Charge transfer resistance and differential capacity of the plasticized PVC membrane|water interface

Ion amperometry at the interface between two immiscible electrolyte solutions in view of realizing the amperometric ion-selective electrode

This article reviews the development in ion amperometry at the interface between two immiscible electrolyte solutions (ITIES) in view of realizing the amperometric ion-selective electrode (ISE). The concept of polarizability of ITIES in a multi-ion system is outlined. Principle aspects of ion amperometry at ITIES are discussed including the use of amperometry as a tool for the clarification of the ion sensing mechanism, and for determining the concentrations of ions in the solution. The reference is made to recent amperometric measurements at the supported liquid membrane (SLM) and polymer composite liquid membranes (PCLM), which, together with the micro-hole supported ITIES, appear to be particularly suitable for realization of the amperometric ISE.

Electrochemical mechanism of ion-ionophore recognition at plasticized polymer membrane/water interfaces

Here, we report on the first electrochemical study that reveals the kinetics and molecular level mechanism of heterogeneous ion-ionophore recognition at plasticized polymer membrane/water interfaces. The new kinetic data provide greater understanding of this important ion-transfer (IT) process, which determines various dynamic characteristics of the current technologies that enable highly selective ion sensing and separation. The theoretical assessment of the reliable voltammetric data confirms that the dynamics of the ionophore-facilitated IT follows the one-step electrochemical (E) mechanism controlled by ion-ionophore complexation at the very interface in contrast to the thermodynamically equivalent two-step electrochemical-chemical (EC) mechanism based on the simple transfer of an aqueous ion followed by its complexation in the bulk membrane. Specifically, cyclic voltammograms of Ag(+), K(+), Ca(2+), Ba(2+), and Pb(2+) transfers facilitated by highly selective ionophores are measured and analyzed numerically using the E mechanism to obtain standard IT rate constants in the range of 10(-2) to 10(-3) cm/s at both plasticized poly(vinyl chloride) membrane/water and 1,2-dichloroethane/water interfaces. We demonstrate that these strongly facilitated IT processes are too fast to be ascribed to the EC mechanism. Moreover, the little effect of the viscosity of nonaqueous media on the IT kinetics excludes the EC mechanism, where the kinetics of simple IT is viscosity-dependent. Finally, we employ molecular level models for the E mechanism to propose three-dimensional ion-ionophore complexation at the two-dimensional interface as the unique kinetic requirement for the thermodynamically facilitated IT.

Stripping analysis of nanomolar perchlorate in drinking water with a voltammetric ion-selective electrode based on thin-layer liquid membrane

A highly sensitive analytical method is required for the assessment of nanomolar perchlorate contamination in drinking water as an emerging environmental problem. We developed the novel approach based on a voltammetric ion-selective electrode to enable the electrochemical detection of "redox-inactive" perchlorate at a nanomolar level without its electrolysis. The perchlorate-selective electrode is based on the submicrometer-thick plasticized poly(vinyl chloride) membrane spin-coated on the poly(3-octylthiophene)-modified gold electrode. The liquid membrane serves as the first thin-layer cell for ion-transfer stripping voltammetry to give low detection limits of 0.2-0.5 nM perchlorate in deionized water, commercial bottled water, and tap water under a rotating electrode configuration. The detection limits are not only much lower than the action limit (approximately 246 nM) set by the U.S. Environmental Protection Agency but also are comparable to the detection limits of the most sensitive analytical methods for detecting perchlorate, that is, ion chromatography coupled with a suppressed conductivity detector (0.55 nM) or electrospray ionization mass spectrometry (0.20-0.25 nM). The mass transfer of perchlorate in the thin-layer liquid membrane and aqueous sample as well as its transfer at the interface between the two phases were studied experimentally and theoretically to achieve the low detection limits. The advantages of ion-transfer stripping voltammetry with a thin-layer liquid membrane against traditional ion-selective potentiometry are demonstrated in terms of a detection limit, a response time, and selectivity.

Potentiometric Sensor for Heparin Polyion: Transient Behavior and Response Mechanism

Chronopotentiometry and electrochemical impedance spectroscopy were used to study the transient behavior and the potentiometric response mechanism of the polymer membrane-based sensor for heparin. Membrane with a composition of 66 wt % poly(vinyl chloride), 33 wt % o-nitrophenyl octyl ether (plasticizer), and 0.05 M tridodecylmethylammonium chloride (ion exchanger) was deposited on the surface of a silver or a glassy carbon (GC) electrode. In the latter case, the membrane contained also 0.1 M 1,1'-dimethylferrocene/1,1'-dimethylferricenium+ couple ensuring the electronic contact between the membrane and GC. The sensor was dipped in an aqueous solution of 0.1 M LiCl, which was stirred with a magnetic stirrer (2-18.2 Hz), and eventually spiked with heparin (0.05-5 U mL-1). Chronopotentiometric measurements were carried out using either the Ag supported membrane with a thickness>100 microm or the GC supported membrane with a defined thickness of 2-30 microm, which was also used in impedance measurements. Remarkable features of the potentiometric response include the linear dependence of the initial slope of the potential transient on the heparin concentration in the aqueous phase and on the square root of the stirring frequency, and the absence of the effect of the membrane thickness. Impedance measurements (0.1 Hz-10 kHz) made it possible to identify and to evaluate the geometric capacitance and the capacitance of the electric double layer at the membrane/solution interface, the bulk membrane and charge-transfer resistances, and the Warburg impedance of the chloride transport. Changes in the membrane bulk and charge-transfer resistances and the Warburg impedance upon spiking the aqueous solution with heparin were found to be consistent with the steady-state response of approximately -25 mV, indicating that the bulk chloride concentration in the membrane decreased to about half of its initial value. A novel theoretical model of the transient behavior was developed based on the balance of the charging and the faradic currents of chloride and heparin, in accordance with the ion-exchange mechanism that has been proposed previously. It was concluded that the initial slope of the potential transient is linked to the charging of the double layer coupled to the chloride ion transfer across the membrane/solution interface and to the diffusion-limited transport of heparin in the solution. The potentiometric assay of heparin could be based on measurements of the initial slope of the potential transient or the potential at a fixed time shortly after the heparin injection.

Partition Coefficients of Ionizable Compounds in o-Nitrophenyl Octyl Ether/Water Measured by the Potentiometric Method

The objective of this study was to investigate the reliability of potentiometric measurements of partition coefficients (log P) in the o-NPOE/water system, o-nitrophenyl octyl ether being an organic solvent widely used for the voltammetric determination of the partition coefficient of ions. Three sets of ionizable model compounds were explored in this study. The results showed that the potentiometric technique gave precise and reliable log P(npoe) values in the explored range of 0.1-4.3.