Differential pulse voltammetric determination of famotidine

A differential pulse voltammetric method for the determination of famotidine in pharmaceutical preparations is described. The method is based on electrochemical oxidation of famotidine at a glassy carbon or platinum electrode. The proposed method shows good reproducibility, and sample preparation is simple.     

Differential pulse voltammetric determination of trace silicon at a glassy carbon electrode

The heteropoly molybdosilicic acid complex produces five well-developed differential pulse voltammetric peaks at a glassy carbon electrode in citrate buffer solutions containing 20% 2-butanone with peak potentials in the neighborhood of +0.05 V, -0.10 V, -0.25 V, -0.50 V and -0.65 V (vs. Ag/AgCl, 0.1 M KCl). The peak current at each peak potential is clearly developed and is proportional to the silicon concentration; the linear range for the most useful peak at +0.05 V is 10^-5–10^-7 M silicon, the lower limit being fixed by the blank conditions. Nickel-base alloy samples and water samples were analyzed with satisfactory results.     

Differential pulse voltammetric assay of lercanidipine in tablets

Lercanidipine in ethanol-0.04M Britton-Robinson buffer (20 + 80) gives an irreversible anodic response on a glassy carbon electrode in a broad pH range (2-12) that depends on pH. This signal can be attributed to oxidation of the 1,4-dihydropyridine ring to give the corresponding pyridine derivative. For analytical purposes, differential pulse voltammetry at pH 4 was selected. Under these conditions, good values of both within- and interday reproducibility were obtained, with coefficient of variation (CV) values of 1.56 and 1.70%, respectively, for 10 successive runs. For quantitation, the calibration curve method was used for lercanidipine concentrations ranging from 1 x 10(-5) to 1 x 10(-4) M. The detection and quantitation limits were 1.39 x 10(-5) and 1.49 x 10(-5), respectively. A liquid chromatographic method with electrochemical detection was used for comparison. The voltammetric method showed good selectivity with respect to both excipients and degradation products. The recovery study exhibited a CV of 0.94% and an average recovery of 98.3%, and it was not necessary to treat the sample before the analysis. The method was successfully applied to the individual tablet assay of lercanidipine in commercial tablets.     

Differential pulse voltammetric oxidation of polyinosinic acid and its components

The differential pulse voltammetric oxidation of polyinosinic acid (poly(I)), inosine-5'-monophosphate, inosine and hypoxanthine at a pyrolytic graphite electrode has been studied. Poly(I) gives a single, pH-dependent voltammetric oxidation peak which is well separated from the single peak observed for very low concentrations of hypoxanthine. An electroanalytical method for the detection and determination of trace amounts of hypoxanthine in poly(I) samples is described.     

Differential pulse adsorptive stripping voltammetric determination of dinoseb and dinoterb at a modified electrode.

A sensitive adsorptive stripping voltammetric method for the determination of dinitrophenolic herbicides, dinoseb (DSB) and dinoterb (DTB) at a bare carbon paste electrode (CPE) and a clay modified carbon paste electrode (CMCPE) was developed. A systematic study of various experimental conditions, such as the pH, accumulation variables and composition of a modifier on the adsorptive stripping response, were examined by using differential pulse voltammetry. A significant improvement was observed in the sensitivity by using the present method with CMCPE. When CMCPE was used, a linear response was obtained over the concentration range 2 x 10(-10) to 3 x 10(-7) M and 6 x 10(-10) to 6 x 10(-7) M with lower detection limits of 1 x 10(-10) M and 5.4 x 10(-10) M for dinoseb and dinoterb, respectively, at an accumulation time of 100 s. The interference from other herbicides and ions on the stripping signals of both compounds was also evaluated. The described method was applied to estimate of the dinoseb and dinoterb in environmental samples.     

