Portugaliae
Electrochimica Acta

This work is concerned with pressure and temperature effects on the limiting conductances and Walden products of tetraethylammonium salts in different primary and secondary alcohols. The results were discussed in terms of both the hydrodynamic model and the transition state theory. . Complementary conclusions were reached by the two sets of parameters resulting from those theories.

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.