Spectroelectrochemical investigations in molten salt and aqueous media

Author

Vincent Eugene Norvell

Date of Award

6-1981

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Chemistry

Major Professor

G. Mamantov

Committee Members

Leon Klatt, William Bull

Abstract

The electroreduction of the manganese oxyanions in alkaline solutions and the electrooxidation of elemental sulfur and electroreduction of Nb(V) in molten AICl3-NaCl mixtures has been investigated in situ by means of spectroelectrochemical techniques and thin-layer coulometry at optically transparent electrodes. The reduction of the manganese oxyanions in alkaline solutions has been found to consist of simple reversible one electron transfers when carried out in solutions in which the reduction product is stable. One or more soluble intermediate species, probably containing Mn(lV) and possibly Mn(V), are formed when reduction is carried out in solutions where the product is unstable and undergoes disproportionation. The effect of varying the pH of the solution on the overall reduction process is discussed. Spectral data on the reduction products are presented.

The UV-visible spectroelectrocheraistry of the reduction of Nb(V) in molten AlCl₃-NaCl mixtures was investigated as a function of the melt composition. In AlCl₃-NaCl (saturated with NaCl) melt, Nb(V) could be reversibly reduced to Nb(lV). Further reduction of Nb(lV) in this melt, or reduction of Nb(V) in AlCl₃-NaCl (63-37 mole %) melt produced a precipitate believed to contain Nb(lll). Nb(V) could be reduced in two steps in AlCl₃-NaCl (65-35 mole %) melt to yield soluble products. The product of the first step appears to contain Nb in an oxidation state 3. Coulometry data suggest that further reduction of this product may lead to the formation of Nb₃⁸⁺ Spectral data are presented for these reduction products.

The oxidation of elemental sulfur in AlCl₃-NaCl (63-37 mole %) melt was investigated by means of UV-visible absorption and electron spin resonance spectroelectrochemical techniques. The initial oxidation step produces several lower oxidation state sulfur species such as S₁₆²⁺ , S₈²⁺, and possibly S₁₉²⁺. Two radical cations are also involved, S₅⁺ and possibly S₈⁺. The interactions of these species as a function of temperature and concentration are discussed. Reactions are proposed describing chemical equilibria between these species. Further oxidation produces S(I), as S₂²⁺ and S(IV), as SCl₃⁺. Spectroelectrochemical evidence for the formation of S₄²⁺ and S(II) as intermediate products during the oxidation is discussed. Spectral data on the oxidation products are presented. An overall reaction sequence for the oxidation of S(0) to S(IV) is propose.

Recommended Citation

Norvell, Vincent Eugene, "Spectroelectrochemical investigations in molten salt and aqueous media. " PhD diss., University of Tennessee, 1981.
https://trace.tennessee.edu/utk_graddiss/13493