Chloroplatinic acid or hexachloroplatinic acid is the chemical compound usually found as the hexahydrate with the formula H2PtCl6•(H2O)6. This is one of the most readily available soluble compounds of platinum. It is rarely obtained in the pure state. The commercial product is the oxonium salt of the hexachloroplatinate(IV) anion. Therefore, the correct formula is [H3O]2[PtCl6]•4H2O.(5,6) The related palladium compound, [H3O]2[PdCl6] is extremely unstable and has not been isolated in pure form.
Chloroplatinic acid is produced by dissolving platinum metal sponge in aqua regia. This reaction is rumored to produce nitrogen-containing platinum compounds, but the product is H2PtCl6. Chloroplatinic acid is brownish-red, and can be isolated by evaporating this solution to a syrup.
Pt + 4 HNO3 + 6 HCl → H2PtCl6 + 4 NO2 + 4 H2O
Alternative methods have been heavily investigated, but the older literature can be unreliable.
When hexachloroplatinic acid is heated, it decomposes through platinum(IV) chloride, platinum(II) chloride, to elemental platinum, although the reactions do not occur stepwise, cleanly:
- (H3O)2PtCl6•n H2O PtCl4 + 2 HCl + (n + 2) H2O
- PtCl4 PtCl2 + Cl2
- PtCl2 Pt + Cl2
All three reactions are reversible.
Chloroplatinic acid was popularized for the determination of potassium. The potassium is selectively precipitated as potassium chloroplatinate. Determinations were done in 85% (v/v) alcohol solutions with excess platinate ions, and the precipitated product was weighed. Potassium could be detected for solutions as dilute as 0.02 to 0.2% (m/v).
This method for determination of potassium was advantageous vs. the cobaltinitrite method used previously, since it required a single precipitation reaction. Today, the concentration of potassium is determined with an ion-selective electrode. These modern methods remain subject to interference.
Treatment with an ammonium salt, such as ammonium chloride, gives ammonium hexachloroplatinate, which is very insoluble in ammonium solutions. Heating the ammonium salt in hydrogen reduces it to elemental platinum. Platinum is often isolated from ores or recycled from residues thus.
Like many platinum compounds, chloroplatinic acid is used in catalysis. This compound was first reported by John Speier and colleagues from Dow Corning Corporation to catalyze the reaction of silyl hydrides with olefins, hydrosilylation.(3) Typical of his reactions, Speier used isopropanol solutions containing trichlorosilane (SiHCl3), and methyldichlorosilane (CH3HSiCl2), with pentenes. Prior work on the addition of silanes to alkenes required radical reactions that were inefficient. It is generally agreed that chloroplatinic acid is a catalyst precursor, and more recent discussions have considered a possible role for colloidal platinum or zero-valent complexes