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Chloroauric acid

Names
Other names Hydrogen tetrachloroaurate Hydrogen tetrachloroaurate(III) Chlorauric acid Tetrachloroauric(III) acid Aurochloric acid Aurate(1−), tetrachloro-, hydrogen, (SP-4-1)- Hydrogen aurichloride brown gold chloride
Identifiers
CAS Number	16903-35-8 16961-25-4 (trihydrate)
3D model (JSmol)	Interactive image
ChemSpider	26171
ECHA InfoCard	100.037.211
EC Number	240-948-4
PubChem CID	28133
UNII	8H372EGX3V 31KV0KH4AY (trihydrate)
CompTox Dashboard (EPA)	DTXSID701014499 DTXSID90156444, DTXSID701014499
InChI InChI=1S/Au.4ClH/h;4*1H/q+3;;;;/p-3Key: VDLSFRRYNGEBEJ-UHFFFAOYSA-K InChI=1/Au.4ClH.Na/h;4*1H;/q+3;;;;;+1/p-4/rAuCl4.Na/c2-1(3,4)5;/q-1;+1Key: IXPWAPCEBHEFOV-ACHCXQQJAP InChI=1/Au.4ClH/h;4*1H/q+3;;;;/p-3/rAuCl4/c2-1(3,4)5/q-1/p+1Key: VDLSFRRYNGEBEJ-ZXMCYSOYAI
SMILES .Cl(Cl)(Cl)Cl
Properties
Chemical formula	H[AuCl4]
Molar mass	339.785 g/mol (anhydrous) 393.833 g/mol (trihydrate) 411.85 g/mol (tetrahydrate)
Appearance	orange-yellow needle-like hygroscopic crystals
Density	3.9 g/cm (anhydrous) 2.89 g/cm (tetrahydrate)
Melting point	254 °C (489 °F; 527 K) (decomposes)
Solubility in water	350 g of H[AuCl4] in 100 g of H2O
Solubility	soluble in alcohol, ester, ether, ketone
log P	2.67510
Conjugate base	Tetrachloroaurate(III)
Structure
Crystal structure	monoclinic
Hazards
GHS labelling:
Pictograms
Signal word	Danger
Hazard statements	H302, H314, H317, H373, H411
Precautionary statements	P260, P261, 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)	3 0 1
Safety data sheet (SDS)	JT Baker
Related compounds
Other anions	Tetrabromoauric acid
Related compounds	Gold(III) chloride
Except where otherwise noted, data are given for materials in their standard state (at 25 °C , 100 kPa). N verify (what is  ?) Infobox references

Chloroauric acid is an inorganic compound with the chemical formula H[AuCl₄]. It forms hydrates H[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 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 [H₅O₂][AuCl₄] and two water molecules.

Structure

The oxidation state of gold in H[AuCl₄] and [AuCl₄] anion is +3. The salts of H[AuCl₄] (tetrachloroauric(III) acid) are tetrachloroaurates(III), containing [AuCl₄] anions (tetrachloroaurate(III) anions), which have square planar molecular geometry. The Au–Cl distances are around 2.28 Å. Other d complexes adopt similar structures, e.g. tetrachloroplatinate(II) [PtCl₄].

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. 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 H[AuCl₄]·nH₂O melts in the water of crystallization, quickly darkens and becomes dark brown.

Chemical reactions

Since [AuCl₄] is prone to hydrolyze, upon treatment with an alkali metal base, chloroauric acid converts to gold(III) hydroxide. The related thallium salt(Tl[AuCl₄]) 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](Cl)₂.

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, CS(NH₂)₂:

[AuCl₄] + 3 CS(NH₂)₂ + H₂O → [Au(CS(NH₂)₂)₂] + CO(NH₂)₂ + S + 2 Cl + 2 HCl

Chloroauric acid is the precursor to gold nanoparticles by precipitation onto mineral supports. Heating of H[AuCl₄]·nH₂O in a stream of chlorine gives gold(III) chloride (Au₂Cl₆). 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.

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:

Au(s) + HNO₃(aq) + 4 HCl(aq) → H[AuCl₄](aq) + NO(g) + 2 H₂O(l)

Under some conditions, oxygen can be used as an oxidant. For higher efficiency, these processes are conducted in autoclaves, which allows greater control of temperature and pressure. Alternatively, a solution of H[AuCl₄] can be produced by electrolysis of gold metal in hydrochloric acid:

2 Au(s) + 8 HCl(aq) → 2 H[AuCl₄](aq) + 3 H₂(g)

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].

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 H[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. 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 Na[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 sections stained with silver.

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

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.

References

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• Gold(III) compounds
• Acids
• Gold–halogen compounds
• Chlorometallates
• Aurates
• Mineral acids