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Electrochimica Acta

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.

A PVC membrane containing the tetra ethyl ester of p-tert-butyl calix[4]arene as ionophore was prepared and tested for sodium ion in solutions with and without albumin (BSA), after confirmative experiments with respect to Nernstian behaviour, limit of detection, selectivity to K+, reproducibility and lifetime. Measurements of Na+ have been taken in solutions of albumin containing constant 0.01 mol dnT" NaCl solution. The protein concentration ranged from 0 to 100 g dm"3 in increments of 20 g dm"3. The reference electrodes were of two types, hypertonic (saturated calomel electrode) and isotonic (Ag, AgCl, 0.15 mol dm"3 KC1). Variation of potential with albumin concentration is larger than expected [2] and bigger for hypertonic than for isotonic reference electrodes. Explanation was found in the presence of sodium contamination of the protein.

Good laboratory practice means among other things, specifically for pH measurements, that measurements are done correctly and to have evidence that this has been done correctly. The several components of the pH System have to be considered. The pH meter has to provide raw data (mV) as well as the sample results. This is especially important during the calibration. The choice of the electrode is important as well as the sample type is critical. Once proved and validated the correct electrode and its correct filling solution the refilling procedure should be a part of the correct SOP (Standard Operating Procedures). Calibration points and frequency together with maintenance procedures also need to be included in the SOP's. Calibration standards must be traceable to international Standards, and therefore identifiable and dated. The various factors to affect the pH need to be understood. In particular, the temperature has different effects which need to be worked out on the proper way to arrive at a reproducible and accurate result. In particular, the patented Ross technology compensates several of the temperature effects, using an innovative galvanic pair, which is proven to have a faster answer to temperature changes and is more precise on the readings. Finally, recording and submitting data in proper records is very important in GLP environments as adherence to these procedures enhance the relevance of documentation.

The electrochemical oxidation of molinate, a thiocarbamate herbicide, has been studied in order to determine it in a phytopharmaceutical product. A square wave voltarnmetry electroanalytical method has been developed and the results were in good agreement with those obtained by the AO AC reference method using gas chromatography. The recovery is 100.2%.

Square wave adsorptive voltammetry at the hanging mercury drop electrode has been applied to the determination of folic acid in pharmaceutical preparations including complex mixtures of vitamins. Folic acid was extracted with an alkali solution in a single step process. One aliquot of this solution was analysed in a boric acid/sodium hidroxide buffer, pH 9.75, sweeping the potential from -0.50 to -0.90 V vs AgCl/Ag at pulse amplitude of 40mV and pulse frequency of 50Hz. The accumulation potential was -0,50V while the accumulation time was 10s. When other substances that might adsorve on the HMDE were present the accumulation potential was -0,75V and the time was 30s. The determination was carried out by using the standard additions method. Recoveries results between 96.0 and 99.7% were obtained.