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This article in SSSAJ

  1. Vol. 45 No. 2, p. 347-353
     
    Received: Apr 29, 1980
    Published: Mar, 1981


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doi:10.2136/sssaj1981.03615995004500020024x

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

  1. R. J. Norman and
  2. J. W. Stucki2

Abstract

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.

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