Shvarev Alexey, Bakker Eric
Department of Chemistry, 179 Chemistry Building, Auburn University, Auburn, AL 36849, USA.
Talanta. 2004 May 10;63(1):195-200. doi: 10.1016/j.talanta.2003.10.010.
A pulsed galvanostatic technique is presented to distinguish free and total levels of calcium with a single membrane electrode by varying the magnitude of the applied current. Pulsed chronopotentiometry creates the possibility of accurate control of ion fluxes across ion-selective plasticized polymeric membranes without ion-exchanger properties (note, however, that a low concentration of ion-exchanger impurities is always present in such membranes). During a constant current pulse ions are forced to extract from the aqueous sample into the contacting membrane phase. Each current pulse is followed by a constant potential pulse to remove the extracted ions from the membrane. The induced concentration gradients are reproducible from pulse to pulse. At relatively small applied currents and in the presence of labile complexes in the sample, the sensor responds to the ion activity, in analogy to a conventional ISE. If a larger current is applied, the flux of complexed ions dominates the sensor response because of its increased magnitude, and the observed potential is now a function of the total ionic concentration. This approach allows one to detect, with the same sensor, the levels of free and total ionic concentration by varying the magnitude of the applied current. The technique utilized here gives much more stable signals than with earlier work demonstrating the principle with zero-current potentiometry. This concept is illustrated with calcium selective membranes based on the ionophore ETH 5234.
本文提出了一种脉冲恒电流技术，通过改变施加电流的大小，利用单个膜电极区分钙的游离态和总含量。脉冲计时电位法使得精确控制离子通过不具有离子交换特性的离子选择性增塑聚合物膜的通量成为可能（然而需要注意的是，此类膜中总是存在低浓度的离子交换杂质）。在恒电流脉冲期间，离子被迫从水样中萃取到与之接触的膜相中。每个电流脉冲之后紧接着一个恒电位脉冲，以去除膜中萃取的离子。每次脉冲所诱导的浓度梯度具有可重复性。在相对较小的施加电流以及样品中存在不稳定络合物的情况下，该传感器对离子活度产生响应，类似于传统的离子选择性电极。如果施加更大的电流，由于络合离子通量的增大，其在传感器响应中占主导地位，此时观察到的电位是总离子浓度的函数。这种方法能够使用同一个传感器，通过改变施加电流的大小来检测游离态和总离子浓度水平。与早期用零电流电位法证明该原理的工作相比，这里所采用的技术给出的信号更加稳定。基于离子载体ETH 5234的钙选择性膜对这一概念进行了说明。
