Pulse Polarograms Research Articles

Determination of metal ion binding parameters for humic substances

Extraction of valid metal ion complexation parameters for heterogeneous ligands from voltammetric data is fraught with difficulties. We apply a straightforward calculation method to pulse polarograms, which overcomes some of the limitations. Our method takes into account the heterogeneity of complexants such as fulvic acids through the use of two parameters which are inherent properties of the substance under study and which allow complexation parameters to be calculated for other metal-loading conditions. The results obtained are used to generate theoretical polarograms, by using a numerical simulation model that allows kinetic effects to be taken into account. Application of this methodology to Cu(II)- and Pb(II)-complexation by Suwannee River fulvic acid showed that RPP polarographic waves for both of these metals are affected to some extent by adsorption of the ligand on the working electrode surface. For Cu(II), the complexation parameters were strongly dependent on the experimental conditions, implying that the overall impact of adsorption is far more significant than for Pb(II).

Determination of hypericin in herbal medicine products by differential pulse polarography

A differential pulse polarographic method is presented for the determination of total hypericin in phytotherapeutic preparations (drops, tablets and capsules). The polarographic behaviours of hypericin and of pseudohypericin were examined in various buffer systems over the pH range 3.5–10.0. In Britton Robinson buffer:methanol solution (at pH 6.0) the differential pulse polarograms exhibited reproducible peaks at Ep−1.02 V vs silver/silver chloride for hypericin, and at −1.00 V for pseudohypericin. Under these conditions, a plot of peak height against concentration of hypericin was found to be linear over the range 0.5–9.0 µg/mL (r = 0.9994) and 9.0–16.0 µg/mL (r = 0.9987). The polarographic method was applied to the determination of the content of total hypericin (hypericin, pseudohypericin, protohypericin and protopseudohypericin) in herbal medicinal products containing Hypericum perforatum. The precursors protohypericin and protopseudohypericin were converted into hypericin and pseudohypericin, respectively, by subjecting them to artificial light or daylight prior to analysis. Under these conditions, no separation step was required for the polarographic analysis. In order to evaluate the total concentration of hypericin, the standard addition method with hypericin as standard was applied. The relative standard deviation involved in analysing various herbal medicinal products ranged from ±1.9 to 2.9%. Copyright © 2000 John Wiley & Sons, Ltd.

Cadmium Succinate and Cadmium Malate Stability Constants Revisited

The complexes formed in the cadmium(II) succinate (suc2-) hydrogen ion and in the cadmium(II) malate (mal2-) hydrogen ion systems in aqueous solution at 37 °C and I = 150 mmol·dm-3 (NaCl) have been characterized by means of glass-electrode potentiometry. The succinate and malate protonation constants were found to be 5.187 ± 0.001, 9.135 ± 0.002 and 4.618 ± 0.001, 7.804 ± 0.002, respectively. The formation constants for the complexes Cdsuc, CdsucH+, and Cdmal were found to be 1.63 ± 0.02, 6.26 ± 0.06, and 1.25 ± 0.05, respectively. Particular attention has been paid to the evaluation of the effect of possible systematic errors on the constant values determined. Reliable standard deviation estimations have been made by applying a Monte Carlo calculation technique. Cyclic voltammograms and differential pulse polarograms for both cadmium(II) systems were consistent with formation of weak, labile complexes.

Determination of 5-hydroxynaphthoquinones in phytotherapeuticDrosera preparations by differential pulse polarography

A differential pulse polarographic method is described for the determination of three pharmacologically active 5-hydroxynaphthoquinones in Drosera-containing phytomedicines available on the Austrian market. The polarographic behaviours of plumbagin, 7-methyljuglone and droserone were examined in various buffer systems in the pH-range from 3.5 to 10.0. The compounds were reduced in a single reversible peak at the dropping mercury electrode. The differential pulse polarograms of the naphthoquinones showed distinct peaks in sodium acetate/acetic acid buffer:methanol solution, and a plot of peak height against concentration was found to be linear over the range 0.16–30.0 µg/ml for droserone, 0.06–8.0 µg/ml for plumbagin and 0.16–25.0 µg/ml for 7-methyljuglone. The polarographic method was applied to determine the amounts of these naphthoquinones in three Drosera-containing phytomedicines. In one preparation the droserone concentrations, depending on the sample taken, were found to be 6.81 and 5.78 µg/ml, whilst in a second preparation the droserone concentrations were 1.87, 3.67 and 0.51 µg/ml. The content of plumbagin and 7-methyljuglone was lower than the polarographic detection limit (0.4 µg/ml of each). In a third preparation neither droserone, plumbagin, nor 7-methyljuglone were polarographically detectable (<0.2 µg/ml). Copyright © 1999 John Wiley & Sons, Ltd.

