|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)
|Melting point||254 °C (489 °F; 527 K) (decomposes)|
|Solubility in water||350 g HAuCl4 / 100 g H2O|
|Solubility||soluble in alcohol, ester, ether, ketone|
|Other anions||Tetrabromoauric acid|
|EU classification||not listed|
|Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)|
|(what is: / ?)|
Chloroauric acid is an inorganic compound with the chemical formula HAuCl
4. Both the trihydrate and tetrahydrate are known. It is an orange-yellow solid, a common precursor to other gold compounds and an intermediate in the purification of gold metal. Both the trihydrate and tetrahydrate are available commercially.
A hydrogen center in the oxidaniumyl group (-OH+
2) in oxidanium cations such as oxonium can separate from the molecule by heterolysis:
2O + H+
Because of this release of the proton (H+
), chloroauric acid has acidic character. Chloroauric acid is a strong monoprotic conjugate acid. Although chloroauric acid forms aqueous solutions, such solutions are unstable due to hydrolysis of the tetrachloridoaurate ion. An aqueous solution consists of an ensemble of anions in equilibrium, with the overall reaction being:
4 + 2 H
3 + 4 HCl
Thus, unless outgassing hydrogen chloride is replenished, aqueous chloroauric acid will decompose to auric hydroxide. However, this decomposition route is unlikely, as HCl is a strong acid, and thus it is very unlikely that Cl- ions will combine with H+ to form HCl in an aqueous solution.
The crystalline tetrahydrate is known to contain H5O2+ AuCl4– and two water molecules. The AuCl4− 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−.
Solid chloroauric acid is a hydrophilic (ionic) protic solute. 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.
Upon treating chloroauric acid with a standard base, it converts to metal tetrachloridoaurate and water. 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.
Partial reduction of chloroauric acid gives oxonium dichloridoaurate(1−). 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):
- AuCl4− + 4 (H2N)2CS + H2O → Au((H2N)2CS)2+ + (NH2)2CO + S + 2 Cl− + 2 HCl
- 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).
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 dibutyl glycol, methyl isobutyl ketone, tributyl phosphate, dichlorodiethyl ether (chlorex).
Health effects and safety
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 the mucous membranes. Gloves are worn when handling the compound. Chloroauric acid is a strong eye, skin, and mucous membrane irritant. Prolonged skin contact with chloroauric acid may result in tissue destruction.
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