Peak Current Intensity Research Articles

Cyclic Voltammetry and Quartz Crystal Microbalance Electrochemical Studies on Thiosalicylic Acid and Dithiodibenzoic Acid

ABSTRACT Cyclic voltammetry (CV) and electrochemical quartz crystal microbalance (EQCM) were used to study the behavior of thiosalicylic acid (KTS) and dithiodibenzois acid (DTDB) at the controlled-growth mercury drop electrode (CGMDE), at the gold electrode, and at the carbon electrode. The CV method was used to study peak current intensities and peak potentials in relation to pH of the solution, to scan rate and to the concentration of the analyte. Optimum measurement parameters were established and stock solutions developed. The electrode activity was found to be primarily due to the oxidation of the sulhydryl group. The currents observed are diffusion controlled. Electrochemical studies on complexes of KTS with Cu(II) were undertaken at varying constituent proportions and the cathodic peak was found to rise by ca. 70% whereby the sensitivity of the determination was considerably increased. The EQCM method allowed to establish that, as KTS is oxidized, the electrode slightly gains in weight (ca. 80 ng). As DTDB is reduced, the electrode slightly looses in weight (ca 40 ng). Studies on the equimolecular KTS-Cu(II) complex showed the electrode to gain about 125 ng more in weight than in the solutions containing KTS alone, a fact evidencing for the formation of the complex and for its deposition on the electrode surface. A procedure was developed to determine KTS and DTDB in the substance by the CV method. Statistical evaluation of data showed the results to be characterized by good accuracy and precision (RSD 0.80% and 0.61%, respectively).

First electrochemical investigation of the redox properties of DOPA–melanins by means of a carbon paste electrode

The electrochemical properties of eumelanins, a cutaneous pigment responsible for skin color, insoluble in any solvent, was studied by using a carbon paste electrode. Eumelanins synthesized by enzymatic oxidation of l-DOPA were included in the carbon paste. The concentration of melanins (10%) and the potential scan rate (10 −4 V s −1) were chosen in ranges for which there is a linear relationship between these parameters and the current peak intensity. In these thin-layer conditions, the voltammogram of eumelanins at pH 5.6 reveals two main peaks in oxidation at +460 and +525 mV versus SCE and in reduction at +20 and −355 mV versus SCE. No variation of peak potential was observed when the pH was varied in the range 2–8. An average of two electrons (three for the first oxidation scan) were exchanged in these redox processes whereas the number of subunits in one molecule of melanin was around 15 as found by MALDI mass spectroscopy experiments. These results support the assumption of an irregular organization of monomer units in the structure of melanins.

Cobalt(II) Tetrasulfophthalocyanine Complex Adsorbed on a Silica Gel Surface Chemically Modified with 3-N-Propylpyridinium Chloride: Oxalic Acid Oxidation Study

Cobalt(II) tetrasulfophthalocyanine complex was adsorbed on the 3-N-propylpyridinium chloride functionalized silica gel surface by means of an ion exchange reaction. The specific surface area of the modified silica was SBET= 320 m2 g–1 and the amount of the complex bonded to the substrate surface was 17 mol g–1. A carbon paste electrode made with electrocatalytically oxidized oxalic acid at an anodic potential E=850 mV in 0.5 M KCl solution at pH 2. The anodic peak current intensities were linearly correlated with oxalic acid concentrations between 3.97 and 19.2×10–2 M. The electrode showed a very reproducible response and was chemically very stable under various oxidation-reduction cycles. The electrode was applied in the determination of oxalic acid in a spinach sample.

Voltammetric and EQCM Studies on Selenium(IV) at Mercury, Gold and Glassy Carbon Electrodes in the Presence of Cu(II), Au(III), Pb(II) and Cd(II)

