Flow injection potentiometric stripping analysis and potentiometric stripping coulometry

Summary Potentiometric Stripping Analysis is used as a detection method for Flow Injection Analysis. A wall-jet glassy carbon disc electrode is applied for the determination of zinc, cadmium, and lead. Microliter samples are sufficient for high-speed determination in the ng-range. The sample throughout is up to 180 per hour depending on the metal ion concentration. The influence of various parameters on the stripping time is investigated.In addition the new concept of Potentiometric Stripping Coulometry is presented. Based on Faraday's law the measured stripping time can be used to calculate the mass of deposited metal. Provided that the deposition is complete the value found is identical to the mass totally present in the sample. Using lead as an example the accuracy of the determination is demonstrated for the wall-jet electrode. The application of a porous carbon felt electrode increases the speed of deposition. Optimum parameters are given for exhaustive depletion.

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Fließinjektion zur potentiometrischen Stripping-Analyse und potentiometrischen Stripping-Coulometrie

Zusammenfassung Die potentiometrische Stripping-Analyse wird zur Detektion in der Fließinjektionsanalyse eingesetzt. Zur Bestimmung von Zink, Cadmium und Blei wird eine Wall-Jet Scheibenelektrode aus Glaskohlenstoff verwendet. Bei Injektion vonl-Mengen sind Schnellbestimmungen im ng-Bereich möglich. Je nach Metallionenkonzentration läßt sich ein Probendurchsatz von bis zu 180 pro Stunde erreichen. Der Einfluß verschiedener Parameter auf die Strippingzeit wird untersucht. Weiterhin wird das neue Konzept der potentiometrischen Stripping-Coulometrie vorgestellt. Die abgeschiedene Metallmasse kann aus der gemessenen Strippingzeit und dem zu berechnenden Oxydationsstrom ermittelt werden. Nach vollständiger Abscheidung ist diese Masse der gesamten in der Probe enthaltenen Masse gleich. Am Beispiel einer Bleibestimmung mit der Wall-Jet-Elektrode wird die Richtigkeit dieser Methode belegt. Durch die Verwendung einer Carbonfilzelektrode wird die Geschwindigkeit der vollständigen Abscheidung erhöht. Der Einfluß von Flußrate und injizierter Menge wird untersucht.

     

Computerized flow injection potentiometric stripping analysis with large-volume wall-jet cell

Summary A microcomputer system for flow injection potentiometric stripping analysis is presented permitting fast digital recording of potential time-curves. A large-volume wall-jet cell simple in design has been used as detector cell. Investigations about the influence of electrode pretreatment and solution delivery mode (i.e. gravity flow, pumping) reveal the strong influence of these parameters on the magnitude and precision of stripping times. The optimized system was applied to the simultaneous determination of cadmium and lead in geological samples.

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Computerisierte Potentiometrische Stripping Analyse in einem Fließinjektionssystem mit großvolumiger Wall-jet Zelle

     

Computerised flow potentiometric stripping analysis

Computerised flow potentiometric stripping analysis simplifies automated electroanalytical determination of several trace elements. It facilitates changing of samples and cleaning of electrodes. It also makes deoxygenation of the samples unnecessary and permits the use of a fresh mercury film surface for each analysis.     

Exploiting flow injection and sequential injection anodic stripping voltammetric systems for simultaneous determination of some metals

Flow injection (FI) and sequential injection (SI) systems with anodic stripping voltammetric detection have been exploited for simultaneous determination of some metals. A pre-plated mercury film on a glassy carbon disc electrode was used as a working electrode in both systems. The same film can be repeatedly applied for at least 50 analysis cycles, thus reducing the mercury consumption and waste. A single line FI voltammetric system using an acetate buffer as a carrier and an electrolyte solution was employed. An injected standard/sample zone was mixed with the buffer in a mixing coil before entering a flow cell. Metal ions were deposited on the working electrode by applying a potential of −1.1 V vs Ag/AgCl reference electrode. The stripping was performed by anodically scanning potential of working electrode to +0.25 V, resulting a voltammogram. Effects of acetate buffer concentration, flow rate and sample volume were investigated. Under the selected condition, detection limits of 1 μg l⁻¹ for Cd(II), 18 μg l⁻¹ for Cu(II), 2 μg l⁻¹ for Pb(II) and 17 μg l⁻¹ for Zn(II) with precisions of 2–5% (n=11) were obtained. The SI voltammetric system was similar to the FI system and using an acetate buffer as a carrier solution. The SI system was operated by a PC via in-house written software and employing an autotitrator as a syringe pump. Standard/sample was aspirated and the zone was then sent to a flow cell for measurement. Detection limits for Cd(II), Cu(II), Pb(II) and Zn(II) were 6, 3, 10 and 470 μg l⁻¹, respectively. Applications to water samples were demonstrated. A homemade UV-digester was used for removing organic matters in the wastewater samples prior to analysis by the proposed voltammetric systems.     

