Determination of Bentazepam by differential pulse polarography and adsorptive stripping voltammetry

Summary A method is described for the determination of Bentazepam using DPP and ADSV with DP. Bentazepam is determined in buffer Britton-Robinson 0.04 mol l^-1 at pH 9 with detection limits of 3.1×10^-9 mol/l and a relative standard deviation of 0.8 DPP was used to determine Bentazepam in Tiadipona, the commercial product. ADSV was used to determine Bentazepam in urine with a detection limit of 2.7 ng ml^-1 (accumulation time 5 min) and a relative standard deviation of 1.5%.     

Spectrophotometric determination of Si(IV) with 3,6-dichloro-2,5-dihydroxy-1,4-benzoquinone

A new rapid spectrophotometric method for the determination of Si is described. Chloranilic acid (CA) reacts with Si(IV) forming a complex, which is stable for several hours. The procedure is suitable for the determination of Si in the concentration range of 0.5–5.0 μmol/ml, at pH 1.25 ± 0.05 measured at 370 nm. The relative standard deviation at the level of 1.0 μmol/ml Si is ± 3.5%. Of the foreign ions investigated, Fe, Ti, Mo, and PO₄³⁻ interfere significantly.     

Determination of trace amounts of morphine in human plasma by anodic adsorptive stripping differential pulse voltammetry

New adsorptive anodic differential pulse stripping voltammetry method for the direct determination of morphine at trace levels in human plasma of addicts is proposed.The procedure involves an adsorptive accumulation of morphine on a HMDE,followed by oxidation of adsorbed morphine by voltammetry scan using differential pulse modulation.The optimum conditions for the analysis of morphine are pH 10.5,Eacc of -100 mV（vs.Ag/AgCl）,and tacc of 120 s.The peak current is proportional to the concentration of morphine,and a Linear calibration graph is obtained at 0.01-3.10μg mL^-1.A relative standard deviation of 1.06%（n=5）was obtained,and the limit of detection was 3 ng mL^-1.The capabiLity of the method for the analysis of real samples was evaluated by the determination of morphine in spiked human plasma and addicts human plasma with satisfactory results.     

New hydrogen-bonded donor-acceptor pairs between dipyridylacetylenes and 2,5-dichloro-3,6-dihydroxyl-1,4-benzoquinone

[structure: see text] The crystalline donor-acceptor hydrogen-bonding complexes between 2,5-dichloro-3,6-dihydroxy-1,4-benzoquinone (chloranilic acid) and dipyridylacetylenes (DPA) [2,2'-DPA, 3,3'-DPA, and 4,4'-DPA] were prepared, and crystal structures were revealed by X-ray analysis. The structures of the complexes are formed by intermolecular hydrogen-bonding interactions and demonstrate three supramolecular architectures based on a new common supramolecular synthon, which allows the formation of a different stacking arrangement and ionicity.     

Studies on the reaction of 2,5-dichloro-3,6-dihydroxy-1,4-benzoquinone with tribenzylamine and diazabicyclo[2.2.2]octane in acetonitrile solvent

The reaction of chloranilic acid with two nitrogen nucleophiles, tribenzylamine and diazabicyclo[2.2.2]octane has been studied. Careful spectroscopic analysis showed that the product in both cases is a radical ion pair of Q^.-, A^.+ type. Stoichiometry of the complex formed is of 1:1 type. Mechanism of the reaction is proposed. This revised version was published online in June 2006 with corrections to the Cover Date.     

Adsorption voltammetric techniques for the determination of uranium(VI) with 2,5-dichloro-3,6-dihydroxy-1,4-benzoquinone as complex forming reagent

Uranium (VI) can be determined by adsorptive voltammetric techniques, as its chloranilic acid complex, over a wide concentration range. Differential pulse polarography is useful for quantification of uranium between 0.1 and 1.5 mg/l; for the range from 10 to 500 g/l differential pulse voltammetry and for ultra-trace analysis between 0.024 and 40 g/l adsorptive stripping voltammetry are preferred. The standard deviation for the 3-detection limit of 24 ng/l uranium was found to be 8%. In the trace analysis of metals in aquatic environmental systems by adsorptive stripping voltammetry it is normally necessary to decompose polluted water samples by UV irradiation or microwave digestion. The advantage of the developed method is the fact that no sample pretreatment is necessary.Dedicated to Professor R. Geyer on the occasion of his 80th birthday     

Infrared and ab initio studies of conducting molecules: 2,5-diamino-3,6-dichloro-1,4-benzoquinone

