Double Step Chronoamperometry Research Articles

A Humidity Sensor Using an Electrochemically Prepared Poly(1,5-Diaminonaphthalene)Film

An electrochemical humidity sensor was fabricated with poly(1,5-diaminonaphthalene) film coated on a gap of two splitted gold electrodes, which were made by vacuum deposition. Response currents according to humidity were measured by the potential sweep method and chronoamperometry. The stability of the polymer film was improved by double step chronoamperometry using the applied voltage of <TEX>${\pm}0.5$</TEX> Vdc. The response time determined by the pulse technique was about <TEX>${\sim}50$</TEX> msec and the relative standard deviation of current response was within <TEX>${\pm}5.0%$</TEX>. The response current of the film was intrinsically humidity dependent. The film exhibited a non-linear but reproducible response in ordinary range of relative humidity. The linear equations were <TEX>$I(nA)=0.28{\times}%RH-1.01$</TEX> between 10 to 70 %RH and <TEX>$I(nA)=6.05{\times}%RH-403.21$</TEX> between 70 to 90 %RH.

Electrochemical study of 7α,12,20- O-trimethyl-conacytone in acetonitrile

An electrochemical study of the electroreduction in anhydrous acetonitrile of 7α,12,20- O-trimethyl-conacytone, an abietane quinoid diterpene derivative from the natural product 7α- O-methyl-conacytone, showed two reduction signals. At the first reduction step, fast chemical reactions involving the loss of the methoxyl group located at C-7 with simultaneous regeneration of the quinoid moiety were observed. This electrogenerated quinone is reduced again, at the same potential used with the former quinone, resulting in a two-electron peak. These results were obtained by cyclic voltammetry and double-step chronoamperometry experiments. The electrolysis under methylating conditions of 7α- O-methyl-conacytone, at potential values of the second electron transfer, generates as major products, methoxy-hydroquinone, where the methoxy group at C-7 is lost, which is in agreement with the proposed mechanism. Therefore, the second reduction signal was attributed to the reduction of semiquinone intermediates by a mechanism not elucidated.

Carbon paste electrode spiked with ferrocene carboxylic acid and its application to the electrocatalytic determination of ascorbic acid

A carbon paste electrode spiked with ferrocene carboxylic acid (FCAMCPE) was constructed by incorporation of ferrocene carboxylic acid in a graphite powder–paraffin oil matrix. It has been shown by direct current cyclic voltammetry and double step chronoamperometry that this electrode can catalyze the oxidation of ascorbic acid in aqueous buffered solution. It has been found that under optimum conditions (pH 5) the oxidation of ascorbic acid at the surface of such an electrode occurs at a potential about 248 mV less positive than at an unmodified carbon paste electrode. The catalytic oxidation peak current was linearly dependent on the ascorbic acid concentration and a linear calibration curve was obtained in the range 3.48×10 −5–0.49×10 −3 M of ascorbic acid with a correlation coefficient of 0.9997. The detection limit (3 σ) was determined as 1.08×10 −5 M.

Polymer chain encapsulation followed by a quartz microbalance during electropolymerization of bithiophene-β-cyclodextrin host–guest compounds in aqueous solution

Abstract Hydroxypropyl-β-cyclodextrins (HPβCD) which form 1:1 host–guest inclusion compounds with bithiophene (BT) are used for the anodic electropolymerization of bithiophene in aqueous solution. Partial encapsulation of the polymer chains by cyclodextrins is suspected from the quartz balance response during polymer reduction by cyclic voltammetry or double-step chronoamperometry. In this case, relatively thin films containing more than one HPβCD molecule for ten BT monomers are formed on platinum electrodes. On the other hand, HPβCD insertion or adsorption in the oxidized form of the polymer, obtained under galvanostatic or potentiostatic conditions, was found to be dependent on experimental conditions (long polymerization time). These results tend to confirm that polymerization occurs outside the CD cavity and that cyclodextrin insertion may occur by chain encapsulation after the polymer formation.

