MECHANISTIC STUDY OF PHOTOELECTROCHEMICAL REACTIONS - PHOTO-TRANSIENT CHARACTERIZATION OF CE PROCESSES

A channel electrode photo-transient experiment is described for the mechanistic characterization of photo-CE processes. This involves monitoring the transport limited current due to the discharge of a photo-generated electroactive species immediately following the stepwise application of light to the system. The theory of the experiment is given and a working surface presented which allows the analysis of experimental transients regardless of the cell geometry or solution flow rate used in their measurement. The approach is applied to the photo-electrochemical oxidation of the aminocarbene complex [(η5-C5H5)Fe(CO) P(p-Tol)3{equal to or precedes}C(NHMe)(Me)BF4 in acetonitrile solution at a platinum electrode. The process, in the presence of light of wavelengths near 330 nm, is confirmed to be of the photo-CE type. © 1995

Sonovoltammetric measurement of the rates of electrode processes with fast coupled homogeneous kinetics: Making macroelectrodes behave like microelectrodes

Voltammetric measurements of the rates of fast homogeneous chemical reactions coupled to heterogeneous electron transfer at macroelectrodes are demonstrated for two systems, the reductive dehalogenations of ortho-bromonitrobenzene and of 3-bromobenzophenone, by the use of power ultrasound to confer fast mass-transport properties so giving the electrodes the kinetic timescale of microelectrodes

The use of ultrasound in the enhancement of the deposition and detection of metals in anodic stripping voltammetry

The fast electrochemical deposition of both mercury and amalgam films, a crucial step in analytical anodic stripping voltammetry, is demonstrated to occur in the presence of power ultrasound emitted from an immersion horn probe which was placed opposite the working electrode in a conventional three electrode cell. Due to considerably enhanced mass transport the rate of deposition is strongly increased compared to 'silent' conditions. The total amount of mercury adhering to a glassy carbon electrode surface, however, is limited to a 'thin film' of microscopically small droplets of mercury as shown by atomic force microscopy and voltammetry. Due to surface forces induced by ultrasound, abrasion of some of the deposit occurs and after a certain period of time the parallel electro-deposition and abrasion processes reach a 'steady state'. The deposition of mercury in the presence of Pb2+ or Cu2+ results in the formation of an amalgam and well-defined, very sharp 'stripping' responses for the oxidation process of the corresponding amalgam were detected. The merit of insonation in analytical electrode preconcentration techniques is dicussed

SONO-ELECTROCHEMISTRY - THE OXIDATION OF BIS(CYCLOPENTADIENYL)MOLYBDENUM DICHLORIDE

The oxidation of [Mo(cp)2Cl2] (cp = η-C5H5) in acetonitrile solution at platinum electrodes has been studied. In the absence of ultrasound a simple one-electron oxidation with a half-wave potential of +0.5V (vs. saturated calomel electrode) is observed together with a much smaller wave with a half-wave potential of +1.05 V. In the presence of ultrasound the first wave involves the passage of greater than one electron per molecule of electroactive species and the second wave disappears. The mechanism in equations (i)-(iii) is proposed to explain the observations, where the chemical (c.) (e. = electrochemical) and the disp. (disproportionation) steps are promoted by ultrasound. e. step [Mo(cp)2Cl2] - e- → [Mo(cp)2Cl2]+ (i), c. step [Mo(cp)2Cl2]+ + MeCN → Cl. + [Mo(cp)2Cl(MeCN)]+ (ii), disp. step [Mo(cp)2Cl2]+ + [Mo(cp)2Cl(MeCN)]+ → [Mo(cp)2Cl(MeCN)]2+ + [Mo(cp)2Cl2] (iii)

A novel approach for the quantitative kinetic study of reactions at solid/liquid interfaces in the presence of power ultrasound.

A novel cell and procedure is described which permits the quantitative mechanistic study of ultrasonically enhanced reactions which occur at solid/liquid interfaces. A model of a controlled and calculable time-average rate of mass transport to and from the interface is used in order to compare experimental results with theoretical predictions based on mechanistic reaction schemes. In this way concentrations of mechanistically significant species near the interface can be related to those in bulk solution and hence the sonochemical effects of ultrasound dissected from those arising purely from mass transport. The effect of ultrasound is demonstrated for the reaction dissolution of p-chloranil in the presence of aqueous base and for the reaction of the same substrate with the aromatic amine, N,N-dimethyl-phenylenediamine, both systems which have been studied previously in the absence of ultrasound. Complementary atomic force microscopy images are also reported

