Modeling of divalent/monovalent ion selectivity of ion-exchanger-based solvent polymeric membranes doped with coexchanger

It is shown that the addition of a coexchanger does not always improve the selectivity of an ion-selective electrode. As a model case, the selectivity toward divalent primary ions I2+ in the presence of monovalent interference J+ is numerically simulated for membranes based on ion-exchanger R- and doped with a coexchanger (ionic additive) S+. It is, for the first time, demonstrated that the addition of a coexchanger, together with a high mobility of ion-exchanger sites, may be either beneficial or harmful to selectivity. Accordingly, two distinctive representatives of systems, i.e., membrane/solution are discussed. The theory is illustrated with experimental data obtained from PVC membranes based on calcium bis(2ethylhexyl)-phosphate plasticized with tris(2ethylhexyl)phosphate and doped with tetradecylammonium. Electrodes with these membranes exhibit both types of selectivity behavior. Interference from lithium with a calcium response may be sufficiently decreased by the addition of tetradecylammonium, while the interference from potassium and cesium was increased in the doped membranes.

Use of ion-selective electrodes for blood-electrolyte analysis. Recommendations for nomenclature, definitions and conventions. International Federation of Clinical Chemistry and Laboratory Medicine (IFCC). Scientific Division Working Group on Selective Electrodes.

This paper will familiarize the reader with the terms used to describe the behavior of ion-selective electrodes, particularly in relation to their use in clinical chemistry for determination of blood electrolyte cations. It serves as an introduction to a series of papers dealing with important cations in blood, namely calcium, sodium, and potassium. The detailed relationships between the ion activity determined by means of ion-selective electrode potentiometry in undiluted specimens, and the total substance concentration measured by flame atomic-emission spectrometry are described by flow chart and equations. Adoption of a convention for reporting results is recommended. The Working Group on Selective Electrodes has taken into account recent revisions of IUPAC recommendations on nomenclature and selectivity coefficient determinations for ion-selective electrodes, and benefited from the experience of a member of the WG, who was also involved in the IUPAC discussions. Nomenclature for determined quantities follows previous IUPAC/IFCC joint recommendations.

Magnesium. An update on physiological, clinical and analytical aspects.

There is an increased interest in the role of magnesium ions in clinical medicine, nutrition and physiology. The characteristics of the binding of magnesium and calcium ions to various components, macromolecules and biological membranes are described. Magnesium affects many cellular functions, including transport of potassium and calcium ions, and modulates signal transduction, energy metabolism and cell proliferation. The mechanism of cellular uptake and efflux of magnesium, its intracellular transport, intestinal absorption, renal excretion and the effect of hormones on these are reviewed. Magnesium deficiency is not uncommon among the general population: its intake has decreased over the years especially in the western world. The magnesium supplementation or intravenous infusion may be beneficial in various diseased states. Of special interest is the magnesium status in alcoholism, eclampsia, hypertension, atherosclerosis, cardiac diseases, diabetes, and asthma. The development of instrumentation for the assay of ionized magnesium is reviewed, as are the analytical procedures for total magnesium in blood and free magnesium in the cytosol. The improved procedures for the assay of different magnesium states are useful in understanding the role of magnesium in health and disease.

IFCC recommended reference method for the determination of the substance concentration of ionized calcium in undiluted serum, plasma or whole blood.

A reference method is described for the determination of the substance concentration of ionized calcium in plasma by which ionized calcium (free or unbound) may be reliably determined on the basis of calibration with aqueous solutions with known concentration of ionized calcium. The composition of the calibration solutions is chosen such that the activity coefficient of the calcium ion is assumed to be identical both in the calibration solutions and in "normal" plasma, i.e. by convention, the ionic strength (Im) is 0.160 mol/kg. The convention is adopted of reporting ionized calcium measurements as concentration expressed as mmol/l. The proposed reference method for ionized calcium measurement in plasma is based on the use of a cell consisting of an external reference electrode with a saturated potassium chloride liquid/liquid junction in combination with a calcium ion-selective membrane electrode of defined construction and performance. Procedures for using the reference cell and a protocol for sample measurement are described. The preparation of the calibration solutions to be used are described in detail in Appendix A, secondary calibration solutions and check standards in Appendix B, and reference cell vessel design in Appendix C.

