The major limitation of diffusion methods for N analysis is the time required to obtain the results — typically from one to several days. Methods were developed that involve a much shorter period for diffusion. In these methods, 10‐ to 100‐mL samples of 2 or 4 M KCl soil extract or water were treated with MgO, or MgO plus Devarda's alloy, in a 473‐mL (1‐pint) wide‐mouth Mason jar to liberate NH 4 ‐N, NO 3 ‐N, and/or NO 2 ‐N as NH 3 . The jar was placed on an electric griddle maintained at 45 to 50°C. Gaseous NH 3 was collected in 5 mL of H 3 BO 3 indicator solution in a petri dish attached to the Mason jar lid, for quantitative determination by acidimetric titration. Complete recovery of 4 mg of N was achieved from 10 or 20 mL in 1.5 to 5 h. Recovery from 50 or 100 mL was complete in 5 to 8.5 h with 1 to 2 mg of N. The methods permit 15 N analysis of the diffused NH 3 ‐N. Isotopic analyses of labeled soil extracts were accurate to within 3%, as determined by isotope‐dilution calculations. Diffusions from 10 to 50 mL were subject to very little, if any, interference from decomposition of alkali‐labile organic‐N compounds.
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Khan et al. (1997) studied this question.