Chloroauric acid

Chloroauric acid is an inorganic compound with the chemical formula HAuCl4. Both the trihydrate and tetrahydrate are known. It is an orange-yellow solid, a common precursor to other gold compounds and an intermediate in the purification of gold metal. Both the trihydrate and tetrahydrate are available commercially. Chloroauric acid is an inorganic compound with the chemical formula HAuCl4. Both the trihydrate and tetrahydrate are known. It is an orange-yellow solid, a common precursor to other gold compounds and an intermediate in the purification of gold metal. Both the trihydrate and tetrahydrate are available commercially. Chloroauric acid is a strong monoprotic acid. The crystalline tetrahydrate is known to contain H5O+2·AuCl−4 and two water molecules. The AuCl−4 anion has square planar molecular geometry. The Au–Cl distances are around 2.28 Å. Other d8 complexes adopt similar structures, e.g. 2−. Solid chloroauric acid is a hydrophilic (ionic) protic solute. It is soluble not only in water, but also in many oxygen-containing solvents, such as alcohols, esters, ethers, and ketones. For example, in dry dibutyl ether of diethylene glycol, the solubility exceeds 1 mol/L. Saturated solutions in the organic solvents often are the liquid solvates of specific stoichiometry. When heated in air, solid HAuCl4·nH2O melts in the water of crystallization, quickly darkens and becomes dark brown. Upon treating chloroauric acid with a standard base, it converts to metal, tetrachloridoaurate and water. The related thallium salt is poorly soluble in all nonreacting solvents. Salts of quaternary ammonium cations are known. Other complex salts include and Cl2. 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. 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, (H2N)2CS: Chloroauric acid is the precursor to gold nanoparticles by precipitation onto mineral supports. Heating of HAuCl4·nH2O in a stream of chlorine gives gold(III) chloride (Au2Cl6).