What substance is commonly used to preserve water samples?

The use of nitric acid as a preservative for water samples is based on its ability to effectively lower the pH of the sample, which helps prevent the growth of microorganisms and stabilizes the sample for various types of chemical analyses. Preservation of water samples is crucial for ensuring the accuracy of subsequent tests, especially in analyzing metals and nutrients. A lower pH can help to minimize the adsorption of certain elements onto containers, reducing the risk of contamination or alteration of sample composition.

Sodium chloride, hydrochloric acid, and calcium carbonate serve different purposes and are not typically used for preserving samples in the same manner as nitric acid. Sodium chloride is more commonly used as a reagent or for salinity measurements, hydrochloric acid, while capable of lowering pH, is generally not suitable for preservation due to its aggressive nature, which can interfere with other analyses. Calcium carbonate acts as a buffering agent and may actually alter water chemistry rather than stabilize it for testing. Therefore, nitric acid is the preferred choice for water sample preservation in many laboratory settings.