Flow-injection approach for the determination of the dynamic response characteristics of ion-selective electrodes. Part 2. Application to tubular solid-state iodide electrode

The flow-injection approach proposed in Part 1 for investigating the dynamic behaviour of flow-through detectors was applied to the study of a tubular iodide electrode prepared from a pressed precipitate of silver iodide. It was found that the electrode response cannot be described by either of the models developed in Part 1 on the basis of the concepts in the literature for the transient response mechanism of ion-selective electrodes. Scratches and microcavities in the electrode bore were observed under the microscope, most probably resulting from bore drilling. These surface non-idealities make the accessibility of the electrode surface to the analyte variable. A mathematical model developed on the basis of the assumption that part of the electrode surface is directly exposed to the flow while the remainder of the surface incorporates all microcavities in which ideal mixing exists, describes fairly successfully the experimentally obtained potential—time transients. Moreover, this study shows that the rate of response of solid-state ion-selective electrodes is in principle comparable to that of other electrochemical sensors.     

Limitation of linear response in flow-injection systems with ion-selective electrodes

The lower limit of linear response in flow-injection systems employing membrane and second-kind ion-selective electrodes as detectors depends on the dispersion in the system and is usually much worse than in batch measurements. Below a certain value of the peak height, linearity of the electrode response is not maintained. The limiting value depends on the process causing the loss of linear response (solubility of the electrode material, contamination, or adsorption at the electrode surface), and varies from 15 to 70 mV. Chloride- iodide-, fluoride- and copper(II)-selective electrodes are discussed.     

Preparation and examination of calcium ion-selective electrodes for flow analysis

The preparation of calcium ion-selective electrodes based on known alkylphenylphosphate exchangers or on the ETH 1001 ionophore, and their use in a flow-through cell in a flow-injection system for the determination of calcium are described. The response and lifetime of the electrodes and the effects of magnesium and sodium ions on the determination of 10^?3?10^?5 M calcium are examined in detail. The ionophore electrode is shown to be most satisfactory.     

Use of ion-selective electrodes in industrial flow analysis

Two cells with ion-selective electrodes are described for industrial flow analysis, involving the flow-injection technique and continuous monitoring. The dependence of the electrode signal on the experimental parameters is discussed using the determination of cyanide as a model. It is shown that sensitive and reproducible measurements are possible even if the industrial application requires that the measuring cell be large, provided that the flow rate of the carrier liquid is sufficiently high. Large cel volumes are advantageous in continuous monitoring, as the signal approaches that obtained under steady-state conditions. Experiments with a calcium ion-selective electrode and with an ammonia gas probe show that the slow response of the sensor severely limits its applicability in flow analysis.     

Solid-contact pH-selective electrode using multi-walled carbon nanotubes

Multi-walled carbon nanotubes (MWCNT) are shown to be efficient transducers of the ionic-to-electronic current. This enables the development of a new solid-contact pH-selective electrode that is based on the deposition of a 35-μm thick layer of MWCNT between the acrylic ion-selective membrane and the glassy carbon rod used as the electrical conductor. The ion-selective membrane was prepared by incorporating tridodecylamine as the ionophore, potassium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate as the lipophilic additive in a polymerized methylmethacrylate and an n-butyl acrylate matrix. The potentiometric response shows Nernstian behaviour and a linear dynamic range between 2.89 and 9.90 pH values. The response time for this electrode was less than 10 s throughout the whole working range. The electrode shows a high selectivity towards interfering ions. Electrochemical impedance spectroscopy and chronopotentiometry techniques were used to characterise the electrochemical behaviour and the stability of the carbon-nanotube-based ion-selective electrodes.     

Potentiometric detection in flow analysis

Special aspects of ion-selective electrodes relevant to applications in flow-through systems are discussed. The predominant role of the dynamic response characteristics of the sensor, especially in flow-injection analysis, is emphasized. Indirectly, these characteristics can affect the linear response range, the detection limit and the selectivity properties of the sensors. As examples, flow-injection methods are described for the determination of fluoride in rain-water samples and of potassium ion activity in blood sera.     

Amperometric determination of hydrogen and hydroxyl ion concentrations in unbuffered solutions in the pH range 5–9

Hydrogen and hydroxyl ion concentrations can be determined by ion-selective amperometry in the pH range 5–9 in unbuffered solutions. The working electrode is a plasticized PVC membrane electrode loaded with the neutral hydrogen ion carrier tridodecylamine. Measurements are made in a flow-injection system.     

Flow-injection system with ion-exchange separation and potentiometric detection for the determination of three halides

The use of ion exchangers in flow-injection systems is reviewed briefly. In the method described, halides are separated on a short column of a strongly basic ion-exchange resin (Dowex 1-X8) placed in the flow-injection conduit, with a laboratory-made tubular silver/silver halide ion-selective electrode as potentiometric sensor. The response capabilities of the different halide-selective electrodes to a wide concentration range (20-5000 mg dm^?3) of single and mixed halide solutions with and without the incorporated ion-exchange column are compared. By careful selection of suitable concentrations of the potassium nitrate carrier/eluent stream to satisfy the requirements of both the ion-exchange column and the halide-selective electrode, it is possible to separate and determine chloride, bromide and iodide in mixed halide solutions with a detection limit of 5 mg dm^?3. The bromide-selective electrode is the most satisfactory detector.     

