Square-wave Cathodic Stripping Voltammetry Research Articles

Modified Square-Wave Cathodic Stripping Voltammetry for Determination of Se(IV) at Trace and Ultratrace Levels

A method for the improvement of the signal-to-noise ratio in square-wave (SW) voltammetry is proposed. It is based on the modification of a square-wave waveform. This method is applied to cathodic stripping voltammetric (CSV) determination of selenium(IV) in nitric acid solutions. Hanging mercury drop electrode (HMDE) was used in the presence of Cu(II) ions. The optimal conditions were chosen: square-wave waveform mode, pH, time and potential of electrodeposition. Kinetic processes of copper selenide reduction with the use of SWCSV, applying different square-wave waveforms, are studied. Quasi-reversible processes are observed. The standard reaction rate constants are evaluated for different square-wave waveforms. It is shown that the determination of Se(IV) by this modified SWCSV technique is possible with an excellent sensitivity (the detection limit 8×10−12 mol L−1 only for 5 min of electrodeposition) and good reproducibility (Sr<8%) in a wide range of concentration (1×10−11−1×10−6 mol L−1) of Se(IV). The presence of Mo(VI), Pb(II), Ni(II), Zn(II), Cr(VI) metal ions with molar excess around 1000 do not prevent the detection of Se(IV) signals.

Speciation of Inorganic Arsenic in Natural Waters by Square-Wave Cathodic Stripping Voltammetry

A fast and sensitive voltammetric method for the determination of arsenite and total inorganic arsenic in natural water systems is described using square-wave cathodic stripping voltammetry at a hanging mercury drop electrode. All the determinations were done in 1 M HCl, in the presence of 5 ppm copper(II) and dissolved oxygen. Discrimination between both oxidation states is achieved just by varying the composition of the supporting electrolyte: reduction of As(V) to As(III) is carried out at room temperature by a mixture of potassium iodide and ascorbic acid and inorganic arsenic determined as As(III)+As(V), while As(III) is measured in the absence of the reducing agent. For an accumulation time of 60 s the detection limits were 0.7 ppb for As(III) in 1 M HCl, and 5 ppm Cu(II), 0.6 ppb for As(III)+As(V) in 1 M HCl, 5 ppm Cu(II) and 0.04 % (w/v) KI+0.02 % (w/v) ascorbic acid. The precision, expressed as relative standard deviation (RSD) at 95 % confidence level, was calculated as 9 % from ten individual measurements taken at 2 ppb standard. The method was tested by the measurement of As(V) and As(III) in real and synthetic natural water samples.

Chemical speciation of sulfur compounds in surface sediments from three bays (Fresnaye, Seine and Authie) in northern France, and identification of some factors controlling their generation

The determination of total reduced sulfur, elemental sulfur, sulfide, thiosulfate and sulfite in porewaters — which were previously extracted from surface sediments collected in three bays, i.e. Seine, Fresnaye and Authie located in northern France — has been undertaken using different electroanalytical techniques (linear sweep cathodic stripping voltammetry (LSCSV), square wave cathodic stripping voltammetry (SWCSV) and differential pulse cathodic stripping voltammetry (DPCSV)) at a static mercury drop electrode. Furthermore, the analyses of sulfur solids present in these sediments have been performed by sequential extraction procedures. Overall, the description of speciation of dissolved sulfur compounds and reduced sulfur solids has shown that the availability of sulfate and reactive iron, the sedimentation rate and probably even the nature and content of organic matter are important factors for controlling the sulfidisation and pyritisation processes involved in the different sedimentary systems studied. Thus, in the Seine-bay sediments, it has been found that reactive iron scavenges the most part of the generated sulfide as a result of a particularly activated sulfate reduction by bacterial activities; as a consequence, this precipitation limits the accumulation in the porewaters of reduced sulfur compounds such as elemental sulfur and polysulfides which ought to be generated in the redox boundary (where the oxydants, viz. oxygen, nitrate and metal oxides exist abundantly) through a partial oxidation of the H 2S and HS − species. This process is further accentuated by a sedimentation rate measured in the Seine estuary. Conversely, in the Authie bay and, to a lesser extent, in the Fresnaye bay the contribution of the sedimentation rate and/or reactive iron scavenging to the sedimentary sulfur processes is weaker. This explains the increase of the built-up of dissolved reduced sulfur in the interstitial waters as well as the existence of elemental sulfur and polysulfides that permit the conversion of FeS into FeS 2.

Cathodic adsorptive stripping square-wave voltammetry of the anti-inflammatory drug meloxicam.

