Fast Scan Rates Research Articles

An examination of the finite difference numerical approach to the solution of electrochemically-induced diffusive transport at stationary solid cylinder electrodes: Single sweep voltammetry of reversible systems

Extensive computational testing of a finite-element Galerkin/B-spline (GBS) numerical model of electrochemical transport recently developed in this laboratory yielded results which agreed with predictions for planar and spherical hydrodynamics but were in disagreement with M.M. Nicholson's explicit finite difference (CEFD) model for the important case of nernstian boundary conditions at stationary solid cylinder electrodes. This disparity prompted a detailed re-examination of both models. We shown in this note that the CEFD approach was imprecise in its formulation of the first-order space derivative and was incorrect in applying established criteria for computational stability. Our re-examination of the same problem using implicit (IFD) and modified explicit finite difference (EFD) methods gave stable solutions which agreed fairly closely with each other; the GBS model agreed with the IFD solution to within ±0.3%. We are thus obliged to conclude that attempts to use the original non-dimensionalized cylindrical CEFD model to calculate concentration profiles, surface fluxes, and current-voltage curves are destined to yield incorrect results except for large radii, fast potential scan rates, or both, in which cases transport occurs essentially by planar diffusion which this model adequately describes.

Practical Algorithms for Mean Velocity Estimation in Pulse Doppler Weather Radars Using a Small Number of Samples

Doppler weather radars with fast scanning rates must estimate spectral moments based on a small number of echo samples. This paper concerns the estimation of mean Doppler velocity in a coherent radar using a short complex time series. Specific results are presented based on 16 samples. A wide range of signal-to-noise ratios are considered, and attention is given to ease of implementation. It is shown that FFT estimators fare poorly in low SNR and/or high spectrum-width situations. Several variants of a vector pulse-pair processor are postulated and an algorithm is developed for the resolution of phase angle ambiguity. This processor is found to be better than conventional processors at very low SNR values. A feasible approximation to the maximum entropy estimator is derived as well as a technique utilizing the maximization of the periodogram. It is found that a vector pulse-pair processor operating with four lags for clear air observation and a single lag (pulse-pair mode) for storm observation may be a good way to estimate Doppler velocities over the entire gamut of weather phenomena.

ON NEOPOLAROGRAPHY III——ANODIC STRIPPING NEOPOLAROGRAPHY WITH HANGING MERCURY DROP ELECTRODE

Anodic stripping neopolarography with HMDE has been carried out for 1.5 th order (e') and 2.Sth order (e") differential electroanalysis. Our experiments show that this method is very sensitive especially at fast scanning rate and reproducible and can be developed for trace analysis as pulse polarography.

The analytical performance of direct current, normal pulse and differential pulse polarography with static mercury drop electrodes

The recently developed static mercury drop electrode (SMDE) provides a fundamentally new approach to electrodes for polarography. An analytical evaluation of the electrode is presented. For a range of electrode processes, current-sampled d.c. polarography at the SMDE is useful down to at least the 10 -7 M concentration level when short drop times and fast potential scan rates are used. The improvement in the limit of detection for d.c. polarography is therefore very substantial. Improvements in sensitivity associated with normal pulse and differential pulse polarography at the SMDE compared with the dropping mercury electrode (DME) are marginal. It is concluded that at the SMDE, the analytical performance and response characteristics of d.c., normal pulse and differential pulse polarography tend to converge.

A computer-controlled X-ray powder diffraction system with an automatic sample changer

A 35-position Philips sample changer has been added to a PDP-8/I computer-controlled X-ray powder diffraction system which has operated in a single-sample mode for the past four years [Medrud, Harter, Stephenson & Kane, (1972), Program and Abstracts Amer. Cryst. Assoc. Annual Meeting, Albuquerque, NM, p. 58]. The system operation, instrumentation and data processing are discussed. The computer controls the diffractometer and the sample changer as well as the data acquisition and on-line processing. Subroutines provide for sample identification, correction of 20 values from an external standard, and shutdown of the system in the event of hardware malfunction. The diffraction data can be recorded on either a teletype or magnetic tape as well as on an analog strip-chart recorder. For ease of interpretation, each sample run is automatically started on a major grid division of the strip chart with an interrupt generated by a recorder-mounted photoelectric device. All essential elements of conventional manual diffractometer operation were included in the system plan. Digital intensity profiles taken at fast scan rates (2 to 4 deg min−1) are more precise than the corresponding analog profiles. As a result, the precision of peak location can be increased up to a factor of five.

