Studies of equilibrium, structure, and dynamics in the aqueous Al(iii)-oxalate-fluoride system by potentiometry, C and F NMR spectroscopy

The AlOx_1-3 (Ox = oxalate) species were identified in 0.6 M aqueous NaCl by ¹³C nuclear magnetic resonance (NMR). Rate constants and activation parameters for intramolecular cis/trans isomerization of the Werner-type AlOx₂⁻ complex (k(298 K) = 5 s⁻¹, ΔH^# = 67 ± 5 kJ mol⁻¹, ΔS^# = −6 ± 6 J mol⁻¹ K⁻¹, the rate determining step could be the breaking of the Al-O(C=O) bond) and a very slow intermolecular ligand exchange reaction of AlOx₃³⁻ complex and the free ligand (k₃₀(298 K) = 6.6 · 10⁻⁵ s⁻¹, ΔH^# = 164 ± 17 kJ mol⁻¹, ΔS^# = 225 ± 51 J mol⁻¹ K⁻¹, D/I_d mechanism) were determined by dynamic 1D and 2D ¹³C NMR measurements. Mixed complexes, AlFOx, AlFOx₂^2-, AlF₂Ox⁻, and AlF₂Ox₂^3-, with overall stability (logβ) of 11.53 ± 0.03, 15.67 ± 0.03, 15.74 ± 0.02, and 19.10 ± 0.04 were measured by potentiometry using pH- and fluoride-selective electrodes and confirmed by ¹³C and¹⁹F NMR. The role of these complexes in gibbsite dissolution was modeled. The mixed Al(III)-Ox^2--F⁻ complexes have to be considered as the chemical speciation of Al(III) in natural waters is discussed.     

Species composition and dietary relationships in a brackish shallow water fish assemblage in the Bothnian Sea,Sweden

A nearshore fish assemblage inhabiting a shallow bay in the southern Bothnian Sea, Sweden, with demanding environmental conditions (c. 5‰; >15°C during 4 months 1980 and 1 month 1981) was studied during a two-year period, in 1980 and 1981. Seasonal distribution patterns, dietary relationships, and growth rates were studied in Pungitius pungitius (L.), Pomatoschistus minutus (Pallas.), Gasterosteus aculeatus (L.), Phoxinus phoxinus (L.), Pomatoschistus microps (Krøyer) and Gobius niger L. The structure of the juvenile populations changed both over seasons and years but the adult populations remained constant. Lower water temperature, fewer individuals, lower and delayed fish growth, and lower interspecific food overlaps were found in 1981 compared to 1980. Few significant correlations were found in both years between the following community parameters: diversity, niche width, food overlap, and the proportion of each species in the fish assemblage.According to these results it is suggested that food competition and resource partitioning were of minor importance for the structuring of the fish community in the present area. Rather, the extreme salinity and the fluctuations in temperature regulated the fish populations. One may conclude that the populations of juveniles were more influenced by short-term changes in the environment than those of the adults.     

Carbon fluxes in the Arctic Ocean—potential impact by climate change

Because of its ice cover the central Arctic Ocean has not been considered as a sink of atmospheric carbon dioxide. With recent observations of decreasing ice cover there is the potential for an increased air–sea carbon dioxide flux. Though the sensitivity of the carbon fluxes to a climate change can at present only be speculated, we know the responses to some of the forcing, including: melting of the sea ice cover make the air–sea flux operate towards equilibrium; increased temperature of the surface water will decrease the solubility and thus the air-sea flux; and an open ocean might increase primary production through better utilization of the nutrients.
The potential change in air-sea CO₂ fluxes caused by different forcing as a result of climate change is quantified based on measured data. If the sea ice melts, the top 100 m water column of the Eurasian Basin has, with the present conditions, a potential to take up close to 50 g C m⁻². The freshening of the surface water caused by a sea ice melt will increase the CO₂ solubility corresponding to an uptake of ∼ g C m⁻², while a temperature increase of 1°C in the same waters will out-gas 8 g C m⁻², and a utilization of all phosphate will increase primary production by 75 g C m⁻².     

Complexation of Gold(III)-Chloride at the Surface of Hematite

The acid/base properties of the hematite/water interface and surfacecomplexation with gold has been studied by potentiometric titrations in thefour component system H⁺ – FeOH –AuCl ₄ ^- – Cl^-. Equilibrium measurementswere performed in NaCl media at 298.2 K. In the evaluation of equilibriummodel from experimental data the constant capacitance model was applied. Theacid/base properties were investigated in 0.1 M NaCl in the range 2.6 pH 7.4. The resulting intrinsic constants for protonation anddeprotonation of hydroxyl groups at the surface were log _1,1,0,0(int) ^s = 7.10 ± 0.06 andlog _{-1,1,0,0(int)} ^s = - 7.80 ±0.06. The density of proton active surface sites was 2.85nm^-2 and the specific capacitance 2.5 C V^-1 m^-2. In the investigation of surface complexation of gold, thepotentiometric titrations were performed in the range 2.0 pH 10.2.Titration data was supplemented with analysis of Au in the aqueous phase byatomic absorbance spectrometry. The equilibrium model proposed consists ofthe following monodentate surface complexes: FeOHAuCl₃(log _0,1,1,-1 ^s = 1.45 ± 0.03),FeOHAuCl₂OH (log _-1,1,1,-2 ^s = -3.89 ± 0.02), FeOHAu(OH)₃ (log _-3,1,1,-4 ^s = -21.94 ± 0.05). Aslightly better fit could be obtained by assuming formation of a bidentatecomplex with the composition(FeO)₂Au(OH)H₂O. However, based onstructural arguments this complex was rejected.     

The application of potentiometric techniques to study complexation reactions at the mineral/water interface

The most widely used technique to characterize the acid/base and complexing properties of hydrous particle surfaces is based on potentiometric titrations. Properly applied it provides accurate data over wide concentration ranges and, with help of automatic measuring and data collecting systems, it has become possible to collect large amounts of data within a reasonably short time period. However, collection and interpretation of experimental data is beset with difficulties. Some of these difficulties are identified and discussed with special reference to some goethite systems.