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Determination of Trace Metals in Sodium Dithionite-Citrate Extracts of Soils and Sediments by Atomic Absorption¹

ABSTRACT

A general method has been developed for the analysis of Co, Cu, Ni, Mo, and Zn in dithionite-citrate extracts of soils and sediments. Because of the high Zn content of dithionite, it is necessary to preclean the dithionite-citrate solution before it is used to extract soils or sediments. This precleaning lowers the detection level significantly for other trace elements as well. Since Fe causes flame interferences in the determination of several first transition series elements by atomic absorption, the size of the individual sample aliquot taken to chelation-solvent concentration is varied to provide a constant quantity of Fe (5,000 µg). The standards and blanks are adjusted to the same Fe content. This approach is necessitated by the wide differences in the Fe concentrations of the dithionitecitrate extracts. Following pH adjustments and addition of ammonium-1-pyrrolidine dithiocarbamate, the trace metals (and Fe) are concentrated via two successive 10-ml methyl isobutyl ketone extractions. Quantities of Fe much greater than 5,000 µg interfere in the solvent concentration step. Natural organics released by the dissolution of free-Fe and -Mn oxides, to the extent that they are represented by fulvic acids, do not cause any significant interference in the ammonium-1-pyrrolidine dithiocarbamate methyl isobutyl ketone extraction of the trace metals due to the releasing effect of the Fe present.

Key Words: Co • Cu • Ni • Mo • Zn • iron oxides • solvent concentration • interferences

¹ Publication authorized by the Director, Geological Survey, U. S. Dept. of the Interior.

² Research Soil Scientist, Chemist, and former Chemist, respectively, Water Resources Div., Geological Survey, U. S. Dept. Interior, Menlo Park, Calif. 94025.

Received for publication September 21, 1973.