Recommendations for measurement of and conventions for reporting sodium and potassium by ion-selective electrodes in undiluted serum, plasma or whole blood. International Federation of Clinical Chemistry and Laboratory Medicine (IFCC). IFCC Scientific Division Working Group on Selective Electrodes.

Ion-selective electrodes (ISEs) respond to ion-activity and therefore do not sense substance concentration directly. However, it is recognized that sodium and potassium in plasma will continue to be expressed for clinical purposes in terms of substance concentration (mmol/l). A convention is proposed whereby for routine clinical purposes results of ISE measurements of sodium and potassium in undiluted plasma should be reported in terms of substance concentration (mmol/l). In specimens with normal concentrations of plasma water, total CO2, lipids, protein and pH, the values will concur with the total substance concentration as determined for example by flame atomic emission spectrometry (FAES) or ISE measurements on diluted samples. In specimens with abnormal concentrations of plasma water, the results will differ. However, under these circumstances, measurements of sodium and potassium by ISE in the undiluted sample will more appropriately reflect the activity of sodium and potassium and are therefore clinically more relevant than the determination in diluted samples. Detailed recommendations are made about practical procedures to achieve this. The recommended name for this quantity is the substance concentration of ionized sodium or ionized potassium in plasma, as opposed to total sodium or total potassium determined by, e.g. FAES, or ISE measurements on diluted samples.

Interferences in a polypyrrole-based amperometric ammonia sensor.

The main focus of this article is on the interferences encountered when an amperometric ammonia sensor is used in matrices that are typical in clinical samples. The sensor is tested in presence of such interferences and ways to suppress them by sensor modification are presented. Two modification procedures are evaluated by comparing the stabilities, detection limits and signal-to-noise ratios of such sensors. Further, the issue of the ammonia-sensing mechanism of the polypyrrole electrode is addressed. Evidence is shown that a mobile counterion may be required for proper sensor operation. Such evidence supports the idea that polypyrrole undergoes a reversible redox reaction when ammonia is detected at submillimolar concentrations.

Polypyrrole-calcion film as a membrane and solid-contact in an indicator electrode for potentiometric titrations.

This paper shows the application of conducting polymers (CPs) for constructing potentiometric indicator electrodes. Two types of polypyrrole (PPy)-based calcium sensors are presented, one sensor with PPy-calcion film as the active part and the other sensor with PPy-calcion as a solid-state contact coated with a conventional membrane selective towards calcium ions. It is shown that the PPy-calcion film, due to the complexing properties of calcion ensuring high loading of the film with calcium, is sufficiently selective to be used as the active part or as a mediating layer of the indicator electrode. The electrode, with PPy-calcion film as the active part, was used as the indicator electrode in potentiometric titrations of calcium in mixed solvents, where conventional PVC-based electrode can not be used. For the first time, the practical applicability of PPy-based electrodes in titrations is demonstrated.

Carbonate ion selective electrodes with trifluoroacetophenone derivatives in potentiometric clinical analyser.

Properties of six derivatives of 1-trifluoroacetylbenzene: [4-(n-butyl)- (1), 4-(n-hexadecyl)- (2), 4-dodecyloxy- (3), 4-(n-dodecylsulfonyl)- (4), N,N-dioctyl-4-trifluoroacetylbenzamide (5), octyl-p-trifluoroacetylbenzoate (6)] as neutral carriers for carbonate ion were examined and compared. The sensitivity towards carbonate ion was for (3) pH dependent. This eliminates (3) from practical applications in clinical analysis. When measuring CO(3)(2-) within the physiological range of human blood using as carriers compounds 1 and 2 the interference of chloride must be taken into account. In the case of carriers 4, 5, 6 this effect is negligible. Electrodes with membranes containing as carriers 2, 4, 5 and 6 were tested in an automatic potentiometric clinical analyser Microlyte 6, KONE. To avoid contamination by atmospheric CO(2) of three aqueous standards (TES, NaCl, NaHCO(3)), pH was adjusted by coulomeric generation of H(+) or OH(-) in a system devoid of carbon dioxide. Recovery of HCO(3)(-) calculated from measured CO(3)(2-) and pH, was investigated in a series of aqueous solutions and spiked bovine serum samples. The correlation between added and recovered concentration of HCO(3)(-) was linear with the intercept close to 0 and slope equal to 1 in aqueous solutions for all ligands and in bovine serum samples only in the case of ligand (2).

