Platinum Metals Review - Volume 15, Issue 3, 1971

The instability sometimes encountered in metal-sheathed noble metal thermocouples used for long periods at temperatures above 1300°C is attributable to the presence within the sheath of mixed platinum and rhodium oxide vapours from which the pure platinum limb can take up rhodium. Such instability can be avoided by removing residual air from the sheath and substituting an inert gas before the assembly is hermetically sealed. Couples sheathed and sealed up in this way have thermoelectric stabilities comparable to those of the normal unsheathed thermocouple where natural convection prevents the accumulation of dangerous concentrations of rhodium oxide vapour. In the absence of oxygen, the deleterious effects of rhodium metal vapour migration upon the thermoelectric output of these devices can be reduced to negligible levels by suitably selecting the alloys from which the thermocouple and its sheath are constructed. This article is based upon a paper presented to the Fifth Symposium on Temperature, Its Measurement and Control in Science and Industry, held last month in Washington, D.C.

The number of parameters affecting the rate and possibly the mechanism of a liquid phase, heterogeneously catalysed reaction is very much larger than that for the analogous gas phase reaction. The hydrogenation of two alkynes has been studied using a palladium oxide catalyst, in a number of solvents, in order to discover the effect of the latter upon the rates of reaction. It was hoped that results of general applicability to other liquid phase heterogeneously catalysed systems might be obtained. In this context, surface- and diffusion-controlled regimes have been identified, pronounced solvent effects were observed and attempts have been made to measure hydrogen concentrations in solution under reaction conditions.