Flow-injection analysis with a coated tubular solid-state copper(II)-selective electrode

The incorporation, behaviour and suitability of a home-made coated tubular solid-state copper(II)-selective electrode into the conduits of a flow-injection system is described. The compact tubular sensor (volume 7.8 μl) is constructed from a copper tube and Tygon tubing, treated with ammonium sulphide to give the copper/copper sulphide electrode and conditioned in an ascorbic acid medium. Interferences of foreign ions are similar to those found in batch analysis, but are less severe in the flow-injection system. Changes in carrier stream pH between 1 and 3 affected the electrode response, but sample pH has no or little influence in the range 1–7 depending on the copper(II) ion concentration in the samples. With 30-μl samples the flow-through electrode system covers a working range up to 5000 mg dm^?3 with a detection limit of 0.5 mg dm^?3. The system is suitable for the determination of copper(II) in effluent and tap water (relative standard deviation < 1.75% for 0.5–912 mg dm^?3 copper) and acidic copper sulphate plating solutions (relative standard deviation < 1.21% for 87–4135 mg dm^?3 copper) at a sample frequency of about 80 h^?1. The results obtained agree well with results by a standard atomic absorption spectrometric method.     

Selective determination of Ni(II) and Co(II) by flow injection analysis and adsorptive cathodic stripping voltammetry on a wall jet mercury film electrode

Ni(II) and Co(II) have been determined simultaneously by means of adsorptive cathodic stripping voltammetry (AdCSV) in a computerised flow injection system. The working electrode was a glassy carbon disk that was fitted in a wall-jet flow cell. The electrode was initially electrochemically coated with a mercury film at -1.0 V by injecting a Hg(II) solution in the flow stream. Then, the sample, containing Ni(II) and Co(II), was mixed on-line with a solution containing dimethylgyoxime (DMG) at pH 9 in order to selectively complex the metal ions and was injected in the flow system. After a number of successive injections during which accumulation took place under controlled potentiostatic conditions, the surface-bound complexes were reduced in ammonia buffer at pH 9 by a cathodic scan of the potential of the working electrode in the square wave mode and the current-potential response was recorded. Finally, the electrode surface was regenerated by a potentiostatic polarisation at -1.4 V in the same buffer. The apparatus could be easily converted for continuous flow accumulation in order to increase the sensitivity; in this mode of operation, instead of performing discrete injections, the sample was continuously pumped through the cell. Various parameters associated with the preconcentration, stripping and regeneration steps were optimised for the determination of Ni(II) and Co(II). The selectivity of the method was demonstrated for the analysis of high purity iron; the accuracy for the determination of Ni(II) and Co(II) was 11 and 3%, respectively while the coefficient of variation was 10 and 8%, respectively.     

Determination of lead in whole blood with a simple flow-injection system and computerized stripping potentiometry

An automated (24 samples/hour) procedure is described for the determination of lead (0–1000 μg l^?1) in human blood based on flow-injection stripping potentiometry. The samples are diluted 20-fold with 0.5 M hydrochloric acid containing 100 mg l^?1 mercury and 40 μg l^?1 cadmium (II), and a 1.1 ml aliquot is injected into the flow system. With a mercury-coated carbon fibre as working electrode, lead (II) is determined by using cadmium (II) as internal standard and a calibration graph prepared from bovine blood. Analyses of two human blood reference samples yielded results of 335±37 and 691±24 μg l^?1 lead, the certified values being 332 and 663 μg l^?1, respectively.     

The trace determination of some heavy metals in waters by flow-injection spectrophotometry and potentiometry

Three automated flow-injection systems are proposed for the determination of traces of manganese(II), lead and copper(II) in waters. The first system utilizes the catalytic effect of manganese(II) on the oxidation of N,N-diethylaniline by potassium periodate at pH 6.86–7.10 (30°C) and is used for spectrophotometric determination at 475 nm in the range 0.02–1.00 μg1^?1; the system involves reagent injection and stopped flow. The determination of lead in the range 0.7–100 μg1^?1 is based on spectrophotometric detection of the lead 4/(2-pyridylazo)resorcinol complex at 525 nm after on-line preconcentration of the sample (5–50 ml) on a minicolumn filled with Chelex-100 or Dowex 1-X8 resin. A potentiometric flow-injection system with a copper ion-selective electrode is applied for the determination of 0.5–1000 μg 1^?1 copper(II) after on-line preconcentration of 50–500 ml of sample on Chelex-100 resin. The procedures are tested on synthetic and real water samples, including sea water and waste-waters.     

