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Analytical and Bioanalytical Chemistry (v.356, #3-4)
Advantages and limitations of combining separation techniques with voltammetry by A. Möller; F. Scholz (pp. 160-168).
In this review the combination of voltammetric methods of analysis with separation techniques, such as electrochemical separations, extractive separations, chromatography, electrochromatography, electrophoresis, volatilization and coprecipitation is critically discussed. Based on available publications and their own experience, the authors try to evaluate different separation techniques in view of future developments, paying special attention to extraction methods. For organic analysis, chromatography and electrophoresis with voltammetric detection are considered to be especially attractive while for stripping analysis, major progress is expected to be made in the development of electrodes and electronic equipment. Extraction, coprecipitation and volatilization will be of minor significance.
Capillary electrophoresis and capillary flow injection analysis with electrochemical detection by F.-M. Matysik; Ulli Backofen (pp. 169-172).
Measurements by capillary flow injection analysis (CFIA) and capillary electrophoresis (CE) in conjunction with electrochemical detection are described. The detection is based on an end-column electrode arrangement. Several novel electrodes, such as a spherical gold electrode and a dual-microdisk electrode, are presented and characterized regarding their analytical utility. In order to improve the selectivity of CFIA, dual-electrode and multiple-pulse detection are studied using couples of cyanometallates or metallocenes. Capillary electrophoretic experiments with amperometric detection are performed using 50 μm i.d. capillaries without any electrical-field decoupler. The practicality and analytical characteristics of this detection strategy are illustrated for the separation of serotonin and some biological precursors and metabolites of neurotransmitter substances.
Simulation of titration curves indicated with two indicator electrodes (biamperometry) by P. Surmann; B. Peter; C. Stark (pp. 173-177).
A rigorous method to simulate titration curves with indication using two indicator electrodes (biamperometry) is presented. Computer simulations can be carried out for reversible as well as for irreversible systems. The different parameters like the area of the individual electrodes, applied potential difference, heterogeneous rate constant, and the kinetic parameter α were varied and investigated as to their influence upon the shape of the titration curves. The theoretically derived effects match with the effects obtained by experiment. Considering the effects described here, it is possible to tailor the shape of the titration curve by the experimental conditions for specific applications in order to get an optimum shape at the end point of the titration.
Improved traceability of pH measurements by P. Spitzer; R. Eberhardt; I. Schmidt; U. Sudmeier (pp. 178-181).
Traceability is a prerequisite for the comparability and uniformity of measurements. Although pH-measurements are carried out on a large scale in laboratory and industry, the problems involved in the traceability of pH values have not adequately been solved in the past. The comparability of pH measurements is limited, among other parameters, by the accuracy of the pH values of the standard buffer solutions used to calibrate the pH meter-electrode assemblies. The measured pH(X) value must be traceable to primary standard pH(PS) values through an unbroken chain of comparisons, all values having stated uncertainties. A new primary standard measurement device for pH is used to certify primary pH reference materials from which these secondary reference materials can be derived.
Partially selective semiconductor redox electrodes by W. Habermann; P. John; H. Matschiner; H. Spähn (pp. 182-186).
Redox electrodes made of passivated metals of the subgroups IV to VI of the periodic system with a surface coating of n-conducting metal oxides, generated by oxidizing agents or anodic oxidation, are well suited for measuring the concentration of oxidizing agents. Neither oxygen nor reducing agents affect the measurements. There is a correlation between the mean conductivity of the passive layer and its selectivity. The aged n-conducting passive layers permit reproducible potential settings, and the occurring changes in potential are greater than what is to be expected according to the Nernst equation. The potentiometric measuring of nitrosyl ions and chlorine is used as an example to demonstrate the selectivity of these electrodes.
Microgalvanics: locally resolved concentration measurements with ion-selective microelectrodes and chronoamperometric microelectrodes by M. Küpper; J. W. Schultze (pp. 187-191).
Localized electrochemical analysis in the micrometer-range is useful for the in situ investigation of galvanic plating processes. Two techniques are presented, a potentiometric and a chronoamperometric one, using xyz-positionable microelectrodes for practical concentration measurements in microstructures during cathodic metal deposition. Locally measured concentration data c xy are related to local current density i xy , which is not obtainable by other means in a system containing excess supporting electrolyte.
