Portugaliae
Electrochimica Acta

A study of temperature effects on the response of ISEs made with membranes constituted by HgS/Hg2Cl2 applied to electrically conductive epoxy supports of several types is reported. Results for operational temperature ranges, 3lope and standard potential variations with temperature, isopotential points, stabilization times and hysteresis curves have been obtained. These results confirm that the equilibria reached by membranes with this sensor are thermodinamically metastable, which explains why the operation of these electrodes Is more troublesome than for AgCl/Ag2S electrodes. It is shown that the decrease in the lower limit of linear response obtained by cooling the electrodes below room temperature is very limited

A novel type of birnolecular liquid-phase reactions entitled "pseudodif fusion-controlled" is described. Rate of a pseudodif fusion reaction is inversely viscosity proportional as for a diffusion-controlled reaction but pseudodif fusion reaction proceeds slower than a diffusion-controlled one. The reason lies in steric constraints to pseudodiffusion reaction. A comparative analysis of diffusion-, activation-controlled, and pseudodiffusion reactions is given. Different boundary conditions for diffusion equations manifest themselves in different reaction types. Examples of pseudodiffusion-controlled reactions are presented.

The polarographic behaviour of adenine and of the copper-ade-nine system was studied in a medium of sulphuric acid. It was found that adenine is irreversibly reduced at dropping mercury electrode (DME) and that its differential pulse polarography (DPP) peaks are suitable for the analytical determination of adenine con centrations. In the presence of adenine, copper(II) ions are reduced in two consecutive steps indicating that adenine may complex with copper (I) ions, forming a scecies sufficiently stable in solution. The influence of pH and of the concentrations either of copper (II) ions or adenine on the electrochemical reduction of the system were also studied.

The reduction process of Eu(III)/Eu(II) in KNO3 medium has been carried out in aqueous solutions. The Kinetic parame -ters have been determined for the reduction of Eu(III) at different nitrate concentrations and pH=2,2 on the dropping mercury electrode. The coordination numbers of the oxidized and reduced -species were calculated.

Lysozyme acts as suppresor of polarographic minima of Cu(II) reduction dc wave. Lysozyme gives 1:1 and 1:2 complexes with the Cu(II) and catalyzes the reduction of copper ion. A structural change of the lysozyme with the pH is detected around pH = 3.5 which is related to carboxilic groups dissociation.

Cyclic voltammetric studies on the reduction of the title complexes were carried out in acetic acid-sodium acetate buffer at the HMDE. The mechanism of these processes is discused. The complex species in solution of this system have been determined, depending on methionine and glycine concentrations, pH values and potential scan rate.

The competition between the anodic oxidation of the illuminated n-GaAs electrode and that of tetramethyl-p-phenylenediamine (TKPD) was studied by rotating ring-disk voltammetry as a function of the photo-current density, the TMPD concentration, the pH and the solvent composition, i.e. the water to acetonitrile ratio. From the results, it is concluded that decreasing the water content of the water-acetonitrile mixture influences the kinetics in the sense of improving the stability of the phctoelectrode, and that the subsequent electrochemical decomposition steps may involve either a free hole or a surface intermediate as the mobile surface species, depending on the composition of the electrolyte. These medium effects lead to a better insight into the structure of the decomposition intermediates of GaAs and into their chemical interactions with the solvent.

The over-all formation constants of the mixed ligand complexes of 1-methylimidazole (l-Melm) and oxalate (Oxal) with cadmium (II) have been studied polarographically at constant ionic strength, p = 2.0, (NaN03) and pH 8 at 25 + 0.1°C. The reduction of the complexes at a dropping mercury electrode,DME, is reversible and diffusion controlled. The constants for three mixed complexes of Cd(II) are log = 4.0 for Cd(l-Melm)(Oxal), log p12 = 4.4 for Cd(l-Melm) (Oxal)2 and log p2± = 5.9 for Cd(l-Melm)2 (Oxal). The tendency of a ligand to add to a complex and to substitute another ligand was compared.

In the present paper we present the results of a study of the complexes of 1,4,8,12-tetra--azacyclopentadecane-N,N',N';N'''-tetracetic acid (cPENTA) which was synthesized in our laboratory. The stability constants of the complexes of Ca, Sr, Ba, Co, Ni, Cu and Zn and the thermodynamic functions for the formation of the complexes of Ni , Cu and Zn have been determined. The sequence of protonation of the ligand is discussed on the basis of the results of a NMR study and the structure of the complexes of nickel and cobalt is analysed in terms of their respective electronic spectra and of the calculated fractions of the ionic and covalent contributions of the enthalpy changes on formation of these complexes.

Reaction of [MoS^][PPh^]^ with methyl isocyanide (CNMe) affords species formulated as [Mo0S,(SCNMe)0][PPh.] or [Mo„S,(CNMe)(CNH)(SCN)][PPh.] in which the isocyanide or the derived ligands undergo cathodic CN bond cleavage to give ammonia, methane and ethylene, in processes involving sulphur as a reductant of the substrate.

The behaviour of ions in organic and mixed solvents is illustrated by a number of examples. Systematic study of the effect of solvent on potentials of cells with ion-selective electrodes led to the development by Kakabadse and co-workers of a new technique in direct potentiometry, viz. the ion isoconcentration technique (IICT). In IICT, concentration of solvent is varied and that of indicator ion is kept constant. Many simple mixtures show under ion isoconcentration conditions a systematic change of cell potential with variation in the concentration of organic solvent, providing a graphical method for the rapid determination of, (a) individual organic solvents in simple mixtures, and (b) residual water in organic solvents. A laboratory prototype monitor for the continuous determination of ethanol in cider and sherry, using the fluoride isoconcentration technique (FICT), is described.

An electrochemical study on the mesoionic heterocycle mentioned above is carried out, using dc polarography, cyclic voltammetry and controlled potential electrolysis . A reduction mechanism with a consumption of four electrons in each polarographic wave is proposed. On the other hand, we detect a kinetic response of CE type caused by an equilibrium between hydrolysis products ot the heterocycle.

The polarographic reduction of Zn(II> and Co(II) complexes with paludrine has been studied in moderated acid media. The Zn(II) 2:1 complex shows two well defined waves. The Co(II) 2:1 complex shows only one wave. Arrhenius activation energies, obtained from diffusion currents of np, dp, aci , aca, KT and K-L- polarograms, show that all cathodic and anodic processes are controlled by diffusion. The mechanism of these processes is proposed.

Cyclic voltammetric studies were performed for six redox couples at a Pt electrode, in the absence and in the presence of Bi, Pb and Cd adatoms. Standard rate constants were evaluated for all the systems in the absence and in the presence of adatoms. Whenever a catalytic effect was obtained the effect of coverage was studied in a quantitative way.It was found that the rate constant increases initially rapidly and then passes through a maximum or reaches a constant value, at coverages of the order of a monolayer. For all the systems the influence of the adatoms decreases in the order Bi > Pb > Cd.