Coulometric Analysis Research Articles

Coulometric study of the redox dynamics of iron in seawater

Controlled potential coulometric analysis has long been recognized as one of the most sensitive electrochemical techniques but is limited in application by the lack of specificity of the method. One way of improving specificity is through electrochemical masking using strong, selective complexing agents. In this paper, a controlled potential coulometric procedure combined with a masking technique is developed and applied to the measurement of low concentrations of oxidizable Fe(II) species in seawater. The technique is particularly appropriate to studies of iron redox processes in seawater since the redox dynamics of the system under investigation are not greatly perturbed by the analytical procedure developed.

CdS solar cells: The formation of non-stoichiometric curpous sulphide by the ion exchange process

The formation of non-stoichiometric cuprous sulphide by the ion exchange process is explained as due either to the incorporation of copper(II) species remaining in the ion exchange solution or to oxidation by the solvent. The hypotheses are based on determinations of the Cu x S composition by solid state coulometric analysis and are supported by thermochemical calculations. For this study sprayed CdS was used as the substrate, but it is shown that the reaction does not depend on the origin of the CdS. In particular, the high oxygen content in sprayed CdS is not the sole cause of the deviation from stoichiometry.

Ligand exchange of metal carbonyls by chain mechanisms. Electrochemical kinetics of electron transfer catalysis

The metal carbonyl derivatives LM(CO) n of manganese, rhenium, molybdenum and tungsten undergo facile ligand substitution by an electrode-mediated process. Phosphine, pyridine and isocyanaide substitutions are shown to be chain reactions by coulometric analysis and by cyclic voltammetry of the metal carbonyl solutions containing the added nucleophiles L . Reversible electrochemical parameters can be obtained for both LM(CO) n and the substitution product L M(CO) n . These allow the digital simulation of the cyclic voltammograms by Feldberg's method, which provides a detailed analysis of the kinetics of the electrocatalytic mechanism for ligand substitution. Generally, the radical cation LM(CO) n + produced initially at the anode undergoes rapid exchange with L to afford the cationic substituted species L M(CO) n +. This step is followed by electron transfer with the reactant LM(CO) n to yield the substitution product L M(CO) n and regenerate the radical cation LM(CO) n +, which completes the chain propagation sequence. Multiple repetition of this cycle is indicated by the high current efficiencies which are obtainable. The second order rate constants for the ligand exchange of the 17-electron radical cations LM(CO) n + with added L are evaluated, and found to be more than 10 6 times larger than that for the neutral, diamagnetic precursor LM(CO) n . The radical cations L M(CO) n + also react readily with phosphine and alkyl isocyanide nucleophiles, leading to a characteristic distortion of the CV waves. Digital simulation of such cyclic voltammograms allows the kinetics of these processes, particularly the redox catalysis in the oxidation of phosphines by LM(CO) n +, to be evaluated quantitatively. The enhanced reactivity of the 17-electron radical cations LM(CO) n + is discussed in relationship to recent reports of the substitution lability in other 17- and 19-electron metal carbonyls.

Synthesis of Mn-Zn ferrite from Na2O-B2O3 flux under CO-CO2 ambients

Synthesis was carried out by dissolving MnO, ZnO, and Fe2O3 in the molar ratio 0.42:0.44:1.07 in fluxes of Na2O:xB2O3 at 900 °C and slow cooling under ambients of CO-CO2 mixtures; x varied from 1.34 to 2.0 and Co-Co2 from 0.06 to 1.0% CO. The composition and oxidation state of the precipitated ferrite were dependent on both CO content and x. The composition of the ferrite ranged between Mn0.44Zn0.40Fe2.16O4 and Mn0.09Zn0.06Fe2.85O4 and the Fe2+ between 0.16 and 0.85 moles/formula unit. By using reactive gas mixtures, the cation oxidation states were maintained constant during cooling. Mössbauer spectroscopy and coulometric analysis were used to establish that Mn3+ was unstable in the flux in the presence of Fe2+. The results obtained by these experiments establish the optimum conditions necessary to grow Mn-Zn ferrite films by liquid phase epitaxy (LPE). The saturation magnetization of the synthesized material was between 4700 and 5200 G; the Curie temperature increased with Fe2+ from 180 to 490 °C. These properties agree with measurements previously made on LPE films of this material.