Modeling potentiometric sensitivity of conducting polymers.

The influence of different polymerization conditions and electrochemical processes provoked by soaking on the potentiometric sensitivity of conducting polymer films is discussed. Poly(pyrrole) doped with hexacyanoferrate(II) is selected as a model polymer. It is shown that, depending on the conditions applied during film deposition and soaking, either anionic or cationic potentiometric responses can be observed and related to the composition of the film. The potentiometric sensitivity of the conducting polymer films is analyzed and interpreted by means of generalized theoretical schema.

Electric potential measured, concentration reported: how to get mmols from mV.

The interdependence between the electric potential of a galvanic cell and concentration of electrolytes will be analyzed in order to show how a clinically relevant report can be obtained. The use of SI units is emphasized.

A polypyrrole-based amperometric ammonia sensor.

An ammonia sensor is described in this work. The sensing membrane is a thin layer of oxidized polypyrrole (PPy) on a platinum substrate. This sensor is used as the working electrode in a conventional three-electrode system for amperometric measurement of ammonia in aqueous solutions in the potential range of + 0.2 to + 0.4 V (vs. Ag/AgCl). Contact with ammonia causes a current to flow through the electrode. This current is proportional to the concentration of free ammonia in the solution and ammonium ions do not contribute to the measured signal. The signal is due to reduction of PPy by ammonia with subsequent oxidation of PPy by the external voltage source. The sensor is able to detect ammonia reproducibly at the muM level. The main interference is the doping effect of small anions such as Cl(-) and NO(3)(-), also giving a response on PPy at the mM level. This anionic response can, to a certain degree, be reduced by covering the polymer surface with dodecyl sulfate. The sensor gradually loses its activity when exposed to ammonia concentrations greater than 1 mM. The sensor has been tested by the flow injection analysis technique.

Enzymatic flow-injection determination of urea in blood serum using potentiometric gas sensor with internal nonactin based ISE.

Ion-selective electrode with cellulose triacetate membrane containing nonactin is employed for the potentiometric detection of ammonia produced in biocatalytic reaction in flow-injection system with enzyme reactor. The elimination of interferences occurring in the presence of alkali metal ions was achieved by covering a nonactin membrane with outer hydrophobic gas permeable membrane. The obtained flow-injection response to ammonia indicates a possibility of ammonia determination down to 10 microM ammonia. In the flow-injection system for urea determination 200 microl of 10-fold diluted blood serum sample was injected into carrier stream of distilled water merged with TRIS buffer, passed through the urease flow-through reactor and then after merging with NaOH stream delivered to the detector. It was found in several series of natural blood serum samples, that the correction for endogenous ammonia in such a determination is not indispensable.

Bifunctionality of chemical sensors based on the conducting polymer polypyrrole.

Responses of polypyrrole based ion-selective chloride electrode were investigated in chloride and redox media. Bifunctional character of the potentiometric response of the polypyrrole films doped with chloride ions was observed being sensitive both to chloride ions and to the redox potential of the solution, however the redox response seems to predominate.

Design and pitfalls of ion selective electrodes.

Design of ion-selective electrodes (ISEs) currently used in clinical analysis is presented. The theory of these sensors is outlined. Problems specific for the application of ISEs in clinical analysis are discussed.

Characterization, standardization and experiences with KONE ISE for Mg2+.

The principles of a fully automated potentiometric determination of ionized magnesium in human blood serum are presented. By incorporating a magnesium selective electrode, into a commercially available six channel electrolyte analyzer (Microlyte 6, KONE Instruments, Finland), determination of the biologically-active, ionized fraction of magnesium has been made possible. In this paper, the analytical performance of the measurement system is presented and determination of ionized magnesium in real patient samples is discussed with special attention to the significance of this new parameter.

