Determination of Nitrite in Water by Ion Chromatography

Nitrite (NO₂⁻) is a nitrogenous species of particular concern in water quality assessment because of its toxicity and its role as a precursor to carcinogenic nitrosamines. 

Ion chromatography (IC) has become the method of choice for nitrite determination in drinking water, surface water, and wastewater, owing to its high sensitivity, selectivity, and capacity for simultaneous anion analysis.

The principle of IC for nitrite is straightforward. A filtered water sample is injected into a stream of eluent and passed through an anion-exchange column. Nitrite ions are retained on the positively charged stationary phase according to their affinity relative to other anions present. 

After separation, the eluent and analyte ions pass through a suppressor, which converts the eluent into a weakly conducting form while enhancing the conductivity of nitrite. The separated nitrite is then quantified by suppressed conductivity detection. Ultraviolet detection at approximately 210 nm may be used as an alternative or complementary approach.

Sample preparation for water analysis is minimal. The sample is typically filtered through a 0.22 or 0.45 µm membrane to remove particulate matter and bacteria, which would otherwise oxidize nitrite to nitrate and alter its concentration. Samples should be analyzed promptly or preserved at low temperature, since nitrite is unstable and readily converts to nitrate under ambient conditions. 

For samples containing high concentrations of chloride, such as seawater or certain industrial waters, interference can be addressed by using a silver-based cartridge or silver column to remove chloride prior to injection. When carbonate-based eluents are employed, preparing standards and diluting samples in the eluent itself can minimize the water dip effect and improve quantitative accuracy.

In practice, the method is widely applied under standard protocols such as EPA Method 300.0/300.1 and ASTM D4327. Typical detection limits for nitrite by suppressed conductivity IC are in the low micrograms-per-liter range, which is well below regulatory limits, and the approach can be extended to the simultaneous determination of nitrate and other inorganic anions from a single injection. Overall, IC offers a reliable, efficient, and versatile means of monitoring nitrite in water quality programs.