Aqua Regia
Also: Royal water, Nitro-hydrochloric acid
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 volume parts of concentrated hydrochloric acid (HCl, 37 %) and one volume part of concentrated nitric acid (HNO₃, 65 %). The name comes from medieval alchemy and refers to the fact that this mixture is able to dissolve the "kings of metals" - gold and platinum - which 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 of the hydrochloric acid form stable chloro complexes that pull the oxidised metal out of equilibrium and thus drive the dissolution.
For gold, the net reaction equation is:
Au + 4 HCl + HNO₃ → HAuCl₄ + NO↑ + 2 H₂O
The resulting tetrachloridoaurate(III) (HAuCl₄) is water-soluble and forms the basis for wet-chemical gold recovery and refining. Platinum reacts analogously to hexachloridoplatinate(IV) (H₂PtCl₆).
Important: Freshly mixed aqua regia decomposes within hours through internal redox reactions into nitrosyl chloride (NOCl) and further decomposition 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 (very resistant) |
| Silver (Ag) | Does not dissolve - AgCl layer passivates |
| 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, in which silver does react.
Applications in the Precious Metals Industry
- Refining and separation: Refineries use aqua regia to break down 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 is used for the complete digestion of ore samples and secondary material (recycling, electronic scrap) prior to the quantitative determination of precious metal contents.
- Authenticity testing: Professional authenticity checks of gold coins and bars use aqua regia as an unambiguous proof: genuine gold scrap dissolves, gold plating on base metal reacts differently or not at all. In addition, the melt value calculation offers a quick numerical plausibility check.
- Catalytic converter recycling: In the recovery of platinum group metals from automotive catalysts, aqua regia is a standard reagent because it selectively dissolves Pt, Pd and Rh from ceramic supports.
Safety Notes
Aqua regia is extremely aggressive and produces toxic nitrogen oxides (NOₓ) and traces of chlorine gas when mixed. It may be handled only in a fume hood and with suitable protective equipment (nitrile gloves, safety goggles, acid-resistant apron). Its use is not advisable for laypersons; professional refineries have the necessary infrastructure.
In Brief
Aqua regia is the key chemical substance of precious metal chemistry: only this acid mixture reliably dissolves gold and platinum, thereby 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 actual material value exactly.