Christian Madec

Phosphorus and nitrogen behaviour in the Mediterranean Sea

Abstract Mediterranean nutrient studies differ from one another by their rates of exchange with the Atlantic Ocean and by atmospheric and terrestrial sources, which sometimes vary by six times. During the Medatlante cruises (in 1988 and 1989), increases of phosphate and nitrate concentrations were confirmed in deep western waters and may be related to increasing agricultural, industrial and urban activities around the sea since the 1960s. In a non-steady-state model, this evolution of deep water concentrations constrains uncertainties in the nutrient budgets; we propose a reduced range for atmospheric and terrestrial sources of nutrients. In the Western Basin in the late 1980s the total atmospheric and terrestrial source amounted to 8–10 × 109 mol y−1 of phosphate, and 190–220 × 109 mol y−1 of nitrate; about two-fold greater than estimates based on measurements of atmospheric and terrestrial inputs (the latter is calculated from the Rhone river concentrations over the 1971–1988 period). Consequently, phosphate budgets suggest that some of the riverine particulate input of phosphorus dissolves when entering the sea and constitutes a main source of phosphate, a scenario proposed by Froelich (1988). Likewise, dissolved inorganic nitrogen budgets require biological fixation of molecular nitrogen by plankton species and seagrasses. This process may constitute the main nitrogen source and explain the peculiar molar ratio N/P in the Mediterranean Sea (about 21–23) instead of about 15 in the global ocean.

Determination of trace amounts of chromium(VI) in water by electrochemical methods

Abstract A sensitive procedure for the determination of chromium(VI) is described. A chelate of chromium with an organic ligand is adsorbed on a hanging mercury drop electrode and the reduction current of the accumulated chelate is measured by differential-pulse voltammetry. The effects of various parameters (pH, ligand concentration, potential and collection time) on the response are discussed. Possible interferences by trace metals and organic matter are considered.

NITROGEN UPTAKE AND ASSIMILATION KINETICS IN ALEXANDRIUM MINUTUM (DYNOPHYCEAE): EFFECT OF N‐LIMITED GROWTH RATE ON NITRATE AND AMMONIUM INTERACTIONS1

Uptake and assimilation kinetics of nitrate and ammonium were investigated along with inhibition of nitrate uptake by ammonium in the harmful dinoflagellate Alexandrium minutum Halim at different nitrogen (N)–limited growth rates. Alexandrium minutum had a strong affinity for nitrate and ammonium (Ks=0.26±0.03 and 0.31±0.04 μmol·L−1, respectively) whatever the degree of N deficiency of the cells. Ammonium was always the preferred form of nitrogen taken up (=0.42–0.50). In the presence of both forms, nitrate uptake was inhibited by ammonium, and inhibition was particularly marked in N‐sufficient cells (Imax∼0.9 and Ki=0.31–0.56 μmol·L−1). In the case of N assimilation, ammonium was also the preferred form in N‐deficient cells (=0.54–0.72), whereas in N‐sufficient cells, both N sources were equally preferred (=0.90–1.00). The comparison of uptake and assimilation rates highlighted the ability of A. minutum to significantly store in 1 h nitrate and ammonium in amounts sufficient to supply twice the daily N requirements of the slowest‐growing N‐deficient cells. Nitrogen uptake kinetic parameters of A. minutum and their ecological implications are discussed.

Voltammetric study of the copper-1,10-phenanthroline complex

The behaviour of the complex of copper with 1,10-phenanthroline at a mercury electrode was investigated in acidic chloride media. Adsorption phenomena were observed by both differential-pulse and normal-pulse polarography. These properties were used in developing a sensitive stripping voltammetric procedurefor the determination of traces of copper. The effects of various operational parameters (pH, ligand concentration, potential and accumulation time) on the reduction current of the adsorbed chelate are discussed. Interferences by other trace metals and organic matter were investigated. A linear current-concentration relationship was observed up to about 1.5 × 10−7 M. The detection limit is 1.2 × 10−10 M copper after a 10-min accumulation time with a stirred solution.

Determination of Humic Substances in Seawater by Electrochemistry (Mechanisms)

Abstract A new method for the measurement of the concentration of humic substances (HS) in seawater is described. The peak obtained by differential cathodic sweep voltammetry after HS adsorption in the presence of traces of molybdenum(VI) can be used for quantitative determination. Measurements using various electrochemical techniques (ACP, CV, ADPV, DPP) would indicate that the film adsorbed onto the electrode is the result of the adsorption of the organic substance in a first step followed in a subsequent step by the formation of a surface complex (Mo(VI)-HS)ads rather than the direct adsorption of the dissolved complex. The electrochemical mechanism corresponds to a one-electron reduction of the adsorbed (Mo(VI)-HS)ads complex to the corresponding (Mo(V)-HS)ads complex.

Électrochimie dans le carbonate de propylène: II. Utilisation du système Ar+/Ar− des hydrocarbures aromatiques alternés comme référence de potentiel indépendante du solvant

Abstract The standard potentials of Ar+/Ar and Ar/Ar− redox couples of some mono- and poly-nuclear aromatic hydrocarbons have been measured in propylene carbonate. It appears that: (a) the difference E0(Ar+/Ar)−E0(Ar/Ar−) is constant in various solvents for a given hydrocarbon; (b) in a medium, the sum E0(Ar+/Ar)+E0(Ar/Ar−) is independent of the hydrocarbon. These results confirm that the Ar+/Ar− system can be used as a solvent-independent reference for potential comparisons.

Electrochemical determination of low levels of residual chlorine dioxide in tap water

The reaction between 1,2-dihydroxyanthraquinone-3-sulphonic acid and chlorine dioxide, in phosphate buffer medium, was studied spectrophotometrically and electrochemically. An electroanalytical method is proposed for the determination of traces of ClO2 with a detection limit of 2 μg l−1. The stoichiometry of the reaction, the effects of various operational parameters and the influence of foreign ions are reported.

Influence de la Matière Organique des Milieux Naturels sur la Redissolution Cathodique [Agrave] l'électrode [Agrave] Goutte Pendante de Mercure

Abstract At pH 2 the cathodic stripping of seawater at the HMDE displays two peaks (-0.520V and -0.260V). If the peak at -0.520V is due to humic and fulvic acids the other one is probably associated to traces of organic compounds (10−7M) having the thiocarboxylic functional group .

Électrochimie dans le carbonate de propylène: I. Oxydation anodique de quelques hydrocarbures aromatiques polynucléaires

Abstract In propylene carbonate, the two consecutive one-electron oxidations of the aromatic compounds: 9,10-diphenyl anthracene, perylene and thianthrene are quasi-reversible. The cation radicals formed during the first step are very stable; on the other hand, the dications react very rapidly with nucleophilic species such as water. These results are similar to those obtained in acetonitrile or dichloromethane.

Conditions D'Exploitation De La Méthode à La Diéthyl-P. Phénylénediamine Au Dosage Du Brome Libre Et/Ou Combiné En Milieu Marin

Abstract Free bromine and bromamine analysis by the diethyl-p. phenyl-lene-diamine (DPD) colorimetric method requires some care when chloride and bromide are present at seawater concentration. If no particular problem is encountered in mono and dibromamine determination, on the contrary the analysis of either free bromine or that of a free bromine and tribromamine mixture can induce errors of at least twenty per cent in the total residual oxidant concentration due to halogen loss. In order to prevent that phenomenon, iodide addition into the phosphate buffer and DPD mixture is necessary before the sample introduction. In these conditions the results are in good agreement with those of the phenol red method.