Hg-Ir (Mercury-Iridium)

Abstract

Experimental information about the Hg-Ir system is sparse but it is sufficient to sketch an assessed phase diagram. [Moffatt] presented the low-temperature part of the diagram at constrained pressure. In this evaluation we present the phase relationships at 0.101 MPa in Fig. 1. Since the melting point of Hg (−38.8290 °C) [ITS-1990] is not measurably shifted by the presence of Ir, it may be concluded that the solids are almost completely immiscible. Virtually pure (Ir) is in equilibrium with virtually pure liquid Hg up to the boiling point of Hg (356.623 °C) and gaseous Hg is in equilibrium with almost pure Ir up to the melting point of Ir (2447 °C) [Massalski2]. [Moffatt]’s version of the diagram is similar to that shown in Fig. 1, except that the liquid, instead of the gas, is present above the boiling point of Hg. Hg may maintain as liquid up to its critical point (1492 °C and 151 MPa) [1992Gum]. [1956Str] attempted to determine a solubility of Ir in liquid Hg at room temperature by chemical analysis of filtrate of the saturated amalgam; its content in Hg was below the analytical detection limit of 0.001 at.% Ir. [1973Jan] performed similar experiments at 500 °C at a sensitivity level of 10 at.% Ir, and also, no Ir was found in the filtrate as a result of spectrophotometric analysis. [1989Gum] predicted the solubility at 25 °C based on a regular solution model (1 × 10 at.% Ir) and the cellular model of Miedema et al. [1983Nie] (2 × 10 at.% Ir). While the predictions are disparate, both indicate values far below detectability. Experimental corroboration comes from [1964Rhy] who found no attack on Ir by liquid Hg after the equilibration for 168 h at 550 °C and from [1987Kou], who observed no detectable solubility of Ir in Hg during electroanalytical measurements at room temperature. No interaction between the metals was observed in x-ray diffraction (XRD) studies of [1973Jan1] and [1981Sut], in the corrosion investigations by [1964Rhy], thermogravimetry by [1995Fer, 2002Fer], scanning electron microscopy [1995Fer, 1998Nol, 2002Fer], atomic force microscopy [1998Nol], energy dispersive x-ray microanalysis [1995Fer, 2002Fer], x-ray photoelectronic spectroscopy of amalgamated Ir as well as Ir-Pt surfaces by [1999Fer], nor in electroanalytical experiments of Hg redox processes on Ir electrodes by [1981Sut], [1986Kou], [1987Kou], [1998Nol], [1999Fer], and [1999Fer]. The effect of incomplete removal (∼95% efficiency) of electrodeposited Hg from Ir surface observed by [2003Mil] seems to be rather related to an interaction of Hg with Ir oxide than to any Hg-Ir interaction with formation of an intermetallic compound or a concentrated solid solution of Hg in Ir. In an attempt to determine Hg solubility in solid Ir, [1973Jan2] equilibrated Ir powder with Hg in a quartz ampoule at 800 °C for 350 h. The lattice