Explosive Rhodium

In 1868 Bunsen accidentally discovered that several of the platinum metals can be obtained in an explosive form. Rhodium is a case in point. If alloyed with excess of zinc or cadmium, and the product treated with hydrochloric acid, the zinc (or cadmium) passes into solution, leaving an insoluble residue of finely divided explosive rhodium.

Bunsen explained the explosive property of the metal on the assumption that an unstable modification or allotrope results from the above method of preparation, and that its conversion into the stable variety is accompanied by explosive violence. This view, however, is open to question. When explosive rhodium is kept at 100° to 200° C. for several days it ceases to be explosive. Furthermore, if the metal is obtained by the foregoing method in entire absence of air, it is not explosive.

This suggests that the explosiveness is due to the union of the occluded oxygen and hydrogen to form water, and not to an inherent change in the metal itself, a supposition which is supported by measurement of the amount of heat developed during explosion.