Determination of 5‐hydroxynaphthoquinones in phytotherapeutic Drosera preparations by differential pulse polarography

A differential pulse polarographic method is described for the determination of three pharmacologically active 5-hydroxynaphthoquinones in Drosera-containing phytomedicines available on the Austrian market. The polarographic behaviours of plumbagin, 7-methyljuglone and droserone were examined in various buffer systems in the pH-range from 3.5 to 10.0. The compounds were reduced in a single reversible peak at the dropping mercury electrode. The differential pulse polarograms of the naphthoquinones showed distinct peaks in sodium acetate/acetic acid buffer:methanol solution, and a plot of peak height against concentration was found to be linear over the range 0.16–30.0 µg/ml for droserone, 0.06–8.0 µg/ml for plumbagin and 0.16–25.0 µg/ml for 7-methyljuglone. The polarographic method was applied to determine the amounts of these naphthoquinones in three Drosera-containing phytomedicines. In one preparation the droserone concentrations, depending on the sample taken, were found to be 6.81 and 5.78 µg/ml, whilst in a second preparation the droserone concentrations were 1.87, 3.67 and 0.51 µg/ml. The content of plumbagin and 7-methyljuglone was lower than the polarographic detection limit (0.4 µg/ml of each). In a third preparation neither droserone, plumbagin, nor 7-methyljuglone were polarographically detectable (<0.2 µg/ml). Copyright © 1999 John Wiley & Sons, Ltd.

Polarographic and voltammetric determination of triazine-based reactive azo dyes with 4-carboxypyridyl and 1,4-diazabicyclo[2,2,2]octanyl (DABCO) leaving groups

Direct current and differential pulse polarograms of reactive triazine-based azo dyes containing 4-carboxypyridyl and 1,4-diazabicyclo[2,2,2]octanyl (DABCO) leaving groups show one wave or peak corresponding to reduction of the azo group and other waves or peaks at more negative potentials corresponding to the reduction of reactive groups. Optimum conditions were found for polarographic and voltammetric determination at submicromolar concentrations of the test dyes based on azo group reduction. The peaks corresponding to the reduction of the reactive group can be used for monitoring the hydrolysis of the test dyes.

Resolution of global signals using ratio differential pulse polarograms: Determination of p-nitroaniline and p-nitrotoluene in their mixture

Ratio differential pulse polarograms obtained by dividing the multianalyte and single analyte signals are proposed as a tool for resolution of global signals and quantification of the analytes from a qualitatively known mixture by differential pulse polarography (DPP) and related electroanalytical techniques. The influences of shape and position of the resolving function (DP polarograms of individual analyte) on the efficiency of resolution are discussed on simulated and experimental results. The method is applied for the determination of p-nitroaniline (NA) and p-nitrotoluene (NT) from their mixture in N,N′-dimethylformamide solutions with 0.1 M tetrabutylammonium iodide as supporting electrolyte, using an external calibration diagram and internal standard addition methods. NA and NT give one-electron DP polarographic peaks with 93 mV of peak separation and, therefore, show significant overlapping which depends on the concentration ratio of NA and NT in the mixture. The method is especially suitable for quantification of one analyte in the presence of a large excess of another analyte, because by division the component in excess is removed and the pseudo-ratio DPP of the minor component is clearly revealed in a way which is not possible by deconvolution using polynomial division or deconvolution by Fourier transforms.

