title: Donnan membrane technique (DMT) for anion measurement.
abstract: Donnan membrane technique (DMT) is developed and tested for determination of free anion concentrations. Time needed to reach the Donnan membrane equilibrium depends on type of ions and the background. The Donnan membrane equilibrium is reached in 1 day for Cl(-), 1-2 days for NO(3)(-), 1-4 days for SO(4)(2-) and SeO(4)(2-), and 1-14 days for H(2)PO(4)(-) in a background of 2-200 mM KCl or K(2)SO(4). The strongest effect of ionic strength on equilibrium time is found for H(2)PO(4)(-), followed by SO(4)(2-) and SeO(4)(2-), and then by Cl(-) and NO(3)(-). The negatively charged organic particles of fulvic and humic acids do not pass the membrane. Two approaches for the measurement of different anion species of the same element, such as SeO(4)(2-) and HSeO(3)(-), using DMT are proposed and tested. These two approaches are based on transport kinetics or response to ionic strength difference. A transport model that was developed previously for cation DMT is applied in this work to analyze the rate-limiting step in the anion DMT. In the absence of mobile/labile complexes, transport tends to be controlled by diffusion in solution at a low ionic strength, whereas at a higher ionic strength, diffusion in the membrane starts to control the transport.
authors.affiliation: Department of Soil Quality, Wageningen University, P.O. Box 47, 6700 AA, Wageningen, The Netherlands.
authors.forename: Flora Alonso
authors.initials: FA
authors.lastname: Vega
authors.forename: Liping
authors.initials: L
authors.lastname: Weng
authors.forename: Erwin J M
authors.initials: EJ
authors.lastname: Temminghoff
authors.forename: Willem H
authors.initials: WH
authors.lastname: Van  Riemsdijk
mesh_terms: D000465:Algorithms; D000838:Anions / Q000032:analysis*; D004058:Diffusion; D007700:Kinetics; D008956:Models, Chemical; D008958:Models, Molecular; D009994:Osmolar Concentration
chemical_list: D000838:Anions