Cyclic staircase voltammetry

The use of cyclic staircase voltammetry was investigated in order to study kinetic mechanisms at low concentration levels. The reversible case for c.s.c.v. was studied along with two common kinetic cases: e.c. and catalytic mechanism. For both the e.c. and catalytic case, it was possible to correlate the peak current ratio with the rate constant with one working curve regardless of the step size, the step time or the time in the step in which the current was measured. For the catalytic mechanism, the increase in the peak current of the forward peak can also be correlated with the rate constant, once again regardless of the experimental parameters. Using this technique, high sensitivity can be obtained at low concentrations and fast scan rates as was shown by comparing c.s.c.v. with cyclic voltammetry for micromolar cadmium solutions. The use of staircase voltammetry for the study of the catalytic reaction between iron (III)-triethanolamine and hydroxylamine was demonstrated and a rate constant of 180 M −1 s −1 was obtained, which is consistent with previous work.

Automatically tunable second-harmonic generation of dye lasers

A new technique which realizes the phase-matching of second harmonic generation (SHG) over a wide spectral range without mechanical or temperature tuning of the SHG crystal is developed. The phase-matching is achieved by changing the angle of the laser beam incident on the SHG crystal using dispersive components such as gratings or prisms. Over the entire spectral range of a Rhodamine-6G dye laser, the phase-matching is realized with a KDP crystal using this technique. This system is capable of producing tunable coherent ultraviolet light from 2300 Å to 3500 Å with an extremely fast scanning rate, if the dispersion of the prism and the SHG crystal is carefully matched.

ChemInform Abstract: LINEAR SWEEP VOLTAMMETRY OF MANGANATE(VII), MANGANATE(VI), AND MANGANATE(V) IN ALKALINE MEDIA

AbstractDas elektrochemische Verhalten von alkalischen MnO4‐‐ und MnO42‐‐Lösungen an Pt‐Elektroden wird voltammetrisch untersucht.

Linear sweep voltammetry of manganate(VII), manganate(VI), and manganate(V) in alkaline media

Electrochemical reactions were assigned to the voltammetric waves obtained in alkaline KMnO 4 and K 2MnO 4 solutions. Plots of i p (AC°ν 1 2 ) −1 and i ap i cp −1 ν s log ν indicate that at slow potential scan rates in MnO − 4 solutions ranging from 0·07 to 0·19 F in NaOH the first step in MnO − 4 reduction is a l-electron reversible charge transfer with no coupled chemical reaction; at fast scan rates (⩾ V s −1 the process becomes quasi-reversible. The standard rate constant and the activation energy for the MnO − 4/MnO 2− 4 charge transfer step were estimated. Plots of i p (AC°ν 1 2 ) −1 and i ap i cp −1 ν s log ν indicate that in 4·O F KOH and at potential scan rates between 0·005 and 2·V s −1 the reduction of MNO 2− 4 to MnO 3− 4 is a reversible charge transfer with no coupled chemical reaction. The diffusion coefficients of MnO − 4 and MnO 2− 4 in alkaline solutions are reported.

