Adsorptive Cathodic Stripping Voltammetry Research Articles

Simultaneous Adsorptive Cathodic Stripping Voltammetric Determination of Nickel(II) and Cobalt(II) at an In Situ Bismuth‐Modified Gold Electrode

AbstractA study on the simultaneous determination of Ni(II) and Co(II) dimethylglyoximates (Ni‐DMG and Co‐DMG) through adsorptive cathodic stripping voltammetry at an in situ bismuth‐modified gold electrode (Bi‐AuE) is reported. The key operational parameters, such as Bi(III) concentration, accumulation potential and accumulation time were optimized and the morphology of the Bi‐microcrystals deposited on the Au‐electrode was studied. The Bi‐AuE allowed convenient analysis of trace concentrations of solely Ni(II) or of Ni(II) and Co(II) together, with cathodic stripping voltammograms characterized by well‐separated stripping peaks. The calculated limit of detection (LOD) was 40 ng L−1 for Ni(II) alone, whereas the LOD was 98 ng L−1 for Ni(II) and 58 ng L−1 for Co(II), when both metal ions were measured together. The optimized method was finally applied to the analysis of certified spring water (NIST1640a) and of natural water sampled in the Lagoon of Venice. The results obtained with the Bi‐AuE were in satisfactory agreement with the certified values and with those provided by complementary techniques, i.e., ICP‐OES and ICP‐MS.

Determination of aluminum in botanical samples by adsorptive cathodic stripping voltammetry as Al-8-hydroxyquinoline complex

A fast and sensitive new procedure for the determination of total aluminum by adsorptive cathodic stripping voltammetry (AdCSV) in plant samples consumed for dietary and/or medicinal purposes was described. The Al(III)-8-hydroxyquinoline (Al-8HQ) complex was adsorbed on a hanging mercury dropping electrode (HMDE) at 0.300V versus Ag/AgCl electrode during a 30s accumulation time. An electrochemical cell containing 0.024molL−1 ammonia–ammonium acetate buffer (pH6.80) as supporting electrolyte and 7.43mgL−1 8-hydroxyquinoline (8-HQ) as ligand for aluminum were employed. A linear response was observed over the 8.54×10−8 to 2.35×10−7molL−1 range with a limit of detection (LOD) of 8.54×10−8molL−1 and a relative standard deviation (RSD; n=3) of 1.86% for 8.54×10−8molL−1 aluminum solution. Aluminum contents in green tea (Camellia sinensis), carqueja (Baccharis genistelloides), cascara sagrada (Rhamnus purshiana) and senna (Cassia acutifolia) were 502.5±14.1, 195.6±21.0, 309.1±11.7, 205.6±16.1μg aluminum per g of dry tea, respectively. The accuracy of the developed method was evaluated by flame atomic absorption spectrometry (FAAS) and all of the results were similar. It indicates that AdCSV could be efficiently applied in the determination of aluminum as 8-HQ complex in real food samples.

Differentiating Homogeneous and Heterogeneous Water Oxidation Catalysis: Confirmation that [Co4(H2O)2(α-PW9O34)2]10– Is a Molecular Water Oxidation Catalyst

Distinguishing between homogeneous and heterogeneous catalysis is not straightforward. In the case of the water oxidation catalyst (WOC) [Co4(H2O)2(PW9O34)2](10-) (Co4POM), initial reports of an efficient, molecular catalyst have been challenged by studies suggesting that formation of cobalt oxide (CoOx) or other byproducts are responsible for the catalytic activity. Thus, we describe a series of experiments for thorough examination of active species under catalytic conditions and apply them to Co4POM. These provide strong evidence that under the conditions initially reported for water oxidation using Co4POM (Yin et al. Science, 2010, 328, 342), this POM anion functions as a molecular catalyst, not a precursor for CoOx. Specifically, we quantify the amount of Co(2+)(aq) released from Co4POM by two methods (cathodic adsorptive stripping voltammetry and inductively coupled plasma mass spectrometry) and show that this amount of cobalt, whatever speciation state it may exist in, cannot account for the observed water oxidation. We document that catalytic O2 evolution by Co4POM, Co(2+)(aq), and CoOx have different dependences on buffers, pH, and WOC concentration. Extraction of Co4POM, but not Co(2+)(aq) or CoOx into toluene from water, and other experiments further confirm that Co4POM is the dominant WOC. Recent studies showing that Co4POM decomposes to a CoOx WOC under electrochemical bias (Stracke and Finke, J. Am. Chem. Soc., 2011, 133, 14872), or displays an increased ability to reduce [Ru(bpy)3](3+) upon aging (Scandola, et al., Chem. Commun., 2012, 48, 8808) help complete the picture of Co4POM behavior under various conditions but do not affect our central conclusions.

