Bidirectional Coupled Diffusion of Glycine Driven by pH Gradients

Stokes magnetically‐stirred diaphragm cells have been used to determine multicomponent diffusion coefficients for aqueous glycine + hydrochloric acid + sodium hydroxide solutions at 25°C. In acidic solutions, pH gradients drive coupled flows of glycine toward the low‐pH region of the solution, countercurrent to the flow of H+ ions. In alkaline solutions, the coupled flow of glycine travels in the opposite direction, “up” the pH gradient. The diffusion measurements also show that gradients in concentration of glycine can produce large coupled flows of H+ and OH− ions. Equations are developed to predict multicomponent transport coefficients for glycine and related ampholytes in acid‐base solutions.