Portugaliae
Electrochimica Acta

Organic molecules with fragments which have a specific affinity for a charge (negative or positive) and, separately, for an electron can form ion-radicals of the special (distonic) type. Such a type is characterized by a scattering of charge and spin densities between different atoms of an organic skeleton. Reasons and results of this phenomenon constitute the aim of this report.

In this work, the various methods that have been used for the calculation of ionic strength are discussed. An iterative process is introduced where, instead of traditional approximations, calculation of ionic strengths through complete equations derived from the acidity constants is performed. Concentrations of the various species involved in the conventional assignment of pH, their distribution and activity coefficients are also obtained. The process prooves particularly useful for poliprotic acids with overlaping acidity constants Ce. g. H^Cit: k1 = 7.447 x 10"*; k2 = 1.734 x l-~5; k3 = 4.018 x 10-7 at 25° C). Finally, and as a consequence, conventionally assigned pH values of reference buffer solutions are recalculated and corrections are introduced whenever required.

The polarographic reduction of the azo dye 4-nitro-2'-hydroxy-5'-tert-butylazobenzene in Britton Robinson Buffer with pH range of 2 and 12 at room temperature has been investigated in aqueous-ethanol solution of ratio 1:1. Two straight lines were obtained from the Em -pH plots for the compound. Diffusion controlled reduction behaviour was observed in acid and alkaline media. In addition, a polarographic determination method for the analysis of 4-nitro-2'-hydroxy-5'-tert-butylazobenzene has. been developed. The detection limit was 50 pbb for this medium.

Manganese dioxide exists in several crystalline forms. Heating MnO 2 in air or, vacuum brings about changes in crystallinity. Heat-treated samples of Mn02 at 373/ 573, 723 and 1023 K were used in fabricating Li-Mn02 cells. These cells were subjected to impedance measurements at 283, 293, 303 and 313 K temperatures. The R and I values were calculated after subjecting to Cdl and R corrections. These results are discussed.

Heat treated electrolytic manganese dioxide is used as the cathode in lithium manganese dioxide primary batteries. Heat treatment at 400°C, removes the occluded water in the Mn02. Also, partial crystal change from pure gama to a mixture of gama- Beta occurs during this heat treatment. In this paper the reduction mechanism of heat treated Mn02 in 0.5 M LiC104/PC - 2 M LiC104/PC have been studied by cyclic voltammetry. The mechanism of Mn02 reduction varies with the concentration of electrolyte.

Measurements are reported of the linear sweep voltammograms of silver on the presence of mixed catalytic oxidation/reduction of hydrogen peroxide on to vitreous carbon microdisk electrode. The catalytic rate constants were determined by performing theoretical simulations of the expected responses of the current-potential profiles and good agreement was found between these calculated profiles and the experimental responses in the cathodic branch for k9°= 2.5. 10"6 mol cm'^s'. The occurrence of a nonelectrochemical pathway reaction associated to the catalytic behaviour of hydrogen peroxide at silver substrates was observed experimentally and the density flux rate constant of this reaction was found to be 3.2. 10~6 mol cm'^s'K.

The electrochemical study of 4,iVii2,l,2,4,Triazole-pentacyanoferrate (II), 1,^/^2,1,2,4, Triazole-pentacyanoferrate (II) and Benzotriazole-pentacyanoferrate (II) complexes was carried out in a platinum rotating electrode at different pH values. For these complexes, values of the electrode rate constant, ko, and the transfer coefficient, ana have been estimated. The results obtained for these complexes were found to be PH-independent in a wide pH range.

Activity coefficients of NaCl and Na2Mal (Sodium Malonate) in their aqueous mixtures, NaCl+Na2Mal+H20 at 298.15 K, were determined from emf measurements of galvanic cells. The measurements were carried out at stoichiometric ionic strengths of 0.025, 0.05, 0.075, 0.1, 0.25, 0.5, 0.75, 1,2, ajid 3 mol kg1 and at series of ionic-strength fractions yB=3mB/(mA+3mB) within each of these ionic strengths. The results obtained were analysed by the Harned rule, Pitzer (s#clMal=0.0380 kg mol"1; i/'NaC1Mai =-0.0257 kg2mof2) and Lim (HOLL) equations.

A novel zinc-N.N'-dichlorodimethylhydantoin (Zn/DDH) organic primary cell system has been studied under various current drains. The attractive feature of this system Is its high open circuit (2.12 V) and closed circuit voltages (2.00 V) and non-toxicity in comparison with conventional aromatic nitro compounds (eg. Zn/m-dltrobenzene OCV: 1.10; CCV: 0.90 V). Several battery parameters viz. Internal resistance, capacity, energy density and coulomblc efficiency have been calculated.

The impedance spectra obtained at short exposure times for Al 7075-T6 immersed in 0.5 N NaCI do not agree with the pitting model proposed by Mansfeld and co-workers. Instead of a transmission line impedance, an inductive loop is observed at low frequencies. This result is due to the non-linearity of the system at short exposure times, when Ecorr is close to Ep^. The application of the anodic portion of the ac signal causes this non-linear behavior. The observed impedance is therefore a system response and not a true impedance. For longer exposure times agreement with the pitting model is observed.