New approach to analysis of voltammetric ligand titration data improves understanding of metal speciation in natural waters

In this study, previously published multiple analytical window datasets of competitive ligand exchange—adsorptive cathodic stripping voltammetry titrations for dissolved copper with salicylaldoxime as the competing ligand were reanalyzed. A new numerical method was applied, the Sander‐Wells (S‐W) method, which is able to simultaneously analyze multiple analytical window titrations as a unified dataset and calculate parameters for one to three ligand classes in the samples. The two datasets used were from separate sampling events at Dumbarton Bridge, San Francisco Bay. The aim of this study was to see if discrete ligand classes can be resolved from these data, and how these compare with results obtained by applying commonly used manual calculations of individual analytical window titrations separately: Scatchard and van den Berg/Ružic linearizations, and the Gerringa non‐linear regression. The united multiple analytical window dataset could only be analyzed simultaneously by the S‐W method, and two or three discrete ligand classes were resolved for each sample. Applying the four different data analysis methods independently to each of the five analytical windows resulted in solutions spanning a continuum of conditional stability constants and ligand concentrations. It was shown that the sensitivity S, as well as data structure and iterative optimization, are the major reasons for different results. This study demonstrates the utility of simultaneous analysis of multiple analytical window titration curves to better determine the distribution of trace metal species in aqueous systems and the advantages of adopting such a ‘unified data set’ approach over fitting individual titration curves.

Copper binding by terrestrial versus marine organic ligands in the coastal plume of River Thurso, North Scotland

It is generally assumed that terrestrial humic matter contains a high density of metal complexing ligands spanning a wide range of stability constants. From equilibrium modelling it would be anticipated that carboxyl groups, which are known to be abundant in humic matter, should be capable of binding Cu quantitatively in humic-rich estuarine systems such as the river plume outflow of River Thurso, on Scotland's north coast, where concentrations along the full mixing gradient typically range for Cu: 1−25 nM and for DOC (Dissolved Organic Carbon): 150−1500 μM. Using a combination of reverse and forward titrations monitored by adsorptive cathodic stripping voltammetry (ACSV), we show that terrestrial humic substances were the likely source of high-affinity (class 1), low abundance ligands (1−4 nM) found throughout the coastal plume, but that weaker (class 2), more abundant ligands (90−170 nM) appeared to be produced largely in situ, probably as a biological response to Cu inputs into the estuary. Conditional stability constants for Cu-binding were respectively log K1′ = 14.9–15.9 and log K2′ = 11.8–12.6. Class 2 ligands were evenly distributed between the low and high molecular weight fractions of dissolved organic matter (DOM), both in March and May. Class 1 ligands were more abundant in the low molecular weight fraction in March, but not in May. The latter could be related to seasonal differences in the nature of peatland derived organic matter carried by River Thurso. Since class 1 ligands (terrestrial, humic) were saturated with Cu throughout the plume, it effectively fell upon class 2 ligands (marine, biological) to control Cu speciation in Thurso Bay, in particular keeping the free Cu concentration [Cu2+] within a relatively narrow range (10−13.6 M to 10−12.4 M). On the other hand, class 1 ligands were responsible for solubilising small amounts of Cu derived from this peatland catchment and effectively transporting Cu, in dissolved and colloidal forms, to the ocean.

