The Determination of Nitrate and Nitrite in Soil Extracts by Ultraviolet Spectrophotometry

Abstract

This work describes an ultraviolet (UV) spectrophotometric difference method for quantitative assay of soil extracts for NO3- and NO2-. The method is rapid, sensitive, and very reliable even in the presence of organic matter and other non-nitrate species that interfere with NO3- determinations and that are often found in soils. The method measures NO3- by first measuring the total absorbance of the soil extract solution at 210 nm, which arises from the presence of both NO3- and non-nitrate species. The absorbance of the non-nitrate species is determined after reducing NO3- to nonabsorbing species using Raney nickel catalyst in acid medium. The absorbance difference is attributed to NO3- alone and is proportional to its concentration. If NO2- is present in the original soil extract, both NO3- and NO2- are measured separately by incorporating an additional step into the procedure, which selectively removes NO2- with sulfamic acid. Soil NO3- determined by this method on extracts from nine Illinois soils were highly correlated (r2 = 0.998) with results obtained by steam distillation. Correlation with results from a direct UV method was poorer. Recovery of standard additions of NO3- and NO2- were 99% and 98.5%, respectively. The minimum detectable concentrations in the soil samples are 0.45 µg NO3--N/g of soil and 0.64 µg NO2--N/g of soil, with the linear ranges extending to 100 and 140 µg/g of soil, respectively. In the absence of NO2-, 50 samples were analyzed in 3 hours' time with coefficients of variation at the 99% confidence interval (CI) of 0.68, 1.2, and 6% for 15 to 100, 5 to 15, and 1 to 5 µg NO3--N/g of soil. When samples are also assayed for NO2-, the time is extended to 4 hours for 50 samples with coefficients of variation at the 99% CI, of 0.71, 1.4, and 10% for 15 to 100, 5 to 15, 1 to 5 µg NO3--N/g of soil.