High‐Sensitivity Determination of Lead(II) and Cadmium(II) Based on the CNTs‐PSS/Bi Composite Film Electrode

A high‐sensitivity sensing platform for lead(II) and cadmium(II) based on the bismuth modified carbon nanotubes (CNTs)‐poly(sodium 4‐styrenesulfonate) composite film electrode (CNTs‐PSS/Bi) was fabricated. The composite film CNTs‐PSS/Bi provided remarkably improved sensitivity and reproducibility compared with previously reported CNTs‐modified electrodes. The detection limits were estimated to be 0.04 ppb for lead(II) and 0.02 ppb for cadmium(II) with a preconcentration time of 120 s, respectively. The linear responses of Cd²⁺ and Pb²⁺ were over the ranges of 0.5–50 ppb and 0.5–90 ppb, respectively. Finally, the practical application of the proposed method was verified in the real water sample with satisfactory results.     

Constant-potential pulse polarographic detection in flow-injection analysis without deaeration of solvent or sample

A constat-potential pulse waveform is applicable for the polacographic analysis of buffered solutions (pH ?= 7) of cathodically active metal ions without voltammetric interference from dissolved oxygen. The technique is demonstrated at a dropping mercury electrode for detection of lead(II) and cadmium(II) in a conventional polarographic Cell (ca. 75 ml) as well as for small samples (2 ml) in a flow-injection system. The flow-injection polarographic technique is recommended for higher sample throughout than conventional polagraphy and is demonstrated for an electroless copper plating solution containing about 1.5 × 10^–2 M copper(II).     

Mercury ion selective poly(aniline) solid contact electrode based on 2-mercaptobenzimidazol ionophore

A mercury(II) ion selective poly(aniline) solid contact electrode based on 2-mercaptobenzimidazol (2MBI) ionophore as a sulfur containing sensing material was successfully developed. The electrode exhibits a good linear response of 29.1 mV/decade (at 20 ± 0.2°C, r ² = 0.997) within the concentration range of 1 × 10^?2?1 × 10^?7 M Hg(II). The composition of this electrode was: ionophore 0.100, polyvinylchloride (PVC) 0.330, dibutylphthalate (DBP) 0.470, potassiumtetrakis(4-chlorophenyl)borate (KTpCIPB) 0.090, and oleic acid (OA) 0.010. A poly(aniline) solid contact electrode based on 2MBI with DBP and OA plasticizers exhibited the best response characteristics of the results obtained for similarly coated wire type electrodes and solid contact electrodes based on only one DBP plasticizer. The electrode shows good selectivity for mercury(II) ions in comparison with alkali, alkaline earth, transition and heavy metal ions. This electrode is suitable for use with aqueous solutions of pH 3.3?C8.0 and the standard deviation in the measured EMF difference was ±0.5 mV in a mercury nitrate sample solution of 1.0 × 10^?2 M and ±1.1 mV in a mercury nitrate sample solution of 1.0 × 10^?3 M. The stabilization time was less than 15 min and the response time was less than 33 s. The electrode was applied as a sensor for the determination of Hg(II) content in a sea water sample and some amalgam alloys. The results show good correlation with data obtained by atomic absorption spectrometry.     

Sequential injection monosegmented flow voltammetric determination of cadmium and lead using a bismuth film working electrode

