# Chloroauric acid

Chloroauric acid is an inorganic compound with the chemical formula H[AuCl$_4$]. It forms hydrates H[AuCl$_4$]·nH$_2$O. Both the trihydrate and tetrahydrate are known. Both are orange-yellow solids consisting of the planar [AuCl$_4$]$^−$ 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 [H$_5$O$_2$]$^+$[AuCl$_4$]$^−$ and two water molecules.

### Structure

The oxidation state of gold in H[AuCl$_4$] and [AuCl$_4$]$^−$ anion is +3. The salts of H[AuCl$_4$] (tetrachloroauric(III) acid) are tetrachloroaurates(III), containing [AuCl$_4$]$^−$ anions (tetrachloroaurate(III) anions), which have square planar molecular geometry. The Au–Cl distances are around 2.28 Å. Other d$^8$ complexes adopt similar structures, e.g. tetrachloroplatinate(II) [PtCl$_4$]$^{2−}$.

### 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[AuCl$_4$]·nH$_2$O melts in the water of crystallization, quickly darkens and becomes dark brown.

### Chemical reactions

Since [AuCl$_4$]$^−$ is prone to hydrolyze, upon treatment with an alkali metal base, chloroauric acid converts to gold(III) hydroxide. The related thallium salt(Tl$^+$[AuCl$_4$]$^−$) is poorly soluble in all nonreacting solvents. Salts of quaternary ammonium cations are known. Other complex salts include [Au(bipy)Cl$_2$]$^+$[AuCl$_4$]$^−$ and [Co(NH$_3$)$_6$]$^{3+}$[AuCl$_4$]$^−$(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(NH$_2$)$_2$:
    [AuCl$_4$]$^−$ + 3 CS(NH$_2$)$_2$ + H$_2$O → [Au(CS(NH$_2$)$_2$)$_2$]$^+$ + CO(NH$_2$)$_2$ + S + 2 Cl$^−$ + 2 HCl
Chloroauric acid is the precursor to gold nanoparticles by precipitation onto mineral supports. Heating of H[AuCl$_4$]·nH$_2$O in a stream of chlorine gives gold(III) chloride (Au$_2$Cl$_6$). 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. [AuCl$_4$]$^−$ 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) + HNO$_3$(aq) + 4 HCl(aq) → H[AuCl$_4$](aq) + NO(g) + 2 H$_2$O(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[AuCl$_4$] can be produced by electrolysis of gold metal in hydrochloric acid:
    2 Au(s) + 8 HCl(aq) → 2 H[AuCl$_4$](aq) + 3 H$_2$(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 [AuCl$_2$]$^−$.

## 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[AuCl$_4$] 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[AuCl$_4$] (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.