Differential pulse voltammetric studies of ethidium bromide binding to DNA

The interaction of ethidium bromide (EtBr) with calf thymus DNA is investigated electrochemically with the use of differential pulse voltammetry (DPV) at two different ionic strengths of a solution (0.154 M and 0.02 M [Na+], pH 7.0). It is revealed that EtBr binds with DNA in more than one way. The appropriate values of constants (K) and number site sizes (n) of EtBr binding to DNA are determined. The values of binding constants are equal to 1.9 x 10(6) and 5.6 x 10(5) M(-1), and number site sizes to 9 and 3.6 for strong interactions at ionic strengths of solutions 0.02 and 0.154 M Na+ at 28 degrees C, respectively. For a weaker interaction, these parameters are equal to 7 x 10(4) and 8 x 10(4) M(-1) and 1.5 and 1 at the mentioned ionic strengths of solutions, respectively. Thus, EtBr interacts with DNA in more than one way--intercalative and electrostatic at low ionic strength, and semi-intercalative and electrostatic at a higher strength of the solution. These results are in good accordance with the ones obtained by spectroscopic (absorption and fluorimetric) methods.     

Differential pulse voltammetric determination of uranium in low concentration streams

A differential pulse voltammetric method has been successfully used for the determination of uranium in low concentration streams of a uranium plant. The method gives a precision of about 13% to 7% in the range of 300 ppb to 15 ppm. The accuracy of the results was ascertained by comparing the values with those obtained by a spectrophotometric method. The method is simple, fast, sensitive, fairly accurate and does not require a preconcentration step.     

Differential pulse voltammetric determination of tin in the presence of noble metals

A voltammetric method for the determination of tin is proposed to minimise interferences from noble metals that are commonly encountered with other analytical techniques. Strong distortions of voltammetric peaks are observed in the presence of platinum. On the basis of a full investigation, the formation of an intermediate Sn(II)–Pt mixed chloro-complex at the electrode surface is identified as being responsible for the platinum interference, as it competes with the normal Sn(IV)→Sn(0)_Hg reduction. The use of a higher scan rate prevents the relatively low reaction kinetics and thus gets rid of this interference. No problems are encountered with other noble metals such as Pd, Ir, Re, Rh and Ru when using the modified method, although a baseline subtraction is necessary for the latter one. The proposed method is validated with real Pt–Sn catalysts.     

Target transform fitting in a voltammetric study of metal complexation

In this work target transform fitting (TTF) is proposed as a hard-model based data analysis method to analyze differential pulse polarograms recorded in a successive metal complexation system. In such cases, equations for both complexation equilibria and shape of the differential pulse polarogram for individual species are available. Utilizing TTF to fit these models in separate stages, stability constants and E_1/2 values of species were estimated. In the first stage, E_1/2 values of all species were acquired using the shape-equation of voltamograms and projection of a test vector into the row space of data matrix. In the second stage, using equations derived from relationships among species’ concentrations in complexation equilibria and a non-linear parameter fitting algorithm, the optimum values of overall formation constants were estimated. Finally, rank annihilation (RA) method was employed for calculation of diffusion coefficients. In spite of the fact that proposed method is a hard-model based approach, obtained results show that analysis can be performed correctly even in presence of unknown species. The reliability of the method was tested analyzing simulated data and also polarograms obtained from Cd(II) and 1, 10-phenanthroline complexation system. This experimental system yields three successive complexes which are relatively inert from electrochemical point of view. The results were in a good agreement with reported results in the literature.     

Differential pulse cathodic stripping voltammetric determination of nanomolar levels of dissolved sulfide applicable to field analysis of groundwater

