Massachusetts Wastewater Practice Exam

Samples collected for heavy metal analysis can be preserved effectively by adding nitric acid. This is crucial because heavy metals can undergo changes in concentration and speciation if not properly preserved, leading to inaccurate measurements. Nitric acid acts as a strong acid that keeps the heavy metals in solution and prevents precipitation or adsorption to container surfaces.

In this context, the addition of nitric acid lowers the pH of the sample, which helps in stabilizing heavy metals in their ionic form and limits microbial growth that could otherwise alter the concentration of metals. Since preservation is essential for the sample integrity until analysis, using nitric acid is a widely accepted practice in laboratories conducting heavy metal testing.

Other methods of preservation, such as adding polymers or lime, do not provide the same level of efficacy for preserving heavy metals specifically, making nitric acid the preferred choice.