Response Amplification of an Ion-Selective Electrode

Abstract

The response of an ion-selective electrode can be amplified by connecting the cell, which is composed of an ion-selective electrode and a reference electrode, in series. For a cell using 1, 2 or 3 valinomycin electrodes connected in series, the response slopes to 1 × 10^?5 ?1 × 10^?1 M K⁺ were 58, 116 and 174 mV/activity decade (a.d.) at 25°C, respectively. This amplification method would especially be useful for accurate determinations with electrodes in the range of low concentrations outside the Nernstian response or for determinations of polyvalent ions, in which both cases exhibit small emf response changes in changing the ion concentration.     

Continous monitoring of copper and cadmium in zinc plat electrolyte usinga microprocessor-based batter-operated data acquisition system,multiple ion-selective electrodes and redundancy principles

Monitoring of both copper and cadmium is essential in the electrolytic production of zinc because these elemetns must be removed in order to improve the efficiency of the zinc electro-refining process. A new high-volume flow-through cell with an assembly of four ion-selective electrodes (four copper, four cadmium or two copper and two cadmium electrodes), reference electrode and temperature probe coupled with microprocessor-based instrumentattion can be used for this purpose. Determinations can be done continuously for 72-h periods without maintenance in an on-line mode. when multiple electrode determinations and redundncy principles are implemented. A microcomputer system incorporating low-power CMOS technology with multichannel and multiplexing capabilities is used for data acquisition. The use of the battery-powered data acquisition system provides excellent signal-to-noise ratios, meets the special demands of the harsh industrial environment, and is preferable to conventional mains-powered monitoring systems based on ion-selective electodes.     

Application of amalgam electrodes in studies of heavy metals under natural water conditions

The application of amalgam electrodes for measuring the degree of complexation of metal ions is described with respect to natural water conditions. The amalgam electrodes are compared with the corresponding capabilities of ion-selective electrodes. A special cell is described for preparing the amalgam and for filling a hanging amalgam drop electrode. Factors affecting the reproducibility of the standard potentials and slopes, the response time and detection limits are discussed. Complexation measurements are described with lead and zinc amalgam electrodes. Triethylenetetramine, carbonate and nitrolotriacetic acid are used as ligands, to test the ability of these electrodes to measure correctly8 the degree of complexation even at low total-metal. concentrations (down to ca. 10^?7 M) and at very low concentrations of free metal ion (10^?15 M). Results obtained with well-characterized fulvinc compound and an algal culture medium (AAP) are also reported. The observed results are in compl;ete accordance with theoretical predictions (based on Nernstian behaviour), evven at the lowest concentrations of tltal and free metal ion used. An important limitation is that any oxidant in the solution can interfere by oxidizing the amalgma. Solutions must be carefully degassed to eliminate oxygen. It is shown that the interfering actin of oxidants can be corrected for by means of equations which are theoretically sound, even when the nature of the oxidant is unknown, provided that its content is not too high. Compared to ion-selective electrodes, amalgam electrodes are more reproducible, inexpensive and readily prepared for various metal ions which cannot be measured with ion-selective electrodes.     

The calibration of glass electrodes and cadmium and copper ion-selective electrodes

The glass electrode is calibrated on the concentration scale against the hydrogen electrode from measurements of the potentials of the two electrodes immersed in the same buffer. The p[H⁺] of the buffer is changed between 2 and 11 by addition of base. The difference in potential between the electrodes is used to correct the glass electrode potentials. Analogously, cadmium and copper ion-selective electrodes (i.s.e.) can be calibrated against the corresponding metal-amalgam electrodes in p[M²⁺] buffers. The response of the copper i.s.e. (Radiometer Selectrode) was always close to Nernstian, whereas the response of the cadmium i.s.e. was variable and larger than Nernstian. The method is independent of errors in equilibrium constants and composition of the buffers, but it assumes ideal behaviour of the hydrogen and amalgam electrodes.     

Potentiometric determination of dialysate urea nitrogen

An enzymatically modified ammonium ion-selective electrode has been applied for the determination of urea in spent dialysate. The biosensor has been used in a simple flow-injection analysis (FIA) system. The system enables one to perform over 25 dialysate urea nitrogen (DUN) determinations per hour. The interferences from other components of posthemodialysis fluid were eliminated by simultaneous measurements with non-modified enzymatically ion-selective electrode. It is possible to use both the sensors in a simplified differential potentiometric system. The results of DUN determination using the biosensor/FIA system and a conventional method of urea determination were comparable. The presented analytical system can potentially find wider biomedical application in the monitoring of hemodialysis progress.     

