Humic substances as interferences in the analysis of nitrite in water

Humic substances are of current interest because of their roles in environmental processes involving pollutants. It is also becoming recognized that humic substances may interfere in the analysis of environmental samples, though the possible adverse effects do not appear to be fully appreciated. The present effort focuses on determining whether humic materials interfere in the analysis of nitrite in water with the use of the Griess reaction. This is a well-known reaction that uses nitrosation to give a diazonium salt, then couples with an appropriate reagent to form a colored product. This colorimetric method continues to be applied in the laboratory and the field for nitrite. It was found that nitrite values at low ppm levels in water may be reduced by 50–60% in the presence of approximately 50-ppm quantities of specific humic acids. The greater the humic acid concentration, the greater the interference effect. However, it is projected, based on experiments with Fluka humic acid, that humic-substance concentrations of 10 ppm or less will have a measurable but small effect. It was shown that the interference is due to molecular association of the Griess dye with the humic acid. The interference results in less color, and, with some humic acids, a shift in the wavelength of maximum absorption. © 2000 John Wiley & Sons, Inc. Field Analyt Chem Technol 4: 134–146, 2000