Simultaneous determination of concentration,diffusion coefficient and number of electrons for electroactive species by combining suitable electroanalytical measurements

The electroanalytical method described for the simultaneous determination of concentration, the number of electrons involved in the redox process and diffusion coefficient is based on evaluation of the ratios between the currents recorded for the analyte and for an easily standardized reference species dissolved in the same medium. Three different electroanalytical techniques are used in which the currents exhibit, for two techniques at least, different dependences on both the diffusion coefficient and electron number. The approach is applicable to diffusion-controlled processes, regardless of the degree of reversibility involved. Reliability tests with electroactive organic compounds dissolved in dimethyl sulphoxide show that both accuracy and precision are within 10% depending on the chosen combination of techniques.     

A non-conventional way to perform voltammetry

In a conventional voltammetric experiment, the electroactive species is dissolved in solution, and then diffuses from the solution phase to the electrode phase. In our proposed non-conventional voltammetric experiment, the electroactive species is trapped in the electrode phase instead of being dissolved in solution. A non-aqueous solvent was first used to trap the organic species in a porous surface layer and the modified electrode then transferred to an aqueous buffer to conduct voltammetry measurements.We tested the non-conventional voltammetric mode using a modified multi-walled carbon nanotube electrode containing mono-, di- and tri-nitroaromatic compounds trapped in the porous three-dimensional network of the CNTs. From these experiments, we conclude that the non-conventional mode produces higher peak currents and displacement of the peak potentials, yielding lower overpotentials. Furthermore, it is possible to obtain more selective voltammograms in the non-conventional mode, showing peaks that could not be resolved in the conventional mode.These results are due to a change in the mass transport regime, with thin layer diffusion being the main transport method in the non-conventional mode, compared to semi-infinite diffusion in the conventional mode.The proposed approach is an excellent alternative for performing voltammetric studies on insoluble or slightly soluble organic compounds.     

Determination of autoprotolysis constants of some non-aqueous solvents using coulometric titration

Autoprotolysis constants of acetonitrile, propionitrile, nitromethane, ethylene carbonate and dimethyl sulphoxide were determined using a coulometric — potentiometric method with a hydrogen/palladium electrode as generator. The method is based on the titration of a strong base, tetrabutylammonium hydroxide, with H⁺ ions generated by anodic oxidation of hydrogen dissolved in palladium. The titration was carried out in a galvanic cell with glass and calomel electrodes at 25°C. The pK_s values for the investigated solvents are: acetonitrile, 28.8; propionitrile, 24.6; nitromethane, 23.7; ethylene carbonate. 21.5; and dimethyl sulphoxide 29.1. These data are in accordance with those reported in the literature.     

Acid-base equilibria in organic solvents : Part 1. Evaluation of solvent basicity by cyclic voltammetry

Cyclic voltammetry is applied to evaluate the relative basicity of some organic polar solvents of electroanalytical interest (nitromethane, acetonitrile, tetrahydrofuran, 1,2-dimethoxyethane, dimethylformamide, dimethyl sulphoxide and pyridine). For this purpose, anhydrous perchloric acid is electrogenerated in these solvents and the half-wave potentials of the redox couple H⁺/H₂ are recorded by referring them to a reference system independent of the nature of the solvent. The proton-basicity scale so obtained is compared with previous basicity series and the observed analogies and differences are discussed. The same procedure is also applied to evaluate, in the solvents mentioned, the relative strengths of some bases which can be used as titrants in nonaqueous media.     

H‐Point Standard Addition Method Applied to Voltammetry of Microparticles. Quantitation of Dyes in Pictorial Samples

A solid‐state electrochemical application of the H‐point standard addition method (HPSAM) for quantifying two electroactive compounds, A, B, that produce strongly overlapped voltammetric peaks is described. It is based on peak current measurement in square‐wave voltammograms recorded for solid samples containing a reference compound R, upon additions of a A‐ (or B‐) containing standard compound. The method allows to the determination of the mass fraction of A and B by applying the H‐point standard addition method to solid state voltammetry. The quotients between the currents measured at two selected potentials and the peak current of R vary linearly with the mass ratio of the added standard and the reference compound, thus providing an electrochemical method for determining the content of A and B in the sample avoiding matrix effects. The method is applied to the simultaneous determination of alizarin and purpurin in madder pigments and pictorial specimens using morin as a reference material.     

