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| Names | |
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| IUPAC name Tetrachloroauric(III) acid | |
Other names
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| Identifiers | |
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3D model (JSmol) | |
| ChemSpider |
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| ECHA InfoCard | 100.037.211 |
| EC Number |
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| UNII |
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| Properties | |
| H[AuCl4] | |
| Molar mass |
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| Appearance | orange-yellow needle-likehygroscopic crystals |
| Density | 3.9 g/cm3 (anhydrous) 2.89 g/cm3 (tetrahydrate) |
| Melting point | 254 °C (489 °F; 527 K) (decomposes) |
| 350 g ofH[AuCl4] in 100 g ofH2O | |
| Solubility | soluble inalcohol,ester,ether,ketone |
| logP | 2.67510[1] |
| Conjugate base | Tetrachloroaurate(III) |
| Structure | |
| monoclinic | |
| Hazards | |
| GHS labelling: | |
    | |
| Danger | |
| H302,H314,H317,H373,H411 | |
| P260,P264,P272,P280,P301+P330+P331,P302+P352,P303+P361+P353,P304+P340,P305+P351+P338,P310,P321,P333+P313,P363,P405,P501 | |
| NFPA 704 (fire diamond) | |
| Safety data sheet (SDS) | JT Baker |
| Related compounds | |
Otheranions | Tetrabromoauric acid |
Related compounds | Gold(III) chloride |
Except where otherwise noted, data are given for materials in theirstandard state (at 25 °C [77 °F], 100 kPa). | |
Chloroauric acid is aninorganic compound with thechemical formulaH[AuCl4]. It forms hydratesH[AuCl4]·nH2O. 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 ofgold inaqua regia. These solutions can be converted to other gold complexes or reduced to metallic gold orgold nanoparticles.
The tetrahydrate crystallizes as[H5O2]+[AuCl4]− and two water molecules.[2][3]
The oxidation state of gold inH[AuCl4] and[AuCl4]− anion is +3. The salts ofH[AuCl4] (tetrachloroauric(III) acid) are tetrachloroaurates(III), containing[AuCl4]− anions (tetrachloroaurate(III) anions), which havesquare planar molecular geometry. The Au–Cl distances are around 2.28 Å. Other d8 complexes adopt similar structures, e.g. tetrachloroplatinate(II)[PtCl4]2−.
Solid chloroauric acid is ahydrophilic (ionic)protic solute. It is soluble in water and other oxygen-containing solvents, such as alcohols, esters, ethers, and ketones. For example, in drydibutyl ether ordiethylene glycol, the solubility exceeds 1 M.[4][5][6] Saturated solutions in the organic solvents often are the liquid solvates of specific stoichiometry. Chloroauric acid is a strongmonoprotic acid.
When heated in air, solidH[AuCl4]·nH2O melts in the water of crystallization, quickly darkens and becomes dark brown.
Since[AuCl4]− is prone to hydrolyze,[7] upon treatment with an alkali metal base, chloroauric acid converts togold(III) hydroxide.[8] The related thallium salt(Tl+[AuCl4]−) is poorly soluble in all nonreacting solvents. Salts ofquaternary ammonium cations are known.[9] Other complex salts include[Au(bipy)Cl2]+[AuCl4]−[10] and[Co(NH3)6]3+[AuCl4]−(Cl−)2.
Partial reduction of chloroauric acid gives oxonium dichloridoaurate(1−).[11] Reduction may also yield other gold(I) complexes, especially with organic ligands. Often the ligand serves as reducing agent as illustrated withthiourea,CS(NH2)2:
Chloroauric acid is the precursor to gold nanoparticles by precipitation onto mineral supports.[12] Heating ofH[AuCl4]·nH2O in a stream of chlorine givesgold(III) chloride (Au2Cl6).[13] 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.[AuCl4]− 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.[14]
Chloroauric acid is produced by dissolving gold inaqua regia (a mixture of concentratednitric andhydrochloric acids) followed by careful evaporation of the solution:[15][16]
Under some conditions, oxygen can be used as an oxidant.[17] For higher efficiency, these processes are conducted inautoclaves, which allows greater control of temperature and pressure. Alternatively, a solution ofH[AuCl4] can be produced by electrolysis of gold metal inhydrochloric acid:
To prevent thedeposition 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]−.[18]
Chloroauric acid is the precursor used in thepurification of gold byelectrolysis.
Liquid–liquid extraction of chloroauric acid is used for the recovery, concentrating, purification, and analytical determinations of gold. Of great importance is the extraction ofH[AuCl4] 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.[4][5][6] Frequently used extractants for this purpose are dibutyl glycol,methyl isobutyl ketone,tributyl phosphate,dichlorodiethyl ether (chlorex).[19]
Inhistology, chlorauric acid is known as "brown gold chloride", and its sodium saltNa[AuCl4] (sodium tetrachloroaurate(III)) 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 sectionsstained with silver.[20]
Inphotography, chloroauric acid can be used as agold toner.[21]
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 iscorrosive 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.[22][23]
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