Conventional batch mode analysis of dissolved sulfide by cathodic stripping voltammetry (CSV) is known to suffer from loss of sulfide in the cell to the waste mercury pool, compromising quantification of sulfide. Here we report a simple alternative approach to batch-mode differential pulse CSV (DPCSV). A fresh aliquot of sample is used for each voltammetric scan to minimize loss of sulfide through reaction with the mercury by limiting the time for sulfide-mercury contact, which is found to be more important in suppressing the sulfide signal than the amount of free mercury in the cell. Our improved batch-mode method exhibited a limit of detection of 1.3 nM, a relative standard deviation of 2.5% in NaOH supporting electrolyte and a linear response to as high a concentration as 1600 nM in a supporting electrolyte composed of Na₂CO₃/NaHCO₃ (pH 8.3) mixed with an equal volume of oxic groundwater. A relative standard deviation of 4.5% was obtained for a groundwater sample in Na₂CO₃/NaHCO₃ (pH 8.3) supporting electrolyte. These values are comparable to previously published results. Compared to other sensitive sulfide analytical techniques such as gas chromatography or high performance liquid chromatography (HPLC), DPCSV is preferred for sulfide analysis in the field due to its simple and portable instrumentation, lack of complex sample preparation, and short analysis time. The method was applied on site to analyze Fe-rich, reducing groundwater samples collected at a landfill site in Winthrop, Maine. Sulfide concentrations ranged from undetectable (<4 nM) to 7340 nM, generally increasing as the oxidation/reduction potential (ORP) of the water became more negative. We also demonstrate, for the first time, that the onset of sulfate reduction as indicated by the presence of small amounts of sulfide (tens to hundreds of nM) occurs in groundwater systems when the ORP value reaches −130 mV.     

Differential pulse voltammetric study of a typical anaerobic adhesive formulation coated on a glassy carbon electrode

The determination of copper (II) and iron (III) added to an anaerobic adhesive formulation was investigated by differential pulse voltammetry after application of a solution of the adhesive in acetone to a glassy carbon electrode. The best supporting electrolyte was 0.1 M sodium dodecyl sulphate, which ensured adequate surface contact with the adhesive coating. Under optimum conditions, copper (II) (as CuEDTA ^2?) could be determined at levels down to 0.1 mg l^?1 and iron (III) (in some complexed form) down to 2.0 mg l^?1. The method is also capable of detecting the presence of poly (ethylene glycol) dimethacrylate, cumene hydroperoxide and N,N-dimethyl-p-toluidine in a typical formulation.     

Differential pulse voltammetric determination of trace rotenone using molecularly imprinted polymer microspheres

A new voltammetric method for the determination of rotenone is described. It is based on the reduction of an electroactive derivative of rotenone on the surface of an electrode. Rotenone in water was pre-concentrated using a new type of molecularly imprinted polymer microspheres and can react with hydrazine chloride to produce the electroactive derivative. The experimental conditions were discussed. Under optimum conditions, it was found that the peak potential (Ep) of the derivative of rotenone is ?1.02 V (vs. Ag/AgCl). Using the proposed procedure rotenone can be determined in the range 0.2–400 μg L^?1. The detection limit for rotenone is 0.1 μg L^?1 and the relative standard deviation for 100 μg L^?1 rotenone is 1.99 %. The method was applied to the determination of rotenone in water samples with satisfactory results.     

Differential pulse polarographic determination of trace levels of iron(III) by using the catalytic current

Trace of iron(III) are determined by differential pulse polarography in a medium of sodium hydroxide and sodium bromate using the catalytic current. Various cations do not interfere. The relative standard deviation is 2%.     

Differential pulse voltammetric simultaneous determination of noradrenalin and acetaminophen using a hematoxylin biosensor

