UV Spectrophotometric Determination of Nitrate Nitrogen in Water – A Simple Method

Nitrate nitrogen is a common nutrient pollutant in surface and ground waters. This short article describes a fast, reagent‑free UV spectrophotometric method suitable for clean water samples. By measuring light absorption at two ultraviolet wavelengths (220 nm and 275 nm) and applying a simple correction, organic interference can be effectively removed. The procedure, interference control, and practical tips are presented without complex formulas.

1. Introduction

Monitoring nitrate nitrogen is essential for assessing water pollution and eutrophication risk. Among available analytical techniques, UV spectrophotometry stands out because it is rapid, requires no toxic reagents, and is easy to perform in both laboratory and field settings. However, the method works best for relatively clean water (e.g., drinking water, groundwater, and surface water with low organic matter).

2. Principle

Nitrate ions absorb ultraviolet light strongly at a wavelength of 220 nm, and the absorbance is proportional to concentration following the Beer‑Lambert law. Dissolved organic matter also absorbs at 220 nm but has negligible absorption at 275 nm. By taking the absorbance difference between the two wavelengths, the specific signal from nitrate can be isolated. The corrected absorbance is calculated as the reading at 220 nm minus twice the reading at 275 nm. This corrected value is then compared with a calibration curve to obtain nitrate nitrogen concentration.

3. Interferences and How to Avoid Them

Several substances can interfere with the measurement.

Organic matter: It increases the background at 220 nm. The dual‑wavelength correction (using 275 nm) largely removes this interference.

Nitrite: It also absorbs at 220 nm. If nitrite is present above a very low level, add a few drops of diluted hydrochloric acid and a small amount of sulfamic acid solution to the sample; let it stand for ten minutes before measurement.

Turbidity: Suspended particles scatter light and cause false readings. Remove turbidity by filtering the sample through a fine membrane filter (0.45 μm pore size) or by centrifugation.

Other absorbing ions: Hexavalent chromium and ferric iron can absorb UV light, but they are rarely found at significant levels in ordinary water samples. If present, more advanced pretreatment (e.g., ion exchange) is needed.