Exchange velocity, dynamic equilibrium

As the dynamic adsorption layer deviates farther fi"om equilibrium, the drop in the surface concentration increases along the surface and leads to the formation of a stagnant cup. The deviation from equilibrium is caused by compressive viscous shear forces and surface convection of adsorbed molecules it grows as the bubble dimensions and velocity increase and it diminishes as the exchange of adsorbed molecules between the solution bulk and surface... 

Dynamic properties of i.s.e.s. differ greatly for various electrode types and constructions. When the capacitance of analyte/active surface interface is the only cause of response delay, then relaxation time (or time constant of first-order step-response characteristic) is in the order of milliseconds. When the transport of ions across the dynamic Prandtl layer to the surface of the i.s.e. is the main factor (i.e., this transport is the slowest process of equilibrium reinstallation), for a mixing velocity of about lOcm/s a relaxation time of several seconds occurs. This is typical of solid-membrane electrodes with the exception of glass ones. On the other hand, the limited rate of the exchange process in the liquid membrane, the small diffusion flux of the tested ions into the membrane, the slow dynamics for the creation of diffusion potential and the solubility of the active component of the membrane in the testing solution are the main reasons for the slow response of liquid ion-exchanger electrodes (time constants 10-30 s or even more). 

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