A highly efficient noradrenalin (NA) biosensor was fabricated on the basis of hematoxylin electrodeposited on a glassy carbon electrode, GCE. The cyclic voltammetric responses of the hematoxylin biosensor at various scan rates, which were obtained in a 0.25 mmol L⁻¹ NA solution, showed the characteristic shape typical of an EC_cat process. The kinetic parameters such as electron transfer coefficient, α, the catalytic electron transfer rate constant, k′, and the standard catalytic electron transfer rate constant, k⁰, for oxidation of NA at the hematoxylin biosensor surface were estimated using cyclic and RDE voltammetry. The peaks of differential pulse voltammetric (DPV) for NA and acetaminophen (AC) oxidation at the hematoxylin biosensor surface were clearly separated from each other when they co-exited in the physiological pH (pH 7.0). It was, therefore, possible to simultaneously determine NA and AC in the samples at a hematoxylin biosensor. Linear calibration curves were obtained for 5.0 × 10⁻¹ to 65.40 μmol L⁻¹ and 65.40-274.20 μmol L⁻¹ of NA, and for 12.00-59.10 μmol L⁻¹ and 59.10-261.70 μmol L⁻¹ of AC. The sensitivities of the biosensor to NA in the absence and presence of AC were found virtually the same, which indicates the fact that the electrocatalytic oxidation processes of NA are independent of AC and, therefore, simultaneous or independent measurements of the two analytes (NA and AC) are possible without any interference. The results of 16 successive measurements show an average voltammetric peak current of 1.13 ± 0.03 μA for an electrolyte solution containing 5.00 μmol L⁻¹ NA. The hematoxylin biosensor has been satisfactorily used for the determination of NA and AC in pharmaceutical formulations. The results obtained, using the biosensor, are in very good agreement with those declared in the label of pharmaceutical inhalation products.     

Differential pulse anodic stripping voltammetric determination of ziram (a dithiocarbamate fungicide)

A d.c. polarographic technique has been used previously for the determination of the pesticide, ziram, in aqueous samples, this paper reports differential pulse anodic stripping voltammetric determination of ziram zinc in rice samples using a static mercury drop electrode. The procedure developed distinguishes inorganic zinc and ziram zinc in sodium acetate-sodium chloride media. The procedure developed is suitable for the determination of concentrations as low as 10 ppb of ziram with a precision of 2.1% for five successive determinations of 150 ppb of ziram.     

Differential pulse voltammetric study of the complexation of Cd(II) by the phytochelatin (γ-Glu---Cys)2Gly assisted by multivariate curve resolution

A multivariate curve resolution method by alternating least-squares (MCR-ALS) is applied to differential pulse voltammograms measured on the Cd(II)+(γ-Glu---Cys)₂Gly system as a model of metal–phytochelatin interactions at concentrations of both components in the range 10⁻⁷–10⁻⁵ mol l⁻¹. The course of complexation is different when peptide is titrated with metal from that when metal is titrated with peptide. The combined analysis of both matrices from titrations of peptide with metal and of metal with peptide allowed the resolution of the system. The analysis of the resulting pure voltammograms and concentration profiles of the resolved components suggested the presence of four different types of bound Cd(II) and made possible the formulation of a complexation model.     

Differential pulse polarographic determination of arsenic, selenium and tellurium at mug levels

An analytical method, based on differential pulse polarography, for determination of arsenic, selenium and tellurium in solid matrices, is described. The method involves decomposition of the matrix with a mixture of nitric, perchloric and hydrofluoric acid, isolation of tellurium from the other analytes by liquid-liquid extraction (from 4M hydrochloric acid with methyl isobutyl ketone), and determination of the analytes. Tellurium is determined separately, and arsenic is determined in the same solution as selenium after determination and oxidation of the selenium and addition of catechol. Graphitized carbon black and chelating resin were used to eliminate the organic solvent in the aqueous solution and avoid interferences due to the other metals of the matrix. The decomposition, the influence of each analyte on the determination of the others, and the extraction process were given particular attention. The method is characterized by>96% recovery, with a relative standard deviation ranging from 2 to 10% at ppm levels.     

Differential pulse voltammetric methods for the simultaneous estimation of uranium-iron and uranium-cadmium mixtures in solution

