Stability of the calcium sulphate ion pair at the ionic strength of seawater by potentiometry

The stability of the ion pair CaSO₄ was determined from measurement of the change in calcium ion activity with medium composition at constant ionic strength. A calcium selective PVC-matrix liquid membrane electrode was used to monitor the calcium ion activity. All measurements were performed at 1 atm, 25 ± 0.1°C and 0.7 M formal ionic strength. The evaluation of the stability constant depends on the degree of complexation between calcium and chloride and between sodium and sulphate. The dependence of K_CaSO4, on K_NaSO4 and K_CaCl can be described by the following relations:K_CaSO4=17.7 K_CaCl+16.5 (K_NaSO4=1.8)K_CaSO4=18.8 K_CaCl+17.3 (K_NaSO4=2) for K_CaCl=0–1A value of K_CaSO4=25.4 is suggested.     

Determination of cadmium(II), copper(II), manganese(II) and nickel(II) species in Antarctic seawater with complexing resins

The strong species of cadmium(II), copper(II), manganese(II) and nickel(II) in an Antarctic seawater sample are investigated by a method based on the sorption of metal ions on complexing resins. The resins compete with the ligands present in the sample to combine with the metal ions. Two resins with different adsorbing strengths were used. Very stable metal complexes were investigated with the strong sorbent Chelex 100 and weaker species with the less strong resin, Amberlite CG-50. Strong species were detected for three of the considered metal ions, but not for Mn(II). Cu(II) is completely linked to species with a side reaction coefficient as high as log α_M(I) = 11.6 at pH = 7.3. The ligand concentration was found to be similar to that of the metal ion, and the conditional stability constant was around 10²⁰ M^− 1. In the considered sample, only a fraction of the metal ions Cd(II) and Ni(II) is bound to the strong ligands, with side reaction coefficients equal to log α_M(I) = 5.5 and 6.5 at pH = 7.3 for Cd(II) and Ni(II), respectively. These findings were confirmed by the test with the weaker sorbent Amberlite CG-50. It can be calculated from the sorption equilibria that neither Mn(II) nor Ni(II) is adsorbed on Amberlite CG-50 under the considered conditions and, in fact, only a negligible fraction of Mn(II) and Ni(II) was adsorbed. A noticeable fraction of Cd(II) was adsorbed on Amberlite CG-50, meaning that cadmium(II) is partially linked to weak ligands, possibly chloride, while no copper(II) was adsorbed on this resin, confirming that copper(II) is only combined in strong species. These results are similar, but not identical, to those obtained for other seawater samples examined in previous investigations.     

The interaction of manganese(II) with the surface of calcite in dilute solutions and seawater

The interaction of Mn²⁺ with the surface of calcite in aqueous solutions is complex. In dilute solutions the Mn²⁺ is rapidly absorbed, MnCO3₃ nucleates on the calcite surface and then grows by a first order reaction with respect to the initial Mn²⁺ concentration. At higher ionic strengths in NaCl solutions, the rate of these processes is slower, but the same general pattern persists. In solutions containing Mg²⁺, at the concentration of seawater and in seawater, the nucleation phase of the uptake process does not appear to occur. The long-term uptake rate of Mn²⁺ on the surface of calcite in seawater is first order with respect to the dissolved Mn²⁺ concentration. The rate constant is over three orders of magnitude smaller than that found in dilute Mg²⁺-free solutions. A probable explanation for the slower growth rate in seawater is that MnCO₃ is not nucleated on the calcite surface due to the presence of high Mg²⁺ concentrations. The Mg²⁺, through site competition, prevents enough Mn²⁺ from being adsorbed to reach a critical concentration for MnCO₃ nucleation. This behavior is similar to that found for orthophosphate with calcite surfaces in dilute solutions and seawater. It indicates that rhodochrosite cannot nucleate in carbonate-rich recent sediments unless the Mg²⁺ concentration is lowered below that of seawater.Measurements of the solubility of rhodochrosite in seawater gave results from an undersaturation approach to equilibrium in excellent agreement with those found in previous studies in dilute solutions. When equilibrium was approached from supersaturation, approximately fifty times more calcium was precipitated than Mn²⁺. The measured solubility was over twice that determined from undersaturation. It is possible that a Mn—calcite containing 25 to 30 mol% MnCO₃ formed on the rhodochrosite from the supersaturated solutions. Consequently, it is doubtful that pure rhodochrosite controls the concentration of Mn²⁺ even in calcium carbonate-poor marine environments.     

