Department of Inorganic and Analytical Chemistry, University of Geneva , Quai Ernest Ansermet 30, CH-1211 Geneva, Switzerland.
Anal Chem. 2017 Aug 1;89(15):7828-7831. doi: 10.1021/acs.analchem.7b01777. Epub 2017 Jul 13.
Finite difference analysis of ion-selective membranes is a valuable tool for understanding a range of time dependent phenomena such as response times, long and medium term potential drifts, determination of selectivity, and (re)conditioning kinetics. It is here shown that an established approach based on the diffusion layer model applied to an ion-exchange membrane fails to use mass transport to account for concentration changes at the membrane side of the phase boundary. Instead, such concentrations are imposed by the ion-exchange equilibrium condition, without taking into account the source of these ions. The limitation is illustrated with a super-Nernstian potential jump, where a membrane initially void of analyte ion is exposed to incremental concentrations of analyte in the sample. To overcome this limitation, the two boundary elements, one at either side of the sample-membrane interface, are treated here as a combined entity and its total concentration change is dictated by diffusional fluxes into and out of the interface. For each time step, the concentration distribution between the two boundary elements is then computed by ion-exchange theory. The resulting finite difference simulation is much more robust than the earlier model and gives a good correlation to experiments.
离子选择性膜的有限差分分析是一种非常有价值的工具，可以帮助我们理解各种时间相关的现象，例如响应时间、长期和中期的电位漂移、选择性的确定以及（再）调理动力学。本文表明，一种基于扩散层模型的成熟方法应用于离子交换膜时，未能利用传质来解释相界面处膜侧的浓度变化。相反，这些浓度是由离子交换平衡条件决定的，而没有考虑这些离子的来源。该限制通过超 Nernst 电位跃变来说明，其中膜最初不含分析物离子，然后暴露于样品中递增的分析物浓度。为了克服这一限制，本文将样品-膜界面两侧的两个边界元素视为一个组合实体，其总浓度变化由扩散通量进出界面决定。然后，通过离子交换理论计算两个边界元素之间的浓度分布。与早期模型相比，这种有限差分模拟更加稳健，并且与实验结果有很好的相关性。
