Donnan Membrane Technique (DMT) for Anion Measurement

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 SO42- and SeO42-, 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 SO42- and SeO42-, 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 SeO42- 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.