How are dissolved ion samples preserved?

• A. Use dirty bottles and acidify with nitric acid to pH less than two
• B. Do not acidify; store at room temperature
• C. Use clean bottles and acidify with sulfuric acid to pH less than two
• D. Use clean bottles and acidify with nitric acid to pH less than two

Answer: (D)

Preserving dissolved ion samples hinges on two ideas: keeping all analytes in solution and preventing contamination or changes while stored. Using clean bottles ensures no stray residues or particulates skew the results. Acidifying to a very low pH (less than 2) with nitric acid keeps metal and other ions from partially precipitating as hydroxides, carbonates, or oxides and also limits microbial activity that could alter the sample over time. Nitric acid is preferred because it adds no problematic sulfate ions and is compatible with common instrumental analyses like ICP-OES/ICP-MS, helping maintain a stable, interference-free sample.

Choosing dirty bottles introduces contaminants that can falsely elevate or alter measured concentrations. Not acidifying would allow slow precipitation or growth that changes dissolved ion levels. Using sulfuric acid would introduce sulfate ions, which can form insoluble sulfates with some metals and can interfere with certain analyses, making results less reliable.