Portugaliae
Electrochimica Acta

The electrochemical behaviour of Vitamin BI2 derivatives with cobalt (III) complex and containing an alkyl chain with a peripherical pyrrole group is analysed by cyclic voltammetry. Adsorption of the derivatives under oxidised and reduced forms appears to allow the oxidation of the pyrrole group with concomitant polymer film formation. The length of the alkyl chain has been seen to play an important role on the polymer properties namely on its conductivity and thus on the possibility of thickening the film. Although a relatively high potential is required for the adequate monomer oxidation the electropolymerization of these new monomers can be envisaged for modified electrodes preparation.

Estudou-se a influência do ião Fe2' na biocorrosão de um aço macio, na presença e na ausência de bactérias redutoras de sulfato (SRB). Os estudos foram realizados por voltametria cíclica, curvas de polarização de estado quasi-estacionário e perda de massa. Os resultados indicaram que o aço é mais susceptível à corrosão quando o ião Fe2+ está presente no meio.

The performance of an ion selective electrode array (ISEAR), designed for the determination of benzoate in the presence of iodide, constituted by all-solid-state electrodes with Aliquat 336 S or tetraocrylammonium salts immobilised in PVC membranes, is described. The response of the ISEAR to both ions was obtained by two treatments: (i) calculation by non-linear least squares (NLS) of the parameters of a model based on the Nikolskii-Eisenmann equation; and (ii) principal components regression (PCR). The results are compared and discussed. Results obtained in the determination of benzoate in pharmaceutical formulations containing iodide are included.

This session intends to celebrate the 5t n anniversary of the Portuguese Electrochemical Society by presenting an account on the establishment of the electrochemistry as a science and on its development at the Portuguese Universities, as well as a reference on the role of that society on the evolution of this science within the Portuguese scientific community. The former aspects will be presented by the invited speakers, whereas the latter are considered now, in the opening of the session.

The electrochemical properties of phenazopyridine hydrochloride (pyridium) were studied at different pH values in Britton-Robinson buffer solutions by differential pulse and direct current polarography, square wave and cyclic voltammetry, controlled potential coulometry techniques. Polarographic reduction of pyridium takes place in a single 4-electron transfer, giving a diffusion-controlled irreversible wave in the pH range 2.00¬10.50. At high pH values (pH 11.00-12.00), reduction of the compound exhibits a single, 2-electron diffusion-controlled reversible wave. A possible mechanism has been suggested on the basis of the number of electrons involved in the reduction process.

New surfaces more resistant to corrosion were produced by ion implantation of W and Ta into aluminium targets. The thermodynamic limitations associated with the low solubility of the solutes were overcome thus increasing the alloying element concentrations to levels necessary to achieve improved corrosion performance. Studies were conducted in solutions of varying acidity, suggested by the E-pH diagrams of the substrate and the alloying elements. The electrochemical results in acid environments (pH 0.5 to 3.0) revealed that passivation of the implanted alloys is pH dependent. The film suffers degradation with Ta dissolution after several polarization cycles. There is no apparent indication of W dissolution according to the polarization characteristics shown by pure W at the same pH. In near neutral chloride solutions, the implanted alloy with higher concentration of the alloying elements showed a marked protective character. A displacement of the pitting potential, Ep, related to aluminium greater than 1.5 V (SCE), was observed. The role of Ta seems to be twofold, contributing to the enrichment of the film in Ta2C>5 and acting as a reservoir of solute to be oxidized under occluded cell conditions whilst the role of W seems to be in agreement with the solute rich interphase model, that implies local passivation of a pit due to the stability of oxidized solute in localized low pH conditions.

Famotidine is an electroactive and a surface-active compound which can be irreversibly reduced at a mercury electrode in moderate acidic media at the potential of about -1.20 V vs Ag/AgCl (KC1 sat.). Square-wave voltammetry of this reaction can be utilised for a quantitative determination of the drug.