Bivoltametric titrations using electrodes with innovative geometry by Peter Surmann; Bernhard Peter; Christine Stark (pp. 192-196).
Electrodes with different surface areas were investigated for the determination of reversible, quasireversible, irreversible or electroinactive substrates. Two kinds of electrodes were constructed, a helical electrode with a given asymmetry and a platinum array electrode with a variable area. These electrodes were applied for the cerimetry of ammonium iron(II) sulfate and for the bromatometry of various organic substances. The theoretically derived effects on the shape of the voltametric titration curve are verified experimentally. It is possible to sharpen one side of the peak and to broaden the other side, depending on the system and the side of the peak one is interested in. It is possible to improve the bivoltametric determination of hydroquinone, benzocaine and sulfaguanidine by bromatometry by the directed employment of electrodes of different areas. For the bromatometric determination of electrochemically irreversible substrates the use of the electrode geometries proposed is a way to obtain a sharp bend and a steep decrease of titration curves with low values of the constant current which is a basic requirement for the accuracy.
A solid composite electrode for the determination of the electrochemical oxygen demand of aqueous samples by R. Rössler-Fromme; F. Scholz (pp. 197-201).
A composite electrode containing graphite, paraffin, AgO and CuO is described for the determination of the electrochemical oxygen demand (EOD) of waste waters. The oxidation of dissolved organic compounds at the electrode is based on a heterogeneous chemical reaction of AgO/CuO with the organic constituents of the waste water. This chemical reaction can be followed by a solid state electrochemical oxidation of the formed Ag2O/Cu2O. The method has been tested with various organic compounds and applied to real samples of sewage water. The EOD values correlate well to BOD and COD values.
The nickel reactrode, a metal specific voltammetric electrode by Modupe A. Jimoh; F. Scholz (pp. 202-203).
The nickel reactive electrode (reactrode) is a solid bulk-modified composite electrode for the selective voltammetric determination of nickel in aqueous solutions. This reactrode offers the possibility for nickel determination in the range from 0.7 ng/ml to 1 μg/ml. The 3σ detection limit is 0.7 ng/ml.
A Prussian blue-based reactive electrode (reactrode) for the determination of thallium ions by Heike Kahlert; Š. Komorsky-Lovrić; Michael Hermes; F. Scholz (pp. 204-208).
A reactive electrode (reactrode) made of Prussian blue (PB), graphite and paraffin can be used for a selective determination of thallium ions down to a concentration of 2 · 10–8 mol 1–1. The working principle of the reactrode is that thallium ions can be pumped into Prussian blue during alternating oxidation-reduction cycles. After a preconcentration of thallium ions in PB, the voltammetric determination follows as usually in anodic stripping voltammetry, i.e. the thallium ions are reduced to thallium metal which is subsequently oxidized to give the anodic stripping signal. The peculiarity of the Prussian blue-thallium system is that the thallium ions are situated in the holes of the PB matrix. When reduced to metallic thallium, they are substituted by potassium ions. Cd2+, Fe3+, Zn2+, Cu2+ and Ni2+ do not interfere up to a hundredfold excess, NH4 + does not interfere up to a thousandfold – and Bi3+ up to tenfold excess. The interference by Pb2+ can be suppressed with EDTA.
High-temperature oxygen sensors for glass-forming melts by Friedrich G. K. Baucke (pp. 209-214).
Electrochemical sensors are reported for the on-line measurement of oxygen partial pressures of oxidic glass-forming melts on a laboratory and technical scale. Based on the principle of solid electrolyte cells without transference, they are principally simple units. The extreme chemical and temperature conditions of their applications, however, demanded extensive fundamental investigations and resulted in specific forms of reference and measuring electrodes, a thermo-dynamic procedure of verifying the correct functioning of such cells, and a method of measuring thermoelectric voltages of non-isothermal glass melts.
Gas-potentiometric method with solid electrolyte oxygen sensors for the investigation of combustion by H. Lorenz; K. Tittmann; L. Sitzki; S. Trippler; H. Rau (pp. 215-220).