Cyclic voltammetry (CV), adsorptive stripping voltammetry (AdSV) and electrochemical quartz crystal microbalance (EQCM) were employed to study the behavior of selenium(IV) at the mercury, gold and glassy carbon electrodes. Optimum measurement parameters and stock solution compositions were established. Selenium(IV) was studied in the presence of copper, gold, lead and cadmium ions. At the glassy carbon electrode, selenium and gold were found to be too slightly electroactive to be determinable. However, in view of the copious release of Se-Au in the EQCM experiment involving the gold electrode, a voltammetric procedure was developed to determine the two elements. Peak current intensities recorded in relation to a Se(IV)/Au(III) mixture concentration yielded a linear relationship over the concentration ranges 3–9 μg mL−1 Se in the presence of 10 μg mL−1 Au, over 3–6 μg mL−1 Au in the presence of 10 μg mL−1 Se, and over 4–10 μg mL−1 Se(IV) in the presence of 16 μg mL−1 Cu(II). Statistical analysis of the results obtained showed the procedures to be useful for determining Se(IV) and Au(III) at the glassy carbon electrode with a satisfactory accuracy and precision. In AdSV method a control growth mercury drop electrode (CGMDE) was used as the working electrode and 0.1 mol L−1 HClO4 solution containg Cu(II) as supporting electrolyte. This method was applied to selective Se(IV) determination in pharmaceutical preparations (tablets) after microwave digestion. The results obtained by AdSV were compared with those obtained by AAS method. Nickel nitrate was used as a modifier in AAS method. The procedure developed enables selenium to be determined quantitatively with a satisfactory accuracy and precision in multicomponent tablets with no need for isolating it from the matrix.

Pulse polarography of soybean trypsin inhibitor in the presence of dithiothreitol

The redox system of reduced and oxidized dithiothreitol, DTT(SH) 2/DTT(S-S), was followed in the presence of bovine pancreatic trypsin inhibitor (BPTI) from buffer solution pH 8.8 using differential pulse polarography (DPP). The reduction of -S-S- bonds in protein with DTT(SH) 2 was investigated either in absence or in the presence of denaturant such as guanidine hydrochloride (Gdn-HCl). From the analysis of -SH and -S-S- DPP peak current intensities, the kinetic of BPTI unfolding was estimated. It was found that the reaction rate is one order of magnitude higher when denaturant is present, indicating that the lifetime of the corresponding intermediary is shortened.

Analysis of Reaction of Luminol at an Indium-tin Oxide Anode by Cyclic Voltammetry.

In the present study, cyclic voltammograms obtained with an indium-tin oxide (ITO) anode were compared with those obtained with a platinum anode. The peak currents generated by luminol oxidation at the platinum anode showed convex curves facing upwards with increasing luminol concentration, as the result of the prevention of the oxidation of the platinum electrode by the presence of luminol. With the ITO anode, no current due to electrode oxidation or reduction was detected. The peak current intensity of luminol oxidation was proportional to the luminol concentration, but the peak current was also in a linear relationship with the square root of the scan rate. However, the intercept current was largely deviated from zero. The deviation suggests radial diffusion of luminol at the ITO anode which has a small active point density. In conclusion, the ITO anode is useful for accurate measurement of the rate of luminol oxidation, but radial diffusion of luminol in the vicinity of the ITO surface should be taken into consideration in analysis of luminol oxidation.

Geomagnetically quiet day ionospheric currents over the Indian sector—II. Equatorial electrojet currents

With 1986 quiet days data of Indian observatories, the equatorial electrojet (EEJ) has been studied in terms of 8 landmark parameters that reveal the structures of hourly latitudinal profiles of EEJ current from 0700 to 1700 h local time. The landmark distances suggest that near dawn, the EEJ is widest and its centre and focus are most northerly before it contracts towards local noon with its centre and focus moving southwards. The seasonal means of peak current intensity and the total forward current seem to peak earlier when the intensity of EEJ is higher than when it is lower. The seasonal order of EEJ intensity is found not to be the same at all hours of the day. This implies that seasonal variation of EEJ is not semiannual at certain daytime hours. The landmark distances of EEJ current have semi-annual variations with minima in the vernal and autumnal equinoxes and maxima at June and December solstices, but annual variations of the measures of EEJ current are just the reverse. Certain properties of the worldwide part of Sq are found to be markedly different from those of EEJ including some key features of diurnal and seasonal variations.

Electrochemical reduction at a mercury electrode and differential-pulse polarographic determination of dibucaine in pharmaceutical ointments

The polarographic behaviour of dibucaine was studied using various electrochemical techniques. In acidic media, a non-reversible diffusion-controlled reduction process involving two electrons takes place. The linear relationship between peak current intensity and dibucaine concentration allowed the differential-pulse polarographic determination of dibucaine over a wide concentration range, from 10–6 to 10–4 mol l–1 at pH 2.6, with a relative standard deviation of 1.9%(nine determinations at the 10–5 mol l–1 level). A simple extraction procedure was employed for the determination of the drug in pharmaceutical formulations and the validity of the method was confirmed by parallel determinations using the official method of the US Pharmacopeia (HPLC).

Experimental point spread function of fm pulse imaging scheme.

