Article pubs.acs.org/ac
Sophisticated Design of PVC Membrane Ion-Selective Electrodes Based on the Mixed Potential Theory Maya Imoto,† Toru Sakaki,‡ and Toshiyuki Osakai*,† †
Department of Chemistry, Graduate School of Science, Kobe University, Nada, Kobe 657-8501, Japan A&T Corporation, 2023-1 Endo, Fujisawa, Kanagawa 252-0816, Japan
‡
S Supporting Information *
ABSTRACT: The mixed potential (MP) theory was successfully utilized to design an ionophore-based polyvinyl chloride (PVC) membrane K+ ion-selective electrode (ISE). Prior to the application of the MP theory, the transfer of K+ and interfering ions (Na+, Li+, and H+) facilitated by bis(benzo-15-crown-5) (BB15C5) or dibenzo-18-crown-6 (DB18C6) at a micro PVC membrane/water interface was studied by ion-transfer voltammetry (ITV). The reversible half-wave potentials were then obtained for the facilitated transfer of the ions. Using such voltammetric data and the literature data about diffusion coefficients of ions, we could well-predict the potential responses of the BB15C5- or DB18C6-based K+ ISE, as the function of the concentrations of primary and interfering ions, and also of the counterion for K+ [e.g., tetrakis(4-chlorophenyl)borate] added to the membrane. Thus, the MP theory has been proven to be useful to optimize the membrane composition for a higher ion selectivity and a lower detection limit. It has also been found that the leaching of ions from an inner solution is too small to affect the detection limit, at least for the designed PVC membrane ISE.
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over, it was shown that the selectivity coefficient of ISEs can be expressed as a function of parameters, including the difference between the standard ion-transfer potentials of the primary and interfering ions, the ratios of the mass-transfer coefficients of ions, and the concentrations of ions.5 Unfortunately, however, such a potentially groundbreaking theory has not been utilized explicitly for developing new ISEs. One of the reasons is that the theory was verified only for unsupported liquid membrane ISEs.6,7 Therefore, we recently have proven that the MP theory is also useful for polyvinyl chloride (PVC)-supported liquid membrane ISEs,10,11 which are the most popular and used in various fields. It has thus been confirmed that the PVC matrix has little effect on the thermodynamics of ion transfer at the PVC membrane/solution interface, as previously described by Langmaier et al.12 Then, it has been shown that the potential response of PVC membrane ISEs can be well-understood by the MP theory as a dynamic model. Meanwhile, Bakker et al. proposed the phase-boundary potential model for the response of ISEs.3,13 The model assumes local equilibria at the membrane/solution interface. In this study, we first showed that the MP theory was also applicable to the most popular ionophore-based ISEs and that the theory was useful for a sophisticated design of a PVC
on-selective electrodes (ISEs) are one of the most important analytical tools for sensing ions.1−3 A practical advantage of ISEs is that the equipment is comparatively small, inexpensive, and easily handled. Because of this, ISEs have now become indispensable, especially in clinical analysis such as the determination of Na+/K+/Cl− in blood serum.4 However, one drawback of ISEs is that their detection limit is usually higher than those of such methods as ion chromatography. Also, another drawback is that only one kind of ion can be submitted for analysis by a single ISE. Therefore, it is desired to provide a sophisticated way to design high-performance ISEs based on a theoretical basis. In a vast majority of studies, ISEs have been developed by screening or trial-and-error (i.e., by changing the membrane components and their concentrations in a systematic manner). In 1984, Kakiuchi and Senda5 and Kihara and Yoshida6 proposed that the potential response of ISEs can be explained on the basis of a concept of the mixed potential (MP) theory. They also confirmed the validity of the theory experimentally for liquid-membrane ISEs.6,7 In the MP theory, the electromotive force (EMF) of a liquid-membrane ISE is regarded as the zero-current potential of the membrane/solution interface, which is determined by the interfacial transfer of at least two ions, including the primary ion and the interfering ion(s). Although this concept had already been described by Camman8 and Koryta,9 the above authors5,6 presented a rigorous analytical formula of EMF for liquid-membrane ISEs. More© 2013 American Chemical Society
Received: February 13, 2013 Accepted: April 1, 2013 Published: April 1, 2013 4753
dx.doi.org/10.1021/ac400427p | Anal. Chem. 2013, 85, 4753−4760
Analytical Chemistry
Article
membrane K+ ISE as a typical example. First, we performed voltammetric measurements with a micro PVC membrane/ water (W) interface and determined the reversible half-wave potentials for the transfer of K+ and the interfering ions (Na+, Li+, and H+) facilitated by two ionophores [bis(benzo-15crown-5); BB15C5 and dibenzo-18-crown-6; DB18C6]. Using these voltammetric data, we theoretically predicted the effects of membrane components and their concentrations on the potential response of the K+ ISE. Then, we could design a K+ ISE that showed a lower detection limit and a higher ion selectivity. It has thus been shown that the MP theory can be applied to ionophore-based ISEs as well, and we will propose a new sophisticated method for the design of high-performance PVC-membrane ISEs.