Gas-potentiometric analysis using oxide-ion-conducting solid electrolytes as stabilized zirconia is a worthwhile method for the investigation of combustion processes. In the case of gas and oil flames specific parameters like the flame contour, the degree of burn-out and mixing can be determined and information about flame turbulence and reaction density can be gained from the temporal resolution of the sensor signal. Measurements carried out with solid electrolyte oxygen sensors in a fluidized bed show that combustion processes of solid fuels are also analyzable. This analysis results in fuel specific burn-out curves finally leading to burn-out times and to parameters of a macrokinetics of the combustion process as well as to ideas about the burn-out mechanism. From the resulting constants of the effective reaction rate a reactivity relative to bituminous coal coke can be given for any solid fuel.
Solid state systems for the potentiometric determination of CO2 by H.-H. Möbius; P. Shuk; W. Zastrow (pp. 221-227).
In a survey of solid state systems for the potentiometric determination of CO2 their different fundamentals, possibilities and problems are demonstrated. Experimental results are given, obtained from sensors in which Na2CO3 reacts electrochemically with an oxide forming the sodium salt thermodynamically stable in contact with it. Sensors with Na2YZr(PO4) as solid electrolyte and MoO3 or SiO2 did not yield long-term stable signals, probably mainly because of chemical reactions between the oxides and the solid electrolyte. Satisfactory results were obtained with galvanic cells represented by the symbol. Their standard cell tensions U* (= –emf*) determined with different test gases are in relatively good agreement with values thermodynamically calculated.
On the analysis of CO2, H2- and CO, H2-mixtures by water-gas potentiometry with solid electrolyte cells by Reinhold Hartung (pp. 228-232).
The potentiometric analysis of CO2, H2 and CO, H2O-mixtures using oxide ion-conducting solid electrolytes requires the adjustment of the water-gas equilibrium without side reactions in the high-temperature galvanic cell. Conventional cell designs suitable for the analysis of H2, H2O and CO, CO2-mixtures are not applicable due to the insufficient gas residence times in the cells and the insufficient catalytical activity of the platinum electrodes. Solid electrolyte cells have to be modified by integrating of suitable catalysts. Under optimized conditions of gas velocity and cell temperature both gas systems can be analyzed only by measuring the cell tension U (=−emf) and temperature in the favorized temperature range around 813 °C. Here systematical errors of the component ratio or the mole fraction were smaller than 6%. Several fundamental requirements for the application of catalysts in solid electrolyte cells for the analysis of reactive water-gas mixtures are pointed out.
Amperometric detection of heavy metal ions in ion pair chromatography at an array of water/nitrobenzene micro interfaces by S. Wilke; H. Wang; M. Muraczewska; Helmut Müller (pp. 233-236).
A novel amperometric detector for heavy metal ions has been developed and successfully applied for ion pair chromatography. The detector is based on the electrochemical transfer of the metal ions across an array of water/nitrobenzene micro interfaces. The ion transfer is facilitated by the neutral ionophores methylenebis(diphenylphosphineoxide) and methylenebis(di- phenylphosphinesulfide). More than eight metals are separated in less than 15 min on an RP18 column using octyl sulfonate as ion pair reagent. For the heavy metals, the limits of decision are 19(Pb2+), 9(Zn2+), 9l (Co2+), 8(Cd2+) and 1.6(Mn2+) μg/L. The applicability of the new method for water samples is demonstrated.
A reactive electrode (reactrode) for the voltammetric determination of heavy metals in laboratories and for use as a passive monitor in remote analysis by I. Helms; F. Scholz (pp. 237-241).