超微电极上半微分电分析法

介绍了超微半球电极上具有简单反应时的半微分循环伏安理论。理论表明,超微电极上半微分电流与电活性物质的浓度成正比,并据此提出了一种利用超微半球电极上半微分曲线进行电化学分析测量的新方法。该法具有灵敏度高、分辨率好等特点。     

Electrocatalytic Oxidation of NADH by Oxidized s‐Adenosyl‐L‐Methionine (SAMe): Application to NADH and SAMe Determinations

s‐Adenosyl‐L ‐methionine (SAMe) is an adenosine analogue with therapeutical activity against affective disorders and liver dysfunctions. It can be oxidized on graphite electrode yielding a strongly adsorbed electroactive oxidation product for which a quinone‐imine structure is proposed. This compound is capable of electrocatalyzing the NADH oxidation at low potentials, lowering the overvoltage by about 300 mV. An amperometric method for NADH determination at +0.1 V (Ag|AgCl|KCl_sat) is developed using an oxidized‐SAMe‐modified electrode in pH 9. Linear calibration plots were obtained with a detection limit of 2.4 nM. The electrode response time and the relative standard deviation of the slope of the calibration plot for 5 different modified electrodes were 12 s and 5.6% respectively. The catalytic scheme also provides the first method to determine SAMe itself by adsorptive differential pulse voltammetry. The linear range was found to be 42.4–424 nM with a reproducibility of 6.9%. The method was applied to SAMe determination in a pharmaceutical formulation.     

Determination of iodide with chloramine-T

A method is described for the estimation of iodide, based on its oxidation to iodate by the addition of excess of chloramine-T, destruction of the excess of chloraniine-T with dimethyl sulphoxide and determination of the iodate iodometrically. In addition, the dimethyl sulphoxide eliminates any interference from bromide or bromate.     

Electrochemical reduction of nicergoline and its analytical determination in dosage forms

Electrochemical reduction of nicergoline was studied at different pH and concentrations using differential pulse polarography and linear sweep voltammetry. Both techniques reveal that the reduction process occurs with strong adsorption of the product. Nicergoline is an excellent model for the previously developed theory related to the effects of strong adsorption of electroactive species in voltammetry. At concentrations below 0.1 mM, the adsorption prepeak is linearly dependent on nicergoline concentration. This peak was used to develop a new differential pulse polarographic method for the determination of the drug in pharmaceutical dosage forms. The method is simple and not time-consuming because nitrogen purging of samples and previous separation of the excipients were not needed. A comparative UV spectrophotometric assay was applied. Recovery data and composite and uniformity content studies for both methods are reported.     

Schwingungsverhalten von dimethylsulfid,dimethyl-sulfoxid,dimethylsulfon und den entsprechenden per-deuterierten verbindungen: 2. Mitt. normalkoordinatenbehandlung

Normal coordinate calculations on dimethyl sulphide, dimethyl sulphoxide, dimethyl sulphone and their perdeuterated derivatives have been carried out. The GF-matrix method of Wilson was applied using the general harmonic valence force field. The calculated frequencies of the normal vibrations agree satisfactorily with the corresponding values obtained from the spectra. In addition the potential energy distributions were calculated and discussed. It has been found that the potential energy of the symmetric SO valence vibration of dimethyl sulphone is located only partly in the SO bond, like in dimethyl sulphoxide.     

Phasor transform to extract glucose and ascorbic acid data in an amperometric sensor

A method for separating the signals from glucose and ascorbic acid on a single recognition surface using an ac immittance technique is presented. It is proposed that each oxidation process can be represented by a unique vector based on psi and YO, and that the concentration of each analyte can be determined by monitoring the change in the admittance magnitude in the direction of the characteristic angle for that particular species. The total Faradaic admittance (YF,total) for all electroactive species present is given by a linear combination of the independent vectors from the different species. In the system tested, the analytes are glucose and ascorbic acid, the former being estimated via the measurand, hydrogen peroxide. Thus, one of the electroactive species (hydrogen peroxide) is not a bulk solution species, but is 'generated' in the enzyme matrix. The admittance measurements from ascorbic acid and the enzyme-generated hydrogen peroxide showed the characteristic phase angles of each oxidation signal, allowing for good spatial resolution. The behaviour of each of these analytes is presented and calibration curves tested. Based on the calibration curves and the basis vectors, samples containing both glucose and ascorbic acid were measured by transforming the measured total admittance from the complex Cartesian space into 'analyte space', where the X-Y axes are given by the basis vectors ?EGHP,GOD and ?AA,GOD, respectively.     