Stripping voltammetry method for determination of manganese as complex with oxine at the carbon paste electrode with and without modification with montmorillonite clay

Oxine (8-hydroxyquinoline) was used as an efficient and selective ligand for stripping voltammetry trace determination of Mn(II). A validated square-wave adsorptive cathodic stripping voltammetry method has been developed for determination of Mn(II) selectively as oxine complex using both the bare carbon paste electrode (CPE) and the modified CPE with 7 % (w/w) montmorillonite-Na clay. Modification of carbon paste with montmorillonite clay was found to greatly enhance its adsorption capacity. Limits of detection of 45 ng l−1 (8.19 × 10−10 mol L−1) and 1.8 ng l−1 (3.28 × 10−11 mol L−1) Mn(II) were achieved using the bare and modified CP electrodes, respectively. The achieved limits of detection of Mn(II) as oxine complex using the modified CPE are much sensitive than the detection limits obtained by most of the reported electrochemical methods. The developed stripping voltammetry method using both electrodes was successfully applied for trace determination of Mn(II) in various water samples without interferences from various organic and inorganic species.

The stabilisation and transportation of dissolved iron from high temperature hydrothermal vent systems

Iron (Fe) binding phases in two hydrothermal plumes in the Southern Ocean were studied using a novel voltammetric technique. This approach, reverse titration–competitive ligand exchange–adsorptive cathodic stripping voltammetry, showed that on average 30±21% of dissolved Fe in the hydrothermal plumes was stabilised by chemically labile binding to ligands. The conditional stability constant (logK′FeL) of the observed complexes was 20.61±0.54 (mean±1 SD) for the two vent sites, intermediate between previous measurements of deep ocean ligands (21.4–23; Kondo et al., 2012) and dissolved weak estuarine ligands (<20; Gerringa et al., 2007).Our results indicate that approximately 7.5% of all hydrothermal Fe was stabilised by complexation with ligands. Furthermore, 47±26% of the dissolved Fe in the plume existed in the colloidal size range (0.02–0.2µm). Our data suggests that a portion (∼7.5%) of hydrothermal Fe is sufficiently stabilised in the dissolved size fraction (<0.2µm) to make an important impact on deep ocean Fe distributions. Lateral deep ocean currents transport this hydrothermal Fe as lenses of enhanced Fe concentrations away from mid ocean ridge spreading centres and back arc basins.

The speciation of copper across active gradients in nitrogen‐cycle processes in the eastern tropical South Pacific

Copper (Cu) complexation and distribution were characterized using competitive ligand exchange adsorptive cathodic stripping voltammetry and isotope dilution inductively coupled plasma mass spectrometry along two transects (20°S and 10°S) in the eastern tropical South Pacific. In the southern and westernmost stations, Cu showed upper water column depletion to values as low as ∼ 0.26 nmol L−1, the lowest concentrations ever reported. However, Cu levels were much higher within the secondary nitrite maxima of the oxygen minimum zone (OMZ) in the northern (10°S) transect. The enrichment of Cu in the reducing conditions of the OMZ has not been reported before and probably reflects remineralization and offshore transport from the shelf. Free [Cu2+] was typically low throughout the water column, ranging from about 3.15 × 10−15 mol L−1 to 1.34 × 10−13 mol L−1, and depth profiles exhibited similar features to those for dissolved Cu, though they showed more variability near the surface. Offshore and beyond the influence of the OMZ, the lowest dissolved and free [Cu2+] was within the primary nitrite maxima (PNM), where ammonia oxidation and nitrate reduction rates are important. This finding is of interest because the two competing explanations for the PNM—iron limitation of diatoms and high rates of ammonia oxidation relative to nitrite oxidation—have high Cu requirements. The low concentrations of free Cu2+ measured here could impose significant constraints on the rates of these processes.

Simultaneous determination of textile dyes by Adsorptive Cathodic Stripping Voltammetry

The Adsorptive Cathodic Stripping Voltammetry (ACSV - differential pulse) proved to be an efficient method in the separation and quantification of two reactive textile dyes, Procion Yellow (PY) and Procion Red (PR), each containing two azo groups as chromophores and two monochlorotriazine reactive groups, at nanomolar level. The linearity of the method obtained for each reactive dye was 30.15 – 633.15 nmol L -1 for PY in B-R buffer pH 8.0, and 62.56 – 982.291 nmol L -1 for PR in B-R buffer pH 8.0, using preconcentration at -0.1 V during 60 s.