Pseudo-derivative d.c. and pulse polarographic methods at short drop times

Short drop time, fast scan rate d.c. polarographic techniques, whilst being superior to conventional methods in many respects, have the disadvantage of a higher charging to faradaic current ratio. Recent theoretical studies have suggested similar considerations, as well as additional problems from faradaic distortion, would apply in normal pulse polarography. In this work derivative readout is examined as a means for providing improvement in the short drop time d.c. and pulse methods. The validity of the Ilkovic (d.c.), Cottrell (pulse) and other polarographic equations are examined at drop times down to 50 ms. At this drop time the limiting current in pulse polarography can be less than in d.c. polarography, contrary to normal expectations. In the 50 ms drop time region, maxima, inhibition and other phenomena present under conventional conditions can be eliminated. With derivative methods, the noise level at a given scan rate decreases, the shorter the drop time. Since the charging current problem is also decreased, short drop time pseudo-derivative methods coupled with fast scan rates provide an attractive analytical technique. The technique of differential pulse polarography at short drop time is also examined and results are compared with the derivative methods. For the reduction of cadmium in 1 M HCl, a limit of detection of between 5×10 −7 and 10 −6 M was found with both the pseudo-derivative and differential pulse methods at a drop time of 0.10 s. This represents an order of magnitude improvement over the d.c. method.

A low noise detection system for the measurement of Auger spectra

An approved electronic detection system for Auger spectroscopy is described which may be used with two, three or four grid conventional display low energy electron diffraction systems. Amplifier noise in this arrangement is reduced so that it is comparable to the shot noise due to 1 mu A electron beam. The improvement in the Auger signal-to-noise ratio allows a much reduced time constant in the lock-in amplifier and hence a faster scan rate.

Principles and Applications of A‐C and D‐C Rapid Polarography with Short Controlled Drop Times

The techniques of a‐c and d‐c rapid polarography employing short controlled drop times and fast scan rates of potential are discussed in detail. Particular emphasis is given to comparing the methods with conventional polarography in electroanalytical applications. The shapes and main characteristics of both a‐c and d‐c polarograms are essentially the same as in conventional polarography. Any differences encountered generally arise from the different time scales of the two techniques. Reversible, quasi‐reversible, and irreversible electrode processes are considered. In analytical applications, the rapid methods are superior to conventional polarography because of the considerable time saving gained from the fast scan rates of potential. The reproducibility is also marginally better. In fundamental studies, such as in the characterization of electrode processes, rapid polarography is also shown to be particularly useful. The analogy of behavior akin to a streaming mercury electrode is considered, as are other phenomena and characteristics introduced by having short controlled drop times. It is concluded that the technique could be given wider usage than presently accorded.

Rapid-scan voltammetry of Ag(I) and Pb(II) in molten sodium nitrate—barium nitrate eutectic

The reduction of Ag(I) and Pb(II) in molten sodium nitrate-barium nitrate eutectic was studied at platinum wire micro-electrodes. The reduction of Ag(I) occurs reversibly and uncomplicated by any reaction of the deposited metal with either the platinum electrode or the nitrate solvent. The reduction of Pb(II), on the other hand, results in the deposition of a liquid metal which reacts with the nitrate solvent to form the oxide. The technique of rapid scan voltammetry was employed to study the above electrode reactions. Scan rates of 20 mV–130 V s −1 were used. It was shown that the reaction between the lead metal and nitrate solvent was negligible at fast scan rates (20–130 V s −1 ).

Rapid-scan voltammetry of Ag(I) and Pb(II) in molten sodium nitrate-barium nitrate eutectic

The reduction of Ag(I) and Pb(II) in molten sodium nitrate-barium nitrate eutectic was studied at platinum wire micro-electrodes. The reduction of Ag(I) occurs reversibly and uncomplicated by any reaction of the deposited metal with either the platinum electrode or the nitrate solvent. The reduction of Pb(II), on the other hand, results in the deposition of a liquid metal which reacts with the nitrate solvent to form the oxide. The technique of rapid scan voltammetry was employed to study the above electrode reactions. Scan rates of 20 mV–130 V s−1 were used. It was shown that the reaction between the lead metal and nitrate solvent was negligible at fast scan rates (20–130 V s−1).

