Aqua Regia
Also: Royal Water, Königswasser
Aqua regia is a strongly oxidising acid mixture of concentrated hydrochloric acid and nitric acid (3:1) that is the only liquid reagent capable of dissolving the precious metals gold and platinum.
Aqua regia (Latin for "royal water") is a freshly prepared mixture of three parts by volume of concentrated hydrochloric acid (HCl, 37%) and one part by volume of concentrated nitric acid (HNO₃, 65%). The name comes from medieval alchemy and refers to the fact that this mixture can dissolve the "kings of metals" — gold and platinum — something that neither hydrochloric acid nor nitric acid can achieve alone.
Chemical Mechanism
The exceptional reactivity arises from the interplay of both acids: the nitric acid acts as a strong oxidising agent and oxidises the metal, while the chloride ions from the hydrochloric acid form stable chloro-complexes that draw the oxidised metal out of equilibrium, thereby driving the dissolution forward.
The net reaction equation for gold is:
Au + 4 HCl + HNO₃ → HAuCl₄ + NO↑ + 2 H₂O
The tetrachloroaurate(III) (HAuCl₄) produced is water-soluble and forms the basis for wet-chemical gold recovery and refining. Platinum reacts analogously to form hexachloroplatinate(IV) (H₂PtCl₆).
Important: Freshly mixed aqua regia decomposes within hours through internal redox reactions to form nitrosyl chloride (NOCl) and other breakdown products. It must therefore always be freshly prepared.
Which Metals Dissolve — and Which Do Not?
| Metal | Behaviour in aqua regia |
|---|---|
| Gold (Au) | Dissolves completely → HAuCl₄ |
| Platinum (Pt) | Dissolves completely → H₂PtCl₆ |
| Palladium (Pd) | Dissolves |
| Rhodium (Rh) | Dissolves very slowly (only when hot) |
| Iridium (Ir) | Barely dissolves (highly resistant) |
| Silver (Ag) | Does not dissolve — AgCl surface layer passivates it |
| Tungsten (W) | Does not dissolve |
| Tantalum (Ta) | Does not dissolve |
Silver immediately forms an insoluble silver chloride protective layer and is therefore resistant to aqua regia. This is a key difference from the acid test with nitric acid, to which silver does react.
Applications in the Precious Metals Industry
- Refining and separation: Refineries use aqua regia to digest gold scrap, doré bars, and alloys. The dissolved gold is then precipitated as fine gold using reducing agents (e.g. sulphur dioxide or sodium sulphite).
- Analysis: In wet chemistry and spectroscopy, aqua regia serves as a complete digest for ore samples and secondary materials (recycling, electronic scrap) prior to quantitative determination of precious metal content.
- Authenticity testing: Professional authenticity testing of gold coins and bars uses aqua regia as a definitive proof: genuine gold dissolves, while gold plating on base metals reacts differently or not at all. The melt value calculator provides a quick numerical plausibility check as a complement.
- Catalytic converter recycling: In the recovery of platinum group metals from catalytic converters, aqua regia is a standard reagent since it selectively dissolves Pt, Pd, and Rh from ceramic substrates.
Safety Information
Aqua regia is extremely aggressive and produces toxic nitrogen oxides (NOₓ) and traces of chlorine gas when mixed. It must only be handled in a fume hood with appropriate protective equipment (nitrile gloves, safety goggles, acid-resistant apron). Use by non-professionals is not recommended; professional refineries have the necessary infrastructure.
In Brief
Aqua regia is the chemical key substance of precious metals chemistry: only this acid mixture reliably dissolves gold and platinum, enabling both precise analysis and large-scale refining of these valuable metals. Anyone who knows the current gold price or platinum price as a valuation basis for scrap metal can — together with a professional aqua regia analysis — determine the exact material value.