Dissociation constants of protonated cysteine species in seawater media

The dissociation constants (pK₁, pK₂ and pK₃) for cysteine have been measured in seawater as a function of temperature (5 to 45 °C) and salinity (S = 5 to 35). The seawater values were lower than the values in NaCl at the same ionic strength. In an attempt to understand these differences, we have made measurements of the constants in Na–Mg–Cl solutions at 25 °C. The measured values have been compared to those calculated from the Pitzer ionic interaction model. The lower values of pK₃ in the Na–Mg–Cl solutions have been attributed to the formation of Mg²⁺ complexes with Cys²⁻ anions

Mg²⁺ + Cys²⁻ = MgCys

The stability constants have been fitted to

after corrections are made for the interaction of Mg²⁺ with H⁺.The pK₁ seawater measurements indicate that H₃Cys⁺ interacts with SO₄²⁻. The Pitzer parameters β⁰(H₃CysSO₄), β¹(H₃CysSO₄) and C(H₃CysSO₄) have been determined for this interaction. The formation of CaCys as well as MgCys are needed to account for the values of pK₂ and pK₃ in seawater.The consideration of the formation of MgCys and CaCys in seawater yields model calculated values of pK₁, pK₂ and pK₃ that agree with the measured values to within the experimental error of the measurements. This study shows that it is important to consider all of the ionic interactions in natural waters when examining the dissociation of organic acids.     

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.     

The effect of ionic interactions on the oxidation of metals in natural waters

The effect of ionic interactions of the major components of natural waters on the oxidation of Cu(I) and Fe(II) has been examined. The various ion pairs of these metals have been shown to have different rates of oxidation. For Fe(II), the chloride and sulfate ion pairs are not easily oxidized. The measured decrease in the rate constant at a fixed pH in chloride and sulfate solutions agrees very well with the values predicted. The effect of pH (6 to 8) on the oxidation of Fe(II) in water and seawater have been shown to follow the rate equation

$\text{-d in [Fe(II)]/dt = k}{}_1\text{β}{}_1\text{α}{}_{\mathrm{Fe}}\text{/[H}{}^{\mathrm{+}}\text{] + k}{}_2\text{β}{}_2\text{α}{}_{\mathrm{Fe}}\text{/[H}{}^{\mathrm{+}}\text{]}{}^2$

where k₁ and k₂ are the pseudo first order rate constants, β₁ and β₂ are the hydrolysis constants for Fe(OH)⁺ and Fe(OH)⁰. The value of α_FE is the fraction of free Fe²⁺. The value of k₁ (2.0 ±0.5 min^?1) in water and seawater are similar within experimental error. The value of k₂ (1.2 × 10⁵ min^?1) in seawater is 28% of its value in water in reasonable agreement with predictions using an ion pairing model.For the oxidation of Cu(I) a rate equation of the form

$\text{?d ln [Cu(I)]/dt = k}{}_0\text{α}{}_{\mathrm{Cu}}\text{+ k}{}_1\text{β}{}_1\text{α}{}_{\mathrm{Cu}}\text{[Cl]}$

was found where k₀ (14.1 sec^?1) and k₁ (3.9 sec^?1) are the pseudo first order rate constants for the oxidation of Cu⁺ and CuCl⁰, β₁ is the formation constant for CuCl⁰ and α_Cu is the fraction of free Cu⁺. Thus, unlike the results for Fe(II), Cu(I) chloride complexes have measurable rates of oxidation.     

Use of Pitzer's equations to determine the media effect on the formation of lead chloro complexes

The spectrophotometric measurements of chloro complexes of lead in aqueous HCl, NaCl, MgCl₂ and CaCl₂ solutions at 25°C have been analyzed using Pitzer's specific interaction equations. Parameters for activity coefficients of the complexes PbCl⁺, PbCl₂⁰ and PbCl₃^? have been determined for the various media. Values of $\text{K}{}_1\text{= 30.0 ± 0.6}$, $\text{K}{}_2\text{= 106.7 ± 2.1}$ and $\text{K}{}_3\text{= 73.0 ± 1.5}$ were obtained for the cumulative formation constants. [$\text{Pb}{}^{\mathrm{2+}}\text{+ nCl}{}^{\mathrm{?}}\text{→ PbCl}{}_{\mathrm{n}}{}^{\mathrm{2?n}}\text{)}$]. These values are in reasonable agreement with literature data. The Pitzer parameters for the PbCl ion pairs in various media were used to calculate the speciation of Pb²⁺ in an artificial seawater solution.     

The Equation of State of Lakes

In recent years, a number of workers have studied the stability of deep lakes such as Lake Tanganyika, Lake Baikal and Lake Malawi. In this paper, the methods that can be used to determine the effect that the components of lakes have on the equation of state are examined. The PVT properties of Lakes have been determined by using apparent molal volume data for the major ionic components of the lake. The estimated PVT properties (densities, expansibility and compressibilities) of the lakes are found to be in good agreement with the PVT properties (P) of seawater diluted to the same salinity. This is similar to earlier work that showed that the PVT properties of rivers and estuarine waters could also be estimated from the properties of seawater.The measured densities of Lake Tanganyika were found to be in good agreement (± 2 × 10^-6 g cm^-3) with the values estimated from partial molal properties and the values of seawater at the same total salinity (S_T = 0.568). The increase in the densities of Lake Tanganyika waters increased due to changes in the composition of the waters. The measured increase in the measured density (45 × 10^-6 g cm^-3) is in good agreement (46 × 10^-6 g cm^-3) with the values calculated for the increase in Na⁺, HCO₃ ^-, Mg²⁺, Ca²⁺ and Si(OH)₄.Methods are described that can be used to determine the conductivity salinity of lakes using the equations developed for seawater. By combining these relationships with apparent molal volume data, one can relate the PVT properties of the lake to those of seawater.     