Simultaneous Determination of Nickel and Cobalt Using a Solid Bismuth Vibrating Electrode by Adsorptive Cathodic Stripping Voltammetry

AbstractA simple, fast, sensitive and greener voltammetric procedure for simultaneous analysis of nickel (Ni) and cobalt (Co) by square wave adsorptive cathodic stripping voltammetry (SW‐AdCSV) using a solid bismuth vibrating electrode is presented for the first time. The procedure enables to determine Ni together with Co, in ammonia buffer 0.1 M (pH 9.2) and in the presence of oxygen, and involves an adsorptive accumulation of metal‐dimethylglyoxime (Ni‐DMG and Co‐DMG) complexes on the electrode surface. For Ni and Co, the detection limits, obtained with 30 s of accumulation time, were 0.6 and 1.0 µg L−1, respectively. The method was free of metals (Cd2+, Cr3+, Cr6+, Cu2+, Fe3+ and Pb2+ up to 50 µg L−1, Al3+ and Mn2+ up to 500 µg L−1; Zn2+ up to 300 µg L−1) interferences up to the concentrations mentioned in brackets. The proposed method was validated for simultaneous determination of Ni and Co in a certified reference surface and river waters with good results.

Sensitive Voltammetric Determination of Titanium(IV) in Catalytic Adsorptive Mandelic Acid‐Chlorate(V) System on Renewable Silver Amalgam Film Electrode

AbstractThe renewable mercury film electrode, applied for the determination of titanium(IV) ultratraces using differential pulse catalytic adsorptive cathodic stripping voltammetry (DP CAdSV) in the presence of mandelic acid as a ligand and chlorate(V) as an oxidant is represented. The calibration graph obtained for Ti(IV) is linear from 0.05 nM (2.39 ng L−1) to 95 nM (4.55 µg L−1) for a preconcentration time of 30 s, with correlation coefficient of 0.9995. For the renewable mercury electrode (Hg(Ag)FE) with a surface area of 8.6 mm2 the detection limit for a preconcentration time of 180 s is as low as 0.006 nM (0.29 ng L−1). The repeatability of the method at a concentration level of the analyte as low as 23.9 ng L−1, expressed as RSD is 1.8 % (n=5). The proposed method was successfully applied and validated by studying the certified reference material NWTM‐26.3 and simultaneously recovery of Ti(IV) from spiked samples.

Quantification of Iron in Seawater at the Low Picomolar Range Based on Optimization of Bromate/Ammonia/Dihydroxynaphtalene System by Catalytic Adsorptive Cathodic Stripping Voltammetry

A new analytical protocol for the challenging analysis of total dissolved iron at the low picomolar level in oceanic waters suitable for onboard analysis is presented. The method is based on the revision of the adsorptive properties of the iron/2,3-dihydroxynaphthalene (Fe/DHN) complexes on the hanging mercury drop electrode with catalytic enhancement by bromate ions. Although it was based on a previously proposed reagent combination, we show here that the addition of an acidification/alkalinization step is essential in order to cancel any organic complexation, and that an extra increment of the pH to 8.6-8.8 leads to the definition of a preconcentration-free procedure with the lowest detection limit described up to now. For total dissolved iron analysis, samples were acidified to pH 2.0 in the presence of 30 μM DHN and left to equilibrate overnight. A 10 mL sample was subsequently buffered to a pH of ∼8.7 in the presence of 20 mM bromate: a 60 s deposition at 0 V led to a sensitivity of 34 nA nM(-1) min(-1), a 4-fold improvement over previous methods, that translated in a limit of detection of 5 pM (2-20 fold improvement). Several tests proved that a nonreversible reaction in the time scale of the analysis, triggered by the acidification/alkalinization step, was behind the signal magnification. The new method was validated onboard via the analysis of reference material and via intercalibration against flow injection analysis-chemiluminescence on Southern Ocean surface samples.

