Determination of nitrate in water containing dissolved organic carbon by ultraviolet spectroscopy

Direct measurement of ultraviolet (UV) light absorption by nitrate in the 200-205 mn wavelength band offers a reliable, accurate and rapid method of nitrate determination in natural waters. However, UV light is also strongly absorbed by organic matter at these wavelengths. A two-wavelength approach was adopted and evaluated for the correction of absorbance that organic matter might have contributed. The measurements were made at 205 nm and 300 nm. The relationship between absorbance at 205 nm and 300 nm due to organic matter was incorporated into traditional nitrate calibration curve to make allowance for the organic matter contribution. The method developed has a detection limit of 0.04 mg NO3-N dm(-3) and is capable of determining nitrate concentration in the range of 0.08-4.0 mg NO3-N dm(-3). In terms of accuracy and reproducibility, the method compares well with the hydrazine-reduction method, with matching precision (1.14% over the concentration range) and accuracy (+/-2%). This two-wavelength approach based method is rapid, low-cost and does not require any reagents or treatment, and is suitable for waters with dissolved organic carbon (DOC) concentration of up to 20-mg dm(-3).