Ultraviolet spectroscopic determination of the stability constants for copper carbonate and bicarbonate complexes up to the ionic strength of seawater

An ultraviolet spectroscopic study at 25°C is used to determine the stoichiometric stability constant of CuCO₃⁰ (β_CuCO3⁰) over a range of ionic strengths; extrapolation to zero ionic strength provides a thermodynamic constant (log β⁰_CuCO3⁰ = 6.89). At 0.69 m ionic strength, log β_CuCO3⁰) is 6.33.This study provides the first experimental evidence for the existence of CuHCO₃⁺. At 0.69 m ionic strength, log β_CuHCO3⁺ is 2.77.The ultraviolet molal absorptivity spectra for the two complexes are also determined. The spectrum for CuCO₃⁰ consists of a single band centered at 276.5 nm and was described well by a Gaussian function. The spectrum of CuHCO₃⁺ consists of at least two overlapping bands.Side reactions which might affect the data interpretation were included in the analysis of the results. The effects of the Cu(II) hydrolysis reactions were accounted for by using data from previous work, and possible effects due to copper-organic species were eliminated by a subtraction procedure.     

Voltammetric study of adsorption of cu(II) species on solid particles added to seawater

The adsorption on solid particles of natural organic ligand in seawater of Cu(II) ions, and of the inert Cu(II) complexes has been studied. Model solids, γ-Al₂O₃, Na⁺-0.392-γ-Al₂O₃, ‘Aerosil 200’, chrysotile, northupite and CaCO₃ were added to seawater. It was observed that at pH 8 natural organic matter was strongly adsorbed on chrysotile and was not adsorbed on Na⁺ -0.392-γ-Al₂O₃; it was also adsorbed on γ-Al₂O₃ over the range of 3 < pH < 7. In this pH range, the complexing capacity and adsorption of Cu is at a minimum because Cu(II) is not adsorbed on γ-Al₂O₃ and natural organic matter is adsorbed. Inert CuL complexes were adsorbed at pH 8.0 on γ-Al₂O₃, ‘Aerosil 200’, CaCO₃, and chrysotile but they were not adsorbed on northupite. The voltammetric method can be recommended for use in natural waters for distinguishing between metal ionic and metal inert complex species which are adsorbed from solution onto various solid particles.     

The effect of pressure on the chemical potentials of seawater components,on ionic species in seawater,and on calcite saturation levels

The activities of most of the major seawater components at 1,001 bars have been estimated, and values for the ions deduced. Equations giving the effect of pressure on the activities of ionic species in seawater ($\text{S = 35‰}$) have been developed. The species covered are: NaSO₄^?, MgSO₄⁰, CaSO₄⁰, H⁺, the free base (NH₃), the HCO₃^?/CO₃² activity ratio and the ion activity product of calcium carbonate. Comparison of the latter with the “ideal” solubility of calcite (pure solid in equilibrium with a mixed electrolyte solution) indicates a degree of saturation compatible with the trends indicated by in situ measurements.     

Calorimetric study of the thermodynamics of formation of MgSO4 and NaSO4− ion pairs at the ionic strength of seawater

The formation of the ion pairs MgSO₄ and NaSO₄^? was investigated calorimetrically at 0.75 M ionic strength, 25°C, 1 atm. Simultaneous determinations of enthalpy changes, ΔH₁⁰, and stability constants, K₁, were not possible, and values of K₁ determined independently had to be introduced for the calculation of ΔH₁⁰. The values of ΔH₁⁰ obtained were 1–3 kJ mol^?1 for MgSO₄ and 0 kJ mol^?1 for NaSO₄^?.     

The effect of pressure on the solubility of amorphous silica in seawater at 0°C

The solubility of amorphous silica in seawater at 0°C and from 1 to 1,220 atm. was found to be a linear function of pressure above 270 atm., but to deviate from linearity below that pressure. Using a quadratic derivation of Planck's equation, ΔV for the dissolution was found to be −16.5 cm³mole⁻¹, and Δk was found to be −4.4 · 10⁻² cm³ mole⁻¹ atm⁻¹∂Δk/∂P was found to be 27.2 · 10⁻⁵ cm³ mole⁻¹ atm⁻² which is too significant a factor to allow the commonly made assumption that ∂Δk/∂P =0. North's (1973) model of hydration suggests that this non-zero ∂Δk/∂P may indicate that the silicic acid molecule is more extensively hydrated at lower pressures.If the pressure in an experiment is suddenly lowered to atmospheric pressure after equilibrium solubility had been attained at the higher pressure, the precipitation that occurs to reduce the resulting supersaturation is complete within one hour in the experimental system used in this study.     

