Which preservation step is recommended for water samples intended for metals determination when analysis cannot be performed immediately?

• A. Filter and preserve with zinc acetate
• B. Add PbCO3
• C. Acidify with 1mL HNO3 to pH < 2
• D. Adjust pH to > 11.5 with NaOH

Answer: (C)

Preserving water samples for metals analysis relies on keeping dissolved metals stable and in solution until analysis. Acidifying the sample to a very low pH—typically below 2—with a strong, appropriate acid like nitric acid prevents metal hydroxide formation and minimizes adsorption to container walls or changes in speciation, while also inhibiting biological activity that could alter concentrations. This approach is standard because nitric acid effectively preserves a wide range of metals without introducing extra metals or interfering with subsequent analytical methods.

That’s why acidifying with nitric acid to pH below 2 is the best choice. The other options don’t fit: filtering and adding zinc acetate isn’t a general-preservation step for dissolved metals; adding carbonate would raise carbonate levels and promote precipitation, and adjusting the pH to above 11.5 would cause most metals to precipitate as hydroxides, removing them from the dissolved phase and biasing results.