In this paper, we have examined the possibility of incorporating pulse compression techniques into a conventional medical B-scan imaging scheme. Linear frequency modulation fm, one form of pulse coding among many others, has been used in this study. With this approach, one can overcome current peak intensity limitations. A theoretical framework that includes medium propagation effects, transducer bandwidth and diffraction effects is presented, which could be used to examine the system point spread function under this imaging scheme. A prototype experimental set-up and signal processing are described and used for simple imaging tasks in attenuating and nonattenuating media. Analysis of the experimental point spread functions shows that resolution similar to conventional short pulse imaging can be achieved. However, the existence of large range side lobe levels usually associated with pulse compression processing can degrade contrast resolution in medical ultrasound. We have considered various different factors that can affect the range side lobe levels and examined their effect either experimentally or through simulations. The technique has the potential for improving signal-to-noise ratio (SNR), maximum penetration depth and resolution without exceeding peak intensity limitations. Some possible applications are discussed that merit further evaluation. Our work demonstrates the feasibility of this technique and presents a theoretical framework that can be used in future studies aimed at evaluating image quality, system performance, and possible artifacts under such an imaging scheme.

Experimental Point Spread Function of FM Pulse Imaging Scheme

In this paper, we have examined the possibility of incorporating pulse compression techniques into a conventional medical B-scan imaging scheme. Linear frequency modulation fm, one form of pulse coding among many others, has been used in this study. With this approach, one can overcome current peak intensity limitations. A theoretical framework that includes medium propagation effects, transducer bandwidth and diffraction effects is presented, which could be used to examine the system point spread function under this imaging scheme. A prototype experimental set-up and signal processing are described and used for simple imaging tasks in attenuating and nonattenuating media. Analysis of the experimental point spread functions shows that resolution similar to conventional short pulse imaging can be achieved. However, the existence of large range side lobe levels usually associated with pulse compression processing can degrade contrast resolution in medical ultrasound. We have considered various different factors that can affect the range side lobe levels and examined their effect either experimentally or through simulations. The technique has the potential for improving signal-to-noise ratio (SNR), maximum penetration depth and resolution without exceeding peak intensity limitations. Some possible applications are discussed that merit further evaluation. Our work demonstrates the feasibility of this technique and presents a theoretical framework that can be used in future studies aimed at evaluating image quality, system performance, and possible artifacts under such an imaging scheme.

Adsorption of environmentally important metal ions on solid particles. Part 1. Anodic stripping voltammetry in the presence of solid particles

The effect of suspended solid particles on the anodic stripping voltammetric signals of Cu2+, Pb2+, Cd2+ and Zn2+ was studied. In the size range between 2 and 30 µm, inorganic suspended particles do not affect the differential capacity of the electrode. The agitation rate usually used for anodic stripping voltammetry (52.4–261.8 rad s–1) does not promote mass transfer. Therefore, the decrease in the anodic stripping voltammetric peak current intensity can be used to study adsorption on surfaces.

Latitudinal and vertical parameters of the equatorial electrojet from an autonomous data set

The magnetic field expressions from the current ribbon and thick current versions of the continuous distribution of current density model and their merits have been presented. For the first time both the latitudinal and vertical parameters of the equatorial electrojet (EEJ) have been derived from the same set of data. The local noon and daytime means of certain key parameters of the EEJ are shown to be in good agreement with those from other sources. Selected local noon means include: peak current density j o , 10.58 ± 0.34 A/km 2; peak current intensity j o , 224 ± 9 A/km; total eastward current I +, 74 ± 5 kA ; EEJ current focal distance w, 300 ± 5 km ; half thickness at half of peak current density p, 7.0 ± 0.1 km; peak westward current location x m , 5.13 ± 0.08° dip latitude; and EEJ latitudinal extent L 1, 12 ± 1° dip latitude. The problem of model calculated landmark distances of EEJ being consistently shorter than observations, encountered by Onwumechili et al. [ J. geomagn. Geoelecl. 41, 443 (1989)] has been solved.

