Chloroauric acid

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Chloroauric acid
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Template:Longitem Template:Chem2
Molar mass Template:Ubl
Appearance orange-yellow needle-like hygroscopic crystals
Density 3.9 g/cm3 (anhydrous)
2.89 g/cm3 (tetrahydrate)
Melting point Template:Chembox CalcTemperatures
Solubility soluble in alcohol, ester, ether, ketone
log P 2.67510 [1]
Template:Longitem monoclinic
NFPA 704 (fire diamond) Template:NFPA 704 diamond
Template:Longitem Tetrabromoauric acid
Template:Longitem Gold(III) chloride

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Chloroauric acid is an inorganic compound with the chemical formula Template:Chem2. It forms hydrates Template:Chem2. Both the trihydrate and tetrahydrate are known. Both are orange-yellow solids consisting of the planar Template:Chem2 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.

Properties

The tetrahydrate crystallizes as Template:Chem2 and two water molecules.[2][3]

Structure

The oxidation state of gold in Template:Chem2 and Template:Chem2 anion is +3. The salts of Template:Chem2 (tetrachloroauric(III) acid) are tetrachloroaurates(III), containing Template:Chem2 anions (tetrachloroaurate(III) anions), which have square planar molecular geometry. The Au–Cl distances are around 2.28 Å. Other d8 complexes adopt similar structures, e.g. tetrachloroplatinate(II) [[Potassium tetrachloroplatinate|Template:Chem2]].

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 M.[4][5][6] 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 Template:Chem2 melts in the water of crystallization, quickly darkens and becomes dark brown.

Chemical reactions

Since Template:Chem2 is prone to hydrolyze,[7] upon treatment with an alkali metal base, chloroauric acid converts to gold(III) hydroxide.[8] The related thallium salt(Template:Chem2) is poorly soluble in all nonreacting solvents. Salts of quaternary ammonium cations are known.[9] Other complex salts include Template:Chem2[10] and Template:Chem2.

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 with thiourea, Template:Chem2:

Template:Chem2

Chloroauric acid is the precursor to gold nanoparticles by precipitation onto mineral supports.[12] Heating of Template:Chem2 in a stream of chlorine gives gold(III) chloride (Template:Chem2).[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. Template:Chem2 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]

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:[15][16]

Template:Chem2

Under some conditions, oxygen can be used as an oxidant.[17] For higher efficiency, these processes are conducted in autoclaves, which allows greater control of temperature and pressure. Alternatively, a solution of Template:Chem2 can be produced by electrolysis of gold metal in hydrochloric acid:

Template:Chem2

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 Template:Chem2.[18]

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 Template:Chem2 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]

In histology, chlorauric acid is known as "brown gold chloride", and its sodium salt Template:Chem2 (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 sections stained with silver.[20]

In photography, chlorauric acid can be used as a gold toner.[21]

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.Script error: No such module "Unsubst".

References

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