Chemical and spectroscopic characterization of insoluble and soluble humic acid fractions at different pH values

Chemical and Biological Technologies in Agriculture

Table 2 Acidic groups’ concentration and distribution of C intensity of ¹³ C NMR spectra of humic fractions

Fraction		Acidic groups^a(meq g⁻¹)			¹³C NMR distribution (%)				HB/HI
		Carboxyl	Phenol	Total acidity	0 to 45 ppm	45 to 95 ppm	95 to 160 ppm	160 to 195 ppm
pH 1	Insoluble	3.8	7.3	11.1	23.0	16.0	33.1	28.0	1.3
pH 1	Soluble	4.0	6.7	10.7	14.2	58.8	2.9	24.2	0.2
pH 3	Insoluble	3.1	5.9	8.9	21.0	18.3	33.9	26.9	1.2
pH 3	Soluble	2.8	6.7	9.4	14.5	42.1	7.5	35.9	0.3
pH 5	Insoluble	nd^b	nd^b		nd^c	nd^c	nd^c	nd^c	-
pH 5	Soluble	nd^b	nd^b		20.8	6.0	38.7	34.5	1.5
pH 7	Insoluble	nd^b	nd^b		nd^c	nd^c	nd^c	nd^c	-
pH 7	Soluble	nd^b	nd^b		19.1	1.3	35.2	44.3	1.2

^aValues are the means of three replicates. The standard error values are < 10%; ^banalytical method not applicable at these pH; ^cnot determinable because of the poor signal-to-noise ratio of the spectra.