Differential pulse voltammetric methods have been developed for the simultaneous estimation of the constituents of uranium-iron and uranium-cadmium mixtures in solution. A mixture of 1M H₃PO₄–1M KH₂PO₄ (with a pH1.5), was found to be the most ideal supporting electrolyte for both methods, among many that were evaluated for their suitability. In uranium-iron mixtures the calibration for iron was found to be linear up to 150 g ml^–1 (r²=0.9986), while that of uranium up to 500 g ml^–1 (r²=0.999). Iron at 6.7 g ml^–1 level could be determined in the presence of 800 fold uranium (wt/wt) without significant interference. Uranium at 21 g ml^–1 level could be analyzed with 5-fold iron (wt/wt). This upper limit of iron was due to the precipitation of iron as phosphate. In the case of uranium — cadmium mixtures, cadmium calibration for cadmium was found to be linear up to 1300 g ml^–1 (r²=0.9993). Concentration levels of 4.6 g ml^–1 Cd could be determined at a 500-fold excess (wt/wt) of uranium. Uranium calibration was linear up to 500 g ml^–1 (r²=0.999) and 21 g ml^–1 uranium could tolerate up to a 1000-fold excess of cadmium (wt/wt). Both procedures could tolerate 10 g ml^–1 levels of metal ions, such as chromium, copper, manganese, molybdenum and vanadium.     

Anionic ligand effect on the nature of epoxidizing intermediates in iron porphyrin complex-catalyzed epoxidation reactions

We have studied an anionic ligand effect in iron porphyrin complex-catalyzed competitive epoxidations of cis- and trans-stilbenes by various terminal oxidants and found that the ratios of cis- to trans-stilbene oxide products formed in competitive epoxidations were markedly dependent on the ligating nature of the anionic ligands. The ratios of cis- to trans-stilbene oxides obtained in the reactions of Fe(TPP)X (TPP = meso-tetraphenylporphinato dianion and X(-) = anionic ligand) and iodosylbenzene (PhIO) were 14 and 0.9 when the X(-) of Fe(TPP)X was Cl(-) and CF(3)SO(3)(-), respectively. An anionic ligand effect was also observed in the reactions of an electron-deficient iron(III) porphyrin complex containing a number of different anionic ligands, Fe(TPFPP)X [TPFPP = meso-tetrakis(pentafluorophenyl)porphinato dianion and X(-) = anionic ligand], and various terminal oxidants such as PhIO, m-chloroperoxybenzoic acid (m-CPBA), tetrabutylammonium oxone (TBAO), and H(2)O(2). While high ratios of cis- to trans-stilbene oxides were obtained in the reactions of iron porphyrin catalysts containing ligating anionic ligands such as Cl(-) and OAc(-), the ratios of cis- to trans-stilbene oxide were low in the reactions of iron porphyrin complexes containing nonligating or weakly ligating anionic ligands such as SbF(6)(-), CF(3)SO(3)(-), and ClO(4)(-). When the anionic ligand was NO(3)(-), the product ratios were found to depend on terminal oxidants and olefin concentrations. We suggest that the dependence of the product ratios on the anionic ligands of iron(III) porphyrin catalysts is due to the involvement of different reactive species in olefin epoxidation reactions. That is, high-valent iron(IV) oxo porphyrin cation radicals are generated as a reactive species in the reactions of iron porphyrin catalysts containing nonligating or weakly ligating anionic ligands such as SbF(6)(-), CF(3)SO(3)(-), and ClO(4)(-), whereas oxidant-iron(III) porphyrin complexes are the reactive intermediates in the reactions of iron porphyrin catalysts containing ligating anionic ligands such as Cl(-) and OAc(-).     

Differential pulse polarography of cadmium-and lead-urate and adsorptive stripping voltammetric determination of uric acid

The complex formation between uric acid and zinc, cadmium and lead ions has been investigated using differential pulse polarography in 0.01M NaNO(3). It is found that the complexes formed by Cd(II) and Pb(II) ions with uric acid have the stoichiometry of 1:2 and the logarithmic values of the apparent stability constant are 9.47 and 11.7, respectively. On the other hand, zinc(II) ions do not give any indication of complexation with uric acid. A sensitive voltammetric method is developed for the quantitative determination of uric acid. This method is based on controlled adsorptive preconcentration of uric acid on the hanging mercury drop electrode (HMDE), followed by tracing the voltammogram in the cathodic going potential scan. The modes used are direct current stripping voltammetry (DCSV) and differential pulse stripping voltammetry (DPSV). The detection limits found were 8 x 10(-9)M (quiescent period 15 sec) by DPSV and 1.6 x 10(-8)M by DCSV.