Potentiometric response of poly(3-octylthiophene), poly(3-methylthiophene) and polythiophene in aqueous solutions.

The potentiometric response of some polythiophenes in aqueous solutions has been investigated. Polythiophene (PT), poly(2,2'-bithiophene) (PBT), poly(3-methylthiophene) (PMT), poly(3-octylthiophene) (POT) and poly(4,4'-dioctyl2,2'-bithiophene) (POTd) were electrochemically deposited on platinum in 0.1M LiBF(4)-propylene carbonate solution containing the corresponding monomer or dimer. Polymer electrodes were also prepared by solution casting of chemically synthesized poly(3-octylthiophene) (POTc) dissolved in chloroform. After film deposition (electrochemical or chemical) the polymer coated electrodes were used as indicator electrodes in potentiometric measurements. The open-circuit potential of the polymer electrodes was measured in aqueous solutions containing inorganic salts (10(-1)-10(-4)M). Interestingly, all the polythiophenes studied were found to give a cationic response to monovalent cations such as H(+), Li(+), Na(+), K(+) and NH(+)(4) (Cl(-) salts). The slope, calculated from the linear part of the response curve, was found to depend on the polythiophene used but always remained lower than that predicted for a Nernstian response. The polythiophenes also showed some sensitivity to divalent cations such as Mg(2+) and Ca(2+) (Cl(-)-salts). POT was used as the polymer to study the influence of the polymerization conditions on the potentiometric response. By investigating different polymers from the polythiophene family it was possible to evaluate how the starting material (monomer or dimer) and the presence of alkyl side-chains influence the potentiometric response of the polymer membranes.

Anionic responses of electrochemically synthesized polypyrrole films.

Conducting polypyrrole films with thickness of about 8 mum were prepared on platinum by continuous scanning in 0.1M pyrrole and 0.1M electrolyte (LiCl, LiBF(4) or NaBF(4)). Scan-rates of 10, 20 and 50 mV/sec with scan-times of 30, 45 and 60 min were used. The potentiometric response of the films was tested over a range of anions. Films were found to be anion-sensitive but very non-selective. The influence of the doping cation was also investigated.

Single-point titration of metal ions and ligands by measuring change in pH.

A single-point titration method is described for the determination of metal ions and ligands. The method is based on deprotonation of a ligand on complexation with a metal. The resultant change in pH is used to calculate the concentration of the analyte. The limits and advantages of the method are discussed. Four selected systems, Pb(2+)-HPO(2-)(4), Cu(2+)-HCO(-)(3), Cd(2+)-EDTA and Ni(2+)-ethylenediamine are used to demonstrate the validity of the equations. The method is applied to determination of total carbonate in serum.

Anionic interferences with copper ion-selective electrodes chloride and bromide interferences.

The effect ofhalide ions on copper ion-selective electrodes is connected with complexation and redox reactions, with the formation of amorphous sulphur, which by blocking the surface causes instability of potential response. It may be eliminated by addition of sodium thiosulphate solution. The electrode behaviour has been explained on the basis of the diffusion model. An equation is proposed for linearization of the calibration curve. The parameters of the semiempirical model which describes the electrode behaviour agree well with the physical meaning presented by the diffusion model. The treatment given enables analytical measurements of copper concentration to be made even in the presence of significant concentrations of chloride or bromide.

Variability of selectivity coefficients of solid-state ion-selective electrodes.

The generalized model for the selectivity mechanism of solid-state ion-selective electrodes has been experimentally verified. The experimental parameters investigated were the concentration of interfering ion, temperature and stirring. Among the systems studied were electrodes sensitive to chloride (bromide, iodide), bromide (chloride, iodide), iodide (chloride, bromide), silver (copper, lead), copper (silver, lead) and lead (silver, copper), the species given in brackets being considered as the interferents. The model has been confirmed except for cases where the concentration of ions formed at the electrode surface by metathesis is too small to be the factor that dictates the electrode potential.