Measurement of iron chemical speciation in seawater at 4 °C: The use of competitive ligand exchange–adsorptive cathodic stripping voltammetry

Iron is mostly bound to poorly characterised organic ligands; thus, organic ligands are paramount in defining Fe biogeochemical cycling and its control on oceanic primary productivity. Since 1994, Fe chemical speciation has been determined by Competitive Ligand Exchange–Adsorptive Cathodic Stripping Voltammetry (CLE–AdCSV) at room temperature. However, chemical speciation is strongly dependent on temperature and some organic ligands can be temperature sensitive. Here, we compare the use of the CLE–AdCSV at room temperature and at 4°C—a temperature closer to that found in the Southern Ocean, one of the largest iron-limited regions. For both temperatures, similar detection limits and total Fe concentrations were found. However, at 4°C the analytical detection window (αFe(TAC)2) was shifted by 1.4-fold towards the detection of weaker ligands, resulting in up to 2-fold lower ligand concentrations as well as a 2- to 5-fold and 10- to70-fold lower conditional stability constants with inorganic Fe (Fe′) and Fe(III), respectively. As a result, the Fe′ concentration at 4°C was 2-fold greater, resulting in direct implication for Fe bioavailability. Results show that difference in Fe chemical speciation at 4°C was not solely explained by temperature effect on thermodynamics with the exchange ligands or the diffusion of the electroactive complex towards the Hg drop. Lowering analytical window during analysis at room temperature is proposed as a first estimate of temperature effect on iron chemical speciation.

Assay of Amlodipine Besylate in Tablets and Human Biological Fluids by Square Wave Adsorptive Cathodic Stripping Voltammetry

Assay of Amlodipine Besylate in Tablets and Human Biological Fluids by Square Wave Adsorptive Cathodic Stripping Voltammetry

Assay of Amlodipine Besylate in Tablets and Human Biological Fluids by Square Wave Adsorptive Cathodic Stripping Voltammetry

The adsorption behavior of the antihypertensive and antianginal agent amlodipine besylate onto a Hanging Mercury Dropping Electrode (HMDE) was explored by square wave and cyclic voltammetry. The drug accumulat ed at HMDE and a well-defined peak was obtained at �1.73V versus Ag/AgCl (saturated KCl) in phosphate buffer at pH 7. A validated simple, rapid and sensitive square -wave voltammetric procedure was described for direct determination of the authentic cited drug. The optimum procedure conditions were accumulation potential E

Characterisation of iron binding ligands in seawater by reverse titration

Here we demonstrate the use of reverse titration - competitive ligand exchange–adsorptive cathodic stripping voltammetry (RT-CLE–ACSV) for the analysis of iron (Fe) binding ligands in seawater. In contrast to the forward titration, which examines excess ligands in solution, RT-CLE–ACSV examines the existing Fe-ligand complexes by increasing the concentration of added (electroactive) ligand (1-nitroso-2-naphthol) and analysis of the proportion of Fe bound to the added ligand. The data manipulation allows the accurate characterisation of ligands at equal or lower concentrations than Fe in seawater, and disregards electrochemically inert dissolved Fe such as some colloidal phases. The method is thus superior to the forward titration in environments with high Fe and low ligand concentrations or high concentrations of inert Fe.We validated the technique using the siderophore ligand ferrioxamine B, and observed a stability constant K′Fe3+FoB of 0.74–4.37×1021mol−1, in agreement with previous results. We also successfully analysed samples from coastal waters and a deep ocean hydrothermal plume. Samples from these environments could not be analysed with confidence using the forward titration, highlighting the effectiveness of the RT-CLE–ACSV technique in waters with high concentrations of inert Fe.

Trace and selective determination of cobalt(II) in water and salt samples using cathodic adsorptive stripping voltammetry in the presence of pyrogallol red

A sensitive and selective procedure is presented for voltammetric determination of cobalt. The procedure involves an adsorptive accumulation of cobalt pyrogallol red (PGR) complex on stationary mercury drop electrode, followed by cathodic stripping voltammetry measurement of reduction current of adsorbed complex at -1.17 V (vs. Ag/AgCl). The optimum conditions for determination of cobalt include pH 11.0, 35 ?M pyrogallol red, an accumulation potential of -0.9 V (vs. Ag/AgCl) and scan rate 80 mVs-1. The peak current is proportional to the concentration of cobalt over the concentration range of 5.0 to 280 ng mL-1 with a detection limit of 1 ng mL-1 with an accumulation time of 140 s. The method was applied for the determination of cobalt in analytical grade NaCl and water samples.