A cost-effective sequential injection monosegmented flow analysis (SI-MSFA) with anodic stripping voltammetric (ASV) detection has been developed for determination of Cd(II) and Pb(II). The bismuth film working electrode (BiFE) was employed for accumulative preconcentration of the metals by applying a fixed potential of −1.10 V versus Ag/AgCl electrode for 90 s. The SI-MSFA provides a convenient means for preparation of a homogeneous solution zone containing sample in an acetate buffer electrolyte solution and Bi(III) solution for in situ plating of BiFE, ready for ASV measurement at a flow through thin layer electrochemical cell. Under the optimum conditions, linear calibration graphs in range of 10-100 μg L⁻¹ of both Cd(II) and Pb(II) were obtained with detection limits of 1.4 and 6.9 μg L⁻¹ of Cd(II) and Pb(II), respectively. Relative standard deviations were 2.7 and 3.1%, for 11 replicate analyses of 25 μg L⁻¹ Cd(II) and 25 μg L⁻¹ Pb(II), respectively. A sample throughput of 12 h⁻¹ was achieved with low consumption of reagent and sample solutions. The system was successfully applied for analysis of water samples collected from a draining pond of zinc mining, validating by inductively coupled plasma-optical emission spectroscopy (ICP-OES) method.     

Voltammetric Determination of Trace Heavy Metals by Sequential-injection Analysis at Plastic Fluidic Chips with Integrated Carbon Fiber-based Electrodes

This work describes a sequential injection analysis (SIA) method for on-line strippping voltammetric determination of Pb(II), Cd(II) and Zn(II) using an injection-moulded electrochemical fluidic chip consisting of 3 conductive carbon fiber-loaded polymer electrodes embedded in a plastic fluidic holder. The sample containing the target metals and a solution containing Bi(III) were aspirated in the holding coil of the SIA manifold. Then, the flow was reversed and the two solutions were directed to the fluidic cell through a mixing coil which induced mixing of the two zones. Upon reaching the cell, simultaneous reduction of the target metals and Bi(III) occurred resulting in the formation of a metal-Bi alloy on the working electrode. Finally, the accumulated metals were stripped off the bismuth-film electrode via a positive potential scan and the oxidation current was recorded. The experimental variables (concentration of the bismuth plating solution, deposition potential, sample volume, stripping mode) were investigated and the potential interferences were assessed. The limits of quantification were 2.8 μg L⁻¹ for Pb(II), 3.6 μg L⁻¹ for Cd(II) and 4.2 μg L⁻¹ for Zn(II) and the the within-chip and between-chip % relative standard deviations were ≤6.3 % and ≤14 %, respectively. Finally, the sensor was applied to the determination of trace metals in a fish food sample.     

Lead-selective electrodes based on lead(IV) oxide

Lead(IV) oxide electrodes are shown to give near-theoretical calibration slopes for lead(II) ions over the range 10^?3–10^?5 mol l^?1, and to have near-theoretical standard potentials in different acidic media. They are compared with lead sulphide-silver sulphide membrane electrodes and shown to be more tolerant of acidity and copper(II), mercury(II) and iron(III) ions. Iron(II) and manganese(II), however, interfere significantly. Some of the advantages of the lead(IV) oxide electrode are brought out in the determination of the solubility product of lead sulphate; implications for constructing phosphate- and sulphate-sensitive electrodes are mentioned.     

Evaluation of Bismuth Modified Carbon Thread Electrode for Simultaneous and Highly Sensitive Cd (II) and Pb (II) Determination

Bismuth film modified and chemically activated carbon micro‐thread electrodes were investigated for the simultaneous determination of Cd(II) and Pb(II) using square wave anodic stripping voltammetry. The carbon thread electrode was characterised using both surface and electrochemical techniques. Electrochemical impedance spectroscopy (EIS) studies demonstrated that the H₂SO₄/IPA‐treated carbon thread electrode showed a much improved resistance response (R_ct=23 Ω) compared to the IPA‐untreated carbon thread (R_ct=8317 Ω). Furthermore, parameters such as the effect of deposition potential, deposition time and Bi(III) concentration were explored using square wave voltammetry. Detection limits (S/N=3) for Cd(II) and Pb(II) were found to be 1.08 µg L^?1 and 0.87 µg L^?1, respectively and response was found to be linear over the range 5–110 µg L^?1. The proposed Bi/IPA‐treated carbon thread electrode exhibited a high selectivity towards Cd(II) and Pb(II) even in the presence of a range of heavy metals and is capable of repetitive and reproducible measurements, being attributed to the high surface area, geometry and electrode treatment characteristics. The proposed metal ion sensor was employed to determine cadmium and lead in river water samples and % RSD was found to be 5.46 % and 5.93 % for Cd(II) and Pb(II) respectively (n=3). Such facile sensing components favour the development of cost effective portable devices for environmental sample analysis and electrochemical applications.     