Experimental Study of Slow-Scan High-Resolution Video Transmission at One-MegaHertz Bandwidth

It is often desirable to transmit graphic information such as a typewritten page or a detailed photograph over band-limited transmission media and to display it at the receiving end on a real-time basis. Since a narrow bandwidth is desirable for this, the video information must be transmitted at a low frame rate. At the receiving station a cathode-ray tube may be used to display the slow-scan information. To overcome flicker resulting from the low frame rate and/or inefficient long persistent phosphors a scan-conversion and frame-repeating technique is described in this paper. At the receiver the transmitted information displayed on a slow-scan cathode-ray tube is optically transferred through a lens to the target of a storage vidicon working at fast-scan rates. A long decay characteristic of stored charge in the photoconductor of the target makes it possible to regenerate the same frame of the video signal many times at the fast-scan rates without conspicuous fading during the slow-scan frame time. The regenerated information is displayed on a high-resolution wideband monitor.

Electronic Scanning Spectrometer for Measurements of Rapidly Changing Spectra

This spectrometer has a resolution of 15 A over the spectral region 0.6-1 micro, and can obtain electronically read out spectra at selectable scanning rates of up to 10,000 scans/sec. The technique of using an image dissector as the detector holds promise of even faster scan rates. The NEI of 4.6 x 10(-5) W/cm(2) at 0.8 micro is comparable with that of ir streak films. The instrument is ruggedized for field and airborne use.

Electrochemical reduction of U(IV) in molten LiF−BeF 2−ZrF 4 (65.6-29.4-5.0 MOLE %). Evidence for adsorption of U(IV)

Summary Voltammetric and chronopotentiometric results at fast scan rates and short transition times, respectively, indicate the presence of weak adsorption of U(IV) at platinum and platinum-rhodium (10%) working electrodes at 500°. Data for concentration-dependence are presented. The variation of the shape of voltammograms with temperature is discussed.

Voltammetry of nickel in molten lithium fluoride-sodium fluoride-potassium fluoride

Results are presented on the rapid scan voltammetry and anodic stripping voltammetry of nickel in molten LiF-NaF-KF. The technique of anodic stripping voltammetry is promising as a means of determining trace quantities of nickel directly in the melt; as little as 1 p.p.m. can be detected. The half-wave potential for the reduction of Ni(II) → Ni(0) occurs at approximately −0.2 V vs. the platinum quasi-reference electrode. Current-voltage curves were recorded at a pyrolytic graphite indicator electrode at voltage scan rates from 50 mV/min-10 V/min. At the faster scan rates, peak-shaped curves resulted which indicated that the transport process was mainly diffusion-controlled. A diffusion coefficient of 1·10 −6 cm 2/sec at 500° was calculated. An activation energy of about 18 kcal/mole was calculated for the current-limiting process which corresponds to the reduction of Ni(II) to the metal.

Rapid scan voltammetry and chronopotentiometric studies of iron in molten fluorides: Fabrication and use of a pyrolytic graphite indicator electrode

Voltammetric and chronopotentiometric studies of iron(II) in molten LiF-NaF-KF and LiF-BeF 2 were conducted. The current-voltage curves were recorded by means of a controlled-potential polarograph capable of measuring cell currents up to 5 mA and providing scan-rates from 50 mV/min to 10 V/min. At faster scan-rates (~1 V/min and greater) peak-shaped curves resulted which indicated that the transport process to the electrode was diffusion-controlled. The diffusion coefficient was calculated to be approximately 1 · 10 −6 cm 2/sec and 5 · 10 −6cm 2/sec in molten LiF-NaF-KF and LiF-BeF 2, respectively. The diffusion coe LiF-BeF 2, calculated from chronopotentiometric data, was too large. This was later found to be due to an alternate current path within the cell. Pyrolytic graphite electrodes prepared from commercially available plates, when properly oriented and insulated in boron nitride, appear to be well suited for electrochemical measurements in corrosive melts.

The effect of sample movement in fault detection using eddy currents

Eddy-current methods of non-destructive testing applied to travelling metal objects on production lines show two advantages over other techniques. These are the comparative ease with which an instrument can be coupled to test objects and the high speed of response, making possible the fast scanning rates required with fast manufacturing process.Experience in recent years has shown that relative motion between test object and measuring head gives results which differ from those of a static system, and may severely limit permissible scanning rates. The paper draws attention to dynamically-induced currents, as causing the speed effect experienced in eddy-current testing. The theoretical arguments advanced have been used to evolve three methods of overcoming the speed effect.