The work described in this thesis is mainly concerned with the accurate measurement of redox potentials for a series of covalently linked porphyrin-quinone molecules (P-Q), in non-aqueous media. These compounds (synthesised by Prof. J.R. Bolton and co-workers of the University of Western Ontario, Canada) are of interest as models for the primary photochemical step in the photosynthetic process. The redox potentials were measured by cyclic voltammetry and differential pulse voltammetry. These electrochemical redox potentials were used to estimate the energy of the excited charge-separated state (P<SUP>.+</SUP>-Q<SUP>.-</SUP>), and confirm that this state is likely to be photochemically produced from the excited singlet state of P-Q rather than the triplet state. As a prelude and postscript to the studies of the P-Q compounds, the electrochemical behaviour of several related but simpler compounds, both porphyrins and quinones, is described. This includes an invesigation of the solvent dependence of quinone reduction potentials in non-aqueous media. A simple linear equation, based on Swain's A+B solvent parameters, is proposed as a means of predicting quinone reduction potentials in different solvents. Finally, the electrode kinetics of some simple metallorporphyrin oxidations are studied using an a.c. technique. A non-linear fitting routine was used to optimise the rate constant values obtained.

Rest potential measurements, cyclic voltammetry, linear sweep voltammetry and constant potential coulometry were used to determine the electrochemical response of arsenopyrite in the absence and in the presence of cyanide and to determine its dissolution chemistry. Surface oxidation of arsenopyrite is proposed to proceed by a two-step reaction sequence. FeOOH, H₂AsO₃⁻, and Sᵒ, are produced in the initial step. Oxidation of Sᵒ to SO₄²⁻, and H₂AsO₃⁻ to HAsO₄²⁻ account for the second step. Coulometric results confirmed that 14 electrons are involved in the overall reaction. The implications of these results on the cyanidation of arsenical gold-bearing concentrates were also investigated. The response of four different concentrates to various cyanidation techniques was examined. Alkaline pressure oxidation in 1 M NaOH, at 200°C and under 500 psi of oxygen overpressure followed by conventional cyanidation resulted in 81% gold extraction from a concentrate that yielded only 2% gold extraction after direct cyanidation.

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