An adsorptive stripping voltammetry of nickel and cobalt at a solid lead electrode

For the first time, a solid lead electrode (PbE) was exploited for adsorptive stripping voltammetric determination of Ni(II) and Co(II) in the presence of nioxime as a complexing agent. The calibration graphs for Ni(II) and Co(II) were linear from 0.059 to 0.59 µg L^?1 and from 0.029 to 0.29 µg L^?1 (accumulation time 120 s), respectively. The analytical parameters such as the detection limit and separation of analytical signals obtained at the solid lead electrode were comparable with those obtained using a lead film electrode while better in comparison to those reported before for the bismuth film or solid bismuth electrodes. Co(II) could be determined in the presence of a large excess of Ni(II) and Zn(II). The proposed electrode was applied to determine Co(II) and Ni(II) traces in certified reference material and a natural water sample with satisfactory results.     

Electrochemical sensor for lead(II) ion using a carbon ionic-liquid electrode modified with a composite consisting of mesoporous carbon, an ionic liquid, and chitosan

A novel electrode was prepared that enables sensing of lead(II) ion. A suspension composed of ordered mesoporous carbon (OMC), an ionic liquid (IL), and chitosan was deposited on the highly conductive surface of a carbon ionic-liquid electrode (CILE). The surface of the sensing electrode was characterized by scanning electron microscopy and cyclic voltammetry. The new electrode can be used to determine lead(II) ion because the hydrophobic ionic liquid of the CILE can extract Pb(II), while the OMC accelerates the electron transfer rate between the electrode and Pb(II) and also strongly adsorbs Pb(II). The resulting electrode displays excellent and synergistic response to Pb(II) which is linear in the range from 0.05 to 1.4?μM, with a correlation coefficient of 0.997 and a detection limit of 25 nM.

The new Micro‐set for Adsorptive Stripping Voltammetric Simultaneous Determination of Nickel and Cobalt Traces in Aqueous Media

This article the first reports on a fabrication and application of an electrochemical three electrode micro‐set containing: in situ plated lead film on carbon fiber working microelectrode, Ag/AgCl reference electrode and a platinum wire counter electrode placed in one casing for simultaneous Ni(II) and Co(II) traces determination by square wave adsorptive stripping voltammetry (SW AdSV). Ni(II) and Co(II) in forms of their complexes with nioxime were accumulated on the lead film plated on a carbon fibers microelectrode during standard procedure of measurement. Thanks to the fact that measurements were performed in micro‐vessel of a volume of 200 μl small amounts of reagents were used to prepare samples for measurements. In addition, because of the use of microelectrode, sample solutions were not mixed during accumulation step of measurements. This fact creates the possibility of conducting fields analysis. The experimental parameters (composition of the supporting electrolyte, potential and time of accumulation) and possible interference effects were investigated. The linear calibration graphs for Ni(II) and Co(II) were in the range from 2×10^?9 to 1×10^?7 mol L^?1 and from 2×10^?10 to 1×10^?8 mol L^?1 for Ni(II) and Co(II), respectively. The correctness of the proposed method was checked by determining Ni(II) and Co(II) in the certified reference material (SPS‐SW1) with satisfactory results.     

Differential pulse cathodic stripping adsorption voltammetric determination of trace amounts of lead using factorial design for optimization

A sensitive cathodic stripping voltammetric method is developed for determination of lead(II), with adsorptive collection of complexes with Pyrogallol red (PGR) on to a hanging mercury drop electrode. After accumulation of the complex at −0.80 V vs. Ag/AgCl reference electrode, the potential is scanned in a negative direction from −0.20 to −0.50 V with differential pulse method. Then the reduction peak current for the lead(II)-PGR complex is measured at −0.39 V. The influence of reagent and instrumental variables was completely studied by factorial design analysis. The optimum analytical conditions for the determination of lead(II) were established. Under optimum conditions, lead(II) determined in the range of 0.1-30.0 ng ml⁻¹ with a limit of detection of 0.06 ng ml⁻¹. The method is successfully applied to determination of lead(II) in water sample.     