Differential pulse polarographic study of the redox centres in pea amine oxidase

Amine oxidase from pea seedlings was investigated using differential pulse polarography (DPP). Quinone cofactor and copper ions, both bound in the active site, are redox centres of this metalloprotein. In order to keep conditions near to those supporting the active state of the enzyme, 50 mmol1 −1 phosphate buffer, pH 7.0, was used as a medium for measurements. Highly purified amine oxidase was reduced at a dropping mercury electrode (DME) at potentials −1.23, −0.60 V and −0.30 V. The assignment of the electrode processes to the DPP peaks was based on selected chemical modifications of the active site of the enzyme. The reaction of pea amine oxidase (PSAO) with diethyldithiocarbamate resulted in the copper-free enzyme and allowed the signal at −1.23 V to be linked with the reduction of bound copper ions. This peak was also replaced by the peaks at −0.1 V of the liberated Cu(II) and Cu(I) ions after modifying reactions with diethylpyrocarbonate and sodium borohydride respectively. The peaks at −0.60 and −0.30 V belong probably to the organic cofactor, as suggested by reactions of the enzyme with carbonyl reagents. The parallel decrease in peaks of PSAO was observed in the polarogram of the acting enzyme, in the presence of substrate and competitive inhibitor. Partial proteolysis of the enzyme by subtilisin at 37 °C served to enhance the current of the cofactor reduction. Differential pulse polarograms of hydrolysed pea amine oxidase were compared with those of a model compound, quinone, prepared by the air oxidation of topa (2,4,5-trihydroxyphenylalanine). These comparisons confirm the quinone nature of the organic cofactor in the active site of PSAO and allow it to be identified with topa quinone.

Complexation of Bi III by nitrogen donor ligands. A polarographic study

Differential pulse polarography was used to determine the formation constants of Bi 3+ at ionic strenght 0.5 and 25°C with the ligands DIEN (1,4,7-triazaheptane), TETREN (1,4,7,10,13-pentaazatridecane), DPA [bis(2-pyridyl)amine], AMPY [2-(amino-methyl)pyridine], and THPED [N,N,N′,N′-tetrakis(2-hydroxypropyl)-1,2-diaminoethane]. The equilibria between Bi 3+ and these ligands were mostly established slowly on the polarographic timescale, so that separate peaks occurred in the differential pulse polarograms for the free metal ion and complexes, simplifying calculation of the formation constants. Values obtained for Bi 3+ were: DIEN, log K 1 = 17.4; log K(ML + H = MLH) = 3.9; log K(ML + OH = MLOH) = 8.1; TETREN, log K 1 = 23.9; log K(ML + OH = MLOH) = 6.9; AMPY, log K 1 = 9.6; THPED, log K 1 = 12.0; log KML + OH = MLOH) = 12.0; DPA, log K 1 = 9.0, log K 2 = 7.4. These results are shown to be reasonable in terms of the formation constant log K 1(NH 3) = 5.0 for Bi 3+ predicted by a dual basicity equation developed previously. A good linear free energy relationship between log K 1 values for Bi III complexes and log K 1 for analogous complexes of the isoelectronic Pb IIion was found, which may be useful in predicting Bi III solution chemistry.

Determination of propazine by differential pulse polarography in micellar and emulsified media

An electroanalytical study of the herbicide propazine's reduction process in micellar solutions and oil-in-water emulsions is reported. The anionic surfactant sodium pentanesulphonate was chosen as the most suitable. The differential pulse polarograms of micellar solutions had two reduction peaks below pH 2.0, whereas only one peak was obtained above pH 2.O. Ethyl acetate was chosen as the organic solvent to form propazine emulsions. Unlike in micellar solutions, the DPP polarograms of propazine emulsions showed only one peak even at pH < 2.0, suggesting that propazine hydrolysis was hindered in the emulsified medium. The limiting current is diffusion-controlled and the electrode process is irreversible. Propazine can be determined by differential pulse polarography over the 1.0 × 10−1 − 1.0 × 10−1moll−1 and 1.0 × 10−15 − 4.0 × 10−1 moll−1 concentration ranges and the limit of detection was 2.8 × 10−1 moll−1. Of the potential interferents simazine, methoprotryne and terbutryn (alls-triazines), thiram (a dithiocarbamate), dinoseb (nitrophenolic), and heptachlor (chlorinated cyclo-diene herbicide), only the first two were significant (10% error for equimolar concentrations). The method was applied to the determination of propazine in spiked drinking water. At a concentration level of 2.0 × 10−1 moll−1 a recovery of 94 ± 6% was obtained, after tenfold concentration on Sep-Pak.

