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Determination of Exchangeable Bases in Soils by Ion Chromatography¹

ABSTRACT

A simple, accurate, and precise ion chromatographic (IC) method for determination of exchangeable bases in soils was developed. It involves extraction of the bases with neutral 1M ammonium acetate (NH₄ OAc), followed by ignition of the soil extract at 400°C for 30 min, dissolution of the residue in 5 mM HCl, and determination of exchangeable bases by using a Dionex Model 10 IC. The IC method is based on separation of the cations by a separator column containing a low-capacity, cation-exchange resin in the H⁺ form and converting the various cations to their corresponding hyroxide forms by a suppressor column in the OH^- form. Solutions of 5mM HCl and 2.5 mM HCl + 2.5 mM m-phenylenediamine dihydrochloride are used as the eluents for determination of monovalent and divalent exchangeable bases, respectively. Electrical conductivity is used as the mode of detection. Results showed that the average exchangeable K in 10 soils by IC, atomic absorption spectrophotometry (AA), and flame photometry (FP) were 0.49, 0.47, and 0.47 cmol(i)/kg soil, respectively. The corresponding average exchangeable Na values were 0.09, 0.09, and 0.08 cmol(i)/kg soil. The average exchangeable Ca values of the same soils by IC and AA methods were 13.5 and 13.5 cmol(i)/kg soil. The corresponding exchangeable Mg values were 3.50 and 3.54. The IC method has excellent sensitivity, with detection limits of 0.1, 0.05, 0.1 and 0.03 mg L^–1 of K, Na, Ca and Mg. respectively. With the IC method, K and Na can be determined simultaneously in 6 min, or Ca and Mg can be determined in 7 min. A single operator can perform about 40 analyses of monovalent or divalent bases in a normal working day.

¹ Journal Paper no. J-11497 of the Iowa Agric. and Home Econ. Exp. Stn., Ames, Iowa. Projects 2112 and 2601. Received 21 June 1984. Approved 25 Sept 1984. Presented before Div. S-2, Soil Sci. Soc. Am. Las Vegas, NV., 25 Nov. 1984.

² Graduate Research Assistant and Professor of Soil Chemistry, respectively. Dept. of Agronomy, Iowa State Univ. Ia. 50011.