UV Spectrophotometric Determination of Nitrate‑Nitrogen in Water

Nitrate‑nitrogen (NO₃⁻‑N) is a key parameter in water quality monitoring, commonly associated with agricultural runoff, wastewater discharge, and potential health risks. UV spectrophotometry offers a rapid, reagent‑free method for direct nitrate determination, based on the strong absorption of nitrate at 220 nm. This article outlines the standard procedure, including background correction for organic interference.

1. Principle

Nitrate ions exhibit a distinct absorption maximum at 220 nm, with absorbance proportional to concentration (Beer–Lambert law). Dissolved organic matter also absorbs UV light, especially near 220 nm; to compensate, a second measurement at 275 nm (where nitrate absorbs negligibly) corrects for organic interference. The corrected absorbance is:A_corr = A₂₂₀ – 2 × A₂₇₅

2. Apparatus and Reagents

UV‑Vis spectrophotometer (capable of 220 nm and 275 nm) with 1 cm quartz cuvettes

Nitrate‑nitrogen stock standard solution (100 mg L⁻¹): dissolve 0.7218 g of KNO₃ (dried at 105 °C) in deionized water and dilute to 1 L.

Working standards (0.2 – 10 mg L⁻¹ NO₃⁻‑N) prepared by serial dilution.

1 M HCl (optional, for pH adjustment)

Deionized water (UV‑transparent, free of nitrate and organics)

3. Procedure

3.1 Sample preparation

Filter the water sample through a 0.45 µm membrane filter to remove suspended particles. If pH is beyond 5–8, adjust to near neutral with 1 M HCl or NaOH.

3.2 Calibration curve

Prepare a series of nitrate‑N standards (e.g., 0, 0.5, 1.0, 2.0, 5.0, 10.0 mg L⁻¹). Measure each at 220 nm and 275 nm using deionized water as a blank. Calculate corrected absorbance (A₂₂₀ – 2A₂₇₅) and plot vs. concentration. The curve should be linear (R² > 0.999).

3.3 Sample measurement

Measure the filtered sample at both wavelengths under the same conditions. Compute corrected absorbance and interpolate nitrate‑N concentration from the calibration curve.

4. Advantages of UV Method

No reagents required, low cost, and fast (5–10 minutes per sample).

Suitable for routine monitoring of clean water (tap water, surface water, groundwater).