Identification of organic sulfur compounds in supercritical extracts of Beypazari lignite using deconvoluted differential pulse polarograms

The objective of this study was the identification of sulfur compounds present in the supercritical ethanol/NaOH extracts of Beypazari lignite by deconvoluting the polarograms of the chromatographically separated fractions of the extracts. Supercritical interaction between coal and ethanol/NaOH was carried out using a microreactor of 15 mL capacity. Liquid product obtained were separated by silica gel liquid chromatography. Differential pulse polarograms of the fractions separated were measured, and the least squares curve fitting techniques were applied to resolve the overlapping bands in the polarograms of the lignite liquids

The affinity of bismuth(III) for nitrogen-donor ligands

Differential pulse polarography has been used to determine the formation constants of Bi3+ at ionic strength 0.5 mol dm–3 and 25 °C with the ligands glycine, trien (triethylenetetramine), bipy (2,2′-bipyridine), [15]aneN4(1,4,8,12-tetraazacyclopentadecane) and triethanolamine. The equilibria between Bi3+ and these ligands are established slowly on the polarographic time-scale, so that separate peaks occur in the differential pulse polarograms for free metal ion and complex, greatly simplifying calculation of the formation constants. Stability constant values (log K1) for the complexes of BiIII with these ligands were 10.0 (glycinate), 21.9 (trien), 4.5 (bipy) and 9.2 (triethanolamine). For the macrocycle [15]aneN4, log K1(M3++ L ⇌ ML3+)= 23.5; for the equilibria ML3++ OH–⇌ ML(OH)2+ and ML3++ H+⇌ M(HL)4+, values of log K were 8.5 and 3.5 respectively. These results are shown to be reasonable in terms of the formation constant log K1(NH3)= 5.0 for Bi3+ predicted by a dual basicity equation developed previously.

Experimental verification of theoretically predicted effects of reactant adsorption in normal pulse polarography

The theoretically predicted behaviour of the limiting current and the maximum current was examined. Normal pulse polarograms of 2-(pyrrolidynomethyl)-7-nitroimidazo-[4,3- b]-2,3-dihydro-oxazole (PNIO) in Britton—Robinson buffer at different bulk concentrations were investigated. The influence of the pulse time t p, the drop lifetime t d and the initial potential E i on the limiting current I 1 and the maximum current I max was investigated. All these dependences are typical of strong reactant adsorption at appropriate choices of E i and the bulk concentration of PNIO. Many theoretically predicted effects of strong reactant adsorption were observed at appropriate choices of E i. Behaviour typical of weak adsorption was found at E i = 0.

Use of a data treatment software for the resolution of overlapping differential pulse polarographic peaks

A procedure based on the use of the Beckman Data Leader Software is proposed for resolving binary mixtures of electroactive analytes when their differential pulse polarographic peaks are overlapping without any separation step. The resolution is made by measuring in the selected “zero-crossings” of the first derivative differential pulse polarograms. For testing the procedure, two well-known systems were used, cadmium (II)-indium (III) and thallium (I)-lead (II) mixtures. The four metallic ions can be determined in diverse ratios with quantification limits lower than 10−7 mol/l.

Anion induced adsorption in pulse polarography

A model of pulse polarograms influenced by anion induced adsorption of metal complexes is developed. The dependence of peak potentials on the logarithm of the ligand concentration is a curve with a flat segment, the slope of which depends on both the number of ligands in a dissolved surface-active complex and the relationship between the surface concentration of the adsorbed ligand and its bulk concentration. This slope can indicate the composition of the adsorbed complex. Some results obtained with lead(II) and cadmium(II) in bromide and iodide media are reported.

Electrochemical reduction mechanism of EDTA complexes of U 4+ and UO 2+2 at HMDE in aqueous solution

The electroreduction of uranium-EDTA (ethylenediaminetetra acetic acid, H 4Y) complexes at the HMDE (hanging mercury drop electrode) was studied in an aqueous 0.5 M NaClO 4 solution. From the cyclic voltammogram and differential pulse polarogram of U 4+-EDTA solution, it was determined that the one electron reversible reduction of UOHY − to UOHY 2− occurs at −1.6V vs. SSCE (sodium chloride-saturated calomel electrode) in the pH range 6.0–7.0.The scan rate and pH dependence suggest that the electroreduction of the UO 2+ 2 -EDTA complex occurs by two different mechanisms depending on the pH of the solution. At the relatively lower pH of 5.5-7.5, the UO 2+ 2 -EDTA complex in the form of UO 2HY − is reduced reversibly to UO 2H 2Y − and then UO 2H 2Y − is further reduced irreversibly to UO 2. The UO 2H 2Y − formed at the first step turns also into UO 2HY − and UY by disproportionation. UO 2 formed at the second step dissolves to become UY, which undergoes a further reduction at −1.6 V. In higher pH range of 7.5–9.0, the reversible reduction of UO 2+ 2 -EDTA complex, as UO 2Y 2−, to UO 2Y 3− is followed by the disproportionation of UO 2Y 3− to UO 2Y 2− and UOY 2−. UOY 2− is further reduced to a U 3+ state at potentials more negative than −1.45 V.

