While no single acid perfectly "cleans" gold in the way soap cleans dirt, nitric acid plays a key role in the purification of gold. Aqua regia, a mixture of nitric and hydrochloric acid, is also used to dissolve gold. It's important to understand the difference between cleaning, purifying, and dissolving in this context.
The Role of Nitric Acid
Nitric acid is used in the gold refining process because it can dissolve most common metals, but not gold. This allows it to separate gold from other metals.
Aqua Regia: Dissolving Gold
Aqua regia, meaning "royal water" in Latin, is a highly corrosive mixture of nitric acid and hydrochloric acid, optimally in a molar ratio of 1:3. It's called "royal" because it can dissolve the noble metals gold and platinum, though not all metals.
Here's why aqua regia works:
- Nitric acid acts as an oxidizer and is used to form gold ions ($Au^{3+}$).
- Hydrochloric acid is then used to react with the gold ions to form tetrachloroaurate(III) anions ($[AuCl_4]^−$), also in solution. The reaction with hydrochloric acid is an equilibrium reaction that favors the formation of tetrachloroaurate(III) anions. This results in the removal of gold ions from solution and allows the further oxidation of gold to take place.
The appropriate reaction equation is:
$Au + 3 HNO_3 + 4 HCl \rightleftharpoons [AuCl_4]^- + 3NO_2 + H_3O^+ + 2 H_2O$
Cleaning vs. Dissolving vs. Purifying
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Cleaning: Refers to removing surface dirt, grime, or tarnish without significantly altering the underlying material. While weak acids might be used in some jewelry cleaning solutions, they don't fundamentally clean gold in the same way as nitric acid or aqua regia work.
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Dissolving: Involves breaking down the gold metal into its ionic form, as aqua regia does. This is a destructive process, not a cleaning one.
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Purifying: Focuses on separating gold from other materials, often through selective dissolution or chemical reactions. Nitric acid purification leans towards removing impurities, not just cleaning the surface.
Summary
While nitric acid and aqua regia are critical in gold processing, they don't "clean" gold in the traditional sense. Nitric acid is used in gold purification to dissolve away base metals, and aqua regia dissolves the gold itself.