2,5-Diamino-3,6-dichloro-1,4-benzoquinone has been synthesized by modifying the procedure reported in literature. Its IR spectrum has been recorded in the solid phase in the range 4000-400cm(-1). Ab initio calculations have been performed using Gaussian '03 program to compute optimized geometry, harmonic vibrational frequencies along with intensities in IR and Raman spectra and atomic charges at RHF/6-31+G*, B3LYP/6-31+G* and B3LYP/6-311++G** levels. To make vibrational analysis Gaussian View software was used. The optimized molecular structure is found to possess C2h point group symmetry. The observed IR frequencies have been assigned to different modes taking C2h molecular symmetry with the help of pictorial view of normal modes. From the magnitude of the observed frequencies corresponding to the NH2 stretching motions an indication of H-bonding is noticed. From geometrical parameters of the molecule it appears that two parallel sets of conjugated strands are formed in this molecule providing a route to conduct charges. The N-H bonds facing towards chlorine atoms are found to be shorter than those facing towards oxygen atoms indicating the presence of H-bonding between hydrogen atom of an NH2 group and carbonyl (quinoid) oxygen atom.     

Determination of traces molybdenum by catalytic adsorptive stripping voltammetry

A reliable and very sensitive procedure for the determination of ultra trace of molybdenum is proposed. Molybdenum was determined by cathodic stripping differential pulse voltammetry based on the adsorption collection of the Mo(VI)-Tiron complex on a hanging mercury drop electrode (HMDE). The variation of peak current with pH, concentration of Tiron and chlorate, plus several instrumental parameters such as accumulation time, accumulation potential and scan rate, were optimized. Under optimized condition, the relationship between the peak current and molybdenum concentration is linear in the range of 0.010-21.0 ng ml⁻¹. The limit of detection was found to be 0.006 ng ml⁻¹. The relative standard deviation for 10 replicates determination of 0.6 and 10 ng ml⁻¹ Mo(VI) is equal to 1.3 and 0.9%, respectively. The method was applied to the determination of molybdenum in river water, tap water, well water, plant foodstuff samples such as cucumber, tomato, carrot, and certified steel reference materials.     

Trace determination of molybdenum by adsorptive cathodic stripping voltammetry

Molybdenum is determined by adsorptive cathodic stripping voltammetry in 0.15 M nitric acid solution containing 15 μM 2′,3,4′,5,7-pentahydroxyflavone (morin) as a ligand. In this medium, molybdenum is preconcentrated on a hanging mercury drop electrode and stripped cathodically in square-wave voltammetry mode, with a peak potential of -350 mV vs. Ag/AgCl (saturated KCl). The effect of various parameters (ligand concentration, supporting electrolyte composition, accumulation potential and collection time) on the sensitivity and linear range of the calibration curve are discussed. With controlled accumulation for 1 min, the detection limit (3σ) was 0.45 ng ml^?1 molybdenum and the calibration curve is linear up to 70 ng ml^?1. The procedure is applied to the determination of molybdenum in real samples with satisfactory results.     

Employment of differential pulse polarography and adsorptive voltammetry for the determination of some sulfonamides

Reduction of sulfonamides in mineral acids was investigated using fast scan differential-pulse polarography (FSDPP). The optimal medium for sulfadimidin, sulfamethoxydin and sulfamethoxypyridazin is 0.01–0.02M HCl, while for phthalazol it is 1M HCl and the latter compound can be selectively determined in the presence of other sulfonamides. Adsorptive properties on the Hg-electrode were analytically used in the case of sulfamethoxypyridazin and phthalazol. Relative standard deviation of all determinations did not exceed 8%.     

Determination of trace procaine hydrochloride by differential pulse adsorptive stripping voltammetry with a Nafion modified glassy carbon electrode.

A sensitive and selective method for the determination of procaine hydrochloride with a Nafion-modified glassy carbon electrode has been developed. The voltammetric behavior of procaine hydrochloride on the Nafion-modified electrode indicated that the modified electrode not only increased the sensitivity of the determination of procaine hydrochloride, but also catalyzed the electrode process. Procaine hydrochloride was accumulated in Britton-Robinson buffer (pH 2.09) at a potential of -0.2 V (vs. SCE) for 180 s, and was then determined by differential pulse adsorptive stripping voltammetry. The effect of various parameters, such as the pH of the medium, the mass of drop-coated Nafion, the accumulation potential, the accumulation time and the scan rate, were investigated. Under the optimum conditions, a linear calibration graph was obtained in the concentration range of 6.0 x 10(-8) to 6.0 x 10(-6) mol l(-1) with a correlation coefficient of 0.9987. The relative standard deviation was 4.18% for eight successive determinations of 1.0 x 10(-7) mol l(-1) procaine hydrochloride, and the detection limit (three times signal to noise) was 7.0 x 10(-9) mol l(-1). A study of interfering substances was also performed, and the method was applied to the direct determinations of procaine hydrochloride in the injection solution of procaine hydrochloride and in rabbit serum.     

