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The first example of the effect of an electric double layer on the reduction of electrochemically generated radical species is reported. The anion radical of methyl 5-(2,4-dichlorophenoxy)-2-nitrobenzoate (pesticide bifenox) is electrochemically reduced in acetonitrile to a phenylhydroxylamine derivative in a process involving three electrons. This heterogeneous reaction is strongly influenced by the concentration and nature of the cation of the indifferent electrolyte. Depending on the type of tetraalkylammonium cation, the redox potential changes by 0.45 V. The kinetic parameters were obtained for five tetraalkylammonium hexafluorophosphate salts. The Frumkin correction, which assumes that the outer Helmholtz plane coincides with the reaction site, was applied to kinetic data of the radical anion reduction. The correction of the apparent rate accounted for the observed effect only in the case of tetramethylammonium salt. The presence of higher tetraalkylammonium homologues causes deviations from the predicted dependence of the electron-transfer rate on the ϕ2 potential of the outer Helmholtz plane. Hence, the nature of the cation of the electrolyte exerts a further effect extending beyond the electrostatic repulsion only. The corrected rate of electron transfer decreases exponentially with increasing size of the alkyl chain of the indifferent electrolyte cation in the order methyl > ethyl > propyl > butyl > hexyl. The rate decay is characterized by an exponent β = 0.83. This confirms that the reaction plane for the reduction of the bifenox radical anion is different for each electrolyte. Due to this fact the Frumkin correction cannot fully account for the observed dependence of the heterogeneous rate on the solution composition. The observed effect is not specific to the bifenox radical. A similar influence of the concentration and nature of the cation of the indifferent electrolyte was observed for other nitro compounds, namely, nitrobenzene, nitrobenzoate, and nitrofen.

The effect of seasonal formation/melting process of pack ice on the PCB level of sea water at Gerlache Inlet and Wood Bay (Ross Sea, Antarctica) was investigated during four Italian expeditions, i.e. 1988-89, 1990-91, 1991-92, and 1993-94. Surface sea water samples from Gerlache Inlet and Wood Bay before pack ice melting showed a typical total PCB concentration of 133 pg/l and 120 pg/l, respectively, which increased by a factor of about 1.3 in both sampling sites during pack ice melting. This effect was attributed to the transfer of PCBs contained in the pack ice to sea water, and salinity was used as a tracer to verify this hypothesis. In this respect, pack ice and sea water samples were collected during the 1993-94 Italian expedition, and both salinity (S) and total PCB content were measured. A fairly good agreement was observed between the experimental PCB concentration and the value calculated by the dilution model which was applied to the mixing process between sea and pack ice melting waters. Although this effect seems to be limited in time and space it is nevertheless significant because it happens during summer when biological species have their highest activity.

For the development of an on-column chromatographic method for the simultaneous separation and detection of selenium(IV) and tellurium(IV), their chromatographic behaviours on cation and anion exchangers as well as on multiphase cation- and anion-exchange/reversed-phase columns, have been investigated. The eluent medium consisted of diluted hydrochloric acid at different concentrations and flow-rates. The detection of selenium(IV) and tellurium(IV) was based on post-column derivatization, of the eluted species, with 1,1,3,3-tetramethylthiourea (TMTU). The TMTU concentration, pH and flow-rate was optimized in order to obtain the best sensitivity for both the analytes. The capacity factor value, k′, as well as the charge of the species eluted on the different columns tested, have been obtained for all experimental conditions. It has been demonstrated that the on-column chromatographic separation between selenium(IV) and tellurium(IV) is achievable by exploiting their different elution patterns on ion exchangers. On the basis of peak symmetry and analyte resolution at different concentration ratios, the cation-exchange mode was chosen. Injecting 50 μl of samples, detection limits of 78 and 83 μg/l for tellurium and selenium, respectively were obtained. Concentrations lower than 10 μg/l could be determined by increasing the injection volume or by means of on-line preconcentration.

The possibility for the determination of some radioisotopes of cesium, strontium, plutonium, uranium and thorium by glow discharge mass spectrometry (GDMS) in soils, sediments and vegetations is investigated. The preparation of samples is described as a combination of the use of a conductive host matrix and a secondary cathode in order to decrease the dilution effect of the blending material for the trace level determination and to gain a stable discharge. Effects of interferences arising from the nature of the conductive host matrix and of the secondary cathode on the sensitivity of the method are discussed. The determination of 137Cs and 90Sr has been attempted and the results obtained were in agreement with those from other analytical techniques. Accuracy, internal and external precisions have been also evaluated. GDMS is shown to be a helpful technique for the determination of radioisotopes in environmental samples. Radioisotopes can be determined according to the matrix of the sample (e.g. grass), also in presence of isobaric interferences. However, limitations still exist on the application of GDMS.

The reduction mechanism of chloroxynil (3,5 dichloro 4 hydroxy- benzonitrile), bromoxynil (3,5 dibromo 4 hydroxybenzonitrile) and ioxynil (3,5-diiodo-4-hydroxybenzonitrile) was studied in dimethylsulfoxide by the electrochemical methods combined with spectroelectrochemical and GC/MS identification of the products. DC polarography shows two irreversible reduction waves for ioxynil and only one reduction wave for bromoxynil and chloroxynil. Cyclic voltammetry at different scan rates and at various concentrations of pesticides is consistent with the radical anion formation followed by the subsequent chemical reactions. The main decomposition pathway includes the cleavage of the halogen atom and the dimerization of the dehalogenated intermediate. The differences in the mechanism of ioxynil, bromoxynil and chloroxynil are caused by the mutual differences in the rates of the bond cleavage and the dimerization process.