The adsorptive behavior of the anti-inflammatory drug meloxicam was studied by cyclic, differentia-pulse and square-wave voltammetry on a hanging mercury drop electrode (HMDE). The drug was accumulated at HMDE and a well-defined stripping peak current was obtained at -1.42 V vs. Ag/AgCl (saturated KCl) electrode in acetate buffer solution (pH 5.0). A voltammetric procedure was developed for the determination of meloxicam using square-wave cathodic adsorptive stripping voltammetry (SW-CASV). The optimum working conditions for the determination of the drug were established. The analysis of meloxicam in human plasma was carried out satisfactorily.

Direct determination of molybdenum in seawater by adsorption cathodic stripping square-wave voltammetry

A reliable and very sensitive procedure for the determination of trace levels of molybdenum in seawater is proposed. The complex of molybdenum with 8-hydroxyquinoline (Oxine) is analyzed by cathodic stripping square-wave voltammetry based on the adsorption collection onto a hanging mercury drop electrode (HMDE). This procedure of molybdenum determination was found to be more favorable than differential pulse cathodic stripping voltammetry because of inherently faster scan rate and much better linearity obtained through the one-peak (instead of one-of-two peaks) calibration. The variation of polarographic peak and peak current with a pH, adsorption time, adsorption potential, and some instrumental parameters such as scan rate and pulse height were optimized. The alteration of polarographic wave and its likely mechanism are also discussed. The relationship between peak current and molybdenum concentration is linear up to 150 μg l −1. Under the optimal analytical conditions, the determination limit of 0.5 μg l −1 Mo was reached after 60 s of the stirred collection. The estimated detection limit is better than 0.1 μg l −1 of Mo. The applicability of this method to analysis of seawater was assessed by the determination of molybdenum in two certified reference seawater samples (CASS-2 and NASS-2) and the comparison of the analytical results for real seawater samples (study on a vertical distribution of Mo in the seawater column) with the results obtained by Zeeman-corrected electrothermal atomization atomic absorption spectrometry (Zeeman ETAAS). A good agreement between two used methods of molybdenum determination was obtained.

Square-Wave Cathodic Stripping Voltammetry of Ultratrace Manganese in the Presence of Ultrasound Irradiation

Square-Wave Cathodic Stripping Voltammetry of Ultratrace Manganese in the Presence of Ultrasound Irradiation

Geochemical factors controlling free Cu ion concentrations in river water

Copper speciation was determined monthly at seven sites on four rivers in southern New England to understand which geochemical factors control free metal ion concentrations in river water. Samples were conventionally filtered (<0.45 μm) and then ultrafiltered (3.000 molecular weight cut-off) to determine Cu speciation in the truly dissolved size fraction. Differential pulse anodic stripping voltammetry (DPASV) was used to quantify natural organic complexation and cathodic stripping square wave voltammetry (CSSWV) to measure directly both Cu sulfide complexes and total EDTA concentrations. The results showed both dissolved organic matter (DOM) and sulfide complexation dominate Cu speciation and control the concentrations of free ion. Free Cu2+ was calculated to be in the subnanomolar range for the majority of the year. Only in the winter months, when concentrations of DOM and metal sulfides complexes were at a minimum were free metal ions directly measurable by DPASV at low nanomolar concentrations. The extent of sulfide complexation appears to be dominated by the size of headwater marshes (upstream sampling sites) and by the amount of sewage treatment plant effluent (downstream sites). DOM complexation was related to the organic matter composition and followed model organic ligands. Indirect evidence suggests variations in river water pH and Ca2+ (metal competition) has only a minor role in Cu complexation. Measured concentrations of total EDTA suggest this synthetic ligand can control Cu speciation in some highly developed watersheds; however, competition from Ni (and possibly Fe) limits the extent of this complexation.

A Cathodic Stripping Square-Wave Voltammetry of a Second-Order Redox Reaction and Its Application to the Mercury-Cysteine System

A model of cathodic stripping square-wave voltammetry of a second-order redox reaction is developed. Both reversible and quasireversible redox reactions are considered. The relationships between the properties of SWV response and both parameters of the redox reaction and the SWV excitation signal are analyzed. The effect of the concentration of the reacting ligand on the SW voltammograms is discussed. The phenomenon of a quasireversible maximum is demonstrated. Numerical simulations are compared qualitatively with the cathodic stripping square-wave voltammetric response of cysteine.