Determination of Captropril using adsorptive cathodic differential pulse stripping voltammetry with the HMDE

A voltammetric method for the determination of 3-mercapto-D-2-methylpropanoyl-L-proline, a hypotensive drug whose pharmaceutical name is Captopril (CPT), in the concentration range from 9.0×10⁻¹⁰M to 3×10⁻⁶M, is described. In this range the peak current increases linearly with drug concentration even when different collection periods are used. A self-cleaning Hanging Mercury Drop Electrode (HMDE) was used and a negative Differential Pulse potential (DP) was applied to the indicator electrode. The stripping peak of CPT splits into two peaks as soon as the concentration is increased over about 10⁻⁵M; in the oxidation DP scan, instead, this splitting is observed at a concentration of 2.0×10⁻⁴M. Some attempts were made to verify the suitability of other techniques such as Alternating Current polarography (AC) and the use of a different electrode, the Wax-Impregnated Graphite Electrode (WIGE).     

Hydrogen bonding and molecular vibrations of 2,5-dihydroxy-1,4-benzoquinone

Intramolecular hydrogen bonding in 2,5-dihydroxy-1,4-benzoquinone has been investigated by means of quantumchemical calculations and vibrational spectroscopy. Both computations at the MP2/6-31+G** level as well as FT-IR and FT-Raman spectra are in agreement with the predominance of the C_2h conformer with two intramolecular hydrogen bonding interactions. The spectra were interpreted with the aid of normal coordinate analysis based on a scaled Becke3–Lee–Yang–Parr/6-31G* density functional force field. The general scale factors introduced recently by Rauhut and Pulay for this theoretical level allowed a reliable assignment of most of the fundamentals. The scale factors for vibrations affected strongly by the intramolecular hydrogen bonding interaction needed some adjustment and hence were re-optimized in the present study. The final scaled force field reproduced the experimental frequencies of the molecule by a weighted mean deviation of 8.8 cm⁻¹. Based on the calculated results, 32 from a total of 36 fundamentals were assigned in the vibrational spectra of 2,5-dihydroxy-1,4-benzoquinone revising and extending the assignments of earlier less sophisticated investigations.     

Determination of trace amounts of nickel by differential pulse adsorptive cathodic stripping voltammetry using calconcarboxylic acid as a chelating agent

A method for the determination of nickel(II) by stripping voltammetry is described. The method is based on the adsorptive accumulation of nickel(II) calconcarboxylic acid complex onto a hanging mercury drop electrode (HMDE), followed by the reduction of the adsorbed complex using differential pulse voltammetry. The optimum operating conditions and parameters were found to be 0.05 M NH₃/NH₄Cl buffer (pH = 9.5) as the supporting electrolyte, a ligand concentration of 1 × 10^?6 M, accumulation potential of ?0.5 V (vs. Ag/AgCl) and accumulation time of 60 s. At the optimized conditions, the peak current is proportional to the concentration of nickel in the range of 1.7 × 10^?9 to 4.7 × 10^?7 M (0.1–28 ng ml^?1) with a detection limit of 0.05 ng ml^?1. The relative standard deviation (n = 10) at nickel concentrations of 2, 10 and 15 ng ml^?1 varies in the range 0.76 to 2.1%. Possible interferences by metal ions, which are of great significance in real matrices, have been studied. The method was successfully applied to the determination of nickel content in a chocolate sample.     

Indirect trace determination of NTA in natural waters by differential pulse anodic stripping voltammetry

Summary Bismuth(III) is added to the water sample in excess to NTA and EDTA to form inert stable complexes with them at pH 2. The uncomplexed Bi(III) is then deposited into a hanging mercury drop electrode at a potential of –0.15 V vs. SCE and subsequently stripped anodically in the differential pulse mode. The peak current of uncomplexed Bi(III) is recorded. By a second deposition at –0.35 V vs. SCE Bi(III) from Bi³⁺ and Bi-NTA is deposited. The concentrations of NTA and EDTA in the sample are determined from the concentration of added Bi(III) and the voltammetrically determined Bi(III) at these two potentials by the standard addition method. Cd, Cu, Pb, and Zn do not interfere. Fe(III) has to be reduced by ascorbic acid or hydroxylamine before the determination. Cu(II) in concentrations larger than 40 g/l has to be removed by preelectrolysis. In samples with chloride contents above 0.05 M the stripping step has to be performed after medium exchange to a perchloric acid solution of pH 2. For a deposition time of 2 min the determination limit is approximately 0.2 g/l NTA and 0.1 g/l EDTA. The relative standard deviation for NTA concentrations at 2 or 10 g/l lies at 10 or 1.3%, respectively. Good accuracy is established by finding within 2% the levels adjusted when water samples are spiked with a standard solutions of NTA or EDTA.