Kinetics of iron(II) oxidation in seawater of various pH

The rate of iron(II) oxidation in North Sea water of pH 5.5–10 in the range 10–25°C has been studied. The oxygenation rate depends linearly on the iron(II) and dissolved oxygen concentrations. The second-order dependence on [OH^?], found by several investigators for synthetic solutions, was confirmed in seawater, but only for pOH > 6.9. For pOH < 5.9 the rate appeared to be independent of the pOH. In the intermediate range, pOH 5.9–6.9, corresponding to the natural pH of seawater, a first-order dependence on the pOH is obeyed. The important discrepancy in the literature between the second-order rate constant for NaCHO₃ solutions and for seawater can be attributed predominantly to the incorrect assumption of a second-order pOH dependence in natural seawater. The results can be useful, for example, in predicting the effect of dumping acidic iron waste from the titanium-dioxide industry into the ocean.     

Stability constants of NaSO4−, MgSO4, MgF+, MgCl+ ion pairs at the ionic strength of seawater by potentiometry

The stability of the ion pair NaSO₄ was determined by measuring the change in sodium activity with medium composition at constant ionic strength, using a sodium-sensitive glass electrode. The stability constants of MgSO₄ and MgCl⁺ were determined indirectly from measurements of the stability of MgF⁺ in different media. All measurements were performed at 1 atm pressure, 25 ± 0.1 °C and 0.7 M formal ionic strength. The stability constants for NaSO₄^?, MgSO₄, MgF⁺ and MgCl⁺ are 1.8 ± 0.1, 6.3 ± 0.1, 22.9 ± 0.1 and 0.34 ± 0.02 M^?1, respectively.     

A note on the polarographic behavior of the Cu(II) ion selective electrode in seawater

The non-Nernstian behavior of a solid-state cupric ion-selective electrode in seawater is attributed to a reduction of Cu²⁺ at the surface in accordance with a suggestion made in the literature. This ‘polarographic’ behaviour was demonstrated by showing that the electrode potential is affected by molecular oxygen in solution.     

Oxidation of copper(I) in seawater at nanomolar levels

The oxidation and reduction of nanomolar levels of copper in air-saturated seawater and NaCl solutions has been measured as a function of pH (7.17–8.49), temperature (5–35 °C) and ionic strength (0.1–0.7 M). The oxidation rates were fitted to an equation valid at different pH and ionic strength conditions in sodium chloride and seawater solutions:

The reduction of Cu(II) was studied in both media for different initial concentrations of copper(II). When the initial Cu(II) concentration was 200 nM, the copper(I) productions were 20% and 9% for NaCl and seawater, respectively. The effect of speciation of copper(I) reduced from Cu(II) on the rates was studied. The Cu(I) speciation is dominated by the CuCl₂⁻ species. On the other hand, the neutral chloride CuCl species dominates the Cu(I) oxidation in the range of 0.1 M to 0.7 M chloride concentrations.     

A review of the effect of salts on the solubility of organic compounds in seawater

The presence of electrolytes (salts) in aqueous solution modifies the solubility and related properties of organic compounds in water. Reported data for salting-out constants (Setschenow constants) which relate solubility to the salt concentration of aromatic and alkane hydrocarbons, and their chlorinated derivatives, and some organic acids have been compiled for 25 aqueous salt solutions at 20–25 °C. The salting-out sequences for various electrolytes are discussed and it is shown that the salting-out effect is greater for organic solutes with large molar volumes. A compilation of salting-out constants for NaCl solutions and seawater (natural or synthetic) with a variety of solutes, shows that the Setschenow constants are similar for natural or artificial seawater (at salinity of 30–35%.) and NaCl solutions (at 3.0–3.5% or 0.5 M). A simple correlation is suggested for estimating the Setschenow constants for a variety of organic solutes in seawater which typically yields a reduction in solubility by a factor of 1.36. The hydrophobicity of organic solutes is therefore increased by this factor, as is the air-water partition coefficient, implying an increased partitioning from aqueous solution into air, organic carbon and lipid phases. The effect must be quantified when comparing the behavior of organic contaminants in freshwater and marine conditions.     

