| Several aspects of redox chemistry were investigated. The measurement of redox potentials with reticulated vitreous carbon electrodes did not improve the reliability of such measurements. A method to simulate reducing conditions in a steady-state, controlled environment was evaluated. Improvements for minimizing the effects of abrasion and organic complexation were used in determining total concentrations of soluble elements. These values were used in a speciation program to determine steady-state activities for comparison with theoretically predicted values. Consistent with previous results that supported the hypothesized presence of the mixed-valence mineral, ferrosic hydroxide (Fe{dollar}sb3{dollar}(OH){dollar}sb8{dollar}), the solubility of Fe(III) exceeded that of amorphous ferric hydroxide at neutral and alkaline pH values. This is important for Fe nutrition in plants and confirmed the importance of pH in the precipitation of mixed-valence Fe minerals.; The relationship of pH to pe and the combined parameter, pe + pH, was investigated in soil and mineral suspensions. The ability of Fe minerals to serve as controls on the redox parameter was suggested by the results of these experiments. A reproducible and linear relationship between pe and pH was observed to vary widely from an ideal slope of minus one, depending on the nature of the soil or mineral system investigated. The observation of dpe/dpH values less steep than minus one (dEh/dpH {dollar}<{dollar} 59 mV) appears to be unique or, at least, unreported in the literature.; Variation in crystallinity or composition will account for the variation from the ideal that was observed for most pe + pH values. This work emphasizes the importance of pH in the relationship between pe + pH and Fe minerals. Natural mixed-valence Fe hydroxides precipitate easily from acid, reduced solutions when neutralized by base additions and show a great capacity for replacement or isomorphous substitution by both cations and anions. Since Fe is ubiquitous in earth materials and rather soluble under acid pH conditions, the precipitation of mixed-valence Fe minerals may be an important control of redox potential, for example, during the neutralization of sulfide mineral oxidation products. |
