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

Abstract

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 $202 (D) and complexant (L) generates =-Fe D three surface complexes that define the dissolution behavior: ~Fe-D, -~Fe-L, and dimeric -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 $20,~ 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.

Extração de ferro de esmectita brasileira com emprego do método ditionito-citrato-bicarbonato

“…A busca de condições operacionais para extração de óxidos de ferro presentes em amostras de solo ou de argila natural, sem alterar de forma significativa a estrutura dos argilominerais, tem sido investigado por alguns pesquisadores [1][2][3][4][5][6][7] . Esse interesse é devido à necessidade de maior confiabilidade nas análises de solos [1][2][3][4] e à necessidade de purificação de argila natural para fins catalíticos [5][6][7] .…”

“…Esse interesse é devido à necessidade de maior confiabilidade nas análises de solos [1][2][3][4] e à necessidade de purificação de argila natural para fins catalíticos [5][6][7] . Argilas naturais podem conter como impurezas diversos tipos de óxidos de ferro cristalinos ou amorfos, tais como: hematita (α-Fe 2 O 3 ), goetita (α-FeOOH) e lepidocrocita (γ-FeOOH), além de outras formas combinadas, como a magnetita (FeO.Fe 2 O 3 ) e a ilmenita (FeO.TiO 2 ) 1,7,8 . No caso das esmectitas brasileiras, o ferro pode estar presente tanto sob a forma de óxidos isolados quanto na forma de substituições isomórficas na rede cristalina.…”

Effective cleaning of rust stained marble

“…Even though the amount of the radical anion is relatively small and can be estimated to 10 −5 M in a 0.1 M dithionite solution, the anion has shown to be the dominant reducing species in the reduction and dissolution of iron oxides [14,15]. From biochemical experiments, the standard reduction potential of the radical anion has been determined to −1.39 V (vs. NHE) in basic solution [18,19], giving a calculated value e°′ = −0.56 V at pH = 7 in accordance with experimentally determined values [18].…”

“…Efficient removal of a slightly soluble material requires a ligand having an overall stability constant comparable to the reciprocal value of the solubility product in order to achieve a favourable equilibrium constant. Based on the solubility product of goethite, efficient removal of rust in Fe(III) stage requires a ligand having a stability constant approaching 10 41 , whereas removal of Fe(OH) 2 only requires a stability constant of 10 14 . Additionally, the ligand should possess low affinity towards Ca(II) to prevent dissolution of calcite.…”

“…Ca(II) and Mg(II), as well as on the identification of a fast reducing agent able to reduce Fe(III) to Fe(II). Among the reducing chemicals, sodium dithionite (SD), Na 2 S 2 O 4 , has been successfully used in combination with different ligands as a dissolving agent for goethite in soil analyses [14,15] and for removal of rust from paper [16]. Furthermore, the use of dithionite in conservation science in general is well described [17].…”