Volatilisation of Platinum

When platinum apparatus is heated to very high temperatures it frequently undergoes a marked alteration in weight, becoming lighter. This is generally attributed to volatilisation of the metal, and several useful investigations have recently been carried out with the object of determining how the loss may best be avoided.

Although it is generally assumed that the loss in weight of pure platinum on heating is due to direct volatilisation of the metal, Roberts points out that this is improbable since platinum does not volatilise in nitrogen, hydrogen, or in vacuo. He determined the lowest temperatures at which platinum volatilises in oxygen by an exceedingly sensitive method, namely, by sudden expansion of gas saturated with moisture, which when any volatilisation has taken place results in the formation of a cloud or mist, because the disintegrated particles constitute condensation nuclei. It was found that platinum begins to disintegrate at 500° C., the extent of volatilisation being roughly proportional to the oxygen pressure. Roberts therefore concludes that the phenomenon is due to the formation of platinum dioxide, PtO₂, which is endothermic and which dissociates at lower temperatures.

Crookes had in 1911 already studied the relative volatilities of the metals of the platinum group at temperatures ranging from 900° C. upwards. The metals were heated in still air in an Herseus furnace, the order of volatilisation proving to be as follows: Ru, Ir, Pd, Pt, and Rh, the last named being the least volatile.

Burgess and Sale show that the presence of iridium in the platinum of crucibles, although added for its stiffening qualities, renders the latter subject to proportionately greater Josses of weight on heating to temperatures above 900° C. Below this temperature, and up to a content of at least 3 per cent, of iridium, the loss on heating is negligible. Below 900° C. also the presence of rhodium and silicon has a negligible effect. The presence of iron appears to lower the heat losses by amounts depending on the quantity of that metal present. Below 900° C. there may even be a slight gain in weight on heating platinum owing to the iron content diffusing to the surface and oxidising. This is a decided disadvantage, however, on account of the solubility of the iron oxide in acids.

Rhodium, like iridium, stiffens platinum; but unlike iridium it reduces the volatilisation of platinum at all temperatures above 900° C. It is suggested, therefore, that a useful alloy for best quality crucibles " would be platinum containing 3 to 5 per cent, of rhodium, practically free from iron and iridium, and containing no other detectable impurities."