Portugaliae
Electrochimica Acta

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.

The passivation and localized corrosion of (a-/3) brass is studied in borate-boric acid buffer + 0.5 M NaCl aqueous solution (pH 9) in the 0 -60 °C range using electrochemical techniques combined with surface microscopy. Passivation of a-fi brass in borate-boric acid buffer is due to the electroformation of a complex ZnO.xH 0 and Cu 0/CuO/Cu(HO) anodic 2 2 2 layer. In borate-boric acid buffer + 0.5 M NaCl passivity breakdown leading to pitting corrosion is observed when the applied potential exceeds E, the b breakdown potential. The E Vs T plot gives a straight line with slope b -5x10 V/ C. At a constant T the localized corrosion resistance of a-0 brass results considerably lower than that of polycrystalline Cu.

Voltammetric investigation of a chemical species after accumulation on a hanging mercury drop electrode can be very interesting chemically. Histidine-containing peptides, for example, are frequently found to have accumulated as copper(I) complexes. The mechanism of adsorption can be altered by modifying the HMDE, for example with polyaminoacids. Chemical transformations can be observed to be occurring on the electrode surface, particularly during repeated cycles in cyclic voltammetry. But is adsorptive stripping voltammetry (AdSV) useful analytically? It is an accepted method for the determination of at least eight trace metals in seawater and other water samples. Its acceptance is partly owing to the ease of destroying organic interferences by means of UV irradiation. But what about the determination of organics? Here, usually, some form of separation is required, and the use of solid phase extraction cartridges seems to be particularly appropriate.

The determination of sulfanilic acid in the colouring matter tartrazine was undertaken. Sulfanilic acid is determined by differential pulse polarography after derivatisation to an azo compound (diazotisation /coupling procedure). Being an azo compound too, tartrazine will interfere in the polarographic determination of derivatized sulfanilic acid. To solve this problem two approaches were tried successfully: a chromatographic separation and a masking procedure for tartrazine. Separation was achieved in a column packed with Amberlite XAD-2. Addition of gelatine caused the elimination of polarographic peak of tartrazine, allowing the determination of derivatized sulfanilic acid without need of a separation procedure.

A method for the determination of ascorbic acid is described, based on the derivatization of dehidroascorbic acid, (oxidazed form of ascorbic acid) with o-phenylenediamine to form aquinoxaline type compound, that is determined using differential-pulse polarography. A linear range between 10ug/L to lOxlO^ug/L was obtained. The method was applied to the determination of ascorbic acid in commercial fruit juices and compared with an established spectrophotometric method.

Square wave voltammetry (SWV) is applied to the anodic reaction of uric acid at a carbon electrode in aqueous solution. The electrochemical oxidation and adsorption characteristics of uric acid under these conditions are presented. In order to evaluate the analytical potentialities of this technique on the oxidative mode on a carbon electrode, reproducibility and sensitivity tests were carried out.

Results of an investigation of electrochemically catalysed nucleophilic substitution of 4-bromobenzophenone in the presence of phenoxide ions, as nucleophile, are reported. The data obtained are analysed by comparing the limiting currents of the reduction waves in the presence and in the absence of the nucleophile, and a value of 1.9 x 105 M"1 s-1 was obtained for the rate constant of the substitution reaction.