The use of a reactive electrode (reactrode) consisting of graphite, a solid ion exchanger (HYPHAN) and paraffin for the batch analysis of Cd2+, Cu2+, Pb2+ and Hg2+ in aqueous samples and as a passive monitor for these metal ions is described. The metal ions are accumulated on the reactrode surface at an open-circuit potential in an ion-exchange reaction. After the accumulation, the ion exchanger-bonded metal ions are reduced to the metals which remain on the electrode surface. In a following step, the metals are anodically dissolved which is recorded by differential-pulse voltammetry. The 3s detection limits for the analysis of drinking water are: 1.1×10-7 mol/l for Pb2+, 5×10-8 mol/l for Hg2+ and 2.4×10-7 mol/l for Cu2+.The reactrode developed can be used for the passive monitoring of heavy metals in aqueous streams if the reactrode is mounted in a wall-jet cell which is part of a flow-through system. Using this arrangement, it has been possible to determine Hg2+, Cu2+ and Pb2+ in drinking water after 20 hours of accumulation.
Selective detection of metal species in HPLC and FIA by means of pulsed amperometric detection (PAD) by Günther Weber (pp. 242-246).
Triple-step pulsed amperometry is used to detect selectively metal complex species after HPLC separation in the presence of coeluting matrix compounds. Optimized pulse parameters enable accurate and selective detection of species even in the presence of electroactive compounds such as phenols or sulfurcontaining peptides. Complexes of tin(IV) with rutin and of platinum(II) with glutathione are presented as examples for these classes of compounds. It is demonstrated that by changing the pulse waveform direct and indirect detection modes can be realized. The method is applied to detect platinum species in a grass extract and iron(II)-lactate in fruit juice. Furthermore it is shown, that pulsed amperometry can be used as a detector in FIA for determining the fraction of metal complex formed after mixing of metal and excess ligand without separation.
Determination of inorganic ionic mercury down to 5×10−14 mol l−1 by differential-pulse anodic stripping voltammetry by S. Meyer; F. Scholz; R. Trittler (pp. 247-252).
A new method is described for the reliable and ultrasensitive determination of inorganic ionic mercury, using differential-pulse anodic stripping voltammetry on a glassy carbon electrode. It has been possible to determine mercury down to a concentration of 5×10−14 mol l-1 (the lowest detection limit ever reported for a voltammetric method). This success was achieved by using a thiocyanate electrolyte and relatively long deposition times. The mercury ions are stabilized in the solution by the formation of strong thiocyanate complexes. This leads to a highly reproducible cathodic plating and anodic dissolution of mercury. A speciation analysis allowing to distinguish between dissolved atomic and ionic mercury in water is possible.
Calibrationless determination of mercury by flow-through stripping coulometry by E. Beinrohr; M. Čakrt; J. Dzurov; P. Kottaš; E. Kozáková (pp. 253-258).
Trace concentrations of Hg were determined in a flow-system by constant current stripping chronopotentiometry in coulometric mode. Mercury was electrodeposited from the flowing sample solution in an electrochemical flow-through cell on a large surface porous electrode plated with a thin layer of gold. The deposited mercury was then stripped with constant current and the potential change of the working electrode was recorded and evaluated. Since complete electrochemical yields were achieved at both the deposition and dissolution steps, the mercury concentration in the sample solution could be calculated from Faraday’s law. The detection limit and reproducibility of the method were about 0.1 ng/ml for 10 ml sample solution and 4%, respectively. The time for a complete analysis was 2 to 5 min. The utility of the method was demonstrated with the analysis of reference materials, water samples, waste materials, plants and charcoal catalysts.
Adsorptive stripping voltammetric determination of vanadium as chloranilic acid complex by Sylvia Sander; Günter Henze (pp. 259-262).
Traces of vanadium(V) can be determined up to 21 ng/l by adsorptive stripping voltammetry using 2,5-dichloro-1,4-dihydroxy-3,6-benzoquinone (chloranilic acid) as complex forming reagent. The complex is stable in a 0.1 mol/l acetate buffer (pH 4.6) solution and adsorbed at the electrode surface in a potential range between −150 mV and −400 mV. The adsorption process is solved by alternating current (ac) voltammetric measurements. The method is applied to different real samples.
Preconcentration and voltammetric behaviour of Ag+ at carbon paste electrodes modified by N-benzoyl-N′,N′-di-i-butyl-thiourea by M. Guttmann; K.-H. Lubert; L. Beyer (pp. 263-266).