AUTOSEP, an automated system for the quantitative ion exchange separation of plutonium from impurities

The anion exchange separation of interfering impurities from plutonium in the Dowex-1 8 N HNO3 system is a reliable purification method. An automated system, AUTOSEP, based on this manual procedure has been developed for greater productivity. Samples, in groups of ten, each containing 5–10 mg of plutonium are processed automatically in this system. AUTOSEP incorporates a means of programming reagent delivery, adjustment of the sample to the plutonium (IV) oxidation state via Fe(II) reduction/HNO3 oxidation, plutonium sorption on the resin in 8 N HNO3, washing of the resin bed with 8 N HNO3 to remove impurities, elution of the purified plutonium with 0.36 N HCl–0.01 N HF, and waste effluent disposal. The reagents are delivered by gravity from a module whose only moving parts are rotary valves. The eluted plutonium solutions are collected for subsequent controlled-potential coulometric analysis. The average recovery of plutonium determined by controlled-potential coulometry using this apparatus was 100.00% with a relative standard deviation of 0.10%.

Extended theory of tandem electron capture detectors

A theoretical model based on second-order kinetics describes the operation of the tandem electron capture detector as an absolute gas-phase coulometer in the fixed pulse model. The previous assumption that linear operation requires the electron concentration to be far in excess of solute is found invalid. Initial growth of electron concentration is independent of sample concentration because of the short ionization interval. Pseudo first-order kinetics therefore apply to sample concentrations up to several orders of magnitude larger than the electron concentration. Equations for coulometric analysis are developed to accommodate tandem detectors having different source strengths, carrier gas and column bleed impurities, and different ion recombination rates.

Electron-capture detector: Theory and practice II

In Part I a simple numerical model of the electron-capture detector (ECD) was described that could account with fair accuracy for the performance of the detector i the constant-current and constant-frequency methods of operation. In this paper, the model is extended to include the effects of electron loss by recombination and by reaction with trace amounts of contaminants in the carrier gas. These refinements appear to improve the predictions of the model to an accuracy of 5% or better. The practical problem of coulometric analysis with the ECD as its own standard is discussed. The environmental problem posed by the presence of atmospheric halocarbons demands an analytical accuracy of 3% or better. Teh theoretical considerations and practical experiments here reproted were made within the context of this need for improved accuracy. To meet it and also to validate the model of the ECD, an exponential dilution chamber was constructed and used to prepare in one step accurate standards with a parts per trillion (10 12) by volume concentration in air.

Nonaqueous Batteries with BiF3 Cathodes

Li cells based on cathodes have been tested. In particular, the effect of cathode formulation and compacting pressure, nature of the solution, temperature, and current density have been evaluated. Through x‐ray analysis, cyclic voltammetry, and coulometric analysis it was possible to ascertain that is reduced to Bi and then to . During the first reduction process, ternary phases of the type are probably formed. cells can give practical energy densities in excess of 200 W‐hr/kg at the rate of 0.5 mA/cm2.

Coulometric trace determination of water by using Karl Fischer reagent and potentiometric end-point detection

A new approach to the determination of water via the Karl Fischer reaction is described. Iodine is coulometrically generated and the end-point corresponding to a slight excess of iodine, is detected potentiometrically with a non-polarized platinum electrode. Samples of 1–500 μl containing 0·05–200 μg of water were analysed with a standard deviation of 0·015 μg in the range 0·05–20 μg of H 2O. A specially constructed electrolysis cell was used in combination with an LKB 16300 Coulometric Analyzer and the time for a complete analysis was 1–4 min, depending on sample size. The reagent composition has been optimized in order to enhance the rate of the main reaction and to minimize the extent of side-reactions. Decreasing the temperature reduced the extent of side-reactions. The displacement of end-point potential on dilution was studied and a correction is discussed.

Electroanalysis and coulometric analysis

Electroanalysis and coulometric analysis

Oxidative coulometric trace determination of sulphur in hydrocarbons

A new combustion system for the oxidative coulometric determination of sulphur in liquid hydrocarbons is described. The conditions were selected so that the recovery as SO 2 was close to 100%. The relative standard deviation was < 1% for sulphur in the range 2–1000 mg/l., with thiophene in cyclohexane as a test substance. Thermodynamic data on the equilibrium between SO 2, O 2 and SO 3 were used to select the operating conditions. To increase the recovery of SO 2 the combustion gas mixture was diluted with an inert gas to lower the partial pressure of oxygen. A temperature of 1000° in the equilibrium zone resulted in a recovery of 99%. The SO 2 was titrated with coulometrically generated iodine, the concentration of which was controlled by a Pt-redox electrode. The response of this electrode has been examined. A rather high concentration of I − was used to suppress iodine losses during the analysis. The time of analysis was 2–5 min, and sample sizes were 3–7 μl. An LKB 16300 Coulometric Analyzer governed the titration procedure.

