DETERMINATION OF STANDARD THERMODYNAMIC PROPERTIES OF SULFIDES IN THE AG-AU-S SYSTEM BY MEANS OF A SOLID-STATE GALVANIC CELL

Abstract

Thermodynamic properties of the following phase reactions in the Ag-Au-S system have been investigated via electromotive force (EMF) measurements in solid-state galvanic cells with Ag4RbI5 as a solid electrolyte: Ag(cr) + Ag3AuS2(cr) = 2Ag2S(cr) + Au(cr) Ag(cr) + 2AgAuS(cr) = Ag3AuS2(cr) + Au(cr) Ag(cr) + Au2S(cr) = AgAuS(cr) + Au(cr) AgAuS(cr) + Ag2S(cr) = Ag3AuS2(cr) The EMF temperature dependences observed for these reactions, together with auxiliary thermodynamic properties for Ag2+S, acanthite, from Robie and Hemingway (1995), result in the following standard thermodynamic properties at 298.15 K and 1 bar (105 Pa): ΔfGo(Ag3AuS2, cr) = -69478(200) J/mol and So(Ag3AuS2, cr) = 273.37(56) J/(mol·K) (uytenbogaardtite), ΔfGo(AgAuS, cr) = -27621(210) J/mol and So(AgAuS, cr) = 128.11(60) J/(mol·K) (petrovskaite), and ΔfGo(Au2S, cr) = 1077(650) J/mol and So(Au2S, cr) = 128.1(2.0) J/(mol·K). Au2S(cr) is a thermodynamically metastable phase, but the values of its thermodynamic functions are open to discussion