Aqua Regia
Also: Royal water, Nitro-hydrochloric acid
Aqua regia is a highly oxidising acid mixture of concentrated hydrochloric and nitric acid (3:1) and is the only liquid reagent capable of dissolving the noble metals gold and platinum.
Aqua regia (Latin for "royal water") is a freshly prepared blend of three volume parts of concentrated hydrochloric acid (HCl, 37 %) and one volume part of concentrated nitric acid (HNO₃, 65 %). The name reaches back to medieval alchemy and reflects the mixture's ability to dissolve the "kings of the metals" — gold and platinum — a feat that neither acid can achieve on its own.
The chemistry behind it
The remarkable reactivity comes from the two acids working together: nitric acid acts as a powerful oxidiser and strips electrons from the metal, while the chloride ions supplied by hydrochloric acid lock the oxidised metal away in stable chloro-complexes. Removing the dissolved metal from the equilibrium keeps the reaction driving forward.
For gold, the net reaction reads:
Au + 4 HCl + HNO₃ → HAuCl₄ + NO↑ + 2 H₂O
The resulting tetrachloroauric acid (HAuCl₄) is water-soluble and underpins wet-chemical gold recovery and refining. Platinum behaves analogously, forming hexachloroplatinic acid (H₂PtCl₆).
Important: freshly mixed aqua regia breaks down within hours through internal redox reactions, yielding nitrosyl chloride (NOCl) and other decomposition products. It must therefore always be prepared just before use.
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 — an AgCl film passivates it |
| Tungsten (W) | Does not dissolve |
| Tantalum (Ta) | Does not dissolve |
Silver instantly forms an insoluble silver-chloride film and is therefore resistant to aqua regia. This is a key distinction from the acid test using nitric acid, where silver does react.
Uses across the precious-metals trade
- Parting and refining: refineries use aqua regia to break down scrap gold, doré bars and alloys. The dissolved gold is then precipitated back out as fine gold using reducing agents such as sulphur dioxide or sodium sulphite.
- Analysis: in wet chemistry and spectroscopy, aqua regia fully digests ore samples and secondary material (recyclate, electronic scrap) prior to quantifying the precious-metal content.
- Authenticity testing: professional authenticity checks on gold coins and bars rely on aqua regia as a definitive proof — genuine scrap gold dissolves, while gold-plated base metal reacts differently or not at all. A quick melt-value calculation offers a complementary numerical sanity check.
- Catalyst recycling: when recovering platinum-group metals from spent car catalysts, aqua regia is a standard reagent because it selectively leaches Pt, Pd and Rh from the ceramic support.
Safety notes
Aqua regia is extremely aggressive and releases toxic nitrogen oxides (NOₓ) plus trace chlorine gas while mixing. It must only be handled in a fume hood with proper protective equipment (nitrile gloves, goggles, acid-resistant apron). It is not suitable for amateur use; professional refineries have the infrastructure required to work with it safely.
In brief
Aqua regia is the chemical cornerstone of precious-metals chemistry: it is the only acid mixture that reliably dissolves gold and platinum, enabling both precise assaying and large-scale refining of these valuable metals. Anyone who knows the current gold price or platinum price as a valuation baseline for scrap can, together with a professional aqua-regia analysis, pin down the true material value exactly.