Essentials of Water Sample Collection for Nitrate-Nitrogen Determination

Nitrate-nitrogen is a key indicator in water quality assessment, and the reliability of its measurement hinges largely on how the sample is handled from the moment of collection. Although nitrate is relatively stable among inorganic nitrogen species, improper collection and preservation can allow microbial activity to alter its true concentration through assimilation or denitrification. Adhering to established sampling protocols is therefore essential for obtaining accurate, defensible analytical results.

Container Selection and Preparation

Samples for nitrate-nitrogen determination should be collected in either polyethylene bottles or hard glass containers. The material must be chemically inert, with no tendency to adsorb or react with nitrate species. Prior to use, containers should be thoroughly cleaned—typically with detergent, tap water, dilute hydrochloric acid, and finally rinsed with distilled or deionized water. Just before sampling, the container should be rinsed two to three times with the water to be sampled, with the rinse water discarded. Dedicated containers are strongly recommended to avoid cross-contamination.

Field Sampling Practices

Sampling should follow established technical guidelines appropriate for the water body in question, such as those for surface water or groundwater monitoring. The sample bottle should be filled completely, leaving minimal headspace to prevent atmospheric oxygen or nitrogen from affecting the sample. For turbid samples or those containing suspended solids, field filtration through a 0.45 μm membrane filter or centrifugation is recommended to remove particulate matter. Filtration equipment must be clean to avoid introducing new contaminants. Prompt processing after collection is critical—delay should be avoided whenever possible.

Preservation and Storage

When immediate analysis is not possible, proper preservation becomes imperative. The two primary preservation measures are low-temperature refrigeration and chemical addition. Samples should be stored in the dark at 0–4°C to suppress microbial metabolism and slow biochemical transformations that could alter nitrate concentrations. 

As a chemical preservative, concentrated sulfuric acid is commonly added at a rate of 0.8 mL per liter of sample, reducing the pH to below 2. This acidification inhibits microbial activity that would otherwise consume or reduce nitrate. Mercuric chloride may also serve as a preservative at 40 mg per liter when combined with refrigeration, but its high toxicity demands strict safety precautions.