High flow-rate cells for continuous monitoring of low concentrations of electroactive species by polarography and stripping voltammetry at the static mercury drop electrode

A high-capacity flow-through cell which can be used at a maximum flow rate of 300 ml min^-1 has been developed for continuous monitoring of electroactive substances. The cell is compatible with the recently developed static mercury drop electrode. Comparative studies with a cell employing a conventional dropping mercury electrode are described. A wide range of polarographic techniques is applied, and it is demonstrated that the static mercury drop electrode improves the limits of detection, that laminar flow conditions are essential for low noise levels of operation, and that solution flow through a sulphite bed is a more effective method of oxygen removal than nitrogen bubbling. The combination of a microprocessor-controlled polarographic system, static mercury drop electrode and high-volume flow cell is very versatile for the determination of trace levels of electroactive species in flow streams. Preliminary results on anodic stripping voltammetry in flow streams are reported.     

Analytical determination of dimethyl sulphoxide by complex formation with the pentacyanoferrate(II) ion

A method for determination of small amounts of dimethyl sulphoxide and non-hindered sulphoxides is proposed, based on selective reaction with the aquapentacyanoferrate(II) ion, leading to stable complexes which can be detected electrochemically.     

Electrodes supported on ion-exchange membranes as sensors in gases and low-conductivity solvents

An electroanalytical sensor is proposed that is suitable for the detection of electroactive analytes present in gases or low-conductivity solvents where supporting electrolytes cannot be introduced. It consists of a porous working electrode supported on one surface of a cationic ion-exchange membrane (Nafion 417), the other surface of which is in contact with an electrolyte solution containing the counter and reference electrodes. Such an ion-exchange membrane replaces a conventional supporting electrolyte dissolved in the analyte sample and can be regarded as a solid polymer electrolyte (SPE) confined in the close neighbourhood of the working electrode. Alternative procedures followed for coating SPE membranes with various materials (Pt, Au, C or Hg) are described, together with the general properties displayed by the resulting composite electrodes in analyte-free gaseous or liquid media. These assemblies have been used as both voltammetric and amperometric sensors for electroactive analytes present in gases and in aqueous or organic solvents with no supporting electrolyte. The results indicate that their performance is similar to that expected on conventional electrodes, the only difference being a slightly lower degree of reversibility for the electrode processes investigated. Detection limits for some analytes were calculated and the use of SPE electrodes as sensors suitable for the continuous monitoring of electroactive analytes dispersed in gases or non-conductive liquids is reported. Preliminary attempts to use these assemblies for the determination of trace metals in low-conductivity solvents by anodic stripping voltammetry are discussed.     

An Alternative Approach to Quantify Partition Processes in Confined Environments: The Electrochemical Behavior of PRODAN in Unilamellar Vesicles

Herein, we investigate the behavior of the electroactive molecular probe 6‐propionyl‐2‐dimethyl amino naphthalene (PRODAN) in large unilamellar vesicles (LUV) formed with the phospholipid 1,2‐di‐oleoyl‐sn‐glycero‐3‐phosphatidylcholine (DOPC) by using cyclic voltammetry (CV). The CV studies in pure water confirm our previous spectroscopic results that PRODAN self‐aggregates due to its low water solubility. Moreover, the electrochemical results also reveal that the PRODAN aggregated species are non‐electroactive within the studied electrochemical potential region. In DOPC LUV media, the redox behavior of PRODAN shows how the LUV bilayer interacts with PRODAN aggregated species to form PRODAN monomer species. Moreover, the electrochemical response of PRODAN allows us to propose a model for explaining the electrochemical experimental results and—in conjunction with our measurements—for calculating the value of the partition constant (K_p) of PRODAN between the water and LUV bilayer pseudophases. This value coincides with that obtained through an independent technique. Moreover, our electrochemical model allows us to calculate the diffusion coefficient (D) for the DOPC LUV, which coincides with the D value obtained through dynamic light scattering (DLS). Thus, our data clearly show that electrochemical measurements could be a powerful alternative approach to investigate the behavior of nonionic electroactive molecules embed in a confined environment such as the LUV bilayer. Moreover, we believe that this approach can be used to investigate the behavior of non‐optical molecular drugs embedded in bilayer media.     