The preconcentration of silver at carbon paste electrodes (CPE) modified with N-benzoyl-N′,N′-di-i-butyl-thiourea and its subsequent voltammetric determination is studied with test solutions. The preconcentration of silver at the surface of the modified CPE succeeds as well with open circuits as with applied potentials. The amount preconcentrated depends on the electrode potential with a maximum at +0.1 V vs. Ag/AgCl, thus enabling the preconcentration of silver without cathodic treatment. The detection limit is in the order of 10–4 mmol/l Ag+. The surface reactions during the preconcentration are discussed. It is assumed that silver complexes of the modifier are formed in a fast reaction followed by a slow reaction with participation of silver and of carbon surface groups.
The quantitative analysis of mixed crystals CuS x Se1-x with abrasive stripping voltammetry and a redetermination of the solubility product of CuSe and the standard potential of the Cu/CuSe electrode by B. Meyer; S. Zhang; Fritz Scholz (pp. 267-270).
Phase identifications and quantitative analyses of mixed crystals by means of abrasive stripping voltammetry are reported using covelliteklockmannite (CuSxSe1-x) and emplectite-chalcostibite (CuBix Sb1-xS2) mixed crystals as examples. The composition of CuSxSe1-x can be determined within a span of x=±0.03. Additionally the standard potential of the Cu/CuSe-electrode is measured to be E⋅ Cu/CuSe= −0.749 V vs. Ag/AgCl.
Determination of cyanide in microsamples by means of capillary flow injection analysis with amperometric detection by Ulli Backofen; Frank-Michael Matysik; G. Werner (pp. 271-273).
A new approach for determining cyanide in microsamples is described. The method is based on capillary flow injection analysis (CFIA) with amperometric detection. The sensing electrode is a silver-plated microdisk electrode, where cyanide can react under formation of a dicyanoargentate complex. A remarkably low mass detection limit of 231 fmol cyanide is obtained for an injection volume of 60 nl. The sample throughput of the CFIA-arrangement is comparable with a conventional sized FIA-system. A practical application is given by analyzing the cyanide (amygdalin) concentration in apple kernels.
Amperometric carbohydrate detection in flow systems by means of a bismuth modified platinum electrode by A. Strübing; G. Wittstock; R. Szargan; G. Werner (pp. 274-278).
Bismuth modified platinum electrodes are used for constant-potential amperometric determination of carbohydrates in flow systems. The monitored response is stable and reproducible over more than two days. An attempt is made to gain more detailed information about the characteristics of the modified layer by electrochemical methods and X-ray photoelectron spectroscopy. The response proved to be linear over the investigated concentration range (1.1–1200 μmol/L) and detection limits for glucose and fructose were found to be 1.1 μmol/L.
Voltammetric determination of Ioxynil and 2-methyl-3-nitroaniline using C18 modified carbon paste electrodes by Christine Faller; Axel Meyer; Günter Henze (pp. 279-283).
The stripping voltammetric behaviour of Ioxynil and 2-methyl-3-nitroaniline has been studied by means of a C18 modified carbon paste electrode. The analytes are preconcentrated under open-circuit conditions (Ioxynil at pH 3.9; 2-methyl-3-nitroaniline at pH 10). For the determination of Ioxynil and 2-methyl-3-nitroaniline 0.01 mol/l HCl and 1 mol/l KOH, respectively, have been used as supporting electrolyte. Under optimized conditions detection limits up to 0.1 μg/ml Ioxynil and 0.3 μg/ml 2-methyl-3-nitroaniline have been obtained. The methods have been applied to the determination of Ioxynil and 2-methyl-3-nitroaniline in drinking water.
Determination of adenine, caffeine, theophylline and theobromine by HPLC with amperometric detection by Axel Meyer; T. Ngiruwonsanga; Günter Henze (pp. 284-287).
A relatively simple method for quantifying caffeine, theobromine, theophylline and adenine by HPLC with amperometric detection was developed. A C18-column and an isocratic elution with phosphate buffer pH 3.5/methanol (90 : 10) were employed for the chromatographic separation of the investigated compounds. The optimal detection potential was +1.4 V. The limits of detection were 0.4 ng for adenine, 1 ng for theophylline and 2.5 ng for caffeine and theobromine. The method was applied to the determination of these purine alkaloids in beverages, tea, coffee and cacao. The determination was carried out directly or after solid-phase extraction.