Sono-electroanalysis: Application to the detection of lead in wine

The quantitative detection of lead in wine is shown to be possible by anodic stripping voltammetry under conditions of insonation. An immersion horn probe is introduced into a thermostatted conventional three-electrode cell opposite a mercury plated platinum disc working electrode. Following acidification of the wine sample lead ions can be reduced at the Hg/Pt electrode surface at -1.0 V (vs SCE). The large mass transport associated with power ultrasound yields efficient pre-concentration of the lead before it is stripped by an anodic linear sweep of the potential. Insonation further offers the crucial benefits of first surface activation and cleaning, helping to prevent electrode fouling by the organic components in wine and second fully equilibrating "free" and "bound" Pb2+ ions in the complex matrix. By use of standard microaddition of lead to the solution the system can be calibrated to give the total amount of lead present. Experiments using samples of a white Italian Chardonnay gave a total lead content of 22 ± 6 μg 1-1. This value was compared with those obtained by independent atomic absorption spectroscopy (AAS) measurements performed by two different laboratories. Quantitative agreement was found. © 1998 Elsevier Science Ltd. All rights reserved

ORGANIC SONOELECTROCHEMISTRY - REDUCTION OF FLUORESCEIN IN THE PRESENCE OF 20 KHZ POWER ULTRASOUND - AN EC' REACTION

The electro-reduction of the dye fluorescein at mercury electrodes in basic aqueous solution is known to produce the stable semi-fluorescein radical. When the electrolysis is exposed to power ultrasound of intensity up to ca. 65 W cm-2 the radicals are shown to be re-oxidised by means of hydroxyl radicals formed through the sonochemical decomposition of the aqueous solvent. The electrode process adopts the mechanistic characteristics of a 'sono-EC′' or 'sono-catalytic' reaction and this constitutes, to the author's knowledge, the first ever mechanistic assignment of this type

Synthesis and characterisation of C-60 derivatives containing functionalised anthraquinone groups and an unusual fullerene-stabilised cation

The ortho-anthraquinonedimethane dienophiles prepared in situ from 1,4-dihydroxy-2,3-bis(bromomethyl)-anthraquinone and 1,4-dimethoxy-2,3-bis(bromomethyl)anthraquinone respectively react with the fullerene C60 to give the anthraquinone derivatives: dihydroxy-anthraquinone-C60 and dimethoxy-anthraquinone-C60. The former compound with sodium butoxide and 15-crown-5 gives the bis(sodium-15-crown-5)dioxo-anthraquinone-C60. The dimethoxy-anthraquinone-C60 reacts with [Ru(PPh3)2(NO)Cl] to form the bis-adduct {[η2-(dimethoxy-anthraquinone-C60)][Ru(PPh3 )2(NO)Cl]}. The electrochemistry of the dihydroxy- and dimethoxy-anthraquinone-C60 derivatives has been studied by cyclic voltammetry and, in contrast to 1,4-dihydroxy-2,3-dimethylanthraquinone, the compound dihydroxy-anthraquinone-C60 shows two separate one electron oxidations at lower potentials strongly suggesting a relatively rare example of a fullerene stablised cation

Dual activation: coupling ultrasound to electrochemistry - an overview

By introducing a powerful immersion horn probe as a source of intense ultrasound into a thermostatted conventional three-electrode cell, dual activation experiments by simultaneously passing a current and applying ultrasound may be undertaken. These sono-voltammetric experiments may be used in order to analyse and quantify the processes induced by ultrasound at the electrode/solution interface. Different effects have been described. First, by applying intense sound fields direct effects of ultrasound on electrode surfaces such as depassivation and erosion can be induced in cavitation events, violent collapses of oscillating bubbles. Second, the huge effect of ultrasound on the mass transport at the electrode surface detected by various voltammetric techniques may be described by the model of an extremely thinned diffusion layer of uniform accessibility. This experimentally verified model may then be used in voltammetric experiments in order to separate pure mass transport from other effects induced by sound waves. Several working electrode geometries have been employed and particularly the use of an electrode embedded in the tip of the ultrasound transducer, a so-called sonotrode, allows extreme conditions to be studied. In aqueous media and under these conditions voltammetry parallel to "classical" hydrodynamic techniques based on the effect of "acoustic streaming" was observed. A wide range of systems including the reduction of a metalloprotein, cytochrome c, are described. In this overview the current state-of-the-art is critically reviewed and the information that has been derived from sonovoltammetric measurements illustrated. © 1997 Elsevier Science Ltd