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Platinum Metals Rev., 1962, 6, (4), 137

Oxides of Iridium and Ruthenium

New Values for Oxygen Pressures of Formation

  • O. L.

In his review of the thermodynamics of the gaseous oxides of the platinum metals, C. B. Alcock (Platinum Metals Rev., 1961, 5, 134) referred to the work of Schafer and his collaborators on the oxides of iridium and ruthenium. Some further experimental work has now been reported by G. Schneidereith in the course of a dissertation from the Harald Schäfer Institute of Münister, Westphalia.

Schäfer and his co-workers had previously found that coarse crystals of IrO2 and RuO2 were produced by heating the metals in a stream of oxygen (760 torr) to 1150 and 1270°C respectively. Both metals form volatile trioxides which, in contact with the colder walls of the quartz vessel, decompose to rough single crystals of blue-black IrO2 and black RuO2. The oxygen pressures of formation of these dioxides have been determined by Schneidereith by different methods and compared with the numerical values given by Alcock. Some of the values reported are set out in the table.

Solid IrO2 dissolves less than 5 atomic per cent of iridium. The enthalphy of formation Δh(298)—calculated from the elements—amounts to —52.42 Kcal for
IrO2 and to -68.1 Kcal for RuO2. The resistivities of single crystals of IrO2 and RuO2 at 22°C were found to be 48 microhm-cm and 50 microhm-cm respectively. The temperature coefficients between —78 and +22°C were approximately +0.0008 for IrO2 and +0.00075 for RuO2.

Oxygen-pressures p in torr for reactions IrO2→Ir+O2 and RuO2→Ru + O2 at temperatures t in °C

t p for IrO2 p for RuO2
900 34
950 76
1000 154
1050 316
1100 572 1.5..4.5
1124 760 ~2
1150 1020 3.3.. 6.9
1200 6.7.12
1250 13.2..23
1300 ~25

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