Surface analysis by scanning electrochemical microscopy: resolution studies and applications to polymer samples by K. Borgwarth; C. Ricken; D. G. Ebling; J. Heinze (pp. 288-294).
Resolution studies of scanning electrochemical microscopy (SECM) have been performed in the feedback mode and in the generator/collector mode at circular model structures. A quantitative correlation of the loss in resolution and the increase in distance between tip and sample is found. Measuring a band electrode of just 500 nm width, the high sensitivity of the SECM in identifying chemically active sites is proven. Applied to polymer samples, the chemical composition was determined in the feedback mode at high lateral resolution. The difference in electrical conductivity allows one to distinguish between doped and undoped parts of a polyaniline film. By scanning above a blend consisting of polypyrrole and polypropylene, a map of the local chemical composition was obtained. In this context, the influence of the tip overpotential on the image is discussed.
A cell for in situ incident-light microscopy for the study of electrochromism of solid state electrochemical reactions by U. Schröder; B. Meyer; F. Scholz (pp. 295-298).
A cell is described which allows in situ incident-light microscopy to be used for the analysis of solid state electrochemical reactions studied by abrasive stripping voltammetry. The cell provides the possibility to screen solid compounds with respect to their electrochromic properties without requiring the preparation of special electrodes on transparent and conducting materials. Silver octacyanomolybdate(IV) and silver octacyanotungstate(IV) have been used to study the performance of the cell because both compounds exhibit a stable electrochromic and voltammetric behaviour.
Polarographic and voltammetric investigations of new catalytic systems of iron for application in trace analysis by Jerzy Zare¸bski (pp. 299-302).
New catalytic systems consisting of complexes of Fe(III)-ions with ligands such as nitrilotriacetic acid, N-(2-hydroxyethyl)iminodiacetic acid, N,N-bis(2-hydroxyethyl)glycine, triisopropanolamine, 2-hydroxy-1,3-diaminopropanetetraacetic acid, tris-(hydroxymethyl)aminomethane, bis-(2-hydroxymethyl) iminotris-(hydroxymethyl)methane, 1,3-bis/tris(hydroxymethyl)methylamino/propane in ammonia buffer and 1,2-dihydroxybenzene in tris-(hydroxymethyl) amino-methane-HCl buffer and ClO- 2-ions as the oxidizing agent were studied by pulse polarography and voltammetry. Optimal conditions such as kind and concentration of the ligand, the composition of supporting electrolyte and concentration of NaClO2 for the determination of traces of iron in the presence of a large excess of other electrochemically active ions were examined. Examples of acceleration and inhibition of catalytic reactions between Fe(II) ⋅ L and ClO- 2-ions by electrochemically inactive ions have been found.
Computer controlled titration with piston burette or peristaltic pump – a comparison by Wolfgang Hoffmann (pp. 303-305).
The advantages and problems of the use of piston burettes and peristaltic pumps for dosage of titrant solutions in automatic titrations are shown. For comparison, only the dosing devices were exchanged and all other components and conditions remained unchanged. The results of continuous acid base titration show good agreement and comparable reproducibility. Potentiometric sensors (glass electrodes) with different equilibration behaviour influence the results. The capability of such electrodes was tested. Conductometric measurements allow a much faster detection because there is no equilibration of electrodes. Piston burettes should be used for titration with very high precision, titration with organic solvents and slow titrations. Peristaltic pumps seem to be more suitable for continuous titrations and long time operation without service.
Kinetics of the alizarine red S surface redox reaction by Ŝebojka Komorsky-Lovrić (pp. 306-309).
Kinetics of the surface redox reaction of alizarine red S adsorbed on mercury is measured by square-wave voltammetry. In 1 mol/l KNO3 buffered to pH 9.22, the standard reaction rate constant of the redox couple anthraquinone/anthrahydroquinone in the adsorbed alizarine red S molecule is ks=100 ±10 s-1 and the cathodic transfer coefficient is α=0.4. At pH 2 in this medium ks is greater than 500 s-1.