Superoxide generation in alkaline dimethyl sulphoxide

The generation of superoxide (O₂) in alkaline dimethyl sulphoxide (DMSO) is investigated by using electron spin resonance and chemiluminescence for the detection of oxygen species. The concentration of superoxide in this system corresponds to the concentration of oxygen dissolved in DMSO. In tightly stoppered vessels, the superoxide is stable for more than 24 h; agitation of the mixture in open vessels rapidly decreases its concentration. The presence of hydroxyl, carbon-centred and superoxide radicals is demonstrated by spin trapping. Alkaline DMSO is not recommended for superoxide dismutase assays.     

Selective solvation of Ag(I) iodate in methanol-dimethyl sulphoxide mixtures

The selective solvation of silver(I) iodate was studied in methanoldimethyl sulohoxide mixtures at 30° C by solubility and EMF measurements. The solubility of the salt increases continuously with the addition of dimethyl sulphoxide after a slight decrease into X_DMSO =0.1. The Gibbs energy of transfer of silver cation (determined on the basis of ferrocene reference method) decreases continuously while that of the iodate ion increases with the addition of dimethyl sulphoxide. The solvent transport number passes through a maximum (Δ = 2.0) around X_DMSO = 0.5. These results were interpreted as arising due to a heteroselective solvation of the salt, the silver ions being preferentially solvated by dimethyl sulphoxide and the iodate ion by methanol in these mixtures.     

Mössbauer study of the after-effects of the converted isomeric transition in 119mSn in frozen solutions of SnCl2 and in solid adducts of SnCl2 and SnBr4 with organic donor molecules

As an after-effect of the converted isomeric transition in ^119mSn, about 30% yield of the oxidised aliovalent species Sn⁴⁺ was observed in Mössbauer spectra of frozen solutions of ^119mSnCl₂ in organic solvents such as dimethyl sulphoxide and methanol. On the other hand, the reduced aliovalent species Sn²⁺ was observed in the emission spectra of crystalline solid adducts of ^119mSnBr₄ with dimethyl sulphoxide and other donors. As a mechanism for the stabilization of the oxidized aliovalent species observed in the emission spectra of the frozen solutions, it is proposed that the electrons released by Auger processes are trapped in imperfections of the glassy matrices of the medium. On the other hand, a possible mechanism for the formation of reduced species from adducts of ^119mSnBr₄ is that reducing radicals are formed by dissociative electron capture in autoradiolysis of the closest ligands within the molecules. The large yields of the oxidized aliovalent species Sn⁴⁺ observed in the emission spectra of the crystalline solid adducts of ^119mSnCl₂ with dimethyl sulphoxide, pyridine N-oxide, and pycoline N-oxides are also to be understood in terms of dissociative electron capture in autoradiolysis of the ligand.     

Spectrophotometric determination of dimethyl sulphoxide

A spectrophotometric method for the determination of milligram quantities of dimethyl sulphoxide in aqueous solutions has been developed. The method is based on the medium effect on the visible absorption spectra of metal salt solutions. Out of several compounds investigated, iron(III) alum was chosen as most suitable for this purpose. Beer's law is obeyed for dimethyl sulphoride concentrations up to 20 mg ml at 410 nm. Dimethyl sulphone does not interfere and dimethyl sulphide can be tolerated in amounts up to 25 % of the sulphoxide present. Possible interferences caused by metal cations can be avoided by passing the sample through a cation-exchanger before adding the reagent. The molar absorptivity is 3 l.mole(-1).cm(-1) at 410 nm, and the average relative error +/- 0.5%.     

Flow injection analysis of ranitidine based on derivatization reaction producing 2-Methylfuran cation as a sensitive and selective amperometric detector

A new electrochemical sensor in a flow injection system is developed for the determination of ranitidine in pharmaceutical formulations. The method is based on the deamination reaction of ranitidine in an acidic environment in the presence of nitrite ions producing the electroactive species 2-methylfuran and releasing nitrosamine [1]. A wide linear range of ranitidine from 4.4 to 6000 mg L⁻¹, with correlation coefficient R² = 0.996 and the detection limit 1.3 mg L⁻¹ was obtained. The method has been applied in tablet analysis and ampoules; the results are within the confidence limits with 95 % reliability compared to the reference method with HPLC.