Electrochemical and in-situ UV-visible-near-IR and FTIR spectroelectrochemical characterisation of the mixed-valence heteropolyanion PMo12O40n− (n=4, 5, 6, 7) in aprotic media

Abstract The mixed-valence heteropolyanion PMo12O40n− (where n=4, 5, 6, 7) in aprotic media was investigated by cyclic voltammetry and in-situ FTIR and UV-visible-near-IR spectroelectrochemical methods. A new optically transparent thin layer cell with adjustable optical path length was designed and characterised by thin layer cyclic voltammetry and double step chronoamperometry using ferrocene as a test redox system. The results indicate that this cell has small ohmic drop and good spectral response. The cell was used for both in-situ FTIR and UV-visible-near-IR spectroelectrochemical measurements. The cyclic voltammetric results indicate that the heteropolyanion PMo12O403− undergoes two reversible one-electron transfer reductions (first and second redox waves) and two quasi-reversible one-electron transfer reductions (third and fourth redox waves). The mixed-valence heteropolyanion PMo12O40n− (where n=4, 5, 6, 7) was formed after electroreduction. In-situ FTIR and UV-visible-near-IR spectroelectrochemical preliminary results indicate that the electronic structure of electrogenerated mixed-valence species PMo12O404− corresponds to class II system in Robin and Day’s classification of mixed-valence compounds. In-situ FTIR spectroelectrochemical studies also suggest that the bond energy of the MoOd double bond and Mo–Ob–Mo and MoOc–Mo bridge bonds was reduced after reduction of the original compound.

Control of the electrochemical reduction of horminone by pH imposition in acetonitrile

An electrochemical study using transient techniques of a quinone-type natural product, horminone, has been performed in acetonitrile in the presence of two buffer solutions of pH = 17.2 and pH = 15.5. Using linear sweep voltammetry, it was found that at both pH values the reduction mechanism of horminone involves a monoelectronic charge-transfer step, followed by a protonation step and homogeneous charge-transfer step due to disproportionation of the protonated intermediate. At pH = 15.5 the mechanism for the homogeneous charge-transfer step was found to be of the DISP1 type (first order disproportionation) from the results of double step chronoamperometry experiments. At pH = 17.2 the latter mechanism was found to be of the DISP2 type (second order disproportionation) since at this pH value the protonation step is minimized, thus causing the disproportionation reaction to be the controlling step of the mechanism.

Anodic polarization in molten silicates

The anodic polarization in binary sodium silicates containing 22–40 mol% Na 2O equilibrated with air was studied at 1000° C by means of voltammetry and double-step chronoamperometry. The electrochemical oxidation of the oxide ions produced by condensation reactions of silicate species is a two-electron process kinetically controlled by mass transport of O 2− to the anode. The oxygen atoms generated by the oxidation rapidly dimerize to produce dissolved oxygen which is further transformed into gaseous oxygen in the saturated bulk liquid. In basic liquids, the voltammetric peak corresponding to oxygen evolution is preceded by a prewave which has been explained by the formation of chemisorbed peroxide ions via an electrochemical reaction which involves dissolved and adsorbed oxide ions.

Kinetics of comproportionation: a spectroelectrochemical approach

A general spectroelectrochemical approach to the kinetics of the comproportionation reaction Y+Y 2− →2Y − is presented which combines both rotating-disc double-step chronoamperometry and quantitative in situ electrochemical ESR spectroscopy. It is shown that when the electroreduction of Y to Y 2− proceeds via two separate one-electron waves and Y 0− is ESR active, the combination of the two techniques allows the unambiguous determination of the kinetics together with the diffusion coefficients of the three species involved. The method is applied to the case where Y=p-chloranil(tetrachloro-p-benzoquinone), and good agreement between theory and experiment is found

Reactivity of carbonylmanganese radicals toward tributyltin hydride: Kinetics analysis of the anodic oxidation of Mn(CO) 3[P(OPh) 3] 2−

The reactivity of the 17-electron carbonylmanganese radical Mn(CO) 3[P(OPh) 3] − 2 toward the hydrogen atom donor tn-n-butyltin hydride is examined by the anodic oxidation of the anionic Mn(CO) 3[P(OPh) 3] 2 −. Quantitative analyses by transient electrochemical methods (cyclic voltammetry and double-step chronoamperometry) yield the second-order rate constant k = 7.7±0.4 M −1 s −1 for the hydrogen atom transfer in tetrahydrofuran which is in accord with that of an earlier study based on photosubstitution. Selectivity in the subsequent homolytic coupling of Mn(CO) 3[P(OPh) 3] − 2 . and Bu 3Sn . is analyzed in detail. The behavior of this carbonylmanganese radical is compared to that of the more sterically encumbered tri- o-tolyl phosphite analogue.