A Cathodic Stripping Square-Wave Voltammetry of a Second-Order Redox Reaction and Its Application to the Mercury-Cysteine System

A Cathodic Stripping Square-Wave Voltammetry of a Second-Order Redox Reaction and Its Application to the Mercury-Cysteine System

Voltammetric determination of elemental sulfur in pore waters

We describe a new simple, reliable, and sensitive method that employs square‐wave cathodic stripping voltammetry (SWCSV) for the measurement of dissolved elemental sulfur, S(0), in pore waters. The method, which requires only small pore‐water samples (~1.5 ml), shows a linear relationship over three orders of magnitude (~10−9−10−6 Eq S liter−1) between peak current or peak area and S(0) concentration; the practical detection limit (3 × 10−9 Eq S liter−1) is below that required for most pore waters. Experiments carried out over a wide range of pH (2.5–11.5) indicate that peak current and peak potential vary with pH and that thiosulfate, sulfite, and thiols (and probably humic substances) would not interfere with the determination of S(0) over this pH range; interference from sulfide is easily removed by acidifying and bubbling with N2. The slopes (nA/Eq S liter−1) obtained by standard additions of S(0) to pore‐water samples containing 10 mg liter−1 dissolved organic carbon were not statistically different from the slopes of calibration curves, indicating that fouling effects due to natural organic matter should be minimal. Porewater concentration profiles obtained at an anoxic site of a Shield lake indicate that S(0) concentrations were greater than those predicted from equilibrium with orthorhombic sulfur.

Determination of Trace Metals (Cu, Pb, Zn and Ni) in Soil Extracts by Flow Analysis with Voltammetric Detection

It is shown here that flow analysis with voltammetric detection can be used to determine Cu, Pb, Zn and Ni in soil extracts. Cu, Pb and Zn were detected by square-wave anodic stripping voltammetry (SWASV), and Ni by square-wave cathodic stripping voltammetry (SWCSV). Advantages of the method are good sensitivity and the possibility to take the instrumentation into the field for on-site measurement. The reagents were: 0.2 M acetate buffer at pH 5.0 for Cu, Pb and Zn, and 0.04 M pH 9.0 ammonia buffer containing 20 μM DMG (dimethylglyoxime) for Ni. The reactive metal fraction was determined in 0.5 M HCl extracts (25 min extraction time) of soil samples from six different sites characterised by varying degrees of heavy metal pollution. The reactive metal fractions amounted to 23–73% of the metals in 5 of the 6 sites, whereas it was much lower (9–30%) at the other site. Organic matter did not interfere with the measurements as it was not extracted significantly by the 0.5 M HCl and as the extracts were diluted prior to analysis. The formation of intermetallic compounds at high levels of Cu and Zn was minimised by determining these metals in separate sample aliquots with calibration using separate standard additions, and by using different plating potentials and short plating times. Good accuracy was verified by analysis of reference materials.

Cathodic stripping voltammetric determination of tellurium(IV) at a Nafion/8-quinolinol mercury film modified electrode

A Nafion/8-quinolinol (oxine) coated mercury film chemically modified (NOMF) electrode for the determination of tellurium(IV) is described. The electrode is prepared by spin-coating a solution of the mixed Nafion/oxine onto the electrode surface. Subsequently, mercury is plated onto the electrode surface. Tellurium(IV) was preconcentrated onto the NOMF electrode from the sample solution containing 0.01 M ethylenediaminetetraacetate (EDTA), and determined by square-wave cathodic stripping voltammetry. This modified electrode showed improved resistance to interferences from surface-active compounds on the determination of tellurium(IV), especially for sodium dodecyl sulphate which is generally considered as a major interferent in the determination of tellurium(IV) at a hanging-mercury drop electrode. The addition of EDTA effectively eliminated the interferences from metal ions, such as molybdenum(VI) and lead(II), which are capable of forming complexes with oxine. Linear calibration graphs were obtained over the range 1–80 μg l −1 of tellurium(IV), with a detection limit (3× blank signal) of 0.2 μg/l.

Iodine speciation in the water column of the Rogoznica Lake (Eastern Adriatic Coast)

Specific concentration profiles of iodate, iodide and organo-iodine in the water column of the Rogoznica Lake, where both oxic and anoxic conditions occur, were observed. Iodate and iodide concentrations were determined directly in water samples by differential pulse voltammetry (DPV) and cathodic stripping square wave voltammetry (CSSWV), respectively. Concentrations of ‘labile’ and ‘stabile’ organo-iodine were determined by DPV in the samples pretreated with chlorine water and UV-irradiation (combined with hydrogen peroxide and followed by chlorine water addition), respectively. In this stratified water column (salinity and dissolved oxygen gradients) with pronounced biological productivity, high concentrations of iodide (up to 0.87 μM) and significant percentages of ‘labile’ and ‘stabile’ organo-iodine, up to 37% and 30% respectively, were found. The formation of ‘labile’ organo-iodine is primarily governed by chemical reactions with dissolved sulphur forms and organic compounds, as well as remineralization processes of ‘stabile’ organo-iodine.