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Indirekte Spurenbestimmung von NTA in natürlichen Wässern durch differentielle Pulsinversvoltammetrie

Zusammenfassung Bismuth(III) wird zur Wasserprobe in Überschuß zur NTA- und EDTA-Konzentration bei pH 2 zugegeben, wobei stabile inerte Komplexe gebildet werden. Nicht komplexiertes Bi(III) wird an der hängenden Quecksilbertropfenelektrode beim Potential –0,15 V (SKE) kathodisch als Amalgam abgeschieden und dann anodisch wieder gelöst. Dabei wird im differentiellen Pulsmodus der Bi(III)-peak registriert. Durch die zweite kathodische Abscheidung beim Potential –0,35 V (SKE) wird Bi(III) aus unkomplexiertem Bi³⁺ und dem BiNTA-Komplex abgeschieden. Die Konzentrationen von NTA und EDTA in der Probe werden aus den voltammetrisch bestimmten Bi(III)-Konzentrationen bei den zwei angegebenen Potentialen und der zugegebenen Konzentration von Bi(III) mit der Standard-Additions-Methode bestimmt. Die Spurenmetalle Cd, Cu, Pb und Zn stören die Bestimmung nicht. Fe(III) muß vor der Bestimmung mit Ascorbinsäure reduziert werden, Cu in der Konzentration von mehr als 40 g/l muß durch Vorelektrolyse entfernt werden. Wenn die Probe mehr als 0.05 M Chlorid enthält, muß der Strippingvorgang nach Elektrolytwechsel in einer Perchlorsäurelösung bei pH 2 durchgeführt werden. Für eine Anreicherungszeit von 2 min liegt die Bestimmungsgrenze bei 0,2 g/l NTA und 0,1 g/l EDTA. Die relative Standardabweichung beträgt bei einer Konzentration von 2, bzw. 10 g/l NTA 10 bzw. 1,3%. Die Richtigkeit ist gut, was aus der Wiederfindungsrate von 2% der zur Probe zugegebenen Lösung des NTA- oder EDTA-Standards hervorgeht.

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Dedicated to Prof. Dr. W. Fresenius on the occasion of his 70th birthday

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On leave from the University of Thessaloniki, Greece     

Determination of trace aluminium by adsorptive stripping voltammetry on a preplated bismuth-film electrode in the presence of cupferron

This work reports the use of adsorptive stripping voltammetry (AdSV) for the determination of aluminium on a rotating-disc bismuth-film electrode (BiFE). Al(III) ions in the non-deoxygenated sample were complexed with cupferron and the complex was accumulated by adsorption on the surface of the preplated BiFE. The stripping step was carried out by using a square-wave (SW) potential-time voltammetric excitation signal. The experimental variables as well as potential interferences were investigated and the figures of merit of the method were established. Using the selected conditions, the 3σ limit of detection for aluminium was 0.5 μg l⁻¹ at a preconcentration time of 240 s and the relative standard deviation was 4.2% at the 5 μg 1⁻¹ level for a preconcentration time of 120 s (n = 8). The accuracy of the method was established by analysing water and metallurgical samples.     

Determination of trace metals in Nile River and ground water by differential pulse stripping voltammetry

The determination of trace metals in river water and ground water by DPSV is seriously disturbed by the presence of organic complexes. The influence of these substances can be eliminated by acidification of the samples with acids. Cd, Pb and Cu were determined at pH 1.1 (HNO₃ medium) and Zn, Cd, Pb and Cu at pH 2 (HCl medium), in both the Nile river and ground water. Zn was determined at pH 3.5 in HCl and pH 4.5 in HNO₃, after neutralizing the samples with NH₃/NH₄Cl buffer. Manganese could then be determined, after further addition of ammoniacal buffer solution up to pH 7.5 and 8.5. Ni and Co were determined in the adsorptive mode after formation of dimethylglyoximates at pH 9.2. The effect of pH on the stripping peaks of manganese was studied. Good agreement was observed between DPSV and AAS results for Zn, Cd, Pb, Cu and Mn, but the concentrations of Ni and Co were below the detection limits for AAS. Good agreement was obtained between DPSV results in HCl and HNO₃ for Ni and Co. The results indicate that decomposition of organic complexes by acidification with HNO₃ is better than in the case with HCl for Zn, Pb, Cu, Ni and Co, but HCl is better than HNO₃ for Cd and Mn.     

