Standard solutions of N-(1-naphthyl) ethylenediamine dihydrochloride (NED), sulfanilamide, glycine, L-aspartic acid, sodium nitrite, urea, ethylenediaminetetraacetic acid disodium salt (EDTA), ammonium chloride, L-glutamic acid sodium salt and L-arginine, each containing 5 mg N/L, were used to determine N recovery (against KN[O.sub.3] standard solution) by the HTCO method.
Regression analyses on relationships between the values of total soluble N measured by the PO and the HTCO methods in various matrices were carried out in STAWSTLX for Window, version 2.2.
Because the chemical and physical properties of the soils in Table 1 vary so widely, they make a good test-bed for comparing the HTCO and the PO methods.
to final concentrations of 0.4M KCl and 0.1 M [K.sub.2]S[O.sub.4]) before measurement of total soluble N by the HTCO method.
Drifts of sensitivity of signals in diluted KCl (0.4 M), [K.sub.2]S[O.sub.4] (0.1 M) and water matrices by the HTCO method were very minor, with <2% in KCl matrix and <3% in [K.sub.2]S[O.sub.4] and water matrices (Table 2).
Nitrogen recoveries from different standard N-containing compounds (5 mg/L) analysed by the HTCO technique (against KN[O.sub.3] standard solution) in all matrices tested (water, [K.sub.2]S[O.sub.4], and KCl) were >94% except for sulfanilamide (c.
Accumulated deposition of salts on the surface of catalyst can gradually reduce oxidation efficiency of the catalyst for the HTCO method.
Comparing the PO and the HTCO methods for the determination of total soluble N
The results of total soluble N measured by the PO and the HTCO methods are shown in Table 4.
The values of hot-water-extractable total N measured by the HTCO method were generally greater than by the PO method, particularly for high N concentrations (Table 2, Fig.