Dichlor-tetrahydroxy-platinic Acid, H₂PtCl₂(OH)₄

Dichlor-tetrahydroxy-platinic Acid, H₂PtCl₂(OH)₄, is readily obtained in aqueous solution by hydrolysing silver tetrachlor-dihydroxy-platinate, Ag₂PtCl₄(OH)₂, with water at 100° C. The reaction takes several hours to complete, but proceeds as follows:

Ag₂PtCl₄(OH)₂ + 2H₂O = 2AgCl + H₂PtCl₂(OH)₄.

The acid is also produced when ammonium hydroxide is added in the requisite amount to a solution of the tetraehlor-dihydroxy acid:

2H₂PtCl₄(OH)₂ + 2NH₄OH = (NH₄)₂PtCl₆ + 2H₂O + H₂PtCl₂(OH)₄.

The solution is evaporated to dryness and the acid extracted with water, ammonium hexachlor-platinate remaining behind as an insoluble residue.

When mixed with excess of dilute hydrochloric acid, hexahydroxy- platinic acid dissolves, yielding the dichlor-tetrahydroxy acid.

When diluted with water this acid yields the hexahydroxy acid in amorphous form, but in a crystalline form if dialysed.

If an excess of ammonium hydroxide is added to a solution of the tetrahydroxy acid the liquid darkens and ultimately deposits a brown flocculent mass of Fulminating Platinum NH₃Pt₂(OH)₁₀, which is soluble in dilute hydrochloric acid. The flocculent mass resembles hydrated ferric oxide in appearance and contains no chlorine. If dried at 100° C. after thorough washing, and then exposed to further desiccation over sulphuric acid, it absorbs moisture upon exposure to air with such avidity that the particles jump about.

When gently warmed to 125° C. the substance darkens in colour and loses the equivalent of a molecule of water. At 180° C. it becomes black, and above 250° C. it decomposes with violent detonation, nitrogen, water vapour, and oxygen being evolved, and a residue of platinum sponge remaining behind.

If pyridine is added to dichlor-tetrahydroxy-platinie acid instead of ammonia, a similar detonating product is obtained, presumably having an analogous composition, namely, C₅H₅N.Pt₂(OH)₁₀.