Chloroauric acid

Chloroauric acid refers to inorganic compounds with the chemical formula HAuCl
4.(H2O)x. Both the trihydrate and tetrahydrate are known. Both are orange-yellow solids consisting of the planar [AuCl4]- anion. Often chloroauric acid is handled as a solution, such as those obtained by dissolution of gold in aqua regia. These solutions can be converted to other gold complexes or reduced to metallic gold or gold nanoparticles.

Chloroauric acid
Names
Other names
Hydrogen tetrachloroaurate,
Chlorauric acid,
Aurochloric acid,
Aurate(1−), tetrachloro-, hydrogen, (SP-4-1)-,
Hydrogen aurichloride
Identifiers
3D model (JSmol)
ChemSpider
ECHA InfoCard 100.037.211
UNII
Properties
HAuCl4
Molar mass 339.785 g/mol (anhydrous)
393.833 g/mol (trihydrate)
411.85 g/mol (tetrahydrate)
Appearance orange-yellow needle-like crystals
hygroscopic
Density 3.9 g/cm3 (anhydrous)
2.89 g/cm3 (tetrahydrate)
Melting point 254 °C (489 °F; 527 K) (decomposes)
350 g HAuCl4 / 100 g H2O
Solubility soluble in alcohol, ester, ether, ketone
log P 2.67510 [1]
Structure
monoclinic
Hazards
Safety data sheet JT Baker
not listed
NFPA 704 (fire diamond)
Flammability code 0: Will not burn. E.g. waterHealth code 3: Short exposure could cause serious temporary or residual injury. E.g. chlorine gasReactivity code 1: Normally stable, but can become unstable at elevated temperatures and pressures. E.g. calciumSpecial hazards (white): no code
0
3
1
Related compounds
Other anions
 Tetrabromoauric acid
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references

Properties

Structure

The tetrahydrate crystallizes as H
5O+
2·AuCl−
4 and two water molecules.[2] The AuCl−
4 anion has square planar molecular geometry. The Au–Cl distances are around 2.28 Å. Other d8 complexes adopt similar structures, e.g. [PtCl4]2−.

Solute properties

Solid chloroauric acid is a hydrophilic (ionic) protic solute. It is soluble in water and other oxygen-containing solvents, such as alcohols, esters, ethers, and ketones. For example, in dry dibutyl ether or diethylene glycol, the solubility exceeds 1 mol/L. Saturated solutions in the organic solvents often are the liquid solvates of specific stoichiometry. Chloroauric acid is a strong monoprotic acid.

When heated in air, solid HAuCl4·nH2O melts in the water of crystallization, quickly darkens and becomes dark brown.

Chemical reactions

Upon treatment with a alkali metal base, chloroauric acid converts to alkali metal salt of tetrachloridoaurate. The related thallium salt is poorly soluble in all nonreacting solvents. Salts of quaternary ammonium cations are known.[3] Other complex salts include [Au(bipy)Cl2][AuCl4][4] and [Co(NH3)6][AuCl4]Cl2.

Gold nanostructures can be made from chloroauric acid in a two-phase redox reaction whereby metallic clusters are amassed through the simultaneous attachment of self-assembled thiol monolayers on the growing nuclei. AuCl−
4 is transferred from aqueous solution to toluene using tetraoctylammonium bromide where it is then reduced with aqueous sodium borohydride in the presence of a thiol.[5]

Partial reduction of chloroauric acid gives oxonium dichloridoaurate(1−).[6] Reduction may also yield other gold(I) complexes, especially with organic ligands. Often the ligand serves as reducing agent as illustrated with thiourea, (H2N)2CS:

AuCl−
4 + 4 (H
2N)
2CS + H
2O → Au[(H
2N)
2CS]+
2 + (H
2N)
2CO + S + 2 Cl−
 + 2 HCl

Chloroauric acid is the precursor to gold nanoparticles by precipitation onto mineral supports.[7] Heating of HAuCl4·nH2O in a stream of chlorine gives gold(III) chloride (Au2Cl6).[8]

Production

Chloroauric acid is produced by dissolving gold in aqua regia (a mixture of concentrated nitric and hydrochloric acids) followed by careful evaporation of the solution:[9]

Au + HNO3 + 4 HCl → HAuCl4 + NO + 2 H2O

Under some conditions, oxygen can be used as the oxidant.[10] 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 + 3H2

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]−.[11]

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·nH2O).

Uses

Chloroauric acid is the precursor used in the purification of gold by electrolysis.

