Spectropotentiometry: A new method for in situ imaging of concentration profiles in ion-selective membranes with simultaneous recording of potential-time transients

A new spectropotentiometric imaging technique is introduced for studying transport processes in connection with cation interference in H+-selective liquid polymer membranes that contain an indicator dye as chromoionophore, Mounted in a specially designed cell under a microscope, the inner and outer rims of a membrane ring are contacted with electrolyte solutions of identical composition. After the system reaches equilibrium, the pH of the inner solution is raised to a value at which cation interference does occur. Pictures are taken with a nominal resolution of 2.25 mu m in selected wavelength ranges using a CCD imaging camera, and potential-time transients are recorded simultaneously. Concentration profiles of the protonated and unprotonated forms of the chromoionophore are calculated from the pictures and plotted as a function of time. On the basis of these results, the rate-determining process is evaluated, and the possible sources of the nonmonotonic potential-time transients are discussed. The diffusion coefficients of the protonated and unprotonated indicator are obtained by fitting the appropriate diffusion equations to the concentration profiles. The new technique allows a more profound comparison of membrane compositions in order to optimize them systematically.