Apparent calcite supersaturation at the ocean surface: Why the present solubility product of pure calcite in seawater does not predict the correct solubility of the salt in nature

It is generally accepted that seawaters near the ocean surface are supersaturated with calcite and that increasing atmospheric CO₂ will not overcome this condition until late in the 21st century. These expectations are based on comparisons of the ion activity products (IAPs) of various seawaters to the solubility product (K) of seawater saturated with pure calcite at 1 atm and a given temperature. It has been shown in recent papers, however, that calcites in contact with natural seawater in the surface oceans are not pure, but are magnesian calcite compositions. As a result, the presently accepted values of the solubility product of calcite in seawater cannot be used to obtain a correct, or even a reasonable estimate of the saturation state of sea water relative to either pure or magnesian calcite. Data are presented demonstrating that use of the currently accepted solubility product of calcite in seawater to determine seawater's relative calcite saturation leads to gross overestimates of its extent.     

Solvent extraction of copper acetylacetonate in studies of copper(II) speciation in seawater

A liquid-liquid partition, ligand exchange procedure involving the formation of copper(II) complexes with acetylacetone is presented for the determination of stability constants and concentrations of copper chelators in seawater. Acetylacetone competes with natural ligands for copper, and the equilibrium concentration of the copper acetylacetonate complex is used in speciation calculations. The concentration of the complex is calculated by partitioning a fraction of it into an organic phase and determining the total Cu concentration in that phase by back extracting with acid, and analyzing by flameless atomic absorption spectroscopy. The concentration of Cu acetylacetonate in seawater in equilibrium with the organic phase is calculated from the partition coefficient. The simple, thermodynamically well characterized procedure offers several advantages over previous techniques. Studies using organic free seawater and model ligands show good agreement between experimental and calculated conditional stability constants. Studies from seawater in Biscayne Bay, Florida, indicate two ligand types are present; type 1, K₁ = 1.2 × 10¹², C_L1 = 5.1 × 10⁻⁹ M; type 2, K₂ = 2.8 × 10¹⁰, C_L2 = 1.1 × 10⁻⁷ M. Speciation is dominated by ligand type 1. Depth profiles of [Cu(II)]_free/[Cu(II)]_total measured with the procedure at ambient copper concentrations show an increase from < 5 × 10⁻⁵ at 50–60 m to > 1 × 10⁻³ at the surface at two stations off the Florida coast.     

The solubility of CaCO3 in seawater at 2°C based upon in-situ sampled pore water composition

Analyses of the concentration product (Ca²⁺) × (CO₃²⁻) in the pore waters of marine sediments have been used to estimate the apparent solubility products of sedimentary calcite (K′_SPc) and aragonite (K′_SPa) in seawater. Regression of the data gives the relation In K′^P_SPc = 1.94 × 10⁻³ δP − 14.59 The 2°C, 1 atm value of K′_SPc is, then, 4.61 × 10⁻⁷ mol² l⁻². The pressure coefficient yields a at 2°C of −43.8 cm³ atm⁻¹. A single station where aragonite is present in the sediments gives a value of K′_SPa = 9.2 × 10⁻⁷ (4°C, 81 atm). The calcite data are very similar to those determined experimentally by Ingle et al. (1973) for K′_SPc at 2°C and 1 atm. The calculated is also indistinguishable from the experimental results of Ingle (1975) if is assumed to be independent of pressure.     

Impact of seawater temperature on growth and recruitment of invasive fouling species at the global scale

Epibenthic fouling communities are dominated by invasive species that are globally distributed and can have substantial ecological and economic impacts in coastal habitats. Little is known about inter‐specific differences in life history strategies that cosmopolitan invasive species employ to acquire space and succeed in invaded habitats. The goal of this study was to examine the impact of seawater temperature on recruitment and growth of several cosmopolitan fouling species including the tunicates Botrylloides violaceus, Botryllus schlosseri and Diplosoma listerianum, as well as the bryozoans Bugula neritina and Watersipora subtorquata. To do this, the iBARGE (Invasive Bryozoan and Ascidian Recruitment and Growth Experiment) program was developed, utilizing a global network of collaborators to examine patterns over a broad geographic scale and a wide range of naturally varying seawater temperatures. This project produced a data set of thousands of photographs from 18 marinas in five countries in summer 2014 and 2015, allowing for recruitment and growth to be tabulated at a variety of temperatures. Thermal growth curves were established for five invasive species, and growth was compared among temperatures across sites, revealing a significant thermal effect. Recruitment was linked to temperature, with generally higher recruitment at warmer seawater temperatures and the highest peak recruitment values for the bryozoan Bugula neritina. Temperature also changed the relative importance of growth and recruitment for several species. These results paint a complex picture of the interactions among invasive fouling species as they relate to seawater temperature.     

