Portugaliae
Electrochimica Acta

The electrochemical behaviours of c/s-[ReCI(NCC6H4Me-4)(dppe)2] and of the hydride complex [ReCIH(NCC6H4CI-4)(dppe)2]+ have been studied by cyclic voltammetry (CV) and controlled potential electrolysis (CPE) in aprotic media and at platinum electrodes. The 16-electron dicationic configuration of the former compound undergoes a facile isomerization to its trans isomer in the time scale of CV whereas the hydride complex undergoes an anodically induced deprotonation through an overall bimolecular process. Quantitative analyses of both processes, by digital simulation of cyclic voltammograms, are outlined. The models of Amavadine [VL2]2" [L = "ON(CH(CH3)COO")2 (HIDPA3-) or "ON(CH2COO")2 (HIDA3-)], act as electron-transfer mediators in the electrocatalytic oxidation of thiols. This redox catalysis process is shown to occur through an unprecedented mechanism involving Michaelis-Menten type kinetics.

Diffusion coefficients in aqueous electrolyte solutions have been calculated according to Onsager-Fuoss and Pikal theories for over 301 electrolytes in aqueous solutions using the most reliable data.

From the polarographic studies of (E-i)t and (i-t)E curves the reduction of chromate ion in the DME has been analysed in order to verify both the irreversibility and the kinetic parameters of the processes. The study has been followed in aqueous and mixed H20-EtOH (15, 30% weight EtOH) media, with 0.1M NaOH at 25°C. A modified version of the OLDHAM-PARRY method is proposed and the possibilities of "sampled current-voltammetry" through instantaneous current, are also emphasized.

Voltammetric studies were carried out using a glassy carbon working electrode to determine the concentration of oxide ion in a CsCI-KCI-NaCI eutectic melt. The mechanism of electrode process and some kinetic parameters were established. The oxidation of O2 is irreversible two-stage process complicated by adsorption. The diffusion coefficient of oxide ions changes from 5.62-106 to 1.99-TO6cm2 s1 at 750-59CPC. The equation for the calculation of oxide ion concentration was derived.

The values of the hydration equilibrium constants of carbonyl and a-dicarbonyl compounds given in the literature are reviewed; the experimental methods used to the calculation of these constants are discussed as well. The values of the correspond mg constants of glyoxal, methylglyoxal and phenylglyoxal are discussed, the last by using a Taft-type correlation.

The redox properties of some tetrakis(pyrazolyl)borate rhenium(V) complexes of the types [ReO{ri3-B(pz)4}L](L= "0^0" type ligand, 1/2L= MeO") and [ReO{r|2-B(pz)4}(OMe)L'] (L'= 0°0" , >To", type ligands) were studied by cyclicvoltammetry. These Re(V) complexes display generally one anodic and one cathodic waves, at values of potential which are discussed in terms of the hapticity of the pyrazolylborate ligand and of the electronic properties of the chelating co-ligands.

The redox properties of complexes of general formulae [{FeCl3L}2], [{CuCl2L}2] and [PdCl2L2] (L=camphor-type ligand) were studied by cyclic voltammetry and controlled potential electrolysis. As a general behaviour, the complexes display at least one cathodic wave which has a marked metal character. The potentials are discussed on the basis of the electronic properties of the camphor-type ligands. Extensive reductive controlled potential electrolysis of one of the diiron complexes allowed the electrosynthesis of a mononuclear product. The electrochemical behaviour of the L ligands [L=A (R=Ph or Pr1) and L=B (R=NH2)] is also reported.

By cyclic voltammetry (CV) in aqueous solutions of KC1, the Amavadine models [NBu4]2[VL2] (L = HIDPA3' - N-hydroxy-a,a'-iminodipropionate - or HIDAV - N-hydroxy-a,a'-iminodiacetate) undergo a single-electron reversible oxidation. The anodically generated Vv species acts as an electron-transfer mediator in the electrocatalytic oxidation of some thiols and evidence is presented for the involvement of an interaction between the substrate (mercaptopropionic acid) and the vanadium complex. The mechanism has been investigated by CV simulation and a preliminary account is now being discussed.

