Platinum Trichloride, PtCl₃

Platinum Trichloride, PtCl₃, results when platinum tetrachloride is heated to 390° C. in a current of pure, dry chlorine for several hours.

It is a greenish black powder, slightly soluble in cold water, but rapidly dissolved by the boiling liquid, yielding a reddish brown acid solution which is possibly trichlor-dihydroxy-platinous acid. H₂PtCl₃(OH)₂, analogous to the tetrachlor-dihydroxy-platinic acid, H₂PtCl₄(OH)₂. obtained when the tetrachloride is dissolved in water. Prolonged boiling with water causes partial hydrolysis, an oxychloride and free hydrochloric acid resulting.

The trichloride is almost insoluble in concentrated hydrochloric acid at room temperature, but on warming the two, decomposition takes place, the di- and tetra-chlorides of platinum resulting.

Pentachlor-platinous Acid has not as yet been isolated in a free state although its presence has been detected in solution when chlorine is passed into a solution of tetrachlor-platinous acid, since it yields with caesium chloride the characteristic green precipitate of caesium penta-chlor-platinite, Cs₂PtCl₅, which is the only salt of this acid at present known.