|Jmol-3D images||Image 1|
|Molar mass||339.785 g/mol (anhydrous)
393.833 g/mol (trihydrate)
411.85 g/mol (tetrahydrate)
|Appearance||orange-yellow needle-like crystals
|Density||3.9 g/cm3 (anhydrous)
2.89 g/cm3 (tetrahydrate)
254 °C, 527 K, 489 °F (decomp)
|Solubility in water||350 g HAuCl4 / 100 g H2O|
|Solubility||soluble in alcohol, ester, ether, ketone|
|Other anions||Tetrabromoauric acid|
|EU classification||not listed|
| (what is: / ?)
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
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.
- Au + HNO3 + 4 HCl → HAuCl4 + NO + 2 H2O
Under some conditions, oxygen can be used as the oxidant. 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]-.
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).
The compound crystallizes 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)
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. [PtCl4]2-.
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. Other complex salts include [Au(bipy)Cl2][AuCl4] and [Co(NH3)6][AuCl4]Cl2.
Chloroauric acid is reduced by many substances. 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 ((H2N)2CS):
- AuCl4- + 4 (H2N)2CS + H2O → Au((H2N)2CS)2+ + (NH2)2CO + S + 2 Cl- + 2 HCl
Chloroauric acid is the precursor to gold nanoparticles by precipitation onto mineral supports.
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. The most frequently used extractants for this purpose are dibuthyl glycol, methyl isobuthyl ketone, tributhyl phosphate, dichlorodiethyl ether (chlorex).
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