Equilibrium Relationships of Zn2+, Fe3+, Ca2+, and H+ with EDTA and DTPA in Soils

Abstract

Abstract A general approach is developed by which the equilibria between chelating agents and metal ions in soils can be predicted from formation constants. Mole‐fraction diagrams are derived for EDTA (ethylenediaminetetraacetic acid) and DTPA (diethylenetriaminepentaacetic acid) in soils when the competing cations are either Fe 3+ , Ca 2+ , and H + , or Zn 2+ , Fe 3+ , Ca 2+ , and H + . These diagrams give the specific metal‐ligand complexes that form in soils and show how the concentration of each complex changes with pH. Evidence is presented to support the hypothesis that Zn reacts with amorphous SiO 2 in soils to form ZnSiO 3 . The concentration of Zn 2+ in equilibrium with solid phase ZnSiO 3 and amorphous SiO 2 is approximately 10 ‐6 M at pH 6, and decreases 100‐fold for each unit increase in pH. This concentration of Zn 2+ is approximately 100,000 times lower than that in equilibrium with ZnO or ZnCO 3 .