Re‐evaluation of Factors Affecting Total Nitrogen Measurements in Soil Extract Solutions Using Microwave‐Assisted Persulfate Oxidation

Abstract

Interest and the use of persulfate oxidation for the measurement of total nitrogen (N) in soil extracts and related solutions has been increasing. Persulfate has a number of advantages relative to Kjeldahl digestion including greater sensitivity, ease of use, opportunities for automation of the procedure or portion of it and reduced requirement for waste disposal. A wide variety of persulfate solution formulations and digestion conditions have been proposed and their effectiveness have been compared to other methods such as Kjeldahl, but not with each other. A study was conducted to re‐examine various components of the persulfate method. The components were the composition of the oxidation solution, digestion conditions and compatibility of the digestion with quantification of the product (nitrate) of the oxidation. Tests suggested that the oxidation step was quite tolerant to the composition of the oxidant (persulfate solution) if there was sufficient to excess quantity of the oxidant to decompose organic compounds present and oxidize all resulting N compounds to nitrate, and if sufficient energy (heat) was applied. It was determined that microwaving the sample with 4% potassium persulfate oxidant in a sealed vessel for 10 minutes at 120°C was adequate. We also found that including boric acid in the oxidant as a pH buffer resulted in the solution being alkaline (pH 9) for a relatively wide range of sample matrices after the digestion was complete. Alkalinity was necessary for quantifying the resulting nitrate using flow injection analysis based on the commonly used colorimetric determination of N as nitrite after cadmium column reduction. Since the apparent recovery of organic N in solutions used for soil extractions was largely due to the effect of the cations and anions on nitrate quantification, it is essential to standardize and calibrate with an appropriate matrix of the solution containing the sample and an organic N standard through both the oxidation and quantification steps of the procedure. With these three precautions, persulfate oxidation can provide effective and efficient measurement of total N in a variety of soil extracts and related solutions.