Which action should be avoided when preserving water samples for metals analysis if immediate analysis is not possible?

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

Answer: (B)

Preserving water samples for metals analysis aims to keep metals in the dissolved form until they can be measured. Acidifying the sample to very low pH (below 2) with nitric acid is a common, effective way to stabilise dissolved metals by preventing adsorption to container walls and suppressing hydrolysis and precipitation. In contrast, making the solution strongly basic by adjusting the pH to above about 11.5 with NaOH causes many metals to form insoluble hydroxides and precipitate out of solution. Once metals precipitate, they’re no longer present in the dissolved fraction that is typically analyzed, leading to inaccurate results. So, raising the pH to a highly alkaline level should be avoided. Other preservation steps, like careful filtration and certain stabilizers, are used in specific protocols, but high pH is consistently contraindicated for preserving dissolved metals.