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Evaluation of Atomic Absorption Methods for Determinations of Aluminum, Iron, and Silicon in Clay and Soil Extracts¹

ABSTRACT

Atomic absorption methods for Al, Fe, and Si determinations were evaluated in clay and soil extracts obtained with 1N NH₄OAc (pH 4.8), 0.1N HCl, 0.2M (NH₄)₂C₂O₄ (pH 3.0), 0.5N NaOH, and buffered Na₂S₂O₄. The evaluation was made either by correlating the results with those of chemical methods or by calculating the percentage recovery of added Al, Fe, or Si from the extracts. Highly significant correlations and regressions were obtained between the atomic absorption and colorimetric methods for NH₄OAc- and HCl-extractable Al and Fe in soils, dithionite-extractable Fe in clays, NaOH-extractable Al in soils and clays, and NaOH-extractable Si in clays. Recoveries of added Al, Fe, and Si from oxalate, dithionite, and dilute alkali media were successful except that the recovery of Fe in soil extracts exceeded 110% in most cases. The presence of a natural humic acid "Leonardite" did not affect the atomic absorption analyses of Al and Si. However, the Fe results were high as was found in soil extracts, indicating organic matter interference in the determination of Fe by this technique. Removal of humic acid by precipitation reduced the Fe but not Al recovery. The coefficients of variation for 10 determinations were below 3% for all three elements with the exceptions of Si at concentrations below 15 ppm and Fe below 2 ppm. Some difficulties were encountered with the oxalate and dithionite as extractants.

¹ Contribution from the Department of Soils, Florida Agr. Exp. Sta. Gainesville 32601, as a Journal Series Paper no. 3094. Presented before Div. S-2, Soil Chemistry, Soil Science Society of America at New Orleans, La., on Nov. 14, 1968.

² Associate Chemist and Associate Soil Chemist.

Received for publication November 18, 1968. Accepted for publication July 30, 1969.