Chloroauric acid
[CAS# 16903-35-8]

Identification

• Classification: Analytical chemistry >> Standard >> Spectrometric standard
• Name: Chloroauric acid
• Synonyms: Tetrachloroauric(III) acid
• Molecular Formula: HAuCl₄
• Molecular Weight: 339.79
• CAS Registry Number: 16903-35-8
• EC Number: 240-948-4
• SMILES: [H+].[Cl-].[Cl-].[Cl-].[Cl-].[Au+3]

Properties

• Melting point: 250 °C (Decomposes) (Expl.)

Safety Data

• Hazard Symbols: GHS05;GHS07;GHS08;GHS09 Danger
• Risk Statements: H290-H302-H314-H317-H318-H373-H411-H412
• Safety Statements: P234-P260-P261-P264-P264+P265-P270-P272-P273-P280-P301+P317-P301+P330+P331-P302+P352-P302+P361+P354-P304+P340-P305+P354+P338-P316-P317-P319-P321-P330-P333+P317-P362+P364-P363-P390-P391-P405-P406-P501

Hazard Classification

Hazard	Class	Category Code	Hazard Statement
Skin corrosion	Skin Corr.	1B	H314
Acute toxicity	Acute Tox.	4	H302
Serious eye damage	Eye Dam.	1	H318
Skin sensitization	Skin Sens.	1	H317
Chronic hazardous to the aquatic environment	Aquatic Chronic	3	H412
Substances or mixtures corrosive to metals	Met. Corr.	1	H290
Chronic hazardous to the aquatic environment	Aquatic Chronic	2	H411
Specific target organ toxicity - repeated exposure	STOT RE	2	H373
Skin corrosion	Skin Corr.	1A	H314

• Transport Information: UN 3260

Discovery and Applications

Chloroauric acid, commonly known as hydrogen tetrachloroaurate(III), has the chemical formula HAuCl₄. It is an inorganic compound consisting of a gold(III) ion coordinated by four chloride ions, with an associated proton to balance the charge. The compound typically exists as a yellow-orange crystalline solid or as a solution in water and is highly soluble in aqueous media.

Chloroauric acid is usually prepared by dissolving elemental gold in aqua regia, a mixture of concentrated nitric acid and hydrochloric acid. This reaction oxidizes gold metal to gold(III) and produces the tetrachloroaurate anion [AuCl₄]⁻, which forms a stable complex with chloride ions in solution. The resulting aqueous solution of chloroauric acid can be concentrated and crystallized to yield the solid form.

This compound is the principal gold-containing species used in gold extraction, refining, and plating processes. Its high solubility and stability make it an important precursor for the synthesis of various gold complexes and nanoparticles. Chloroauric acid serves as a starting material in the preparation of gold catalysts used in oxidation reactions, carbon-carbon coupling, and other homogeneous catalytic applications.

In nanotechnology, chloroauric acid is widely used to produce gold nanoparticles through chemical reduction methods. Reduction of HAuCl₄ with agents such as sodium citrate or sodium borohydride leads to the formation of colloidal gold with controlled size and shape, which finds applications in electronics, sensing, biomedicine, and catalysis.

The compound also plays a role in analytical chemistry as a reagent for gold determination and in histology for staining purposes. It is involved in various synthetic routes to organogold compounds and complexes with diverse ligands, expanding the scope of gold chemistry in research and industrial fields.

Chloroauric acid is corrosive and should be handled with caution, using appropriate personal protective equipment. It is reactive with reducing agents and bases and should be stored in tightly closed containers in a cool, dry place.

In summary, chloroauric acid is a yellow-orange gold(III) chloride complex widely used as a precursor in gold refining, catalysis, and nanoparticle synthesis. Its aqueous solubility and chemical versatility make it fundamental in both industrial and research applications involving gold chemistry.

References

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