Abstract
An empirical solution to abnormal potential responses, showing peaks of emf, of commercial Cu2+- and Cd2+-selective electrodes with solid-state membranes was proposed for aqueous solutions of CuCl2 and CdI2. The two-step processes of Mn+ + Yn? (s: solid phase) MY(s) and MY(s) + 2X? X2MY2?(s) (n = 1, 2) at a test solution/electrode-interface were considered as a model. Here, Mn+, Yn?, and X? refer to a divalent or univalent cation, functional groups of electrode materials, and a halide ion (X? = Cl?, Br?, I?), respectively. By applying electrochemical potentials to these processes at n = 2, we derived an equation. Regression analyses based on the equation reproduced well the plots of emf versus log 2(*[M]t) for the Cd(II) and Cu(II) systems: *[M]t denotes a total concentration of species relevant to M2+ in a bulk of the aqueous solution. Also, we obtained log Ks(CdBr2) = 4.28 ? 0.22, log Ks(CdI2) = 6.98 ? 0.05, log Ks(CuCl2) = 3.96 ? 0.09, and log Ks(CuBr2) = 11.4 at 25?C. The magnitude in ?log Ks reflected that in the logarithmic solubility product, log {*[M2+](*[X?])2}, for bulk water, where *[M2+] or *[X?] denotes a molar concentration of the bulk solu-tion of M2+ or X? at equilibrium, respectively. Moreover, a mixture of CuSO4 with NaCl at the molar ratio of 1:1 yielded a plot similar to that of CuCl2.
Highlights
Many ion-pair formation constants of divalent metal salts (MX2) in water (w) have been determined so far [1,2,3,4]
An empirical solution to abnormal potential responses, showing peaks of emf, of commercial Cu2+- and Cd2+-selective electrodes with solid-state membranes was proposed for aqueous solutions of CuCl2 and CdI2
It was demonstrated that the present model based on the balances among the electrochemical potentials reproduce well the potential responses of the commercial Cd2+ and Cu2+ electrodes with the solid-state membranes in the presence of only the counter halide ions
Summary
Many ion-pair (or complex) formation constants of divalent metal salts (MX2) in water (w) have been determined so far [1,2,3,4]. The Cd2+ electrode could not be used for the determination of the formation constants Such plots with the peaks, namely -shaped plots, have been reported in the cases of potential responses of cation-selective electrodes with liquid membranes to some anions, such as F , SCN and ClO4 [5,6,7]. Similar potential responses were observed in the case of a commercial Cu2+ electrode with a solid-state membrane, when the ion-pair formation of CuCl2 in w had been studied for the application to solvent-extraction experiments of some Cu(II) salts by crown compounds into 1,2-dichloroethane. The equation was extended to potential responses of M+-selective electrodes
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