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Chloroplatinic acid

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Title: Chloroplatinic acid  
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Chloroplatinic acid

Chloroplatinic acid
Dihydrogen hexachloroplatinate (IV) hexahydrate
Identifiers
CAS number  YesY
PubChem
ChemSpider  YesY
UNII  YesY
EC number
RTECS number TP1510000
Jmol-3D images Image 1
Properties
Molecular formula H2PtCl6
Molar mass 409.81 g/mol
Appearance Reddish brown solid
Density 2.431 g/cm3
Melting point 60 °C (140 °F; 333 K)
Boiling point decomposes
Solubility in water highly soluble
Structure
Crystal structure Anti-fluorite.
Coordination
geometry
octahedral
Dipole moment 0 D
Related compounds
Other anions Hexachloropalladic acid
Other cations Potassium hexachloroplatinate,
Ammonium hexachloroplatinate,
Rubidium hexachloroplatinate,
Caesium hexachloroplatinate
Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
 YesY   YesY/N?)

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 hydronium salt of the hexachloroplatinate(IV) anion. Therefore, the correct formula is [H3O]2[PtCl6]·4H2O.[1][2] The related palladium compound, [H3O]2[PdCl6], is extremely unstable and has not been isolated in pure form.[3]

Production

Platinum is being dissolved in hot aqua regia

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.[4]

Pt + 4 HNO3 + 6 HCl → H2PtCl6 + 4 NO2 + 4 H2O

Alternative methods have been heavily investigated, but the older literature can be unreliable.[5]

Reactions

When hexachloroplatinic acid is heated, it decomposes through platinum(IV) chloride and platinum(II) chloride to elemental platinum, although the reactions do not occur stepwise, cleanly:[1]

(H3O)2PtCl6·n H2O is in equilibrium with PtCl4 + 2 HCl + (n + 2) H2O
PtCl4 is in equilibrium with PtCl2 + Cl2
PtCl2 is in equilibrium with Pt + Cl2

All three reactions are reversible.

Applications

Potassium determination

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).[6]

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.

Purification of platinum

Treatment with an ammonium salt, such as ammonium chloride, gives ammonium hexachloroplatinate,[7] 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.[8]

Catalysis

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. 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.[9][10] It is generally agreed that chloroplatinic acid is a catalyst precursor. A possible role for colloidal platinum or zero-valent complexes has also been considered.[11]

Related compounds

Chloroplatinic acid prepared from aqua regia is occasionally contaminated with nitrosonium hexachloroplatinate, (NO)2PtCl6. This species is obtained by the reaction of nitrosyl chloride, NOCl, and Pt metal.[12]

References

  1. ^ a b A. E. Schweizer, G. T. Kerr (1978). "Thermal Decomposition of Hexachloroplatinic Acid".  
  2. ^ Holleman, Wiberg (2001). Inorganic Chemistry (First ed.). New York: Academic Press.  
  3. ^ Greenwood, N.N.; Earnshaw, A. (1997). Chemistry of the Elements (Second ed.). New York: Elsevier Butterworth-Heinemann.  
  4. ^ George B. Kauffman; Thurner, Joseph J.; Zatko, David A. (1967). "Ammonium Hexachloroplatinate(IV)".  
  5. ^ Paul Rudnick and R. D. Cooke (1917). "The Preparation of Hydrochloroplatinic Acid by means of Hydrogen Peroxide".  
  6. ^ G. F. Smith, J. L. Gring (1933). "The Separation and Determination of the Alkali Metals Using Perchloric Acid. V. Perchloric Acid and Chloroplatinic Acid in the Determination of Small Amounts of Potassium in the Presence of Large Amounts of Sodium".  
  7. ^  
  8. ^ Cotton, S. A. Chemistry of Precious Metals, Chapman and Hall (London): 1997. ISBN 0-7514-0413-6.
  9. ^ J. L. Speier, J. A. Webster, G. H. Barnes (1957). "The Addition of Silicon Hydrides to Olefinic Double Bonds. Part II. The Use of Group VIII Metal Catalysts".  
  10. ^ John C. Saam, John L. Speier (1958). "The Addition of Silicon Hydrides to Olefinic Double Bonds. Part III. The Addition to Non-terminal Olefins in the Presence of Chloroplatinic Acid".  
  11. ^ L. N. Lewis, K. G. Sy, G. L. Bryant and P. E. Donahue (1991). "Platinum-catalyzed hydrosilylation of alkynes".  
  12. ^ R. T. Moravek, G. B. Kauffman and T. Mahmood (1967). "Nitrosyl Hexachloroplatinate(IV)".  


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