The electrum-tarnish method for the determination of the fugacity of sulfur in laboratory sulfide systems

A new method for the determination of the fugacity of sulfur in laboratory systems consists of visual observation of the development and decomposition of a sulfide tarnish phase on silver-gold alloy (electrum) of precisely known composition. The alloy system is calibrated against pure sulfur. The method has the following advantages: simple apparatus; ability to cover a large range of fugacity of S₂; ability to cover a large temperature range by permitting runs of long duration; ability to tolerate other components in the gas phase; and ease of recovery of the quenched charges for determinations of phases and compositions. Results obtained by the electrum-tarnish method are in satisfactory agreement with those obtained by other workers for the f_s2 vs. T curves for the assemblage Ni_(1–x)S + NiS₂. The electrum-tarnish method shows promise for investigating many other reactions.

Univariant reactions studied by this method can be represented as lines forming a genetic grid in terms of the environmental parameters f_s2 and T, The slopes of such lines can yield valuable thermodynamic data for the phases involved, but activity coefficients must be known for phases of variable composition.