Lead determination in wine by potentiometric stripping analysis

Summary Computerized potentiometric stripping analysis (PSA) was used for lead determination in wine samples. In this method sample preparation before determination is limited to dilution with an appropriate supporting electrolyte (0.5 mol/l HCl and 40 mg/l Hg²⁺). Due to the very high sensitivity, the time needed for one determination can be reduced to 2 min. The method was tested by lead determination in different wines, different concentration ranges and in one European interlaboratory comparison organized by BCR. Using PSA it is possible to simultaneously determine cadmium, lead and copper in wine. For other elements such as Bi, Tl, Hg, Sn, As, Zn the determination procedures are under development. The PSA equipment is available on the market, small in size, relatively cheap and it does not require any special installation such as gas or cooling water.     

Carbon fibre electrodes in flow potentiometric stripping analysis

The construction of carbon fibre flow electrodes suitable for use in connection with potentiometric and constant-current stripping is described, and the fibre electrodes are compared with a glassy carbon disc thin layer cell. The signal-to-background ratio is approximately 1.6 times higher for an 8–10 μm carbon fibre compared to the glassy carbon disc electrode. If an Ag/AgCl tube is used as both counter and reference electrode, the signal-to-noise ratio of the fibre electrode is approximately five times better than for a glassy carbon disc electrode with a calomel reference; the latter electrode design, however, gives slightly better precision. The dead volume and internal potential drop of the fibre electrodes are more than one order of magnitude smaller than for the glassy carbon disc electrode. Because of the simplicity of the manufacturing process and low material cost, the fibre cells can be used as disposable electrodes and the polishing process necessary in connection with glassy carbon disc electrodes can be omitted.     

Evaluation of single-point calibration in flow potentiometric stripping analysis

The use of a single-point calibration in flow potentiometric stripping analysis has been evaluated. For a number of sample matrices, the results obtained by calibration methods agree with those obtained by using standard addition. The most serious source of systematic error is inhibition in the deposition step, caused by organic surfactants and chelating agents in the sample matrix. Consequently, the use of the calibration method should be restricted to samples which have been totally mineralized. It is shown that most sources of systematic error in the calibration method will result in underestimation of the analyte concentration.     

Determination of germanium by potentiometric stripping analysis and adsorption potentiometric stripping analysis

Potentiometric stripping analysis was applied to determination of germanium(IV) in 0.2M NH₃-NH₄Cl (pH 8.4) buffer solution at −1.8 V (vs. Ag/AgCl), with dissolved oxygen or Hg(II) as oxidant. The sensitivity was 8.5 × 10⁻⁹M in the presence of 2.0 × 10⁻⁵M Hg(II) with plating for 15 min after deaeration for 20 min. Cyclic voltammetry revealed that GE(IV) å Ge at the surface of the mercury-film electrode in a one-step irreversible reduction reaction, and the Ge at the electrode was oxidized by dissolved oxygen in the solution. The presence of complexing agents such as Alizarin Red (ARS), which forms a Ge(IV) complex adsorbed at the electrode, improved the sensitivity by one order of magnitude. The presence of adsorption was revealed by the temperature coefficient, the electrocapillary curve and cyclic voltammetry. Ge-containing samples were analysed by the proposed methods and agreement with the results obtained by other methods was excellent.     

Interlaboratory quality control for lead determination in wine by potentiometric stripping analysis

Summary Computerized potentiometric stripping analysis (PSA) was used for the determination of lead in wine samples. With this analytical method the sample preparation is limited to dilution with an appropriate supporting electrolyte (0.5 mol/l HCl and 40 mg/l Hg²⁺). Due to very high sensitivity the time needed for one determination can be reduced to 2 min. This method for lead determination in wine was compared with results from voltammetry and ETAAS from various laboratories participating in two BCR studies. With PSA it is possible to determine cadmium, lead and copper in wine after a single dilution with an appropriate supporting electrolyte. For other elements such as Bi, Tl, Hg, Sn, As, Zn determination procedures are under development.     

An alternating pulse current technique for potentiometric stripping analysis (APC-PSA)

The response signal in PSA measurements – the transition time – can be considerably amplified by using a special pulse technique which we call alternating pulse current (APC). The sensitivity of the method is thereby considerably increased. This is achieved by modulating the current with a high frequency in a square wave mode during the stripping phase in such a way, that cathodic and anodic pulses alternate, so that the species to be determined is stripped and – at least partially – redeposited during the procedure. The transition time is not affected by the absolute values of the currents and of the times they flow. The amplification is determined by their respective ratios only. The equations holding are derived and proven experimentally by measurements on very dilute Pb solutions. The APC-PSA technique has also been applied for systems termed irreversible such as U and Tc, where also a considerable amplification could be achieved. The probable reasons are discussed and future developments and investigations are depicted.     