Liquid–liquid extraction of chloroauric acid is used for the recovery, concentrating, purification, and analytical determinations of gold. Of great importance is the extraction of HAuCl4 from hydrochloric medium by oxygen-containing extractants, such as alcohols, ketones, ethers and esters. The concentration of gold(III) in the extracts may exceed 1 mol/L.[12][13][14] The most frequently used extractants for this purpose are dibutyl glycol, methyl isobutyl ketone, tributyl phosphate, dichlorodiethyl ether (chlorex).

In histology, chlorauric acid is known as "brown gold chloride", and its sodium salt NaAuCl4 as "gold chloride", "sodium gold chloride" or "yellow gold chloride". The sodium salt is used in a process called "toning" to improve the optical definition of tissue sections stained with silver.[15]

Health effects and safety

Chloroauric acid is a strong eye, skin, and mucous membrane irritant. Prolonged skin contact with chloroauric acid may result in tissue destruction. Concentrated chloroauric acid is corrosive to skin and must, therefore, be handled with appropriate care, since it can cause skin burns, permanent eye damage, and irritation to mucous membranes. Gloves are worn when handling the compound. It can stain skin purple for several days after contact.

References
  1. "hydrogen tetrachloroaurate(iii)_msds".
  2. Williams, Jack Marvin; Peterson, Selmer Wiefred (1969). "Example of the [H5O2]+ ion. Neutron diffraction study of tetrachloroauric acid tetrahydrate". Journal of the American Chemical Society. 91 (3): 776–777. doi:10.1021/ja01031a062. ISSN 0002-7863.
  3. Makotchenko, E. V.; Kokovkin, V. V. (2010). "Solid contact [AuCl4]−-selective electrode and its application for evaluation of gold(III) in solutions". Russian Journal of General Chemistry. 80 (9): 1733. doi:10.1134/S1070363210090021.
  4. Mironov, I. V.; Tsvelodub, L. D. (2001). "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. 46: 143–148.
  5. Brust, Mathias; Walker, Merryl; Bethell, Donald; Schiffrin, David J.; Whyman, Robin (1994). "Synthesis of Thiol-derivatised Gold Nanoparticles in a Two-phase Liquid-Liquid System". J. Chem. Soc., Chem. Commun. Royal Society of Chemistry (7): 801–802. doi:10.1039/C39940000801.
  6. Huang, Xiaohua; Peng, Xianghong; Wang, Yiqing; Wang, Yuxiang; Shin, Dong M.; El-Sayed, Mostafa A.; Nie, Shuming (26 October 2010). "A reexamination of active and passive tumor targeting by using rod-shaped gold nanocrystals and covalently conjugated peptide ligands". ACS Nano. ACS Publications. 4 (10): 5887–5896. doi:10.1021/nn102055s. PMC 2964428. PMID 20863096.
  7. Gunanathan, C.; Ben-David, Y.; Milstein, D. (2007). "Direct Synthesis of Amides from Alcohols and Amines with Liberation of H2". Science. 317 (5839): 790–792. doi:10.1126/science.1145295. PMID 17690291.
  8. Mellor, J. W. (1946). A Comprehensive Treatise on Inorganic and Theoretical Chemistry. vol. 3, p. 593.
  9. Brauer, G., ed. (1963). Handbook of Preparative Inorganic Chemistry (2nd ed.). New York: Academic Press.
  10. Novoselov, R. I.; Makotchenko, E. V. (1999). "Application of oxygen as ecologically pure reagent for the oxidizing of non-ferrous and precious metals, sulphide minerals". Chemistry for Sustainable Development. 7: 321–330.
  11. Belevantsev, V. I.; Peschevitskii, B. I.; Zemskov, S. V. (1976). "New data on chemistry of gold compounds in solutions". Izvestiya Sibirskogo Otdeleniya AN SSSR, Ser. Khim. Nauk. 4 (2): 24–45.
  12. Mironov, I. V.; Natorkhina, K. I. (2012). "On the selection of extractant for the preparation of high-purity gold". Russian Journal of Inorganic Chemistry. 57 (4): 610. doi:10.1134/S0036023612040195.
  13. Feather, A.; Sole, K. C.; Bryson, L. J. (July 1997). "Gold refining by solvent extraction—the minataur™ process" (PDF). Journal of the Southern African Institute of Mining and Metallurgy: 169–173. Retrieved 2013-03-17.
  14. Morris, D. F. C.; Khan, M. A. (1968). "Application of solvent extraction to the refining of precious metals, Part 3: purification of gold". Talanta. 15: 1301–1305. doi:10.1016/0039-9140(68)80053-0.
  15. "Silver Impregnation". Retrieved April 14, 2016.
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