Catalytic adsorptive stripping voltammetry determination of ultra trace amount of molybdenum using factorial design for optimization

A highly sensitive procedure is presented for the determination of ultra-trace concentration of molybdenum by catalytic adsorptive stripping voltammetry. The method is based on adsorptive accumulation of the molybdenum (Mo)-pyrocatechol violet (PCV) complex on to a hanging mercury drop electrode, followed by reduction of the adsorbed species by voltammetric scan using differential pulse modulation. The reduction current is enhanced catalytically by chlorate. The influence of variables was completely studied by factorial design analysis. Optimum analytical conditions for the determination of molybdenum were established. Molybdenum can be determined in the range 1.0 × 10⁻³-100.0 ng ml⁻¹ with a limit of detection of 0.2 pg ml⁻¹. The influence of potential interfering ions on the determination of molybdenum was studied. The procedure was applied to the determination of molybdenum in mineral water and some analytical grade substances with satisfactory results.     

Ultratrace determination of adenine in the presence of copper by adsorptive stripping voltammetry

An electrochemical stripping procedure for ultra-trace measurements of adenine is described based on the adsorption of the adenine-copper complex on a static mercury drop electrode. Cyclic voltammetry was used to characterize the interfacial and redox behavior. Optimum experimental conditions were found when using a 0.005 M NaOH solution containing 0.4 ppm of copper, an accumulation potential of -0.30 V, a scan rate of 100 mV s(-1) and a linear scan mode. There is a linear response to adenine concentration in the range of 0.1-1.0 ppb and the detection limit for 6 min accumulation time was of 4.0 ppt (3.0x10(-11) M). Proper conditions for measuring the adenine in presence of guanine, thymine and cytosine were also investigated. The method was applied for the determination of adenine in a sample of single-stranded calf thymus DNA.     

Economic bismuth-film microsensor for anodic stripping analysis of trace heavy metals using differential pulse voltammetry

Stripping analysis has been widely recognised as a powerful tool in trace metal analysis. Its remarkable sensitivity is attributed to the combination of a preconcentration step coupled with pulse measurements that generate an extremely high signal-to-background ratio. Mercury-based electrodes have traditionally been used to achieve high reproducibility and sensitivity in the stripping technique. Because of the toxicity of mercury, however, new alternative electrode materials are highly desired, particularly for on-site monitoring. Use of thin films of bismuth deposited on platinum or glassy-carbon substrates has recently been proposed as a possible alternative to mercury—bismuth is “environmentally friendly”, of low toxicity, and is in widespread pharmaceutical use. In this paper the preparation of economic bismuth-film microelectrodes by electrodeposition on a copper substrate and their application to heavy metal analysis are described. Bismuth-film electrodes were prepared by potentiostatic electrodeposition. Optimum conditions for chemical and electrochemical deposition to obtain an adherent, reproducible, and robust deposit were determined. The suitability of such microelectrodes for analysis of heavy metals was evaluated by anodic stripping voltammetry of cadmium. The analytical performance of bismuth-film electrodes for anodic stripping voltammetry of heavy metals was evaluated for non-deaerated solutions containing Cd²⁺, Pb²⁺, and Zn²⁺ ions. Well-defined peaks with low background current were obtained by use of differential pulse voltammetry. Linear calibration plots were obtained for Cd²⁺ in acidified tap water at concentrations ranging from 2×10⁻⁸ to 1×10⁻⁷ mol L⁻¹ and from 1×10⁻⁷ to 1×10⁻⁶ mol L⁻¹ with relative standard deviations of 5% (n=15) at the 1×10⁻⁷ mol L⁻¹ level. The method was then successfully used to monitor the Cd²⁺content of plant extracts and validated by polarographic and ICP−MS measurements. These results open the possibility of using bismuth-coated copper electrodes as an alternative to mercury-based electrodes for analysis of heavy metals. The main problem remaining, which prevents on-site monitoring of heavy metals, is the need to use slightly acidic media, because formation of bismuth hydroxide on the film surface above pH 4.3 leads to non-reproducible measurements. Further experiments will be performed to discover whether electrode conditioning can be used to enable reproducible measurement in on-site monitoring of cadmium in natural waters. Moreover, further study should be conducted to evaluate the potential of BiFE for analysis of several pollutants of interest that are usually determined electrochemically by using mercury-based electrodes. Presented at the 9th FECS Conference on Chemistry and the Environment, Bordeaux , 29 August to 1 September, 2004