A study of rocket measurements of ionospheric currents-V. Modelling rocket profiles of low-latitude ionospheric current

Some 25 rocket profiles of the current density of the eastward ionospheric lower current layer close to the magnetic dip equator, one profile of the reverse westward lower current layer at about 5° dip latitude and five profiles of the eastward upper current layer within 0° to about 8° dip latitude have been modelled giving the following results. The parameters of the eastward lower current layer within 0° to 2° dip latitude around local noon are; peak current density normalized to dip equator 10.4 ± 6 A km-2, peak current intensity normalized to dip equator 145 ± 86 A km-2, and half thickness at half of the peak current density 5.7 ± 1 km. The differences in these parameters between the equatorial electrojet in India and Peru are negligible. The parameters of the ionospheric eastward upper current layer around local noon are; peak current density normalized to the dip equator 2.37 ± 0.5 A km-2, peak current intensity normalized to the dip equator 22.1 ± 7 A km-2, and half thickness at half of the peak current density 4.1 ± 2 km. The parameters of the model fitting the rocket profiles extremely well have been provided. The suggestion that the lower eastward and the lower westward current layers constitute the equatorial electrojet, while the upper eastward current layer is the global part of the Sq currents, has been substantiated. This leads to results consistent with spherical harmonic analysis of geomagnetic Sq variations. The first contours of the ionospheric eastward upper current layer density have been presented and they are consistent with the features of the worldwide Sq.

Surface functionalization with oxometalates entrapped in polymeric matrices: Part I. New and remarkably stable poly(4-vinylpyridine) based redox coatings

Entrapment of oxometalates in poly(4-vinylpyridine)(PVP) matrices on electrode surfaces results in new materials which show remarkable stability. The system is described mainly with [SiW 12O 40] 4−/PVP in acid media. Incorporation of the heteropolyanion (HPA) in the polymer is obtained readily but the cyclic voltammogram of the assembly shows its maximum peak current intensity after variable stabilization periods depending on the anions in the supporting electrolyte. The stabilization to this maximum value is fastest in the presence of Cl − or CF 3COO −. Other anions like ClO − 4 and SO 2− 4 behave differently, but several procedures have been devised to prepare stable and reproducible electrodes. Solvent effects have also been studied. It appears that HPAs included in PVP films can serve as probes to follow the dynamic behaviour of this polymer in several supporting electrolytes up to the stabilization of the electrochemical response of the electrodes. The [SiW 12O 40] 4−/PVP assembly has been shown to be stable, which makes it very attractive for theoretical studies as well as practical purposes. An attempt is made to clarify several factors that affect the electrochemical responses of the present systems.

Repetitive firing properties of phrenic motoneurons in the cat.

1. Using both rectangular- and ramp-shaped intracellularly injected currents, the repetitive firing properties of 23 antidromically identified phrenic motoneurons were determined in anesthetized, paralyzed, and artificially ventilated cats during hypocapnic apnea. In response to rectangular depolarizing current injections, regular repetitive firing was observed in all cells. 2. At the beginning of a rectangular current pulse, the firing pattern was characterized by high frequency of firing that rapidly adapted to a much lower steady-state value. The relationship between the reciprocal of the first interspike interval (F1-2) and injected current was described by an initial linear portion of shallow slope, followed by a much steeper segment that smoothly reached a plateau value. The plateau value of F1-2 did not change with further increase in injected current. 3. The steady-state firing frequency versus injected current relationship was represented by a line of shallow slope over the entire range of injected currents. The slope of this line ranged between 1.1 and 4.5 Hz/nA. 4. A weaker correlation between minimal firing frequency for continuous activity and the reciprocal of the after hyperpolarization duration (1/AHPdur) was found for phrenic motoneurons than exists for lumbosacral motoneurons (26). Comparison of AHP shape at different levels of repetitive firing revealed that the slopes of the ascending portions of the AHP were similar except at the higher injected currents. Further, in the same cells during natural inspiratory activity the ascending part of the AHP was similar to that observed during current injection. 5. Depolarizing current ramps (approximately 1-s duration) were injected into 11 phrenic motoneurons. Instantaneous firing frequency was directly proportional to the intensity of the instantaneous injected current and independent of the rate of change of current for the range of ramp slopes tested (5-80 nA/s). Ramp-and-hold current injections were done in three motoneurons, and the peak instantaneous firing frequency showed little adaptation during the hold maneuver. 6. During hypocapnic apnea, we mimicked the expiratory-phase inhibition and inspiratory-phase excitation of phrenic motoneurons by injecting a 1-s depolarizing current ramp that was immediately preceded by a 1-s hyperpolarizing current ramp of the same absolute peak current intensity. Compared with the effects of positive current ramps alone the spike onsets during the negative-positive current ramp paradigm were either facilitated or retarded. Various ionic mechanisms are suggested for these effects as well as their function in determining the onset of firing during natur

Diurnal variations of equatorial electrojet parameters derived from POGO solstitial data

