Badr Ibrahim H A
Department of Chemistry, Faculty of Science, Ain Shams University, P.O. Box 11566, Cairo, Egypt.
Anal Chim Acta. 2006 Jun 16;570(2):176-85. doi: 10.1016/j.aca.2006.04.021. Epub 2006 Apr 25.
Metallo-salens of cobalt(II) (Co-Sal), chromium(III) (Cr-Sal), and aluminum(III) (Al-Sal) are used as the active ionophores within plasticized poly(vinyl chloride) membranes. It is shown that central metal-ion plays a critical role in directing the ionophore selectivity. Polymer-membrane electrodes based on Co-Sal, Cr-Sal, and Al-Sal are demonstrated to exhibit enhanced responses and selectivity toward nitrite/thiocyanate, thiocyanate, and fluoride anions, respectively. The improved anion selectivity of the three ionophore systems is shown to deviate significantly from the classical Hofmeister pattern that is based only on ion lipophilicity. For example, optimized membrane electrodes for nitrite ion based on Co-Sal exhibit logK(Nitrite,Anion)(pot) values of -5.22, -4.66, -4.48, -2.5 towards bromide, perchlorate, nitrate, and iodide anions, respectively. Optimized membrane electrodes based on Co-Sal and Cr-Sal show near-Nernstian responses towards nitrite (-57.9+/-0.9 mV/decade) and thiocyanate (-56.9+/-0.8 mV/decade), respectively, with fast response and recovery times. In contrast, Al-Sal based membrane electrodes respond to fluoride ion in a super-Nernstian (-70+/-3 mV/decade) and nearly an irreversible mode. The operative response mechanism of Co-Sal, Cr-Sal, and Al-Sal membrane electrodes is examined using the effect of added ionic sites on the potentiometric response characteristics. It is demonstrated that addition of lipophilic anionic sites to membrane electrodes based on the utilized metallo-salens enhances the selectivity towards the primary ion, while addition of cationic sites resulted in Hofmeister selectivity patterns suggesting that the operative response mechanism is of the charged carrier type. Electron spin resonance (ESR) data indicates that Co(II) metal-ion center of Co-Sal ionophore undergoes oxidation to Co(III). This process leads to formation of a charged anion-carrier that is consistent with the response behavior obtained for Co-Sal based membrane electrodes.
钴（II）（Co-Sal）、铬（III）（Cr-Sal）和铝（III）（Al-Sal）的金属-萨伦配合物被用作增塑聚氯乙烯膜中的活性离子载体。结果表明，中心金属离子在指导离子载体选择性方面起着关键作用。基于Co-Sal、Cr-Sal和Al-Sal的聚合物膜电极分别对亚硝酸根/硫氰酸根、硫氰酸根和氟离子表现出增强的响应和选择性。这三种离子载体体系改善后的阴离子选择性明显偏离仅基于离子亲脂性的经典霍夫迈斯特模式。例如，基于Co-Sal的亚硝酸根离子优化膜电极对溴离子、高氯酸根离子、硝酸根离子和碘离子的logK（亚硝酸根，阴离子）（电位）值分别为-5.22、-4.66、-4.48、-2.5。基于Co-Sal和Cr-Sal的优化膜电极分别对亚硝酸根（-57.9±0.9 mV/十倍浓度变化）和硫氰酸根（-56.9±0.8 mV/十倍浓度变化）表现出近能斯特响应，响应和恢复时间快。相比之下，基于Al-Sal的膜电极以超能斯特（-70±3 mV/十倍浓度变化）且几乎不可逆的模式对氟离子作出响应。利用添加离子位点对电位响应特性的影响，研究了Co-Sal、Cr-Sal和Al-Sal膜电极的作用响应机制。结果表明，向基于所用金属-萨伦配合物的膜电极中添加亲脂性阴离子位点可提高对主要离子的选择性，而添加阳离子位点则导致霍夫迈斯特选择性模式，这表明作用响应机制是带电载流子类型。电子自旋共振（ESR）数据表明，Co-Sal离子载体的Co（II）金属离子中心被氧化为Co（III）。这一过程导致形成带电荷的阴离子载体，这与基于Co-Sal的膜电极获得的响应行为一致。
