Advances in electroanalytical chemistry

This thesis concerns several advances in electroanalytical chemistry which are separated into four parts: the electrochemical investigation of diffusional behaviour, the mechanistic and kinetic study of electrochemistry with room temperature ionic liquids (RTILs), the study of weakly-supported electrochemistry and a comparison of the Butler-Volmer and Marcus-Hush kinetic theories of electron transfer.
A study of the diffusional behaviour of electroactive species is essential for further studies, especially in the case when electrochemistry is complicated through ion-pairing interactions between the electroactive species and other electrolytes. In Part II of this thesis, the possibility of the ferricenium ion-paired with perchlorate and hexauorophosphate in acetonitrile was discussed firstly employing chronoamperometric technique. Afterwards, the hexaammineruthenium III/II couple supported by chloride, nitrate and sulfate respectively was studied by a similar method.
In order to avoid unwanted ion-pairing effects, room temperature ionic liquids can be applied as solvent, which provide high conductivity by their own ionic nature so that experiments can be conducted without adding additional supporting ions. Because of RTILs have distinctive properties, for example, high viscosity, high conductivity and ionic nature, electrochemistry could be greatly changed compared to those in conventional solvents. Part III of this thesis gives a detailed description of this topic. First, a study of the reduction of 1,4-benzonquinone in 1-ethyl-3-methylimidazolium bis(triuoromethanesulfonyl)imide is presented to show the new mechanistic insight into comproportionation in a electrochemical process. Second, a discussion of the oxidation of hydroquinone in the same RTIL is introduced to suggest a possible ECE scheme which was never reported before.
The interest of weakly supported electrochemistry is also well-established, which not only provides another alternative strategy to avert ion-pairings but also offers more physical insights into electrochemical processes. Quantitative methods analysing voltammetries without an excess amount of supporting electrolyte are developed by introducing a migration term into the mass transport equation. In Part IV, new mechanistic insights into the reduction of 2-nitrobromobenzene and the dimerisation of 2,6-diphenylpyrylium in acetonitrile were provide by using weakly-supported cyclic voltammetry. Also, pulse techniques was also adopted to investigate the reduction of cobaltocenium and cobalt(III) sepulchrate, giving an alternative way for electrochemical analysis.
A major application of electroanalytical chemistry is investigating electrochemical kinetics. Two kinetic models mostly concerned by electrochemists are Butler-Volmer and Marcus-Hush formalisms. The classic phenomenological model, Butler-Volmer formalism successfully describes most common electron transfer kinetics but shows little reference with nature of the involved species, solution and electrode material, while a more physically insightful theory, the Marcus-Hush formalism, takes species natural properties, for instance, a change of distances or geometry in the solvation or coordination shells of the redox, into account although it requires more complex formulations. Comparative studies of these two theories are presented in Part V in order to improve our understanding of the electron transfer kinetics under different circumstances. First, comparison of cyclic voltammograms of the reduction of europium(III) and 2-methyl-2-nitropropane at mercury microhemispherical electrodes was carried out. Second, square wave and differential pulse voltammetric techniques were also employed to further discriminate the two kinetic models. These studies all find that the symetric Marcus-Hush theory assuming the reactants and products have identical force constant dose not satisfactorily agree with the experimental results. Hence, the introduction of asymmetric Marcus-Hush theory was presented considering different oxidative and reductive reorganization energies, which gives reasonable agreement with experiments and makes this theory more insightful.

Keywords: supporting electrolyte
electroanalysis
Electrochemistry
weak support
electrode kinetics
Advisor: Compton, R