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The Determination of Nitrate and Nitrite in Soil Extracts by Ultraviolet Spectrophotometry¹

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.

¹ Research supported by the Illinois Agric. Exp. Stn.

² Graduate Research Assistant and Assistant Professor of Soil Chemistry, respectively, Dep. of Agronomy, Univ. of Illinois, Urbana, IL 61801.

Received for publication April 29, 1980. Accepted for publication November 24, 1980.

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