Diffusion of weak acids in salt solutions Maleic acid+NaCl+water

Abstract

A Taylor dispersion tube has been used to measure binary mutual diffusion in aqueous solutions of partially dissociated maleic acid (H₂MH⁺+HM^-). The diffusion coefficients of the molecular acid and the hydrogen maleate ion are estimated from the results. To investigate the effects of added salt on the transport of a weak electrolyte, quaternary mutual diffusion coefficients are reported for aqueous maleic acid+NaCl solutions. The cross-diffusion coefficients for this system indicate that the gradient in maleic acid drives large coupled flows of NaCl and HCl components. Nernst–Planck equations are used to interpret these results. The analysis shows that the addition of NaCl leads to a dramatic increase in the rate of diffusion of H⁺ ions because they are no longer required to diffuse at the same speed as the less-mobile HM^- ions to maintain electroneutrality. Also, the electric field generated by the diffusing acid drives a large counter-current coupled flow of Na⁺ ions and a large co-current coupled flow of Cl^- ions. Each mole of diffusing maleic acid co-transports up to 5 mol HCl and counter-transports up to 1.5 mol NaCl. The results for the aqueous maleic acid+NaCl solutions provide a guide to the diffusion behaviour of the environmentally important SO₂+seawater system.