Dithionite as a Dissolving Reagent for Goethite in the Presence of EDTA and Citrate. Application to Soil Analysis

Elsa H. Rueda, María C. Ballesteros, Reynaldo L. Grassi and Miguel A. Blesa
Departamento de Química e Ingeniería Química, Universidad Nacional del Sur Avenida Alem 1253, (8000) Bahía Blanca, Argentina
Departamento Química de Reactores, Comisión Nacional de Energía Atómica Avenida del Libertador 8250, (1429) Buenos Aires, Argentina

Abstract: A synergistic effect of reductant and complexant is observed in the dissolution of goethite by dithionite and citrate or EDTA. The rate data are interpreted using the surface complexation approach to describe the interface of the reacting oxide. Adsorption of both S 2 O 4 2 - (D) and complexant (L) generates three surface complexes that define the dissolution behavior: ≡ Fe-D, ≡ Fe-L, and dimeric | ≡Fe D ≡Fe L surface complexes. The initial rate increases at lower pH values because of increased surface complexation conditional formation constants. At pH values below 4, however, the fast decomposition of S 2 O 4 2 - gives rise to a rapid depletion of reductant, and total dissolution is not observed. It is shown that for best analytical results in soil analysis, EDTA is a better complexant than citrate; the iron extracted in one dithionite-EDTA treatment at pH 5–6, under N2 at 315 K is not increased by increasing the number of extractions, and is equivalent to the total extractable iron found by previous procedures.

Key Words: Dithionite • Goethite • Iron analysis • Iron dissolution

Clays and Clay Minerals; October 1992 v. 40; no. 5; p. 575-585; DOI: 10.1346/CCMN.1992.0400512
© 1992, The Clay Minerals Society
Clay Minerals Society (www.clays.org)