Electrochemical reduction of methylcobalamin and 5'-deoxyadenosylcobalamin on mercury in basic medium

Electrochemical reduction of methylcobalamin and 5′-deoxyadenosylcobalamin (coenzyme B 12 ) was studied at Hg electrodes, in pH 11.7 basic media, by means of dc polarography, differential pulse polarography, cyclic voltammetry, controlled potential electrolysis, and chronoamperometry. A surfactant (Triton X-100) was used to minimize adsorption of the compounds on the mercury cathode. Methylcobalamin was found to exist as an equilibrium mixture of its base-off and base-on forms with K eq =3.6 calculated by an analysis of its two reduction waves at E 1/2 =−1.20 V and E 1/2 =−1.50 V (vs. SCE). The value of the forward homogeneous rate constant was found to be 2×10 3 s −1 . A third, ill-defined, broad, catalytic H 2 reduction wave is found at E 1/2 =−1.6 V. The first reduction wave is reversible and is assigned to the formation of a one-electron adduct intermediate from the base-off cobalamin which subsequently undergoes a cleavage of the Co-C bond (i.e., EC process), and the second wave is irreversible and is assigned to a one-electron reduction of the base-on cobalamin. The standard heterogeneous rate constant for the first reduction is 2×10 −2 cm s −1 and the forward heterogeneous rate constant for the second reduction is 2×10 −6 cm s −1 . The homogeneous rate constant for the follow-up chemical reaction of the first reduction process, as determined by chronoamperometric methods, is k =0.37 s −1 with a reversible half-wave potential of E 1/2 r =−1.24 V. In contrast, 5′-deoxyadenosylcobalamin, which appears to exist as the base-off form only, exhibits one irreversible reduction wave at E 1/2 =−1.26 V and a broad, catalytic H 2 reduction wave in the same potential region as the methylcobalamin. The irreversible wave at −1.26 V is due to formation of one electron intermediate which undergoes Co-C bond cleavage much faster than that of methylcobalamin. Double step chronoamperometry showed this rate constant to be k =2.7×10 3 s −1 .

Electrochemical reduction of methylcobalamin and 5′-deoxyadenosylcobalamin on mercury in basic medium

Electrochemical reduction of methylcobalamin and 5′-deoxyadenosylcobalamin (coenzyme B 12) was studied at Hg electrodes, in pH 11.7 basic media, by means of dc polarography, differential pulse polarography, cyclic voltammetry, controlled potential electrolysis, and chronoamperometry. A surfactant (Triton X-100) was used to minimize adsorption of the compounds on the mercury cathode. Methylcobalamin was found to exist as an equilibrium mixture of its base-off and base-on forms with K eq=3.6 calculated by an analysis of its two reduction waves at E 1/2=−1.20 V and E 1/2=−1.50 V (vs. SCE). The value of the forward homogeneous rate constant was found to be 2×10 3 s −1. A third, ill-defined, broad, catalytic H 2 reduction wave is found at E 1/2=−1.6 V. The first reduction wave is reversible and is assigned to the formation of a one-electron adduct intermediate from the base-off cobalamin which subsequently undergoes a cleavage of the Co-C bond (i.e., EC process), and the second wave is irreversible and is assigned to a one-electron reduction of the base-on cobalamin. The standard heterogeneous rate constant for the first reduction is 2×10 −2cm s −1 and the forward heterogeneous rate constant for the second reduction is 2×10 −6 cm s −1. The homogeneous rate constant for the follow-up chemical reaction of the first reduction process, as determined by chronoamperometric methods, is k=0.37 s −1 with a reversible half-wave potential of E 1/2 r=−1.24 V. In contrast, 5′-deoxyadenosylcobalamin, which appears to exist as the base-off form only, exhibits one irreversible reduction wave at E 1/2=−1.26 V and a broad, catalytic H 2 reduction wave in the same potential region as the methylcobalamin. The irreversible wave at −1.26 V is due to formation of one electron intermediate which undergoes Co-C bond cleavage much faster than that of methylcobalamin. Double step chronoamperometry showed this rate constant to be k=2.7×10 3 s −1.