Chloroplatinic acid (also known as hexachloroplatinic acid) is an

inorganic compound An inorganic compound is typically a chemical compound that lacks carbon–hydrogen bonds⁠that is, a compound that is not an organic compound. The study of inorganic compounds is a subfield of chemistry known as ''inorganic chemistry''. Inorgan ...

with the formula ₃Osub>2 tCl₆H₂O)_''x'' (0 ≤ ''x'' ≤ 6). A red solid, it is an important commercial source of

platinum Platinum is a chemical element; it has Symbol (chemistry), symbol Pt and atomic number 78. It is a density, dense, malleable, ductility, ductile, highly unreactive, precious metal, precious, silverish-white transition metal. Its name origina ...

, usually as an

aqueous solution An aqueous solution is a solution in which the solvent is water. It is mostly shown in chemical equations by appending (aq) to the relevant chemical formula. For example, a solution of table salt, also known as sodium chloride (NaCl), in water ...

. Although often written in shorthand as H₂PtCl₆, it is the

hydronium In chemistry, hydronium (hydroxonium in traditional British English) is the cation , also written as , the type of oxonium ion produced by protonation of water. It is often viewed as the positive ion present when an Arrhenius acid is dissolved ...

(H₃O⁺) salt of the hexachloroplatinate anion (). Hexachloroplatinic acid is highly

hygroscopic Hygroscopy is the phenomenon of attracting and holding water molecules via either absorption (chemistry), absorption or adsorption from the surrounding Natural environment, environment, which is usually at normal or room temperature. If water mol ...

Production

Hexachloroplatinic acid may be produced via a variety of methods. The most common of these methods involves dissolution of platinum in

aqua regia Aqua regia (; from Latin, "regal water" or "royal water") is a mixture of nitric acid and hydrochloric acid, optimally in a molar concentration, molar ratio of 1:3. Aqua regia is a fuming liquid. Freshly prepared aqua regia is colorless, but i ...

. Other methods include exposing an aqueous suspension of platinum particles to chlorine gas, or via electrolysis. When produced by the aqua regia route, hexachloroplatinic acid is thought to arise by the following equation: The resulting orange/red solution can be evaporated to produce brownish red crystals. Some authors suggest that hexachloroplatinic acid produced using this method is contaminated with nitrosonium hexachloroplatinate. Newer literature indicates that this is not the case, and that once the nitric acid has been driven off, samples prepared via this method contain no detectable nitrogen. Alternative methods have been investigated and described, often motivated by the avoidance of nitrogen contamination.

Structure

left, Structure of solid hexachloroplatinic acid According to

X-ray crystallography X-ray crystallography is the experimental science of determining the atomic and molecular structure of a crystal, in which the crystalline structure causes a beam of incident X-rays to Diffraction, diffract in specific directions. By measuring th ...

, hexachloroplatinic acid consists of octahedral PtCl ions linked by hydrogen bonding. The cubic array of these octahedra encase water molecules.

Reactions

When heated, hexachloroplatinic acid decomposes to platinum(IV) chloride.

Applications

Potassium determination

Chloroplatinic acid was popularized for the quantitative analysis of potassium. The potassium is selectively precipitated from solution as potassium hexachloroplatinate. 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 compared to the sodium cobaltinitrite method used previously, since it required a single

precipitation In meteorology, precipitation is any product of the condensation of atmospheric water vapor that falls from clouds due to gravitational pull. The main forms of precipitation include drizzle, rain, rain and snow mixed ("sleet" in Commonwe ...

reaction. Gravimetric analysis of precipitated products has been supplanted by modern

instrumental analysis Instrumental analysis is a field of analytical chemistry that investigates analytes using scientific instruments. Spectroscopy Spectroscopy measures the interaction of the molecules with electromagnetic spectrum, electromagnetic radiation. Spec ...

methods such as

ion selective electrode An ion-selective electrode (ISE), also known as a specific ion electrode (SIE), is a simple membrane-based potentiometric device which measures the activity of ions in solution. It is a transducer (or sensor) that converts the change in the concent ...

s, flame photometry, ICP-AES or

ICP-MS Inductively coupled plasma mass spectrometry (ICP-MS) is a type of mass spectrometry that uses an inductively coupled plasma to ionize the sample. It atomizes the sample and creates atomic and small polyatomic ions, which are then detected. It ...

Purification of platinum

Upon treatment with an ammonium salt, such as

ammonium chloride Ammonium chloride is an inorganic chemical compound with the chemical formula , also written as . It is an ammonium salt of hydrogen chloride. It consists of ammonium cations and chloride anions . It is a white crystalline salt (chemistry), sal ...

, chloroplatinic acid converts to ammonium hexachloroplatinate, which precipitates as a solid. Upon heating in an atmosphere of

hydrogen Hydrogen is a chemical element; it has chemical symbol, symbol H and atomic number 1. It is the lightest and abundance of the chemical elements, most abundant chemical element in the universe, constituting about 75% of all baryon, normal matter ...

, the ammonium salt converts to elemental platinum. Platinum is often isolated from ores or recycled from residues using this method.

Catalysis

Like many platinum compounds, chloroplatinic acid is a catalyst (or precatalyst) for

hydrogenation Hydrogenation is a chemical reaction between molecular hydrogen (H2) and another compound or element, usually in the presence of a catalyst such as nickel, palladium or platinum. The process is commonly employed to redox, reduce or Saturated ...

and related reactions. As first reported by John Speier and colleagues from

Dow Corning Dow Corning Corporation, was an American multinational corporation headquartered in Midland, Michigan, United States, and was originally established as a joint venture between The Dow Chemical Company and Corning Inc., Corning Incorporated. In 20 ...

, it catalyzes the addition of hydrosilanes to olefins, i.e. hydrosilylation. Early demonstration reactions used isopropanol solutions of trichlorosilane (SiHCl₃) with pentenes. Prior work on the addition of silanes to alkenes required radical reactions that were inefficient. As well as with Karstedt's catalyst, Speier's catalyst enjoys widespread use for hydrosilylation, the main drawback is the deliquescent properties of the catalyst. It is generally agreed that chloroplatinic acid is a precursor to the actual catalyst. A possible role for colloidal platinum or zero-valent complexes has also been considered.

Related compounds

Chloroplatinic acid prepared from

is proposed to contain nitrosonium hexachloroplatinate, (NO)₂PtCl₆. Nitrosonium hexachloroplatinate is obtained by the reaction of nitrosyl chloride (NOCl) and platinum metal. Nitrosonium hexachloroplatinate has been found to react vigorously with water and hydrochloric acid, making contamination of chloroplatinic acid prepared with aqua regia with nitrosonium hexachloroplatinate unlikely.

References

{{Hydrogen compounds Hydrogen compounds Hexachloroplatinates