Spatial and temporal visualization of a pH-dependent complexation equilibrium by nuclear magnetic resonance imaging

Abstract

Alteration of a chemical equilibrium, ethylenediaminetetraacetic acid chelation of aqueous copper, in response to a pH gradient that protonates the complexing agent, has been observed spatially and temporally by 1H nuclear magnetic resonance. Protonation of the complexing agent shifted the complexation equilibrium towards free copper ion. Free and bound copper ions alter the spin lattice relaxation time (T1) of water in the host polyacrylamide gel to different extents. Based on this difference, a T1 weighted (fast repetition time, short echo time) two-dimensional spin-warp imaging sequence mapped out the distribution of the free and bound species. Addition of a pH 3 aqueous solution to the gel was insufficient to alter the equilibrium; a pH 1 aqueous solution liberated the complexed copper. Quantitative one-dimensional experiments gave a T1 weighted profile that showed the reaction front is displaced a distance proportional to the square root of time.