Computer system and flow cell for automated potentiometric and constant-current stripping analysis

A flow cell and a personal computer-based system for potentiometric and constant-current stripping analysis are described. The thin-layer cell is designed for teflon-embedded glassy carbon electrodes in a spring-regulated holder, allowing simple instalment and leak-free behaviour. The personal computer is interfaced to hardware components such as potentiostat, amplifier, peristaltic pump, six inlet valves for different solutions, autosampler and strip-chart recorder. User-accessible commands allow electrolysis, stripping current, pump flow rates, valves, stripping transient recording, derivation, filtration and integration to be controlled, and through a number of user-defined programs, to be performed in unattended operation. This is described by an example. Through a software interface to a high-level language (BASIC), results from any number of analytical runs can be further evaluated by standard addition, or from calibration curves, or by statistical methods.     

Computerized flow potentiometric stripping analysis with nafion-modified carbon fibre electrodes

Five to ten carbon fibres (10-μm diameter) are mounted perpendicularly to the flow direction in polyethylene tubes (0.8 mm inner diameter) and modified simultaneously by sucking five 10-μl portions of Nation dissolved in ethanol through the flow electrodes; after each portion, air is sucked through the tubes for some minutes. The modified electrodes can be used for several hundred electrolysis/stripping cycles in solutions containing albumin or Triton X-100. Analysis of a urine reference sample shows that zinc(II), cadmium(II) and lead(II) can be determined after acidification and electrolysis for 5, 35 and 100 s, respectively. For the determination of lead(II), 2 M hydrochloric acid is used as stripping medium but for the two other elements the sample itself is used. The electrochemically available fraction of the trace elements can be assessed by analyzing unacidified samples.     

电位溶出分析法同时测定锡, 铅, 铜

本文应用电位溶出分析法进行同时测定锡、铅和铜的研究。结果发现在草酸溶液中锡、铅和铜有分离清晰的平台,其溶出讯号与浓度呈线性关系,可用于定量分析。用这一方法测定了某些罐头食品中痕量锡、铅和铜,获得满意的结果。     

A micro-computer system for potentiometric stripping analysis

In potentiometric stripping analysis, trace metals are first amalgamated into a mercury film electrode and then oxidized back into solution by an oxidant. The potential vs. time curve is recorded. Microcomputer-controlled equipment for automation of the potentiometric stripping analysis is described. The higher resolution of time compared to manual equipment increases the sensitivity of the method. A resident, 3-kB BASIC interpreter with routines for handling the hardware makes the programming simple and the instrumentation flexible for changes in the analytical scheme. Printer and keyboard are incorporated in the equipment which makes the whole analyser compact.     

Double-layer effects in potentiometric stripping analysis

A model for prediction of the background signal (E vs. t) in potentiometric stripping analysis is presented and shown to be consistent with experimental measurements. In the absence of oxidizable material on the working electrode, it is the double-layer capacity which determines the E vs. t function. Surfactants present in solution give rise to marked interferences, because of their effect on the double-layer capacity.     

Automated determination of lead in urine by means of computerized flow potentiometric stripping analysis with a carbon-fibre electrode

Five different digestion procedures have been investigated. In the recommended procedure 5 ml of concentrated hydrochloric acid-nitric acid mixture (1:1) were added to a 5-ml urine sample. After 1 min the sample was diluted to 100 ml with distilled water. The sample was divided into two 50-ml samples and to one of them a standard addition of 10 microg/l. lead(II) was made. The two samples were analysed fully automatically by means of computerized flow potentiometric stripping analysis, the main features of the procedure being mercury-film precoating, electrolysis of the sample for 90 sec and subsequent stripping in 1M calcium chloride/0.1M hydrochloric acid. Tin and copper were found not to interfere if present at concentrations below 50 mg/l. but high concentrations of tin had to be masked by the addition of copper in order to form a copper-tin intermetallic compound. Complexing agents used in lead poisoning therapy did not interfere with the determination. The lead concentration in a Seronorm reference urine sample was found to be 93 microg/l. with a standard deviation of 8 microg/l. (n = 5), the certified value being 88 microg/l.     

A flow injection analysis technique for the determination of chloride using reflectance detection

A flow injection (FI) determination for chloride has been developed using the light reflectance of the precipitate formed by the reaction of chloride with silver(I) as the method of detection rather than turbidimetry, as in the previous FI method using this reaction. The dynamic range of the analysis is increased to 0-10 mM chloride with a 10 mM silver(I) reagent and to 0-50 mM chloride with a 50 mM silver(I) reagent by using this mode of detection. The ability to select the injected reagent from an option of two concentrations via the control program is incorporated into the FI system, enhancing the versatility of the analysis. The dynamic range is further extended to 100 mM chloride by measuring the signal levels at the trailing portion of the response curve. The consumption of reagent is kept to a minimum by merging injected zones of sample and reagent instead of using a constant reagent stream.     

Anodic stripping voltammetry with flow injection analysis

A flow cell with a wall-jet electrode design is used for anodic stripping voltammetry of lead at concentrations of about 10^?7 mol dm^?3. Maximum peak heights are obtained for narrow nozzle diameters and short nozzle-to-electrode distances. Linear calibration plots are obtainedfor almost four decades of change in concentration and can be extended by judicious choice of sample volume. Increasing sample throughput rates by increasing the solution flow rate decreases the analytical signal. Square wave voltammetry provides shorter analysis times and better sensitivity than differential pulse voltammetry.