Amemiya S, Bühlmann P, Umezawa Y
Department of Chemistry, School of Science, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113, Japan.
Anal Chem. 1998 Feb 1;70(3):445-54. doi: 10.1021/ac9710184.
A model that describes divalent cation responses of liquid membrane ion-selective electrodes based on acidic ionophores and ionic sites is presented. Response slopes for membranes with ionophore and anionic sites are predicted to change from Nernstian to apparently "twice-Nernstian" and then back to Nernstian again as the pH of the sample solution decreases. A maximum measuring range for apparently "twice-Nernstian" responses is expected for membranes with 50 mol % anionic sites relative to the ionophore. On the other hand, membranes with ionophore and cationic sites are expected to give only Nernstian responses, either to divalent cations at high pH or to H(+) at low pH. The validity of the present model has been confirmed experimentally with the two Ba(2+)-selective carboxylate ionophores monensin and lasalocid and the Ca(2+)-selective organophosphate ionophore bis(2-heptylundecyl) phosphate. Addition of anionic sites gave apparently "twice-Nernstian" slopes for monensin at pH 7.0 (56.6 mV/decade), for lasalocid at pH 4.0 (53.3 mV/decade), and for bis(2-heptylundecyl) phosphate at pH 3.5 (53.6 mV/decade). Membranes with cationic sites showed only pH responses at the respective pH. The apparently "twice-Nernstian" responses as discussed here are the first examples of super-Nernstian responses that can be explained with a quantitative model based on thermodynamic equilibria.
本文提出了一种基于酸性离子载体和离子位点的液膜离子选择性电极二价阳离子响应模型。随着样品溶液pH值降低，具有离子载体和阴离子位点的膜的响应斜率预计将从能斯特响应变为明显的“两倍能斯特响应”，然后再次变回能斯特响应。相对于离子载体，具有50 mol%阴离子位点的膜预期具有明显“两倍能斯特响应”的最大测量范围。另一方面，具有离子载体和阳离子位点的膜预期仅给出能斯特响应，在高pH值下对二价阳离子，或在低pH值下对H(+)。用两种Ba(2+)选择性羧酸离子载体莫能菌素和拉沙洛西以及Ca(2+)选择性有机磷酸离子载体双(2-庚基十一烷基)磷酸通过实验证实了本模型的有效性。在pH 7.0时，莫能菌素添加阴离子位点后给出明显的“两倍能斯特响应”斜率(56.6 mV/十倍)，在pH 4.0时拉沙洛西为(53.3 mV/十倍)，在pH 3.5时双(2-庚基十一烷基)磷酸为(53.6 mV/十倍)。具有阳离子位点的膜在各自的pH值下仅显示pH响应。本文讨论的明显“两倍能斯特响应”是基于热力学平衡的定量模型能够解释的超能斯特响应的首个实例。
