Chloroauric acid refers to inorganic compounds with the chemical formula .x. Both the trihydrate and tetrahydrate are known. Both are orange-yellow solids consisting of the planar − 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
Structure
The tetrahydrate crystallizes as · and two water molecules. The anion has square planar molecular geometry. The Au–Cl distances are around 2.28 Å. Other d8 complexes adopt similar structures, e.g. Potassium tetrachloroplatinate|2−.
Solute properties
chloroauric acid is a hydrophilicprotic 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 mol/L. 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 HAuCl4·H2O melts in the water of crystallization, quickly darkens and becomes dark brown.
Chemical reactions
Upon treatment with an alkali metal base, chloroauric acid converts to an alkali metal salt of tetrachloridoaurate. The related thallium salt is poorly soluble in all nonreacting solvents. Salts of quaternary ammonium cations are known. Other complex salts include and Cl2. Partial reduction of chloroauric acid gives oxonium dichloridoaurate. Reduction may also yield other gold complexes, especially with organic ligands. Often the ligand serves as reducing agent as illustrated with thiourea, 2CS: Chloroauric acid is the precursor to gold nanoparticles by precipitation onto mineral supports. Heating of HAuCl4·H2O in a stream of chlorine gives gold chloride. 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. 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 followed by careful evaporation of the solution: 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: 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 −. A solution of HAuCl4 can also be obtained by the action of chlorine or chlorine water on metallic gold in hydrochloric acid: 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 or variations of conditions. It is possible also to convert the trichloride or the oxide.
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 HAuCl4 from hydrochloric medium by oxygen-containing extractants, such as alcohols, ketones, ethers and esters. The concentration of gold 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. In histology, chlorauric acid is known as "brown gold chloride", and its sodium salt NaAuCl4 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.
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. It can stain skin purple for several days after contact.