The coordination chemistry of cyanamide (N=CNH-2 ) or derived compounds remains virtually unexplored and, within our interest on the activation of such species by transition metal centres [1], we have prepared some complexes of Pd(II) or Pt(II) of the following types: irara^MC^L^] ( 1, M = Pd or Pt; L = NCNMe2 or NCNEt2 ), trans-[Pt(CF3)L(PPh3)2][BF4] [2, L = NCNMe2, NCNEt2 or NCNC(NH2)2 (cyanoguanidine)], trans- or cw-[PtClL(PPh3)2][BPh4] [3, L = NCNH2, NCNMe2, NCNEt2 or NCNC(NH2)2]-. Although their syntheses and spectroscopic properties are reported separately, we now present their electrochemical behaviour, as well as that of the free cyanamides, as investigated by cyclic voltammetry (CV) and controlled potential electrolysis ( CPE ) in 0.2 M [NBU4KBF4] / NCMe at a Pt-disc or -gauze electrode, respectively.

The electrochemical behaviour, in aprotic medium, of the 18-electron octahedral-type nitrile complexes fr

In the present work we have studied the behaviour of sodium nitroprusside in the presence of Cu(II) using differential pulse voltammetry. For a nitroprusside solution at pH 7.7 three polarographic waves were observed. The cathodic stripping peak observed at - 0.60 V vs. Ag/AgCl for the nitroprusside-copper species, which was formed after accumulation at 0.00 V, can be used for a quantitative determination of copper in the concentration range of 10-7 - 10-6 mol/dm3.

Batch injection analysis in conjunction with preconcentration by anodic stripping voltammetry and adsorptive stripping voltammetry has been investigated and optimised. Mercury films are pre-formed in situ within the cell on glassy carbon substrates by injection of l00 µl electrolyte containing mercury ions. Increased electrolysis efficiency in the stripping experiment is obtained by continuing preconcentration after injection of <100 µl of the metal-ion-containing analyte. Detection limits are in the nanomolar range. This procedure should find widespread application to all types of stripping voltammetry.

The electrochemical reduction of the pentacyanonitrosylferrate(II) ion ([Fe(CN)5NO]2", nitroprusside ion) has been studied in aqueous solutions of low pH by polarography and cyclic voltammetry. There are consistent evidences for the existence of the protonated ion, [Fe(CN)5NOH]2-, electrochemically generated at the potential of the first reduction of the nitroprusside ion. Moreover, a fast regeneration of the reactant(s) of the second reduction process must be assumed, which, at high nitroprusside concentration, can be viewed as a comproportionation reaction between nitroprusside and the product of the second reduction in acidic media. The reduction products adsorb at the mercury electrode.

The influence of anions on the electro-oxidation of D-sorbitol, at a platinum polycrystalline electrode, was studied in HCIO4 and H2SO4 solutions. Results show that Pt has a higher activity in perchloric than in sulfuric medium for the oxidation of D-sorbitol. Based on the rates of oxidation of D-sorbitol, the following sequence has been found for D-sorbitol oxidation in presence of adsorbed anions: Cl04- ~ F- > HS04- (S042-) » CI- » Br-.

Rubredoxin (Rd), desulforedoxin (Dx), rubrerythrin (Rr) and desulfoferrodoxin (Dfx) are bacterial proteins containing the simplest iron-sulfur cluster found in biology: a mononuclear iron center coordinated by four cysteinyl sulfur atoms (FeCys4 or Rd type centers). Although believed to be involved in electron transfer, their function in anaerobic bacteria is still unknown. The reversibility of redox reactions of these metalloproteins systems was studied by cyclic voltammetry, differential pulse voltammetry and square-wave voltammetry. Rd and Dx have been found to exhibit quasireversibility with E°'s in the range -50 to +20 mV versus S.H.E.

Electrochemical studies on Desulfovibrio desulfuricans ATCC 27774 Aldehyde Oxido-Reductase (AOR) were carried out in order to optimize the conditions for the development of an enzymatic sensor for aldehydes. In presence of both aliphatic and aromatic aldehydes, AOR develops a catalytic activity with a characteristic Michaelis-Menten type behavior.