Nitric Acid and Copper: Understanding Interactions and Safety Considerations

Nitric Acid and Copper Interaction

Copper’s reaction with nitric acid varies markedly with the acid’s concentration and the physical form of copper. Concentrated nitric acid tends to protect copper surface due to a passivating copper nitrate layer, while diluted nitric acid more readily dissolves the metal.

Effect of Nitric Acid Concentration on Copper Dissolution

• Concentrated Nitric Acid (Approx. 68%): At this concentration, nitric acid contains sufficient water to dissolve copper.
• In absence of adequate water, concentrated nitric acid causes a copper nitrate film to form on the metal surface, acting as a protective barrier.
• This film impedes further corrosion by preventing the acid from reaching underlying copper.
• Diluted Nitric Acid: When nitric acid is diluted, the copper nitrate film dissolves, exposing copper to continued attack.
• Therefore, copper is more susceptible to dissolution in diluted nitric acid solutions than in highly concentrated acid.

Adding water to concentrated nitric acid reduces its concentration and enhances copper dissolution by preventing the formation of an impermeable nitrate layer.

Reaction Mechanism and Products

The initial formation of copper nitrate on copper surfaces occurs when exposed to nitric acid, creating a passivating film.

Upon copper dissolution, nitric acid oxidizes copper, producing nitrogen oxides gases such as nitric oxide (NO) and nitrogen dioxide (NO2).

The generation of NO2 gas is notable, accompanied by an increase in solution temperature due to the exothermic nature of the reaction.

Influence of Copper Form

• Reaction rate depends on copper’s physical form.
• Copper powder, with its high surface area, reacts faster than copper shot or sheets.
• Powdered copper allows more efficient and rapid dissolution due to enhanced contact with acid.

Safety and Practical Tips

Using lower concentrations of nitric acid is safer, as the copper acid reaction is strongly exothermic, potentially causing temperature spikes.

A recommended approach is to dilute concentrated nitric acid sufficiently with water to submerge copper powder, then adding a slight excess of acid to ensure complete dissolution.

Summary of Key Points

• Nitric acid concentration regulates copper dissolution; dilute acid dissolves copper better than concentrated acid.
• A protective copper nitrate film forms on copper surface in concentrated acid, inhibiting corrosion.
• Nitrogen oxides (NO, NO2) evolve during the reaction alongside heat generation.
• Copper’s physical form affects dissolution rate; powder reacts fastest.
• Appropriate dilution enhances safety and reaction control.