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Iridium and other refractory metals and a ll oys of the platinum group can be easily prepared for analysis, or for small-scale refin ing, by solu tion with hydrochloric acid and suitable oxidants in sealed tubes at temperatures up to 300 0 C . The effects on the rate of solution of iri d ium caused by variations in temperature. in concentrations of hydrochloric acid and of the oxidant, and in the nature of the oxidant, a re reported . The rate increases rapidly with increa e in temperature. It also increases with increasing concentration of hydrochloric acid, up to nearly anhydrous hydrogen chloride. With variation in the concentration of chlorine or equivalent oxidants, the rate passes through a maximum at a relatively low concentration.

This paper describes special technics suita ble for the use of sealed tubes in treating refractory materials with hydrochloric acid and other acid mixtures at eievated temperatures. Methods for filli ng, fo r sealing, and for opening tubes are discussed. The feasibility of re use of tubes of Pyrex glass is discussed with some observed phenomena of the attack of the glass by the acids. A protecting shell is described together with a method of using solid carbon dioxide to provide the compensating pressures needed to protect the sealed t ubes from bursting. The pressures developed on heating hydrochloric acid and mixtures of hydrochloric acid with nitric or perchloric acids were measured in a special gage, which is described.