Gravimetric and coulometric analysis of beryllium samples using 2-methyl-8-quinolinol

Gravimetric and coulometric analysis of beryllium samples using 2-methyl-8-quinolinol

Electroanalysis and coulometric analysis

Electroanalysis and coulometric analysis

Controlled Potential Coulometric Analysis of Amine-Oxides of Pharmaceutical Interest

Controlled potential coulometry was shown to be a valuable technique for the quantitative determination of certain amine-oxides. From 98 to 100% of chlordiazepoxide and the N-oxides of atropine, brucine, 3-bromopyridine, scopolamine, and strychnine was recovered.

The chemistry of vitamin B12. Part XV. Catalysis of alkyl halide reduction by vitamin B12a: studies using controlled potential reduction

Controlled potential coulometric analysis has been used to investigate the reduction of alkyl halides in the presence of vitamin B12a. At voltages greater than the half-wave potential of the alkyl halide the reduction is a two-electron process. Below this half-wave potential the reduction is a one-electron reaction. The importance of these observations lies in considerations of the reaction mechanisms of reductions catalysed by vitamin B12 in both model and biological systems.

Chronopotentiometry

I. INTRODUCTIONBy the term chronopotentiometry we mean an electrochemical technique in which a controlled current, usually a constant current, is caused to flow between two electrodes; the potential of one electrode is monitored as a function of time with respect to a suitable reference electrode. The solution is usually, but not necessarily, unstirred and contains an excess of a supporting electrolyte so that diffusion is the principal mechanism of mass transport. This technique is distinguished from constant-current coulometric analysis and coulometric titrimetry 1 in that the current in chronopotentiometry is sufficiently large that the current efficiency for the reduction of the species of interest falls below 100%, usually within a few seconds. It is distinguished from galvanostatics, a constant-current technique useful for the determination of heterogeneous electron-transfer rates,2, 3 in that the chronopotentiometric potential-time trace spans several hundred millivolts whereas the galvanostatic potential-time trace extends less than ten millivolts from the equilibrium potential.

Electroanalysis and coulometric analysis

Electroanalysis and coulometric analysis

Participation of Oxygen Species in Periodic Anodic Processes

Galvanostatic periodic potential transients of the half element , were measured. A combination of coulometric analysis of the transients and of estimates of surface coverages of the electrodes with organic and oxygen species provided a useful method to study the mechanism of the periodic anodic process over the whole oscillation domain. Three regions with marked differences in mechanism were found experimentally: (i) the region of the “valve” mechanism, from 0.15 to 0.92V vs. SHE, where anodic oxidation of formaldehyde with readsorption of the latter as the coupling step occurs; (ii) the region of a mixed mechanism, from 0.18 to 1.1V vs. SHE, where anodic oxidation of the platinum surface occurs in addition to the anodic oxidation of the organic species; chemical reduction of the surface oxide leads to readsorption of formaldehyde at the oxide free surface; and (iii) the region of the oxide formation‐reduction mechanism, from 0.6 to 1.15V vs. SHE where solely anodic oxidation of the platinum surface occurs, followed by chemical reduction with formaldehyde, the coupling step. Based on mechanistic considerations the nonlinear relationship between process frequency and the process variables was established over the whole frequency domain.

A simple low-current potentiostat coulometric analysis

Presents a circuit diagram for a potentiostat and describes its use to conduct a coulometric analysis of 2,2-dinitropropane.

High sensitivity coulometric analysis in acetonitrile

Summary Cuprous copper was quantitatively coulometrically generated in acetonitrile. No catalytic or induced current contribution was found. Uncomplexed Cu(I) was stable in acetonitrile since it could be kept in solution for at least an hour and subsequently reduced quantitatively to the amalgam. The slightly high results obtained for the reduction of Cu(I) to the amalgam were attributed to relatively high blank corrections and to any cumulative errors from the previous reductions of Cu(II) to Cu(I). In the determination of sodium and lithium, a solution pretreatment and electrolysis with 100 μequiv. of sodium or lithium perchlorate was necessary to eliminate a catalytic current contribution caused by residual oxygen. The results for the quantitative controlled-potential coulometric analysis of sodium and lithium agreed favorably in precision and accuracy with other microquantitative methods of analysis. Under the conditions employed, the method is applicable between a lower limit of 2 μequiv. governed by error in the blank determination and an upper limit governed by solubility of the metals in the pool used.