Abstract
Abstract A pH-titration study is reported at 25·0 ± 0·1°C and μ = 0·1 M (NaClO 4 ) of the equilibria and distribution of complexes in the 2-ligand systems, relevant to soils, (i) M 2+ -fulvate-NTA, (ii) M 2+ -fulvate-phosphate and (iii) M 2+ -NTA-phosphate, where M 2+ = Pb, Cd or Zn, NTA = nitrolotriacetic acid and phosphate = H n PO 4 . Equilibria in the Pb and Cd systems were confirmed using metal-ion-selective electrodes. In (i), M (NTA) and (NTA) M (fulvate) predominate in weakly acidic or neutral solution. In systems (ii) and (iii), (phosphate) M ′(fulvate) and (phosphate) M ′(NTA) predominate, where M ′ = Cd 2+ or Zn 2+ . However, Pb(fulvate) and (phosphate)Pb(fulvate) complexes are insoluble, which could account for the low mobility of Pb in soils and the low uptake of Pb by plants. The role of mixed-ligand complexes in soil chemistry is discussed.
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