DIFFUSION OF WEAK ACIDS IN SALT-SOLUTIONS - MALEIC-ACID PLUS NACL PLUS WATER

A Taylor dispersion tube has been used to measure binary mutual diffus ion in aqueous solutions of partially dissociated maleic acid (H2M rev ersible arrow H+ + HM-). The diffusion coefficients of the molecular a cid and the hydrogen maleate ion are estimated from the results. To in vestigate the effects of added salt on the transport of a weak electro lyte, quaternary mutual diffusion coefficients are reported for aqueou s maleic acid + NaCl solutions. The cross-diffusion coefficients for t his system indicate that the gradient in maleic acid drives large coup led flows of NaCl and HCl components. Nernst-Planck equations are used to interpret these results. The analysis shows that the addition of N aCl leads to a dramatic increase in the rate of diffusion of H+ ions b ecause 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 Nat ions and a large co-current coupled flow of Cl- io ns. Each mole of diffusing maleic acid co-transports up to 5 mol HCl a nd counter-transports up to 1.5 mol NaCl. The results for the aqueous maleic acid + NaCl solutions provide a guide to the diffusion behaviou r of the environmentally important SO2 + seawater system.