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:

 
\[Ag(cr)\ +\ Ag_{3}AuS_{2}(cr)\ =\ 2Ag_{2}S(cr)\ +\ Au(cr)\]
 
\[Ag(cr)\ +\ 2AgAuS(cr)\ =\ Ag_{3}AuS_{2}(cr)\ +\ Au(cr)\]
 
\[Ag(cr)\ +\ Au_{2}S(cr)\ =\ AgAuS(cr)\ +\ Au(cr)\]
 
\[AgAuS(cr)\ +\ Ag_{2}S(cr)\ =\ Ag_{3}AuS_{2}(cr)\]

The EMF temperature dependences observed for these reactions, together with auxiliary thermodynamic properties for Ag2S, 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.

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