The interaction of Np(V)O2+ with common mineral surfaces in dilute aqueous solutions and seawater

The extent and kinetics of Np(V)O₂⁺ adsorption from dilute aqueous solutions and seawater onto a variety of synthetic and natural solids were determined at 25°C and 1 atm total pressure. Extensive and complex adsorption reactions were found, contrary to speculations in the literature that NpO₂⁺ should behave as a simple monovalent ion with a low affinity for surfaces. When normalized to adsorption per unit solid surface area, the ranking for the synthetic solids was aragonite ? calcite > goethite ? MnO₂ ≈ clays. Natural materials generally followed the same behavior patterns as their synthetic counterparts. The dissolved/adsorbed ratio was found to be constant over a wide range (10^?13–10^?7M) of NpO₂⁺ concentrations. At higher concentrations the extent of adsorption decreased until a solubility limit was reached at approximately 10^?5 M.Solution composition had the most significant influence for NpO₂⁺ adsorption on goethite, where much more extensive adsorption occurs in dilute solutions than in seawater. When seawater is added to a dilute solution, extensive desorption of NpO₂⁺ from goethite occurs. Tests conducted on NpO₂⁺ adsorbed on carbonates indicated that it remained in the V oxidation state.There is a growing consensus that Pu dissolved in natural waters also occurs dominantly in the V oxidation state as PuO₂⁺ ion. Consequently, these results for NpO₂⁺ may serve as a guide for Pu behavior when also in the V oxidation state. The fact that most adsorbed Pu is found in the III or IV oxidation states indicates that reduction of Pu may occur subsequent to adsorption in the V oxidation state.     

An ion-association model for estimating acidity constants (at 25°C and 1 ATM total pressure) in electrolyte mixtures related to seawater (ionic strength < 1 mol kg−1 H2O)

The ion-association model of Garrels and Thompson (1962) has been extended in order to incorporate the major acid—base systems present in seawater. The data for the activity coefficients of the various species considered have been presented as a function of ionic strength, thus making it possible to apply the model to solutions with ionic strengths in the range 0–1 mol kg^?1 H₂O. The model therefore allows the prediction of a variety of equilibrium thermodynamic data for natural waters.The model was used to estimate the stoichiometric acidity constants of the major weak acids present in seawater; the conditions simulated correspond to actual measurements made in seawater media at a variety of ionic strengths (salinities). The estimates obtained are generally in good agreement with the experimental results (all within 10% and usually within 5%), thus confirming the usefulness of this model.     

Arctic ocean shelf–basin interaction: An active continental shelf CO2 pump and its impact on the degree of calcium carbonate solubility

The Arctic Ocean has wide shelf areas with extensive biological activity including a high primary productivity and an active microbial loop within the surface sediment. This in combination with brine production during sea ice formation result in the decay products exiting from the shelf into the deep basin typically at a depth of about 150 m and over a wide salinity range centered around S ～33. We present data from the Beringia cruise in 2005 along a section in the Canada Basin from the continental margin north of Alaska towards the north and from the International Siberian Shelf Study in 2008 (ISSS-08) to illustrate the impact of these processes. The water rich in decay products, nutrients and dissolved inorganic carbon (DIC), exits the shelf not only from the Chukchi Sea, as has been shown earlier, but also from the East Siberian Sea. The excess of DIC found in the Canada Basin in a depth range of about 50–250 m amounts to 90±40 g C m^?2. If this excess is integrated over the whole Canadian Basin the excess equals 320±140×10¹² g C. The high DIC concentration layer also has low pH and consequently a low degree of calcium carbonate saturation, with minimum aragonite values of 60% saturation and calcite values just below saturation. The mean age of the waters in the top 300 m was calculated using the transit time distribution method. By applying a future exponential increase of atmospheric CO₂ the invasion of anthropogenic carbon into these waters will result in an under-saturated surface water with respect to aragonite by the year 2050, even without any freshening caused by melting sea ice or increased river discharge.