Electrolytically generated manganese(III) sulphate as a redox titrant: potentiometric determination of thiosemicarbazide, its metal complexes and thiosemicarbazones

Abstract

Conditions have been established for the electrogeneration of Mn^III from Mn^II in a sulphuric acid medium. The Mn^III formed was identified and characterized by ultraviolet spectra. The stability of standard solutions of manganese(III) sulphate at various concentrations of H⁺, Mn^II and acetic acid and in the presence of the complexing agents P₂O₇^4–, Cl^– and F^– was studied and conditions for the potentiometric determination of thiosemicarbazide (TSC), its metal complexes, M(TSC)₂SO₄, and its thiosemicarbazone derivatives, propanal thiosemicarbazone (PTSC) and benzaldehyde thiosemicarbazone (BTSC), have been investigated. Thiosemicarbazide in H₂SO₄ underwent a six-electron oxidation. The M(TSC)₂SO₄ complex participates in a 12-electron redox process, indicative of the number of TSC ligands present in the complex. The PTSC in an H₂SO₄–HOAc (20% v/v) medium underwent oxidation in two stages, namely, six-electron oxidation of TSC with the regeneration of propionaldehyde and two-electron oxidation of the aldehyde to the corresponding carboxylic acid. The benzaldehyde regenerated from BTSC did not undergo further oxidation. Above 40% HOAc, a reproducible four-electron stoichiometry was achieved thereby showing the selectivity of Mn^III as an oxidizing agent. The formal redox potential of the Mn^III–Mn^II couple with and without the complexing agents and at various concentrations of H⁺ was determined. Addition of complexing agents decreased the formal redox potential of the Mn^III–Mn^II couple, thereby reducing the oxidation rate, but did not alter the over-all stoichiometry of the electron transfer process.