Which acid is used to preserve water samples intended for metals analysis?

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

Answer: (B)

Preserving water samples for metals analysis relies on acidifying the sample to a very low pH so metals stay dissolved and don't adsorb to container walls or precipitate as hydroxides or sulfates.

Nitric acid is the best choice because it is a strong inorganic acid that reliably lowers pH without introducing troublesome complexing species. It helps oxidize organic matter that might bind metals, ensuring metals remain in the dissolved phase. Nitrates are highly soluble and generally do not form insoluble compounds with common metals, so the metals stay in solution until analysis and the sample remains compatible with common instrumental techniques.

Other acids have drawbacks. Sulfuric acid can cause the formation of insoluble sulfates with some metals and adds sulfate to the matrix, potentially interfering with measurements. Hydrochloric acid can form metal chlorides and chloride complexes that alter metal speciation and can introduce interferences in certain analytical methods or corrosion issues. Acetic acid is a weak organic acid and won’t reliably reduce pH to the level needed to prevent precipitation or microbial activity, making it unsuitable for preserving trace metals.

For these reasons, nitric acid is the standard choice for preserving water samples intended for metals analysis.