Which acid is used to preserve metal samples?

• A. Hydrochloric acid
• B. Nitric acid
• C. Sulfuric acid
• D. Acetic acid

Answer: (B)

Preserving metal samples relies on keeping metals dissolved and unchanged from collection to analysis. A strong inorganic acid is used for this purpose, with nitric acid being the preferred choice. Adding nitric acid lowers the sample’s pH quickly and converts metals to their nitrate forms, which stay in solution and resist precipitation, adsorption to container walls, or microbial alterations.

Nitric acid is favored because it provides a stable, chloride-free environment and is compatible with common metal analysis techniques like ICP-OES, ICP-MS, and AAS. It avoids introducing chloride ions that can form troublesome metal chlorides or corrode containers, and it minimizes interference with the measurement.

Other acids have drawbacks: hydrochloric acid adds chloride that can complex or recollect certain metals and may cause container or instrument issues; sulfuric acid can cause sulfate precipitation with some metals and isn’t as universally inert for preservation; acetic acid is weaker and adds organic material that can interfere with instrumentation and doesn’t reliably keep metals in solution.

So, nitric acid is used to preserve metal samples.