Aqua regia

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Aqua regia (often abbreviated as AR) is a mixture of hydrochloric acid and nitric acid capable of dissolving many noble metals like gold, platinum and palladium. There are a number of metals that resists aqua regia, they are titanium, iridium, ruthenium, tantalum, niobium, hafnium, osmium, rhodium and tungsten. Some alloys with high amounts of silver is also resistant to aqua regia as there forma a protective surface of silver chloride, halting the reaction. Some gold alloys have high enough silver content (over 5%) to become inert in aqua regia, requiring inquartation as a prior step when dissolving the gold.

Contents

Variations

The classical formula of aqua regia is 1 part concentrated nitric acid to three parts of concentrated hydrochloric acid, but there exists a number of variations to it with various pros and cons.

Incremental added nitric acid

When refining gold one of the problems are getting rid of unused nitric acid, or denoxing a solution. By adding the nitric incrementally the amount of nitric acid left after dissolving some metal could be minimized. The extra hydrochloric acid left in the solution doesn't harm the refining process. This means that the aqua regia will have a lot less than 25% of nitric acid that classically is used but it will still dissolve metals.

Poor mans aqua regia

As nitric acid can be hard to come by for some people, a version with hydrochloric acid and a nitrate salt was developed. Basically, the nitrates dissolved combines with the hydrochloric acid and creates the aqua regia. This will work for most refining work but it will add metal ions from the salt to the mixture and in the end there will be more salts in the solution. Any metal contamination in the nitrate salt will add to the pregnant solution and when the precious metals are extracted it will drag down some of the contaminants. This can be lessened by proper washing procedures.

NaNO3 + HCl --> HNO3 + NaCl

High nitric aqua regia

Some metals are resistant to nitric acid as they form a protective oxide surface, for example aluminum and stainless steel. This effect can be used to remove gold on aluminum or gold plated stainless steel without dissolving the base metal.

To a nitric acid solution a few percent hydrochloric acid is added. This will dissolve the gold while nitric acid will create a passivated oxide surface once the gold is gone. The gold can be reclaimed as gold chloride by evaporating the liquid, leaving metal salts behind.

Decomposition of aqua regia

When hydrochloric acid and nitric acid is mixed there is a chemical reaction that creates chlorine and nitrosyl chloride, both gases dissolved in the liquid phase. The gases are volatile and escapes from the aqua regia and new nitrosyl chloride and chlorine is created and the solution is slowly weakening.

HNO3 + 3 HCl → NOCl + 2 Cl + 2 H2O

Heat and sunlight accelerates the process. That is why aqua regia never should be mixed before usage.

Metal reactions in aqua regia

Some of the reactions of aqua regia.

Gold in aqua regia

Au + 3 HNO3 + 4 HCl → AuCl4 + 3 NO2 + H3O+ + 2 H2O
or
Au + HNO3 + 4 HCl → AuCl4 + NO + H3O+ + {{H2O}

Platinum in aqua regia

There are experimental results that points to the reaction of platinum in aqua regia to be quite complex. A simplified view is shown below.

Pt + 4 NO3- + 8 H+ → Pt4+ + 4 NO2 + 4 H2O
or
3Pt + 4 NO3- + 16 H+ → 3 Pt4+ + 4 NO + 8 H2O

Then the platinum ion reacts with the free chloride ions in solution

Pt4+ + 6 Cl- → PtCl62−

The initial reaction creates a mixture of chloroplatinous acid (H2PtCl4) and nitrosoplatinic chloride ((NO)2PtCl4). As nitrosoplatinic chloride is a solid product there could be a need to leach the remaining solids with concentrated hydrochloric acid to get most of the platinum into solution.

Tin in aqua regia

When tin is dissolved in aqua regia it is oxidized to it's 4+ state, creating tin(IV) chloride. This should not be mistaken with tin(II) chloride that is made by dissolving tin in hydrochloric acid.

4 HCl + 2 HNO3 + Sn → SnCl4 + NO2 + NO + 3 H2O

References

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