Comparison of the Diffusion of Aqueous Glycine Hydrochloride and Aqueous Glycine

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A Taylor dispersion tube has been used to measure mutual diffusion in aqueous solutions of glycine hydrochloride at 25°C and concentrations from 0.0005 to 0.5 M. Analysis of the dispersion profiles shows that the diffusion of glycine hydrochloride (GlyHCl) produces a subtantial additional flow of hydrochloric acid that is liberated by the dissociation: GlyH+ + Cl− ⇄ Gly + H+ + Cl−. Diffusion in this system is, therefore, a ternary process described by the equations J1(GlyHCl) = − D11∇C1 − D12∇C2 and J2(HCl) = −D21∇C1 − D22∇C2 for the coupled fluxes of total glycine hydrochloride (1) and hydrochloric acid (2) components. The ratio D21/D11 of measured diffusion coefficients indicates that up to two moles of HCl are cotransported per mole of GlyHCl. Although protonated glycine diffuses with relatively mobile Cl− counterions, the main diffusion coefficient of glycine hydrochloride, D11, is lower than or nearly identical to the diffusion coefficient of aqueous glycine. A model for the diffusion of protonated solutes is developed to interpret this result and the large coupled flows of HCl. Diffusion coefficients are also reported for the aqueous hydrochlorides of 3- and 4-aminobenzoic acids.

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