This paper describes the development of a technique for evaluating the transference number of mobile species in polymer electrolytes. The procedure used originates in a modified Hittorf method [1] with the application of an appropriate cell configuration, electrolyte support, assembly strategy and analytical method. In accordance with the convention applied to poly(ethylene oxide) based polymer electrolytes, the value of n indicates the number of repeat units of the polymer per guest salt cation in an electrolyte composition given by EOnLiC104. Measurements were carried out on the system based on poly(ethylene oxide), PEO, and lithium perchlorate, with compositions of n between 5 and 100.

Polymer electrolytes based on a poly(ethylene oxide) host with trivalent cation guest species have been prepared by the solvent casting method and characterised by conductivity measurements and thermal analysis. Electrolytes with compositions in the range of n between 5 and 120 were found to behave in a manner similar to that previously observed with lanthanum trifluoromethanesulphonate based systems. The total ionic conductivity of these materials is close to other trivalent cation based systems (approximately 10"3 Q. cm-1) and, as expected, the thermal stability of the electrolyte is limited by the presence of the perchlorate anion.

The mixing parameters, ©CI.AC and yNa,ci,Ac, included in the Pitzer equation to evaluate the chloride ion activity coefficient, were determined from electromotive force data, at 25° C. The ionic strength of the mixed electrolyte, sodium chloride and sodium acetate, varied from 0.05 to 2.5 mol kg-1. The activity coefficient of the chloride ion, in acetate buffer solutions, was calculated using the Pitzer model and the Debye-Hiickel theory, with the Bates-Guggenheim convention, and the difference, in pH, has been discussed.

The pH value of the potassium hydrogen phthalate 0.05 mol.kg-1 has been determined, using the Bates-Guggenheim convention and the Pitzer model for the calculation of the chloride ion activity coefficient. A comparison has been made between the two approaches.

The electrochemical reduction of benznidazole have been studied in mixed aqueous-organic media by cyclic voltammetry and differential pulse polarography as a function of the pH. Between the pH range 4 to 8, the compound undergoes a irreversible four electron reduction step. From the data a reduction mechanism is proposed.

The one-electron electrochemical reduction of the nitroprusside ion (NP) in aqueous solution of different pH was studied by cyclic voltammetry and by coulometry. The UV-VIS spectra of the electrolysis final solutions show conclusively that, at pH 5.1, the ion [Fe(CN)4NO]2" is the final product of the first one-electron reduction of NP although it decomposes through some process favoured by the decrease of pH. Also, the analysis of the cyclic voltammograms and of the curves electrolysis current vs. charge for the pH range under consideration shows that there is a chemical reaction, involving If, following the electron transfer step. Indeed, HCN was detected as a product of the first one-electron reduction of NP and at pH 3 .0, the variation of pH at the end of the electrolysis was evaluated and it corresponds to the expected for the consumption of 1 FT; in order to produce 1 HCN. Hence we conclude that [Fe(CN)4NO]2" is the final product of the 1-electron electrochemical reduction of NP.

The electrochemical behaviour of some Pt(II) complexes of the type trans-[Pt(PPh3)2LL']n [n = 0, L = CH3 , L' = C=CC6H4Me-4 (1); n = 1, L = CH3, L' = C(OCH,CH7CnCH,GJLMe-4 (2a), C(OCH2CH2Br)CH2C6H4Me-4 (2b) or c6cH2CH2CHC6H4Me-4 (3); n = 1, L = CO, L' = C^CCeFLMe^ (4a) or CH=C(OCH2CH2Cl)C6H4Me-4 (4b)] and [PtH(CH2CF3)(PPh3)(C=NC6H40Me-4)] (5) have been studied by cyclic voltammetry (CV) and controlled potential electrolysis (CPE) in aprotic media and at platinum electrodes. While the cationic compounds exhibit irreversible cathodic reductions and most of the above complexes undergo anodic processes, also irreversible, whose peak potentials can be related to the electronic properties of the unsaturated carbon ligands and of their binding metal centres, the oxidation of the neutral isocyanide compound could only be achieved by means of an electron-transfer mediator, N(C6H4Br-4)3. The mechanism of the electrocatalytic process thus observed was studied by digital simulation of the cyclic voltammograms.

