Eliminating Nitrogen Ion Interference in Dissolved Oxygen Measurement of Water Quality

Dissolved oxygen (DO) is a fundamental parameter in water quality assessment, yet its accurate determination is frequently compromised by the presence of nitrogenous species in water samples. Among these, nitrite (NO₂⁻) and ammonia (NH₃/NH₄⁺) are the primary interferents, each affecting different analytical techniques through distinct mechanisms.

 In the classical Winkler iodometric method, nitrite ions oxidize iodide to free iodine under acidic conditions, producing a positive interference that leads to overestimation of DO concentrations. In electrochemical probe methods, certain gases including ammonia can diffuse through the membrane and generate interfering currents. Optical fluorescence-based sensors may also suffer from cross-sensitivity to ammonia. Fortunately, a range of well-established strategies exist to eliminate or mitigate these interferences.

Chemical Modification Approaches for the Winkler Method

The most widely adopted approach for eliminating nitrite interference in the Winkler method is the azide modification. This technique involves replacing the conventional alkaline iodide reagent with an alkaline iodide–azide solution. Under acidic conditions, sodium azide (NaN₃) reacts with nitrite to produce nitrogen gas and water, effectively decomposing the interfering species. The reaction mechanism ensures that nitrite is destroyed before it can oxidize iodide, thereby eliminating the positive bias. 

This modification is applicable to most wastewater and biologically treated samples, and is recognized as the standard procedure for nitrite interference removal in freshwater and wastewater analysis. Importantly, nitrite interference may be safely ignored only when its concentration falls below approximately 0.3 μM; at higher concentrations, azide addition becomes essential. It should be noted, however, that sodium azide is highly toxic and potentially explosive, requiring careful handling—particularly avoiding direct acidification of the alkaline azide solution, which could generate toxic hydrazoic acid fumes.

When samples contain ferrous iron at concentrations exceeding 1 mg/L, the azide modification becomes unsuitable. In such cases, the permanganate modification offers an effective alternative. This method employs potassium permanganate as an oxidizing agent to oxidize both nitrite and ferrous iron, thereby eliminating their interference with the iodometric endpoint determination. Excess permanganate is subsequently removed with sodium oxalate. This approach serves as a valuable complement to the azide method for samples with elevated levels of reducing substances.