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Does acid affect gold?

Published in Chemistry of Gold 2 mins read

Yes, acid can affect gold, but it's not a simple case of just any acid causing a reaction.

Gold's Resistance

Gold is known for its exceptional resistance to corrosion and chemical reactions. This is why it's used in jewelry and electronics - it doesn't tarnish or rust easily. According to the reference material, gold "is unaffected by air, water, alkalis and all acids except aqua regia." This indicates a high level of stability in most common environments.

Understanding the Exception

The crucial exception is aqua regia, a highly corrosive mixture of concentrated nitric acid and hydrochloric acid. This mixture is capable of dissolving gold, unlike most other acids.

Property Gold
Reactivity Very Low
Reaction with Air No reaction
Reaction with Water No reaction
Reaction with Alkalis No reaction
Reaction with Most Acids No reaction
Reaction with Aqua Regia Dissolves

How Aqua Regia Works

  • Nitric acid (HNO3): Acts as the oxidizer, forcing gold ions into solution.
  • Hydrochloric acid (HCl): Reacts with the gold ions to form tetrachloroaurate anions ([AuCl4]−), further driving the dissolution process.

Example

The overall chemical reaction can be represented as:

Au + 3 HNO3 + 4 HCl → [AuCl4]− + 3 NO2 + H3O+ + 2 H2O

Practical Implications

  • Gold refining: Aqua regia is used in the process of refining gold, separating it from other metals.
  • Laboratory use: Chemists employ aqua regia when they need to dissolve gold for analysis or synthesis purposes.
  • Jewelry: Everyday acids, sweat, and household cleaners typically do not affect gold jewelry. However, extremely strong acid exposure should be avoided.

Therefore, while most acids do not affect gold, aqua regia is a notable exception, demonstrating that the interaction between gold and acid is highly dependent on the specific acid involved.