Coupled diffusion in aqueous citric acid + calcium citrate solutions

Abstract

Taylor dispersion is used to measure ternary mutual diffusion in aqueous citric acid(1) + calcium citrate(2) solutions at 25 °C. Main diffusion coefficients D11 and D22 reach values 25% to 60% larger than the respective binary diffusion coefficients of aqueous citric acid and calcium citrate. Of additional interest, cross-coefficient D12 is large and negative, indicating that each mole of diffusing calcium citrate counter-transports up to two moles of citric acid. Nernst-Planck (NP) equations are used to relate the fluxes of the citric acid and calcium citrate components to the fluxes of the solution species: molecular citric acid (H3Cit) and its dissociation products (H+, H2Cit−, HCit2− and Cit3− ions), Ca2+ ions, and calcium complexes produced by ion association (CaCit−, CaHCit, CaH2Cit+). The NP analysis identifies key roles in the coupled diffusion of citric acid and calcium citrate played by ion association and by ion migration in the electric field generated by hydrogen-ion concentration gradients.