The activity coefficients of Fe(III) hydroxide complexes in NaCl and NaClO4 solutions

The osmotic coefficients of FeCl₃ at 25 °C from 0.15 to 1.7 m [Rumyantsev et al., Z. Phys. Chem., 218, 1089-1127, 2004] have been used to determine the Pitzer parameters (β⁽⁰⁾, β⁽¹⁾ and C^?) for FeCl₃. Since the differences in the Pitzer coefficients of rare earths in NaCl and NaClO₄ are small, the values of Fe(ClO₄)₃ have been estimated using the differences between La(ClO₄)₃ and LaCl₃. The Pitzer coefficients for FeCl₃ combined with enthalpy and heat capacity data for the rare earths can be used to estimate the activity coefficients of Fe³⁺ in NaCl over a wide range of temperatures (0 to 50 °C) and ionic strength (0 to 6 m).The activity coefficients of Fe³⁺ in NaCl and NaClO₄ solutions have been used to determine the activity coefficients of Fe(OH)²⁺ in these solutions from the measured first hydrolysis constants of Fe³⁺ [Byrne et al., Mar. Chem., 97, 34-48, 2005]. The activity coefficients of , Fe(OH)₃ and from 0 to 50 °C have also been determined from the solubility measurements of Fe(III) in NaCl solutions [Liu and Millero, Geochim. Cosmochim Acta, 63, 3487-3497, 1999]. These activity coefficients have been fitted to the Pitzer equations. These results can be used to estimate the speciation of Fe(III) with OH⁻ in natural waters with high concentrations of NaCl from 0 to 50 °C.     

Density and conductivity properties of Na−CO3−Cl−SO4 brine from Mono Lake, California, USA

Relationships among conductivity, temperature, total dissolved solids (TDS), and density were determined for hypersaline brine (TDS, 83 g kg⁻¹) from Mono Lake. A theoretical estimate of density based on ionic composition and calculated partial molar volumes compared well with actual measurements (difference, 390 ppm). However, an empirically derived equation of state based on temperature and conductivity provides the most accurate density estimates (residual errors, 190 ppm) and can be used with data collected with CTD profilers.     

An Equation of State for Hypersaline Water in Great Salt Lake,Utah, USA

Great Salt Lake (GSL) is one of the largest and most saline lakes in the world. In order to accurately model limnological processes in GSL, hydrodynamic calculations require the precise estimation of water density (ρ) under a variety of environmental conditions. An equation of state was developed with water samples collected from GSL to estimate density as a function of salinity and water temperature. The ρ of water samples from the south arm of GSL was measured as a function of temperature ranging from 278 to 323 degrees Kelvin (^oK) and conductivity salinities ranging from 23 to 182 g L⁻¹ using an Anton Paar density meter. These results have been used to develop the following equation of state for GSL (σ = ± 0.32 kg m⁻³):

The dissociation constants of carbonic acid in seawater at salinities 5 to 45 and temperatures 0 to 45°C

The pK₁^* and pK₂^* for the dissociation of carbonic acid in seawater have been determined from 0 to 45°C and S = 5 to 45. The values of pK₁^* have been determined from emf measurements for the cell:

Pt](1 − X)H2 + XCO2|NaHCO3, CO2 in synthetic seawater|AgC1; Ag

where X is the mole fraction of CO₂ in the gas. The values of pK₂^* have been determined from emf measurements on the cell:

Pt, H2(g, 1 atm)|Na2CO3, NaHCO3 in synthethic seawater|AgC1; Ag

The results have been fitted to the equations:

lnK^*₁ = 2.83655 − 2307.1266/T − 1.5529413 lnT + (−0.20760841 − 4.0484/T)S^0.5 + 0.08468345S − 0.00654208S¹

InK^*₂ = −9.226508 − 3351.6106/T− 0.2005743 lnT + (−0.106901773 − 23.9722/T)S^0.5 + 0.1130822S − 0.00846934S^1.5

where T is the temperature in K, S is the salinity, and the standard deviations of the fits are σ = 0.0048 in lnK₁^* and σ = 0.0070 in lnK₂^*.Our new results are in good agreement at S = 35 (±0.002 in pK₁^*and ±0.005 in pK₂^*) from 0 to 45°C with the earlier results of Goyet and Poisson (1989). Since our measurements are more precise than the earlier measurements due to the use of the Pt, H₂|AgCl, Ag electrode system, we feel that our equations should be used to calculate the components of the carbonate system in seawater.     

Chromium(VI) reduction by sulphur(IV) in aqueous solutions

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 ΔH_app 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