Voltammetric Characterization of Ion鈥揑onophore Complexation Using Thin Polymeric Membranes: Asymmetric Thin-Layer Responses

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• 作者：Peter J. Greenawalt ; Mohammed B. Garada ; Shigeru Amemiya
• 刊名：Analytical Chemistry
• 出版年：2015
• 出版时间：August 18, 2015
• 年：2015
• 卷：87
• 期：16
• 页码：8564-8572
• 全文大小：417K
• ISSN：1520-6882

文摘

Selective ion鈥搃onophore complexation in a polymeric membrane is crucial to various sensing applications. In this work, we report on a novel voltammetric approach based on a thin polymeric membrane to determine the stoichiometry and overall formation constant of an ion鈥搃onophore complex. With this approach, a 鈭?.6 渭m thick ionophore-doped membrane contacts an aqueous solution containing an excess amount of a target ion to facilitate voltammetric ion transfer across the membrane/water interface. Advantageously, the resultant thin-layer voltammogram shows no diffusional effect, which simplifies the theoretical modeling and quantitative analysis of the voltammogram. We predict theoretically that the complexation stoichiometry affects not only the peak current and peak potential of the thin-layer voltammogram, but also the symmetry of the peak shape with respect to the peak potential. Experimentally, a symmetric voltammogram ensures the formation of a 1:1 complex for a Na⁺-selective ionophore. By contrast, the asymmetric shape and peak current of voltammograms are used to demonstrate that a Ca²⁺-selective ionophore forms 1:3 and 1:2 complexes with calcium and magnesium ions, respectively. The complexation stoichiometry is needed to yield the formation constants that are consistent with those determined previously by potentiometry. In addition, both 1:2 and 1:1 complexes are voltammetrically observed with another Na⁺-selective ionophore, which was assumed to form only a 1:2 complex in previous potentiometric studies. The formation constants of both complexes are determined from a single voltammogram to reveal that the preceding formation of a 1:2 complex thermodynamically hampers the voltammetric observation of a 1:1 complex.