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1.
The hydrolysis of silicic acid, Si(OH)4, was studied in a simplified seawater medium (0.6 M Na(Cl)) at 25°C. The measurements were performed as potentiometric titrations (hydrogen electrode) in which OH? was generated coulometrically. The total concentration of Si(OH)4, B, and log[H+] were varied within the limits 0.00075 ? B ? 0.008 M and 2.5 ? -log[H+] ? 11.7, respectively. Within these ranges the formation of SiO(OH)3? and SiO2(OH)22? with formation constants log β?11(Si(OH)4 ? SiO(OH)3? + H+) = ?9.472 ±0.002 and log β?21(Si(OH)4 ? SiO2(OH)22? + 2H+) = ?22.07 ± 0.01 was established. With B > 0.003 M polysilicate complexes are formed, however, with their formation does not significantly affect the evaluated formation constants. Data were analyzed with the least squares computer program LETAGROPVRID. 相似文献
2.
Coastal upwelling systems are regions with highly variable physical processes and very high rates of primary production and very little is known about the effect of these factors on the short-term variations of CO2 fugacity in seawater (fCO2w). This paper presents the effect of short-term variability (<1 week) of upwelling–downwelling events on CO2 fugacity in seawater (fCO2w), oxygen, temperature and salinity fields in the Ría de Vigo (a coastal upwelling ecosystem). The magnitude of fCO2w values is physically and biologically modulated and ranges from 285 μatm in July to 615 μatm in October. There is a sharp gradient in fCO2w between the inner and the outer zone of the Ría during almost all the sampling dates, with a landward increase in fCO2w.CO2 fluxes calculated from local wind speed and air–sea fCO2 differences indicate that the inner zone is a sink for atmospheric CO2 in December only (−0.30 mmol m−2 day−1). The middle zone absorbs CO2 in December and July (−0.05 and −0.27 mmol·m−2 day−1, respectively). The oceanic zone only emits CO2 in October (0.36 mmol·m−2 day−1) and absorbs at the highest rate in December (−1.53 mmol·m−2 day−1). 相似文献
3.
Pasquale Crea Concetta De Stefano Demetrio Milea Silvio Sammartano 《Marine Chemistry》2008,112(3-4):142-148
The results of a potentiometric investigation (by ISE-H+, glass electrode) on the speciation of phytate ion (Phy12−) in an ionic medium simulating the major components (Na+, K+, Ca2+, Mg2+, Cl− and SO42−) of natural seawater, at different salinities and t = 25 °C, are reported. The work was particularly aimed at determining the possible formation of mixed Ca2+–Mg2+–phytate ion pairs, and to establish how including the formation of these mixed species would affect the speciation modeling in seawater media. After testing various speciation models, that considering the formation of the MgCaH3Phy5−, MgCaH4Phy4−, Mg2CaH3Phy3− and Mg2CaH4Phy2− species was accepted, and corresponding stability constants were determined at two salinities (S = 5, 10). A discussion is reported both on the choice of the experimental conditions and on the possibility to extend these results to those typical of real seawater. A detailed procedure is also described to demonstrate that the stability of these species is higher than that statistically predicted. As reported in literature, a parameter, namely log X, has been determined in order to quantify this extra stability for the formation of each mixed species at various salinities. For example, at S = 10, log X113 = 2.67 and log X114 = 1.37 for MgCaH3Phy5− and MgCaH4Phy4− (statistical value is log Xstat = 0.60), and log X213 = 6.11 and log X214 = 2.15 for Mg2CaH3Phy3− and Mg2CaH4Phy2− (log Xstat = 1.43), respectively. Results obtained also showed that the formation of these species may occur even in conditions of low salinity (i.e. low concentration of alkaline earth cations) and low pH (i.e., more protonated ligand). 相似文献
4.
The rates of the reduction of Cr(VI) with S(IV) were measured in deaerated NaCl solution as a function of pH, temperature and ionic strength. The rates of the reaction were found to be first order with respect to Cr(VI) and second order with respect to S(IV), in agreement with previous results obtained at concentrations two order higher than the present study. The reaction also showed a first-order dependence of the rates on the concentration of the proton and a small influence of temperature with an apparent energy of activation ΔHapp of 22.8 ± 3.4 kJ/mol. The rates were independent of ionic strength from 0.01 to 1 M. The rate of Cr(VI) reduction is described by the general expression
−d[Cr(VI)]/dt=k[Cr(VI)][S(IV)]2