Publications

Abstract The standard (p(o) = 0.1 MPa) molar enthalpies of combustion in oxygen, at T = 298.15 K, for crystalline 3,5-dimethylpyrazole (Me(2)Pyr), 3,5-dimethyl-4-nitrosopyrazole (Me(2)PyrNO), 1,3,5-trimethyl-4-nitrosopyrazole (Me(3)PyrNO), and 3,5-dimethyl-1-phenyl-4-nitrosopyrazole (Me(2)PhPyrNO) were measured by static-bomb calorimetry. These values were used to derive the standard molar enthalpies of formation of the crystalline compounds. The standard molar enthalpies of sublimation for these four compounds were measured by microcalorimetry. [GRAPHICS] The experimental results obtained allow us to derive the values of the standard molar enthalpies of formation, in the gaseous state, for the monomeric compounds involved in this work. These last values are discussed comparatively with results previously obtained for some aromatic nitroso derivatives. (C) 2001 Academic Press.

Abstract The standard (p(o) = 0.1 MPa) molar enthalpies of combustion in oxygen, at T = 298.15 K, for crystalline picolinamide (2-NH2COPy), nicotinamide (3-NH2COPy), isonicotinamide (4-NH2COPy), nicotinamide N-oxide (3-NH2COPyNO), and isonicotinamide N-oxide (4-NH2COPyNO) were measured by static-bomb calorimetry. These values were used to derive the standard molar enthalpies of formation of the crystalline compounds. The standard molar enthalpies of sublimation, at T = 298.15 K, for the three pyridinecarboxamide isomers were measured by microcalorimetry and the standard molar enthalpies of sublimation for the two pyridinecarboxamide N-oxide compounds were measured by a mass-loss effusion technique. From the enthalpies of formation of the gaseous compounds, the molar dissociation enthalpies D-m(o) of the (N+-O-) covalent bonds were derived. Comparison has been made with D-m(o) (N-O) values in pyridine N-oxide derivatives. [GRAPHICS] (C) 2001 Academic Press.

Abstract The electrochemical behavior of the hydrolysis products of oxadiazon was studied by cyclic and square-wave voltammetry using a glassy carbon electrode. Maximum currents were obtained at pH 12.8 in an aqueous electrolyte solution containing 30% ethanol and the current did not decrease with time showing that there was little adsorption of the reaction products on the electrode surface. The hydrolysis products of oxadiazon were identified after isolation and purification, as 1-trimethylacetyl-2-(2,4-dichloro-5-isopropoxyphenyl)-2-ethoxy-carbonylhydrazine (Oxa1) and 1-trimethylacetyl-2-(2.4-dichloro-5-isopropoxyphenyl) hydrazine (Oxa2) with redox potentials + 0.6V and -0.1V (vs. Ag/AgCl), respectively. Based on the electrochemical behavior of 1-trimethylacetyl-2-(2,4-dichloro-5-isopropoxyphenyl) hydrazine (Oxa2) a simple electroanalytical procedure was developed for the determination of oxadiazon in commercial products used in the: treatment of rice crops in Portugal that contain oxadiazon as the active ingredient. The detection limit was 1 x 10 (-4) M, the mean content and relative standard deviation obtained for seven samples of two different commercial products by the electrochemical method were 28.4 +/- 0.8 % (Ronstar) and 1.9 +/- 0.2 % (Ronstar GR), and the recoveries were 100.3 +/- 5.4 % and 101.1 +/- 5.3 %, respectively.

Abstract The electrochemical behaviour of propanil and related N-substituted amides (acetanilide and N,N-diphenylacetamide) was studied by cyclic and square wave voltammetry using a glassy carbon electrode. Propanil has been found to have chemical stability under the established analytical conditions and showed an oxidation peak at +1.27 V versus Ag/AgCl at pH 7.5. N,N-diphenylacetamide has a higher oxidation potential than the other compounds of +1.49 V versus Ag/AgCl. Acetanilide oxidation occurred at a potential similar to that of propanil, +1.24 V versus Ag/AgCl. These results are in agreement with the substitution pattern of the nitrogen atom of the amide. A degradation product of propanil, 3,4-dichloroaniline (DCA), was also studied, and showed an oxidation peak at +0.66 V versus Ag/AgCl. A simple and specific quantitative electroanalytical method is described for the analysis of propanil in commercial products that contain propanil as the active ingredient, used in the treatment of rice crops in Portugal.

Abstract The oxidative stability of refined sunflower oil in the presence and in the absence of propyl caffeate (PC), propyl hydrocaffeate (PHC), propyl ferulate (PF), and propyl isoferulate (PI) has been evaluated according to the Rancimat method. The antioxidant activity of the phenolic derivatives was compared with that obtained with native [alpha -tocopherol (alpha -TOH)] and synthetic [propyl gallate (PG)] antioxidants. The results allow the establishment of a decreasing order of antioxidant power: PG > PHC > PC much greater than alpha -TOH > PI > PF. The oxidative stability was improved neither by the addition of PF nor by a supplement of alpha -TOH. Moreover, a positive antioxidant effect was obtained for PC that was placed between those of alpha -TOH and PG. The antioxidant activity of PHC was higher than that of its analogue (PC). A dose-dependent effect was observed for PG, PHC, and PC. A chain-breaking mechanism was proposed for the antioxidant activity of propyl phenolic esters because the same ranking order of efficacy was obtained for their antiradical activities evaluated by using the 2,2-diphenyl-1-picrylhydrazyl radical method.

Abstract Square-wave voltammetry (SWV) has been used to study the transfer of zinc(II) ion under static conditions, assisted by 5,6-diphenyl-3-(2-pyridyl)-1,2,4-triazine (DPT), through gel-supported microinterfaces. Microhole arrays created by laser photoablation of thin polyester films were used to support an organic gel phase prepared by addition of 1,3 : 2,4-dibenzylidenesorbitol (DBS) to a solution of o-nitrophenyloctyl ether (o-NPOE) with the appropriate supporting electrolyte. The results show that SWV can be used with the gel-supported microinterfaces if a gelified aqueous reference is used for the organic phase. Under such conditions a preliminary estimate of the detection limit for the determination of Zn2+ is 5 x 10(-8) mol L-1.

Abstract Determination of total sulfur in environmental samples can be accomplished using automated dry combustion techniques. The analytical performance of an elemental analyzer (CE Instruments) was compared with a method involving dry ashing followed by ion chromatographic detection of sulfate (IC). Samples included certified reference materials, confirmation materials with a known content of sulfur, and several soils and plants. There was a close agreement between both methods. Superior accuracy, precision and detection limits were obtained using the dry combustion method. Current results suggest that the automated dry combustion technique has developed into an adequate method for the determination of the relatively low total sulfur contents commonly encountered in soils and plants.

Abstract The capacitance of the polarisable aqueous \ organic interface was measured with the impedance method, varying both the solvent and the electrolyte of the organic phase. The results were analysed using the idea of interfacial ion-pairing presented earlier (J. Chem. Soc. Faraday Trans. 87 (1991) 107). It appeared that this model was capable of explaining the measured capacitances quite well for 2-heptanone, 2-octanone and 1,3-dibromopropane as the organic solvent, while for 1,2-dichloroethane - which is perhaps the most common solvent used in liquid \ liquid electrochemistry - the model needed to be modified, assuming a mixed layer between the bulk solvent phases as suggested earlier in the literature.