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Chloroauric Acid: The Yellow Compound That Forms When Gold Dissolves in Aqua Regia

Chloroauric Acid: The Yellow Compound That Forms When Gold Dissolves in Aqua Regia

Chloroauric acid (HAuCl₄) is the soluble gold compound formed when aqua regia dissolves metallic gold, reduced by sodium metabisulfite to precipitate pure gold.

Chloroauric acid (HAuCl₄) is the yellow crystalline compound formed when metallic gold dissolves in Aqua Regia: The Only Acid That Dissolves Gold. The nitric acid component oxidizes gold atoms while hydrochloric acid provides chloride ions, forming the soluble tetrachloroaurate complex. In gold refining, the dissolved gold in chloroauric acid solution is recovered by adding a reducing agent — most commonly Sodium Metabisulfite: The Gold Precipitant and Food Preservative, which reduces Au³⁺ to metallic Au⁰, precipitating fine gold powder while leaving base metal chlorides in solution. Beyond refining, chloroauric acid is a key precursor for gold nanoparticle synthesis: reducing agents like sodium citrate or sodium borohydride produce nanoparticles of controllable size for applications in catalysis, medical diagnostics (lateral-flow immunoassays), and electronics. It is also used in gold electroplating baths and as a histological staining agent.

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April 13, 2026

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