Screen-printed multienzyme arrays for use in amperometric batch and flow systems

Screen-printing technology for electrode fabrication enables construction of amperometric devices suitable for combination of several enzyme electrodes. To develop a biosensor array for characterisation of wastewaters, tyrosinase and horseradish peroxidase (HRP) or cholinesterase-modified electrodes were combined on the same array. The behaviour of the tyrosinase-modified electrode in the presence of hydrogen peroxide (required co-substrate for the HRP-modified electrode) and acetylthiocholine chloride (required co-substrate for cholinesterase) was studied. Performance of bi-enzyme biosensor arrays in the batch mode and in the flow-injection system are discussed.     

How Far the Ion-Selectivity Should be Improved; a Task Oriented Limit of Ion-Selective Electrode Design

Abstract

A simple formula is proposed for the definition and the calculation of the highest acceptable selectivity coefficient values of ion-selective electrodes. On the basis of the formula which is based on the consideration of the precision of the direct potentiometric measuring techniques one can easily decide whether the selectivity of an electrode is sufficient or not for solving a well defined analytical task correctly. On the other, hand it indicates clearly for ion-selective electrode developing teams or manufacturers how far the ion-selectivity of an electrode should be improved for a certain type of analytical application.

Characteristic selectivity values for a few concrete analytical sample types are given in table together with the selectivity coefficients of the corresponding, existing ion-selective electrodes.     

Evaluation of an overdetermined system based on multiple ion-selective electrodes of the same type

Possible advantages of using multiple ion-selective electrodes of the same type for the determination of analyte ions in multicomponent mixtures are investigated. Specifically, potassium and ammonium ions are determined in model multicomponent mixtures with a combination of four valinomycin and four nonactin polymeric membrane electrodes. The electrode potentials are monitored by a specially designed data-acquisition system, and conventional matrix operations are used to calculate the ion concentrations. The results indicate that greater confidence in the analyses can be obtained from an overdetermined system of this type.     

Preparation of an ion-selective electrode by chemical treatment of copper wire for the measurement of copper(II) and iodide by batch and flow-injection potentiometry

The preparation of an ion-selective electrode by chemical treatment of copper wire and its application for the measurements of copper (II) and iodide ions is described. The proposed reaction mechanism at the sensing surface, which explains the response of the electrode to Cu²⁺ and iodide ions, is discussed. The prepared electrode was suitable for direct potentiometric measurements of iodide and copper (II) in batch experiments down to concentrations of 1 × 10^–5 mol L^–1. A tubular electrode, prepared in the same way, may be used as a potentiometric sensor in a flow-injection analysis for Cu (II) and/or iodide determinations.     

Study of the potential response of solid-state chloride electrodes at low concentration ranges

Ion-selective electrodes based on silver chloride precipitates have been investigated in the low concentration range, by use of a specially designed cell of small volume. Electrode potential measurements and silver determinations in the corresponding solutions by atomic-absorption spectrometry were made. The results prove that the potential response of these ion-selective electrodes in the low concentration ranges is governed by inequality of the ion concentrations in the boundary zone of the test solution contacting the electrode membrane. This is a result of adsorption-desorption processes, a dissolution process followed by recrystallization of the silver chloride at the electrode membrane surface, and photoreduction of silver ions at the electrode surface.     

Use of a flow-injection system in the evaluation of the characteristic behavior of neutral carriers in lithium ion-selective electrodes

A flow-injection system based on microconduits is used to investigate electrode characteristics such as selectivity, detection limit, and response and equilibrium times of the new ionophore, N,N,N′,N′-tetraisobutyl-5,5-dimethyl-3,7-dioxanonane diamide, in lithium ion-selective electrodes. These characteristics were compared with those of the ionophore N,N′-diheptyl-N,N′,5,5-tetramethyl-3,7-dioxanone diamide. The new ionophore has superior detection limits and shorter response and equilibrium time, but the other exhibits better selectivity for lithium with respect to sodium. Values of K^Pot_LiNa for the new ionophore vary from 0.0450 to 0.566, depending on the methods of measurement and solution conditions. This phenomenon is discussed. Stop-flow experiments effectively demonstrated the response and equilibrium time differences between these two ionophore membranes.     

Transient signals with an Antimony(V) ion-selective electrode: The relative signal return rate as a selectivity parameter

The dynamic response of a Sb(5+) ion-selective electrode based on 1,2,4,6-tetraphenylpyridinium hexachloroantimonate(V) to different ions, ClO(-)(4), HgCl3(-)(3), AuCl(-)(4),TlCl(-)(4), SbCl(-)(6) and tetraphenylborate, in a flow-injection analysis system using a flow-through cell, is studied. The transient signals obtained are different for every ion. A parameter called relative return rate, defined as the ratio of the return rate to peak height, was found to be characteristic for each ion. It was constant up to a concentration range of three decades for those ions with selectivity coefficients K(pot)(sb,j) 1, thus providing a new source of analytical selectivity. Flow injection variables were studied in order to ascertain their influence on this new parameter.