Which are best collected in separate clean bottles and acidified with nitric acid to a pH below 2 to minimize precipitation and adsorption on container walls?

• A. Dissolved Solids
• B. Dissolved Ions
• C. Particulate Matter
• D. Salts

Answer: (B)

The main idea is preserving dissolved ions in a water sample by keeping them in solution and preventing losses to the container walls or through precipitation. Lowering the pH to below 2 with nitric acid creates strongly acidic conditions that suppress the formation of insoluble hydroxides or carbonates and reduce adsorption of metal ions to the container surfaces. Nitric acid is commonly used because it stabilizes the ions without introducing interferences for many analyses and doesn’t cause problematic redox changes at these concentrations. This approach targets ions that are truly dissolved, not solids or particulates. Particulate matter would be removed by filtration, salts are solids or dissolved salts but the goal here is to quantify individual dissolved ions, which is best preserved with acidification.