from z = 5 (S ₂O ₆ ^2-, which does not appear to have been synthesized) to 1.67 for S ₆O ₆ ^2-. Polythiosulphates: S _xO ₃ ^2-; SoS (z) = 4/x (z < 4), ranging from z = 2 (S ₂O ₃ ^2-, "thiosulphate") to 0.57 for S ₇O ₃ ^2-. Elemental sulphur, S ⁰, z = 0; and Polysulphides: S _x ^2-; SoS (z) = -2/x (z < 0), ranging from -2 (x = 1, S ^2-) to -0.33 (x = 6). All of these species can, potentially donate atomic sulphur to a metal surface, thereby destroying passivity (in the case of Cu) and activating the metal toward corrosion. One of the vehicles chosen in this work to explore copper activation is the Volt-Equivalent Diagram (VED). Volt equivalent for a species is the equilibrium potential for the reduction reaction of the species with respect to elemental sulphur multiplied by the average SoS. The VED is then formed by plotting the VEs for the various species versus the average SoS. The volt equivalent describes the reactivity of a species and fortells the various chemical transformations that may occur between different sulphur-containing species in the system.