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phases, respectively). The solution resistance being determined using a conductivity meter (MY-9, Yanaco) was very large (ca. 5 MΩ); however, most (though not all) could be compensated for by means of a positive feedback circuit attached to the potentiostat.16 Potentiometry. K+ ISEs containing an inner solution were prepared by the following method. A PVC membrane of the composition shown below was prepared in a similar manner as that for voltammetry and was cut to 5 mm ϕ. The membrane was then glued with THF to the opening of a PVC tip [available as a part of the commercial kit for PVC membrane ISEs (7904L, DKK-TOA)]. The PVC membrane was then conditioned in 1.0 mM KCl overnight.17 Unless noted otherwise, the electrochemical cell for the K+ ISE containing an inner solution (1.0 mM KCl) is shown as
EXPERIMENTAL SECTION
Chemicals. Tetrapentylammonium tetraphenylborate (TPnATPB) was prepared in a similar manner as reported previously14 and, finally, recrystallized twice with acetone. Potassium tetraphenylborate (KTPB) was prepared by metathesis of NaTPB (Dojindo Laboratories) with KCl.15 Tetrapentylammonium chloride (TPnACl) and potassium tetrakis(4-chlorophenyl)borate (KTClPB, ≥98%) were purchased from Tokyo Chemical Industry and used as received. Other electrolytes, including KCl, NaCl, LiCl, HCl, MgCl2, MgSO4, and lithium acetate (LiOAc) were of the highest grade commercially available and used as received. Bis(benzo-15crown-5) (BB15C5, ≥98%) was purchased from Dojindo Laboratories and used as received. Dibenzo-18-crown-6 (DB18C6) was purchased from Wako Pure Chemical Industries and purified by recrystallization from benzene before use. PVC (degree of polymerization = 1100, Wako) was purified as described previously.10 ο-Nitrophenyl octyl ether (NPOE, ≥99%) was the product of Dojindo Laboratories and purified by shaking three times with 0.1 M NaOH, followed by washing five times with distilled water. Voltammetry. PVC membranes (ca. 0.2 mm thick) were prepared by dissolving 100 mg PVC, 250 mg NPOE, and given amounts of ionophore and electrolyte in ca. 5 mL tetrahydrofuran (THF; stabilizer free) and then evaporating THF under a hood. For further details, see the previous paper.11 A micro PVC membrane/W interface was formed by pressing the end of a glass pipet (ca. 100 μm i.d.) on the PVC membrane. The glass pipet was filled with a W-phase solution containing a supporting electrolyte (25 mM MgCl2) and a transferring ion (X+). The two-electrode electrolytic cell used can be expressed as
where a = 10−9.0 − 10−1.0; b = 0, 2, 10, 50; c = 1, 10; L = BB15C5 or DB18C6; and Y = TPB or TClPB. A double junction reference electrode with two sintered glass filters (shown by dashed lines) was used. The effect of interfering ions was evaluated by the fixed interference method (FIM).18,19 In order to prevent leaching of Li+ from the reference electrode to the sample solution, the following procedure was performed: The measurement for a comparably low K+ concentration (