Determination of Paraquat by Square-Wave Voltammetry at a Perfluorosulfonated Ionomer/Clay-Modified Electrode

A novel Nafion/clay-modified electrode (NCME) was developed for the determination of paraquat by square-wave cathodic stripping voltammetry. The clay that showed the best performance for the fabrication of the NCME is nontronite (SWa-1, ferruginous smectite). The electrochemical behavior of paraquat showed that the cathodic peak at -0.70 V vs Ag/AgCl permits adequate quantification of the analyte. Linear calibration curves are obtained over the 0-80 ppb range, with a detection limit of 0.5 ppb in pH 8 phosphate buffer solution for 4 min preconcentration time. Various factors influencing the determination of paraquat were thoroughly investigated in this study. The practical analytical utility is illustrated by selective measurements of paraquat in real water samples.

Electrochemistry of the nitroprusside ion. From mechanistic studies to electrochemical analysis

The electrochemical reduction of the nitroprusside ion (NP) was examined by differential-pulse polarography and square-wave voltammetry. The effect of pH on the electrochemical behaviour of NP was studied and experimental evidence for the occurrence of adsorption of the products of the first one-electron reduction of NP is shown. Owing to the adsorption of these products, which are the reactants in the second reduction step of NP, the peak current of the second process is highly enhanced compared with that of the first process, especially at low concentration levels (< 1 × 10–5 mol dm–3). For acidic solutions the enhancement is considerably higher than at pH 7. A surface reaction leading to the regeneration of the reactants of the second reduction process can be advanced as a possible explanation. Square-wave cathodic stripping voltammetry with adsorptive accumulation was used for the determination of NP. The detection limit depends on the square wave parameters and can be as low as 2.3 nmol dm–3 for a delay time of 60 s.

Square-wave cathodic stripping voltammetry of hydrolyzed uranyl species

Adsorption of uranyl-hydroxo species has been studied using square-wave cathodic stripping voltammetry (SWCSV). The peak of the uranyl ion between pH 5 and 8 has the characteristic features of a reduction complicated with reactant adsorption. The reduction process is totally irreversible, with an average transfer coefficient α = 0.47. The adsorbed species was found to be most probably UO 2(OH) 0 2, after correlation of the experimental results with the calculated ionic distribution in the solution. The experimental conditions for the lowest detection limit are given and it amounts to 2 × 10 −8 mol l −1 of uranyl hydroxo species. The maximum surface concentration is determined, to be Γ = (2.6 ± 0.1) × 10 −10 mol cm −2.

Direct determination of dissolved uranyl(VI) in sea-water by cathodic stripping voltammetry

The effect of the adsorption of uranyl species on the static mercury drop electrode is used for the electrochemical determination of dissolved uranium concentration in untreated and estuarine sea-water. The measurements were performed by square-wave cathodic stripping voltammetry. The standard additions method is successfully applied for measurements of natural uranyl concentrations in water samples which were found to be in the range between 4.3 and 15 nmol l–1 depending on the sampling location with detection limits of 2.4 and 10 nmol l–1, respectively.

Determination of 2-amino-5-nitrothiazole by square-wave cathodic stripping voltammetry

An electrochemical method has been developed for the detection and determination of 2-amino-5-nitrothiazole (2,5-ANT) by adsorption square-wave voltammetric stripping. The best sensitivity/resolution ratio was obtained by adsorption at pH 8.0 using a phosphate buffer, an accumulation potential of −10 mV (vs. Ag/AgCl 3 mol/l) and an accumulation time of 15 s. Under these conditions, the proposed method provides a linear electrode response over the 2,5-ANT concentration range 5–300 ng ml−1, and a detection and determination limit of 4 and 7.5 ng ml−1, respectively. The method was applied to the determination of 2,5-ANT in bacon.

Determination of selenium in soils with square-wave cathodic stripping voltammetry

A new voltammetric method is elaborated for the trace determination of selenium in soils. The use of the square-wave cathodic stripping voltammetry in the determination step efficiently eliminates the interference of electroactive components. The detection limit is 0.75 μg/l.

Determination of thyroxine in urine by cathodic stripping square-wave voltammetry.

A square-wave polarographic method for the determination of the amino acid thyroxine in urine, after isolation from the matrix on a C18 Sep-Pak cartridge and subsequent elution through a column packed with Amberlite IRA-400 anionic resin, was developed. The optimal working conditions for the application of the method were found to be pH 10, provided by a carbonate buffer, and a potential of -0.690 V during 10 min. The equilibration time was applied during 5 s, and potential scans were performed at a square-wave amplitude of 100 mV and a scan rate of 300 mV s-1, with a square-wave frequency of 100 Hz. The measuring-system response was linear over the thyroxine concentration range from 10 to 60 ng ml-1 and the detection limit achieved was 2.71 ng ml-1. The relative error and relative standard deviation obtained were 1.18 and 4.67%, respectively.