Silver Chlor-platinate, Ag₂PtCl₆

Silver Chlor-platinate, Ag₂PtCl₆, is obtained as a yellow precipitate by adding silver nitrate in excess to a cold solution of chlor-platinic acid. It is not quite pure, since it is slowly decomposed even by cold water, yielding silver chloride and a solution of platinum tetrachloride, which is dihydroxy-tetrachlor-platinic acid, PtCl₄.2H₂O or H₂PtCl₄(OH)₂.

Upon filtering off the silver chloride and concentrating the solution over sulphuric acid, the pentahydrate, PtCl₄.5H₂O, crystallises out. On warming, however, with excess of silver nitrate, the silver salt of dihydroxy-tetrachlor-platinic acid is obtained, thus:

H₂PtCl₆ + 2AgNO₃ = Ag₂PtCl₆ + 2HNO₃
Ag₂PtCl₆ + 2H₂O = 2AgCl + PtCl₄
2H₂O PtCl₄.2H₂O + 2AgNO₃ = Ag₂PtCl₄(OH)₂ + 2HNO₃.

The heat of formation of silver chlor-platinate is as follows:

[PtCl₄] + 2[AgCl] = [Ag₂PtCl₆] + 7.7 Cals.