Chloroauric acid
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| Other names
Hydrogen tetrachloroaurate,
Chlorauric acid, Aurochloric acid, Aurate(1-), tetrachloro-, hydrogen, (SP-4-1)-, Hydrogen aurichloride | |
| Identifiers | |
3D model (JSmol)
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| ChemSpider | |
| ECHA InfoCard | 100.037.211 |
PubChem CID
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CompTox Dashboard (EPA)
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| Properties | |
| HAuCl4 | |
| Molar mass | 339.785 g/mol (anhydrous) 393.833 g/mol (trihydrate) 411.85 g/mol (tetrahydrate) |
| Appearance | orange-yellow crystals hygroscopic |
| Density | 2.89 g/cm3 (tetrahydrate) |
| Melting point | 254°C (decomposes) |
| 350 g HAuCl4 / 100 g H2O | |
| Solubility | soluble in alcohols,esters, ethers, ketones |
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Chloroauric acid is the inorganic compound with the formula HAuCl4. It is an orange-yellow solid, a common precursor to other gold compounds and an intermediate in the purification of gold metal. Commercial samples have the composition HAuCl4•n H2O with n = 3 or 4.
Preparation
It is produced by dissolving gold in aqua regia (a mixture of concentrated nitric and hydrochloric acids) followed by careful evaporation of the solution:[1]
- Au + HNO3 + 4 HCl → HAuCl4 + NO + 2 H2O
Under some conditions, oxygen can be used as the oxidant.[2] For higher efficiency, these processes are conducted in autoclaves, which allows greater control of temperature and pressure. Alternatively, a solution of HAuCl4 can be produced by electrolysis of gold metal in hydrochloric acid:
- 2 Au + 8 HCl → 2 HAuCl4 + 3 H2
To prevent the deposition of gold on the cathode, the electrolysis is carried out in a cell equipped with a membrane. This method is used for refining gold. Some gold remains in solution in the form of [AuCl2]-.[3]
A solution of HAuCl4 can also be obtained by the action of chlorine or chlorine water on metallic gold in hydrochloric acid:
- 2 Au + 3 Cl2 + 2 HCl → 2 HAuCl4
This reaction is widely used for extracting gold from electronic and other "rich" materials.
In addition to the above routes, many other ways exist to dissolve gold, differing in the choice of the oxidant (hydrogen peroxide, hypochlorites) or variations of conditions. It is possible also to convert the trichloride (Au2Cl6) or the oxide (Au2O3•x H2O).
Properties
The compounds crystallises as a yellow-orange hygroscopic needles. It is soluble not only in water, but also in many oxygen-containing solvents, such as alcohols, esters, ethers, and ketones. For example, in dry dibutyl ether of diethylene glycol, the solubility exceeds 1 mol/L. Saturated solutions in the organic solvents often are the liquid solvates of specific stoichiometry.
When heated in air of solid HAuCl4•n H2O, it melts in the water of crystallization, quickly darkens and becomes dark brown. Heating of HAuCl4•n H2O in a stream of chlorine gives gold(III) chloride (Au2Cl6)[4]
The AuCl4- anion has square planar molecular geometry. The Au-Cl distances are around 2.28 Å. Other d8 complexes adopt similar structrures, e.g. tetrachloroplatinate.
As a strong acid, HAuCl4 converts to the alkali metal salts MAuCl4 (M = Li, Na, K, etc.), which are soluble. The related thallium salt is poorly soluble in all nonreacting solvents. Salts of quaternary ammonium cations are known.[5] Other complex salts include [Au(bipy)Cl2][AuCl4][6] and [Co(NH3)6][AuCl4]Cl2.
Chloroauric acid is reduced by many substances to give gold(I) or gold(0). This redox is used in many syntheses of gold(I) complexes, especially with organic ligands. Often the ligand serves as reducing agent as illustrated with thiourea:
- AuCl4- + 4 (H3N)2CS → Au((H3N)2CS)2+ + (NH2(NH)CS)22+ + 4 Cl-
Liquid-liquid extraction processes
Liquid-liquid extraction is used for the recovery, concentrating, purification, analytical determinations of gold. Of the great importance is the extraction of HAuCl4 from hydrochloric medium by oxygen-containing extractants, i.e. alcohols, ketones, ethers and esters. The concentration of gold(III) in the extracts may exceed 1 mol/L.[7][8][9] The most frequently used extractants for this purpose are dibuthyl glycol, methyl isobuthyl ketone, tributhyl phosphate, dichlorodiethyl ether (chlorex).
References
- ^ Handbook of Preparative Inorganic Chemistry, 2nd Ed. Edited by G. Brauer, Academic Press, 1963, NY.
- ^ Novoselov, R. I.; Makotchenko, E. V. "Application of oxygen as ecologically pure reagent for the oxidizing of non-ferrous and precious metals, sulphide minerals" Chemistry for sustainable development, 1999, vol. 7, p. 321—330.
- ^ Belevantsev V. I., Peschevitskii, B. I.; Zemskov, S. V. "New data on chemistry of gold compounds in solutions" Izvestiya Sibirskogo Otdeleniya AN SSSR, ser. khim. nauk. 1976. N4. Issue 2. P. 24-45.
- ^ Mellor J. W. A comprehensive treatise on inorganic and theoretical chemistry. Vol. 3. 1946. p. 593.
- ^ Makotchenko E. V.; Kokovkin V. V. "Solid Contact [AuCl4]--Selective Electrode and Its Application for Evaluation of Gold(III) in Solutions" Russian Journal of General Chemistry 2010, Volume 80, p. 1733-1737.
- ^ Mironov, I. V.; Tsvelodub, L. D. "Equilibria of the substitution of pyridine, 2,2 '-bipyridyl, and 1,10-phenanthroline for Cl- in AuCl4- in aqueous solution" Russian Journal of Inorganic Chemistry. 2001, vol. 46, p. 143-148.
- ^ Mironov I. V., Natorkhina K. I. "On the Selection of Extractant for the Preparation of High Purity Gold" Russian Journal of Inorganic Chemistry, 2012, Vol. 57, No. 4, pp. 610–615.
- ^ Feather A., Sole K. C., Bryson L. J. "Gold refining by solvent extraction—the Minataur Process" Journal of the South African Institute of Mining and Metallurgy. 1997, July/August, p.169—173.
- ^ Morris D. F. C., Khan M. A. Application of solvent extraction to the refining of precious metals, Part 3: purification of gold" Talanta, 1968. vol. 15, pp. 1301—1305.