Square-Wave Adsorptive Cathodic Stripping Voltammeteric Determination of Manganese (II) Using a Carbon Paste Electrode Modified with Montmorillonite Clay

Manganese is an essential micronutrient for all organisms; however at high concentrations it has a toxic effect. Manganese toxicity is a serious constraint to crop cultivation since it is taken-up by plants and can easily be passed into the food chain again causing symptoms of Parkinson’s disease. A fully validated square-wave adsorptive cathodic stripping voltammetry method has been developed for determination of Mn (II) as a complex with 2-(5’-bromo-2’-pyridylazo) 5-diethylaminophenol in aqueous solutions using a carbon paste electrode (CPE) modified with montmorillonite-Na clay. The results showed that the modified CPE (90% (w/w) graphite powder and 10% (w/w) montmorillonite-Na clay) exhibited excellent electrochemical activity towards the investigated Mn (II) complex in acetate buffer of pH = 5.0. Factors affecting the performance of the modified carbon paste electrode and the sensitivity of the described square- wave stripping voltammetry method, including the electrode composition, concentration of ligand, pulse parameters and preconcentration conditions were examined. A detection limit (S/N = 3) of 0.015μg·L-1 (2.73 × 10-10 mol·L-1) Mn (II) was achieved when a preconcentration time of 240 s was applied. Insignificant interferences from various inorganic and organic species were estimated. The described square-wave adsorptive cathodic stripping voltammetry method coupled with the modified carbon paste electrode has been successfully applied to Mn (II) analysis in different water samples.

Determination of Total Germanium in Chinese Herbal Remedies by Square-Wave Catalytic Adsorptive Cathodic Stripping Voltammetry at an Improved Bismuth Film Electrode

A catalytic adsorptive cathodic stripping voltammetric method on an improved bismuth film electrode (BiFE) for the determination of trace germanium in the presence of pyrogallol has been investigated. A well-defined and sensitive stripping peak of Ge(IV)-pyrogallol complex was observed at −0.79 V (versus SCE) in a 0.1 M acetate buffer solution (pH 4.8) at a deposition potential of −0.34 V. The reduction current is catalytically enhanced by adding KBrO3. The experimental variables and potential interference were studied. Compared with the BiFE plated in the solution prepared based on HAc-NaAc without trisodium citrate, the improved BiFE electrodeposited in the solution of HAc-NaAc containing trisodium citrate displayed a better electroanalytical performance for the determination of germanium(IV). Under the optimized conditions, the detection limit of Ge(IV) was 60 ng L−1, and the relative standard deviation (RSD) was 3.73% at 5 μg L−1level (n=9). This method was successfully applied to determine the total germanium in several Chinese herbal remedies.

Determination of Trace Chromium by Square-Wave Adsorptive Cathodic Stripping Voltammetry at an Improved Bismuth Film Electrode

A square-wave adsorptive cathodic stripping voltammetric protocol coupled with an improved bismuth film electrode (BiFE) for the determination of trace chromium in the presence of 4-(2-pyridylazo) redorcinol (PAR) has been investigated. A well-defined and sensitive stripping peak of the Cr(III)-PAR complex was observed at −0.57 V (versus SCE) in a 0.1 mol/L acetate buffer solution (pH 5.0) at a deposition potential of −0.45 V. The experimental variables as well as potential interferences were studied. Compared with the BiFE plated in the solution prepared based on HAc-NaAc without trisodium citrate, the improved BiFE electrodeposited in the solution of HAc-NaAc prepared based on containing trisodium citrate displayed a better electroanalytical performance for the determination of chromium(III). Under the optimized conditions, the limit of detection for chromium was 2.0 × 10−10 mol/L and the relative standard deviation was 4.35% at the 1.0 × 10−8 mol/L level (n = 7). This method was applied to determine the trace amount of chromium in several environmental samples.