Chemical speciation of sulfur compounds in surface sediments from three bays (Fresnaye, Seine and Authie) in northern France, and identification of some factors controlling their generation.

The determination of total reduced sulfur, elemental sulfur, sulfide, thiosulfate and sulfite in porewaters--which were previously extracted from surface sediments collected in three bays, i.e. Seine, Fresnaye and Authie, located in northern France--has been undertaken using different electroanalytical techniques (linear sweep cathodic stripping voltammetry (LSCSV), square wave cathodic stripping voltammetry (SWCSV) and differential pulse cathodic stripping voltammetry (DPCSV)) at a static mercury drop electrode. Furthermore, the analyses of sulfur solids present in these sediments have been performed by sequential extraction procedures. Overall, the description of speciation of dissolved sulfur compounds and reduced sulfur solids has shown that the availability of sulfate and reactive iron, the sedimentation rate and probably even the nature and content of organic matter are important factors for controlling the sulfidisation and pyritisation processes involved in the different sedimentary systems studied. Thus, in the Seine-bay sediments, it has been found that reactive iron scavenges the most part of the generated sulfide as a result of a particularly activated sulfate reduction by bacterial activities; as a consequence, this precipitation limits the accumulation in the porewaters of reduced sulfur compounds such as elemental sulfur and polysulfides which ought to be generated in the redox boundary (where the oxydants, viz. oxygen, nitrate and metal oxides exist abundantly) through a partial oxidation of the H(2)S and HS(-) species. This process is further accentuated by a sedimentation rate measured in the Seine estuary. Conversely, in the Authie bay and, to a lesser extent, in the Fresnaye bay the contribution of the sedimentation rate and/or reactive iron scavenging to the sedimentary sulfur processes is weaker. This explains the increase of the built-up of dissolved reduced sulfur in the interstitial waters as well as the existence of elemental sulfur and polysulfides that permit the conversion of FeS into FeS(2).

Calcium-induced condensation-reorganization phenomena in multilamellar vesicles of phosphatidic acid. pH potentiometric and 31P-NMR, Raman and ESR spectroscopic studies.

In biological membranes, the anionic characteristics of the polar headgroup of phosphatidic acids are responsible for structural changes induced by Ca2+ in many cellular processes. The very simple headgroup structure of dipalmitoylphosphatidic acid (DPPA) offers particular advantages as a model to study the interactions between Ca2+ and natural phosphatidic acids such as cardiolipin and phosphatidylserine. The effects of calcium ions on DPPA membranes have been studied as a function of temperature by potentiometry and by Raman, ESR and 31P-NMR spectroscopies. The protons in monosodic DPPA liposomes have been considered as a probe to detect pH variations resulting from introduction of Ca2+ inside the membrane. This method has also allowed us to determine the stoichiometry of this reaction: 2 DPPA(H) + Ca2+----Ca(DPPA)2 + 2H+. 31P-NMR spectroscopy has been used to detect reorganization-condensation phenomena in multilamellar vesicles of DPPA under the influence of calcium and temperature. Furthermore, the temperature profiles obtained from Raman spectra for Ca(DPPA)2 membranes provide conclusive evidence that Ca2+ induces major reorganization of the phosphatidic acid component into a highly ordered phase. Quantitative estimates of the degree of motional restriction of spin-labeled soaps embedded inside membranes composed of DPPA with or without Ca2+ have been made using ESR technique. These results are discussed and compared to those found previously for a natural phosphatidic acids such as phosphatidylserine.

Chemical and electrochemical behaviour of nitric and nitrous acids in sulpholane.

With a glass electrode filled with mercury as indicator electrode for hydrogen ions, the acidity constant of nitric acid and the stability constants of the complexes HA(-)(2) (A Cl(-), NO(-)(3)) in sulpholane at 303 K have been determined potentiometrically. The acidity constant of nitrous acid has also been determined in supholane medium because of the low stability of this acid in aprotic media. On the basis of the results, some aspects of the electrochemical and thermodynamic properties of oxygen-nitrogen compounds are discussed in order to explain the catalytic effect of NO(+) in aromatic nitration by N(2)O(4) and/or N(2)O(3) in aprotic solvents.

[Not Available]. / Comportement electrochimique du tetraoxyde de diazote dans le sulfolane.

Knowing the values of the equilibrium constants corresponding to the reactions N(2)O(4) right harpoon over left harpoon 2NO(2) and N(2)O(4) right harpoon over left harpoon NO(+) + NO(3)(-) in sulpholane, we have undertaken the electrochemical study of N(2)O(4) by means of linear and cyclic voltammetry at the platinum electrode. The N(2)O(4) species undergoes one oxidation step N(2)O(4) right harpoon over left harpoon 2NO(2) right harpoon over left harpoon 2NO(2)(+) + 2e and two reduction steps NO(2) + N(2)O(4) + e(-)right harpoon over left harpoon N(2)O(3) + NO(3)(-) (1st wave), followed by 3N(2)O(4) + 2e(-) right harpoon over left harpoon 2N(2)O(3) + 2NO(3)(-), N(2)O(4) + e(-) right harpoon over left harpoon NO + NO(3)(-), 2N(2)O(3) + e(-) right harpoon over left harpoon 3NO + NO(3)(-) (2nd wave). The redox properties of N(2)O(4) are complicated by trace quantities of water because of the formation of the electroactive species N(2)O(3), HNO(3) and HNO(2) according to N(2)O(4) + H(2)O right harpoon over left harpoon HNO(2) + HNO(3) and N(2)O(4) + HNO(2) right harpoon over left harpoon N(2)O(3) + HNO(3). The standard potentials of the couples concerned have been evaluated and are discussed. sont discutés et évalués.

[Not Available]. / Etude electrochimique des sels de nitryle en milieu aprotonique.

The electrochemical reduction of NO(2)ClO(4) solutions at a platinum electrode in sulpholane has been studied. The results suggest the following scheme for the reduction of NO(+)(2) : 2NO(+)(2) + 2e(-) <==> 2NO(2) <==> N(2)O(4). A fast dimerization of nitrogen dioxide, which is favoured by the high donor number of sulpholane, follows the charge-transfer step. The NO(+)(2) ion is much more oxidative than the NO(+) ion in sulpholane, whereas the oxidizing power of the two species is nearly the same in nitromethane or acetonitrile solution. This result can be used to explain the role of water present at trace level.