Flow potentiometric stripping analysis for mercury(II)

The optimum experimental conditions, with respect to sample and stripping solution composition, in computerised flow potentiometric stripping analysis for mercury(II) with a gold working electrode are described. When pre-electrolysis -was done in a sample to which ammonia and iodide had been added and stripping was done in an acidified bromide solution containing chromium(VI), a detection limit of 2 nM (0.4 μg kg^-1) was obtained after 90 s of pre-electrolysis, the dynamic range being almost three decades. Copper(II) interfered when present in a 1000-fold excess and silver(I) when present in a 5-fold excess over mercury(II).     

Chemiluminescence for the determination of traces of cobalt(II) by continuous flow and flow injection methods

Flow injection analysis, with chemiluminescence detection, is used to determine traces of cobalt(II) by means of the gallic acid—hydrogen peroxide—sodium hydroxide system containing a small amount of methanol to increase the sensitivity. This permits the determination of cobalt(II) more selectively than any other chemiluminescent system with a detection limit of 0.04 μg l^-1 (continuous sample flow) or 0.04 ng (10-μl sample injection). The linear range is 3 orders of magnitude, the sampling rate is 20 h^-1, and the relative standard deviation is 5.9% for 0.06 ng Co(II) (n = 10). Silver(I), the strongest enhancer after cobalt(II), provides a signal 1.3% of that for Co(II). A few precipitants and complexing agents suppress the signal.     

Vergleich von Potentiometrischer Stripping-Analyse und Anodischer Stripping-Voltammetrie in der Flie?injektionsanalyse. St?rungen bei der Bleibestimmung

Summary The interferences caused by inorganic ions, organic anions and Triton X-100 in the determination of lead by flow injection potentiometric and voltammetric stripping analysis at a mercury film electrode were investigated. The experiments were performed in the concentration range of 1–10 mg/l Pb²⁺. For both methods linear calibration plots were obtained in the presence of an excess of nitrate, chloride, perchlorate and sulphate. Iodide strongly interferes due to mercury complexation.Metal ion interference caused by formation of irreversible amalgams (as in the case of codeposition of Ni, Co, Fe and Cr) is avoided by proper choice of deposition potential. No effect of water soluble reduced species, i.e. Fe(II) and Cr(II), on the stripping signals was observed. The presence of acetate, citrate and tartrate does not limit the determination of lead by potentiometric stripping analysis (PSA) but has a marked effect on anodic stripping voltammetric (ASV) signals. No influence of Triton X-100 on the determination of lead by PSA was found. In ASV 10^–3% Triton X-100 diminished the peak current by 15%.

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Herrn Prof. Dr. Rolf Neeb aus Anlaß seines 60. Geburtstags gewidmet     

An enzyme reactor electrode for urea determinations.

An enzyme reactor electrode system for the determination of urea is described. A buffer is pumped through an enzyme reactor (0.4 ml) containing urease immobilized with glutaraldehyde to glass. The effluent is mixed with sodium hydroxide pumped through a second channel and fed through an ammonia gas electrode. Samples are introduced via a third flow channel and mixed with the buffer. The conversion of urea to ammonia is quantitative for sample concentrations of less than 0.03 M for a flow rate of 40 ml h^-1. The reactor electrode shows a Nernstian slope of 57 mV/decade for 5·10^-5–3·10^-2 M urea. The response is independent of variations in the flow rate, enzyme activity or temperature of the reactor.     

Preconcentration of copper(II) on immobilized 8-quinolinol in a flow-injection system with an ion-selective electrode detector

A column containing 8-quinolinol, immobilized on porous glass, is used for preconcentration and medium exchange in a flow-injection system with a copper ion-selective electrode detector. The metal ions are bound to the chelating ion exchanger while the anions and inert sample components pass to waste without contacting the electrode. Acid is then injected to elute the ions into a neutralizing buffer passing the electrode. Matrix effects are thus reduced because all measurements are made in the same buffer. The detection limits are 10^?7 and 3 × 10^?8 M copper(II) for sample volumes of 5 and 25 ml, respectively. The maximum throughput is 12 and 5 samples h^?1 for the two stated injection volumes.