Polarography and stripping voltammetry of metal–polycarboxylate complexes: Complexes of cadmium and zinc with polyacrylic and polymethacrylic acids

AbstractComplex formation of Zn(II) and Cd(II) with polymethacrylic acid (PMA) and polyacrylic acid (PAA) is studied by sampled direct current, normal pulse, reverse pulse, and differential pulse polarography and by differential pulse anodic stripping voltammetry (DPASV). Titrations of Zn(II) or Cd(II) with partially neutralized PMA or PAA by DPASV allow the precise determination of the apparent formation constant K of the four systems. The two Zn(II) systems appear to be labile on the different time scales of the several techniques, without remarkable complications due to secondary phenomena. Their K values, determined by all techniques, agree satisfactorily. The two Cd(II) systems, and especially the Cd/PAA system, show rather involved behavior, which tends to normality when the degree of neutralization αn of the polycarboxylates increases. Both slow kinetics and ligand adsorption coupled with induced Cd(II) adsorption (evidenced by maxima on the normal pulse polarograms) seem to be present. For all αn values, DPASV yields higher estimates of K than do polarographic techniques.

Detection and resolution enhancement of two close electrochemical processes

The polarograms of two electrode processes whose half-wave potentials are separated by 15–40 mV have been simulated. Under such conditions the polarogram obtained by differential pulse polarography scarcely shows the composite nature of the response. The normalized peak-shaped response is compared to a single process occurring exactly between the two former ones. The shapes of the responses are identical; only their half-peak widths differ. Fourier transforms of the two signals (for a single and two energetically close processes) were performed and they also differ slightly. In the case of a real experiment, this slight difference could easily be attributed to experimental imperfection. By deconvolution of the same data with a similar peak-shaped function (in the Fourier domain this is the simple division of two functions), it is possible to identify already in the Fourier domain whether a single or two energetically close processes are taking place. Inverse transformations give a clear distinction between single and double processes. As an experimental example, differential pulse polarograms of a mixture of Pb 2+ and Tl + in 4 M NaCl were measured. Detection of the composite solution and the necessary conditions are discussed.

Synthesis, spectroscopic and electrochemical properties and X-ray studies of bis(2,2′-bipyridyl)(3-(2-hydroxy-phenyl)-5-(pyridin-2-yl)- 1,2,4-triazole)-ruthenium(II)hexafluorphosphate

A new ruthenium(II) bis(2,2′-bipyridyl) compound with 3-(2-hydroxy-phenyl)-5-(pyridin-2-yl)- 1,2,4-triazole (H 2L) as co-ligand has been prepared and characterised by X-ray structure determination, NMR spectroscopy, electrochemical measurements, and UV-Vis absorption and emission spectroscopy. The compound of formula [Ru(bpy) 2(HL)]PF 6· CH 3COCH 3 crystallises in the monoclinic space group P2 1/ n, with a=14.242(1), b= 14.277(1), c= 17.968(1) Å, β=97.27(1)° and Z=4. The Ru(bpy) 2 moiety is bound to (HL) via N1 of the triazole ring (Ru-N=2.051(3) Å) and N1 of the pyridine ring (Ru-N=2.085(3) Å). The phenol group is connected to N4 of the triazole ring via intramolecular hydrogen bonding. p K a titrations reveal that for free H 2L the pyridine ring, the triazole ring and the phenol group can be (de)protonated, but for the coordinated ligand, only a triazole- and a phenol-based protonation are observed. The cyclic voltammogram and differential pulse polarogram of the oxidation of the deprotonated compound show the presence of three oxidation steps; one ruthenium based and two phenol based. Protonation of the complex causes a large positive shift potential for oxidation of the ruthenium centre. All measurements agree with a phenolpyridyltriazole ligand having strong σ-donor properties.

Differential Pulse Polarographic Determination of Flavonoids, I: Total Flavonoids in Forsythia viridissima Lindl

AbstractA differential pulse polarographic method is described for the determination of total flavonoids expressed in terms of rutin which is obtained from the dried blossoms of Forsythia viridissima Lindl, cultivated in parks. Differential pulse polarograms of rutin show a distinct peak at ‐ 1.83 V (vs Ag/AgCl) at pH 6.0 Sörensen buffer. The detection limit found was 10 ppm and the response was linear up to 100 ppm with a relative standard deviation (at the 20 ppm level) of 0.75%.