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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 Ag 4 RbI 5 as a solid electrolyte: Ag(cr) + Ag 3 AuS 2 (cr) = 2Ag 2 S(cr) + Au(cr) Ag(cr) + 2AgAuS(cr) = Ag 3 AuS 2 (cr) + Au(cr) Ag(cr) + Au 2 S(cr) = AgAuS(cr) + Au(cr) AgAuS(cr) + Ag 2 S(cr) = Ag 3 AuS 2 (cr)The EMF temperature dependences observed for these reactions, together with auxiliary thermodynamic properties for Ag 2 S, acanthite, from Robie and Hemingway (1995), result in the following standard thermodynamic properties at 298.15 K and 1 bar (10 5 Pa): Δ f G o (Ag 3 AuS 2 , cr) = -69478(200) J/mol and S o (Ag 3 AuS 2 , cr) = 273.37(56) J/(mol⋅K) (uytenbogaardtite), Δ f G o (AgAuS, cr) = -27621(210) J/mol and S o (AgAuS, cr) = 128.11(60) J/(mol⋅K) (petrovskaite), and Δ f G o (Au 2 S, cr) = 1077(650) J/mol and S o (Au 2 S, cr) = 128.1(2.0) J/(mol⋅K). Au 2 S(cr) is a thermodynamically metastable phase, but the values of its thermodynamic functions are open to discussion.

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