The bis(cyanoimido) complex rra/tt-[Mo(NCN)2(dppe)2] (1, dppe = Ph2PCH2CH2PPh2) is susceptible to electrophilic attack to form protonated compounds trans-[Mo(NCNH)(NCN)(dppe)2]+ (2) and írara-[Mo(NCNH)2(dppe)2]2+ (3),' as well as the alkylated derivative íra«í-[Mo(NCNEt)(NCN)(dppe)2]+ (4) which in turn can be protonated to give trans-[Mo(NCNEt)(NCNH)(dppe)2]2+ (5). A preliminary description of the electrochemical behaviour of these complexes is presented and the observed redox induced interconversion of some of them is discussed in terms of their activation upon electron transfer.

The low temperature electrochemical study of the 16-electron thiolate isocyanide complex c/s-[Mo(SC6H2Pr3-2,4,6)2(CNBu)4] indicated the occurrence of a single-electron anodically-induced isomerization according to an EC-square type mechanism for which rate and equilibrium constants have been estimated by digital simulation of cyclic voltammetric data.

The electrochemical behaviour of the complexes rra/zs-[MCl(NCOCH2l)(dppe)2][BPh4] (M = Mo or W, dppe = Ph2PCH2CH2PPh2)2) was investigated by cyclic voltammetry and controlled potential electrolysis. The complexes undergoe two successive monoelectronic reductions. The first reduction process follows a well-defined EC mechanism, leading to iodide loss. The second reduction is reversible and is associated with the novel complex rra/75-[MCl(NCOCH3)(dppe)2]+ produced after the first reduction process.

The redox properties of [{L2M}2(ir-Pr>)2] (M=Pd, L2= C(C02Me)=C(C02Me) (1); M=Ni, L=CO (2) or Bu'NC (3); P P= PPh2CH2C(But)=NN=C(Bu')CH2PPh2, [Pd{n3-CH2C(CH3)=CH2}(P'P)]C1 (4), [PdCl{PPh2CH=C(But)NN=C(But)CH2PPh2)] (5), or [{Pda^r)2-CH2C(CH3)=CH2)}2(u-P P)] (6), as well as rNi{C(C02Me)=C(C02Me}(P'P)] (7), were studied by cyclic voltammmetry and controlled potential electrolysis, in 0.2 M fNBuJfBFJ /THF. By reductive electron transfer chemical reactivity was induced at complex (1) and the new species [Pd(PPh2CH2C(But)=N-NC(But)=C(H)P,Ph2)(MeC02C=C(H)C02Me)] was formed.

Polyurethane has been used lately as an alternative to PVC in the preparation of ISE membranes for clinical applications. This is due both to its biocompatibility [1] and to the claimed elimination of the protein asymmetry effect produced by serum at conventional PVC membranes [2]. Following previous work [3-7] electrodes have been prepared with TECOFLEX SG - 80A polyurethane and K+ measurements have been performed in Bovine Serum Albumin (BSA) containing solutions. Effects such as albumin concentration, sequence and age of solutions have been tested. Simultaneous readings, in the same solution, have been taken with PVC based membranes and with polyurethane based ones vs. Saturated Calomel Electrode (SCE) and isotonic KC1 electrodes (Ag/AgCl and Modified CE). The results follow similar trends, without significant distinction being observed. Therefore, the effects of protein on the potentiometric measurements, due to interaction with the membrane, if any, seem to be independent of the polymers tested. These observations add evidence to the importance of the liquid-liquid junction potential contribution to the overall measurements.