Effect of potassium dihydrogen citrate on the diffusion behaviour of citric acid

Abstract

Ternary mutual diffusion coefficients (D11, D22, D12, and D21) in aqueous solutions of citric acid (H3Cit) (component 1) plus potassium dihydrogen citrate (KH2Cit) (component 2) at 298.15 K and concentrations up to 0.040 mol dm􀀀 3 were measured using a Taylor scattering method. The obtaining of the negative values for D12 and D21 cross coefficients, mainly in dilute solutions and for the first ones, indicates that there is a strongly coupled diffusion of these components. In fact, the concentration gradients of KH2Cit drive substantial counter current coupled flows of H3Cit citric acid, indicating that each mol of diffusing potassium dihydrogen citrate can counteract the transport of up to 0.74 mol of citric acid, approximately. A better interpretation of the electrostatic mechanism for the coupled diffusion of KH2Cit and H3Cit was achieved based on the values predicted by the Nernst-Planck equations, having been verified as a good agreement between them and experimental data.