El-Sayed M. Mabrouk

Thermodynamics of the complexation of some transition metal ions with trithiocarbodiglycolic acid. Potentiometric and voltammetric studies

SummaryProton-ligand dissociation and metal-ligand stability constants of trithiocarbodiglycolic acid (TCGA) with some transition metal ions were calculated potentiometrically. The order of stability constants was found to be Fe3+ > Cu2+ > Ni2+ > Co2+. The effect of temperature on the stability of the complexes formed was studied and the corresponding thermodynamic functions were derived and discussed. Polarographic and cyclic voltammetric behaviours ofTCGA were investigated in B.R. buffer solutions ofpH 2–12. In solutions ofpH<8, the polarograms and cyclo-voltammograms exhibited a single 2-electron diffusion-controlled irreversible step, corresponding to the reduction of the C=S. The mechanism of the reduction process was postulated and the kinetic parameters of the electrode process were evaluated.ZusammenfassungDie Protonendissoziation vom Liganden und die Metall-Ligand-Stabilitätskonstanten von Trithiocarbodiglykolsäure (TCGA) mit einigen Übergangsmetallionen wurden potentiometrisch bestimmt. Die Reihung der Stabilitäten ist Fe3+ > Cu2+ > Ni2+ > Co2+. Die Temperaturabhängigkeit der Stabilitäten wurde gemessen und die entsprechenden thermodynamischen Funktionen abgeleitet. Das polarographische und cyclovoltammetrische Verhalten vonTCGA wurde in B.R.-Puffer beipH 2–12 ermittelt. In Lösungen vonpH<8 zeigten die Polarogramme und Cyclovoltagramme eine einzelne diffusionskontrollierte irreversible 2-Elektronen-Stufe, die der Reduktion von C=S entspricht. Es wurde ein Mechanismus postuliert und die kinetischen Parameter des Elektrodenprozesses wurden bestimmt.

The corrosion behaviour of zinc metal in acidic solutions of polyvinylpyrrolidones and polyvinylpyridines

Abstract The effect of polyvinylpyrrolidone (PVP) of various average molecular weights ( M = 10 000, 24 500 and 40 000), poly-2-vinylpyridine ( M = 2.85 × 10 5 ) and poly-4-vinylpyridine as inhibitors of the corrosion behaviour of zinc metal in 1.0 M H 2 SO 4 solutions has been studied. It was shown using the weight-loss technique that the polymers studied impart a significant inhibiting effect on the corrosion rate of zinc metal. The protection efficiency in the presence of such polymers reached about 87% at an inhibitor concentration of 0.1 M. It was also observed that the inhibitive effect of PVP was increased by decreasing its molecular weight; the protection increased in the order: PVP (10 000) > PVP (24 500) > PVP (40 000) where the figures in paremtheses represent the respective molecular weights. The corrosion rate of zinc using poly-4-vinylpyridine as an inhibitor is lower than in the case using a comparable concentration of poly-2-vinylpyridine. From potential-time measurements it is concluded that the corrosion process is partially cathodically controlled. The results are analysed in terms of the formation of a protective film on the metal surface.

Mixed ligand complexes of copper with amino-acids and tartrate using differential pulse polarography. Part I

SummaryThe differential pulse (d.p.) polarography technique was used to study simple and mixed ligand complexes of copper(II) with tartrate anions and amino-acids (threonine, valine and isoleucine). The reduction of simple and mixed complexes was reversible and diffusion controlled, one mixed complex being formed in each case. The formation constants of simple and mixed complexes have been calculated. The mixing constants of ternary systems as well as the stabilization constants were also calculated. The formation constants of the ternary complexes were calculated statistically and compared with the observed values.

Thermodynamic and Voltammetric Studies of Salicylaldehyde-4-(2-pyridyl)-3-thiosemicarbazone

SummaryUV spectra, dc polarograms, cyclic voltammetry and coulometry measurements of salicylaldehyde-4-(2-pyridyl)-3-thiosemicarbazone (H2SPT) were recorded inBritton-Robinson buffer solutions ofpH 2–12. The dissociation constants of H2SPT evaluated spectrophotometrically and potentiometrically in a 5% (v/v)DMF-water mixture are concordant. The dc-polarograms in solutions ofpH<8.5 exhibit a single 4-electron diffusion-controlled polarographic wave, whereas in solutions ofpH>8.5 two waves are observed. The cyclic voltammograms give a single cathodic peak in acidic and neutral solutions, whereas two cathodic peaks can be identified in alkaline ones. An electrode mechanism is proposed and discussed. The effect of temperature on the dissociation constants and the polarographic data was also investigated; the corresponding thermodynamic parameters were derived and are discussed.ZusammenfassungUV-Spektroskopische, dc-polarographische, voltammetrische und coulometrische Messungen an Salicylaldehyd-4-(2-pyridyl)-3-thiosemicarbazon (H2SPT) inBritton-Robinson-Pufferlösungen (pH 2–12) wurden durchgeführt. Die spektrophotometrisch und potentiometrisch ermittelten Dissoziationskonstanten von H2SPT in 5% (v/v)DMF/Wasser stimmen überein. In Lösungen mit einempH-Wert unter 8.5 zeigen die dc-Polarogramme eine einzelne diffusionskontrollierte polarographische Welle (4e−), während in Lösungen mit einempH-Wert von über 8.5 zwei Wellen zu beobachten sind. Die cyclischen Voltammogramme weisen in sauren und neutralen Lösungen einen, in basischen Lösungen hingegen zwei kathodische Peaks auf. Ein Elektrodenmechanismus wird vorgeschlagen und diskutiert. Der Effekt der Temperatur auf die Dissoziationskonstanten und die polarographischen Ergebnisse wurde ebenfalls untersucht. Die entsprechenden thermodynamischen Größen wurden bestimmt und werden diskutiert.