The POGO electrojet data have been analysed for the winter and summer solstitial seasons of the two years, 1968 and 1969, respectively. Our analysis yielded a very large number of values (about 432), each of the electrojet half width, w, its peak current intensity, J 0, and its total eastward current, I +, at 0900–1400 LT in December, and at 1000–1500 LT in June solstitial seasons, respectively. The all-longitude daytime values of the parameters are 246 ± 48 km for w,216 ± 60 A km −1 for J 0, and (58 ± 8) × 10 3A for I +, in December of 1968 and 218 ± 19 km for w, 187 ± 20 A km −1 for J 0, and (45 ± 3) × 10 3 A for I +, in June of 1969, respectively. We therefore present a diurnal study covering the entire Earth, in which for the first time, morning data earlier than 1000 LT are incorporated in the analysis. This has enabled us to chart a clearer picture of the temporal variations of electrojet parameters at two different solstices. This shows that all of the three parameters vary substantially with local time, in such a manner that J 0 and I + attain maximal values around local noon, while w is a minimum then, and therefore confirms the finding of Agu and Onwumechili.

Electrogenerated Chemiluminescence. 50. Electrochemistry and Electrogenerated Chemiluminescence of Micelle Solubilized Os(bpy)32+

Abstract The oxidative electrochemistry and electrogenerated chemiluminescence (ECL) of Os(bpy)32+ were studied in the presence of sodium dodecyl sulfate (SDS), Triton X-100, and cetyltrimethylammonium bromide (CTAB) in aqueous solutions. Variation of supporting electrolyte (NaCl) concentration affected the interaction between Os(bpy)32+ and the surfactants. The measurement of oxidation peak current (ipa) and ECL intensity at different NaCl concentrations suggested a strong interaction of Os(bpy)32+ with SDS. Above the critical micelle concentration (cmc) the strong hydrophobic interaction of Os(bpy)32+ and the SDS micelle core greatly suppresses both ipa and the ECL intensity. The interaction of Os(bpy)32+ with CTAB micelle is much weaker because of electrostatic effects. Triton X-100 micelles do not interact with Os(bpy)32+.

Triggered lightning in New Mexico

Triggered lightning experiments with rockets pulling a grounded wire were performed at Langmuir Laboratory in 1981. Out of 56 firings, 35 events were triggered among which 29 had a peak current intensity above 2 kA and 27 were hybrid flashes. The highest current peak is 40 kA±10%. The median value of the transported charge is 35 C, including a contribution of 28 C from the continuous component. The largest charge is in a flash carrying a mixture of +235 and −240 C±10%. The median number of pulses is 15 above 2 kA, 10 above 3 kA, and 2 above 10 kA. Comparison with natural lightning reveals a large excess of small current pulses in the triggered case, but there is a great similarity in shape, size, and number between the subsequent strokes of natural flashes and the largest among the pulses in the triggered events.

Thermally-stimulated-depolarization study on the highly viscous isotropic mesophase of w/o microemulsions

Highly viscous, optically isotropic water-in-oil microemulsion samples (dodecane, hexanol, potassium oleate plus water) with water content in the concentration interval 0.68≤C<0.8 (C, mass fraction) were studied by means of the thermally-stimulated-depolarization (TSD) method. Different polarizing temperatures were tested from 293 K down to 188 K. Depending on the polarizing temperature, the intensity of the static electric field was increased from 33 V/cm up to 10 kV/cm. The temperature dependence of the field-induced orientation processes was investigated and the activation energy and the relaxation time of the depolarization processes that were found to follow a first-order relaxation kinetic were evaluated. The temperature domain within which the viscous, isotopic mesophase may exist was also identified. The low-temperature TSD spectra of highly viscous, isotropic microemulsion samples are characterized by a current peak atT=198 K which follows a first-order kinetic and whose maximum current peak intensity is an inverse function of the polarizing temperature.

Temporal variations of POGO equatorial electrojet parameters

An analysis of the POGO satellites observations of the magnetic field of the equatorial electroject for the September equinoctial months of the years 1967, 1968 and 1969, provided about 500 values each of the electrojet half-width w, its peak current intensity J 0, its total eastward current I + at 11, 12, 13, 14 and 15 h LT. The all-sector daytime average values of the parameters for the three years are 232 ± 47 A km −1 for J 0, 234 ± 6 km for w and (55± 8)×10 3 A for I +. This first coverage of all sectors of the globe gives the first study of the diurnal variation of the total current and shows that all the three parameters vary substantially with local time; that w has minimal values around local noon; that J 0 has a pronounced peak around local noon as may be expected from the diurnal variation of H; and that I + has a broad maximum around 11 h and 12 h LT.