Donnan Membrane Technique (DMT) for Anion Measurement

• Published in: Analytical chemistry (Washington) Vol. 82; no. 7; pp. 2932 - 2939
• Main Authors: Vega, Flora Alonso, Weng, Liping, Temminghoff, Erwin J. M, Riemsdijk, Willem H. Van
• Format: Journal Article
• Language: English
• Published: Washington, DC American Chemical Society 01.04.2010
• Subjects: activity model; Algorithms; Analytical chemistry; Anions - analysis; aqueous-solutions; chemical speciation; Chemistry; colloidal particles; Diffusion; dissolved organic-matter; Effects; Exact sciences and technology; heavy-metals; Ions; Kinetics; Membranes; metal-ion concentrations; Models, Chemical; Models, Molecular; Osmolar Concentration; sandy soil; soil solution; water extracts; Ionic strength; Charged particle; Membrane; Cations; Time; Kinetics; Concentration; Technique; Diffusion; Humic acid
• ISSN: 0003-2700; 1520-6882
• DOI: 10.1021/ac9029339

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 NO3 −, 1−4 days for SO4 2− and SeO4 2−, and 1−14 days for H2PO4 − in a background of 2−200 mM KCl or K2SO4. The strongest effect of ionic strength on equilibrium time is found for H2PO4 −, followed by SO4 2− and SeO4 2−, and then by Cl− and NO3 −. 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 SeO4 2− and HSeO3 −, 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.