Determination of thiourea and some of its organic derivatives with sodium vanadate, hexacyanoferrate(III), Cerium(IV) sulphate, manganese(III) and manganese(IV).
The determination of thiourea and some of its organic derivatives with sodium vanadate, hexacyanoferrate(III), cerium(IV) sulphate, manganese(III) and manganese(IV) is described. A mixture of iodate and iodide is used as catalyst. Ferroin, N-phenylanthranilic acid and p-ethoxychrysoidine can be used as indicators.
Electrolytically generated manganese(III) sulphate as a redox titrant: potentiometric determination of thiosemicarbazide, its metal complexes and thiosemicarbazones
Conditions have been established for the electrogeneration of MnIII from MnII in a sulphuric acid medium. The MnIII formed was identified and characterized by ultraviolet spectra. The stability of standard solutions of manganese(III) sulphate at various concentrations of H+, MnII and acetic acid and in the presence of the complexing agents P2O74–, 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 MnIII as an oxidizing agent. The formal redox potential of the MnIII–MnII 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 MnIII–MnII couple, thereby reducing the oxidation rate, but did not alter the over-all stoichiometry of the electron transfer process.
Diagnosis of the σ-, π- and (σ+π)-Aromaticity by the Shape of the NICSzz-Scan Curves and Symmetry-Based Selection Rules
The NICSzz-scan curves of aromatic organic, inorganic and “all-metal” molecules in conjunction with symmetry-based selection rules provide efficient diagnostic tools of the σ-, π- and/or double (σ + π)-aromaticity. The NICSzz-scan curves of σ-aromatic molecules are symmetric around the z-axis, having half-band widths approximately less than 3 A with the induced diatropic ring current arising from Tx,y-allowed transitions involving exclusively σ-type molecular orbitals. Broad NICSzz-scan curves (half-band width approximately higher than 3 A) characterize double (σ + π)-aromaticity, the chief contribution to the induced diatropic ring current arising from Tx,y-allowed transitions involving both σ- and π-type molecular orbitals. NICSzz-scan curves exhibiting two maxima at a certain distance above and below the molecular plane are typical for (σ + π)-aromatics where the π-diatropic ring current overwhelms the σ-type one. In the absence of any contribution from the σ-diatropic ring current, the NICSzz(0) value is close to zero and the molecule exhibits pure π-aromaticity.