Dhanpat Rai

The solubility of (Ba,Sr)SO4 precipitates: Thermodynamic equilibrium and reaction path analysis

The solubility of (Ba,Sr)SO4 precipitates, varying in SrSO4 mole fraction from 0.05–0.90, was investigated at room temperature with an equilibration period extending to almost three years. The data show that on or before 315 days of equilibration the precipitates reach a reversible equilibrium with the aqueous solution. The reversibility of this equilibrium was verified both by the attainment of steady-state concentrations with time and by heating the samples to perturb the equilibrium and then observing the slow return to the initial equilibrium state. n nThe dissolution of the (Ba,Sr)SO4 precipitates does not, in general, follow limiting reaction paths as defined by the Lippmann solutus or stoichiometric dissolution curves. In addition, activity coefficient calculations for the BaSO4 and SrSO4 components of the solid phase, using either total bulk analysis or near-surface analysis of the component mole fractions, do not satisfy the Gibbs-Duhem equation, demonstrating that a single solid-solution phase does not control both the aqueous Ba and Sr concentrations. Instead, our long-term equilibration data can be explained by the unavoidable formation of small amounts of barite and substitution of Sr into a solid-solution phase with the BaSO4 component of the solid-solution phase never reaching thermodynamic equilibrium with the aqueous phase.

The solubility of barite and celestite in sodium sulfate: Evaluation of thermodynamic data

The solubilities of barite [BaSO4(c)] and celestite [SrSO4(c)] in Na2SO4 were studied and found to be significantly lower than the experimental values reported in the literature. Our new solubility data are in excellent agreement with the predictions of ion interaction models, which have previously been parameterized primarily from solubility data obtained in chloride media. Our solubility data were analyzed both in terms of aqueous thermodynamic models that included ion association species and in terms of ion interaction models that did not require the explicit recognition of such species. In the case of SrSO4, although both ion association and ion interaction models can accurately model our solubility data, the ion interaction approach is preferred because it is easier to extend to higher concentrations. In the case of BaSO4, the aqueous ion interactions appear to be stronger than those for SrSO4, and so the explicit recognition of a BaSO4(aq) ion association species is preferred. The logarithms of the thermodynamic solubility products (log Ksp) for celestite and barite were −6.62±0.02 and −10.05±0.05, respectively. When the data were analyzed using models that include ion association species, the logarithms of the thermodynamic equilibrium constants for the SrSO4(aq) and BaSO4(aq) association reactions were 1.86±0.03 and 2.72±0.09, respectively.

Thermodynamics of the U(VI)-Ca2+-Cl- -OH- -H2O System: Solubility Product of Becquerelite

Summary The solubility of synthetic becquerelite (Ca(UO2)6O4(OH)6·8H2O) was determined in 0.02, 0.1, and 0.5 M CaCl2 solutions and at pCH+ values ranging from approximately 4 to 11. The presence of becquerelite in equilibrated samples was confirmed by a combination of techniques involving X-ray diffraction, total chemical composition, and analyses of solubility data. The solubility data were interpreted using Pitzers aqueous thermodynamic model and the thermodynamic data for U(VI) species available in the literature. The log of the solubility product for becquerelite [Ca(UO2)6O4(OH)6·8H2O + 14H+ ⇌ Ca2+ + 6UO22+ + 18H2O] was determined to be 41.4 ± 0.2. This value is similar to the values previously reported for other synthetic becquerelites, but is drastically different from a value reported for a natural sample.

Solubility of Crystalline Calcium Isosaccharinate

The solubility of calcium isosaccharinate Ca(ISA)2(c) was determined at 23°C as a function of pH (1–14) and calcium ion molality (0.03–0.52). The similarity of solubility from the over- and undersaturation directions for different equilibration periods indicated that equilibrium in these solutions was reached rapidly (< 7 days) and that these data can be used to develop thermodynamic equilibrium constants. The solubility data were interpreted using the Pitzer ion–interaction model. The logarithms of the thermodynamic equilibrium constants determined from these data were 1.30 for the dominant reaction at pH < 4.5 [Ca(ISA)2(c) + 2H+ ⇌ Ca2+ + 2HISA(aq)], and −2.22 for the dominant reaction at 4.5 [Ca(ISA)2(c)+ ⇌ Ca(ISA)2(aq)]. In addition, the logarithm of the dissociation constant of HISA [HISA(aq) ⇌ ISA- + H+] was calculated to be −4.46.

Hydrolysis constants and ion-interaction parameters for Cd(II) in zero to high concentrations of NaOH−KOH, and the solubility product of crystalline Cd(OH)2

AbstractThe solubility of Cd(OH)2(c) was studied in 0.01M NaClO4 solutions, from both the over- and the undersaturation directions, with OH− ion concentration ranging from 10−6 to 1.0 mol-L−1, and the equilibration period ranging from 2 to 28 days. Equilibrium Cd concentrations were reached in less than 2 days. The Cd(OH)2(c) solubility showed an amphoteric behavior. In the entire range of OH−/H+ investigated, the only dominant aqueous Cd(II) species required to explain the solubility of Cd(OH)2(c) are Cd2+, Cd(OH)20, and Cd(OH)42−. The logarithms of the thermodynamic equilibrium constants of the Cd(OH)2(c) solubility reactions involving these species, that is, the reactionsn

Strontium isotopic characterization of soils and coal ashes

begin{gathered} {text{ }}Cd(OH)_2 (c) rightleftarrows Cd^{2 + } + 20H^ - ,{text{ }}Cd(OH)_2 (c) rightleftarrows Cd(OH)_2^0 , hfill and Cd(OH)_2 (c) + 20H^ - rightleftarrows Cd(OH)_4^{2 - } hfill end{gathered}

Thermodynamic model for aqueous Cd2+−CO32− ionic interactions in high-ionic-strength carbonate solutions, and the solubility product of crystalline CdCO3

n were found to be −14.14±0.21, −7.04±0.21, and −5.62±0.32, respectively. The ion-interaction parameters reported in the literature, in conjunction with the values for Cd(OH)20−Na+(−0.20), Cd(OH)42−−Na+ (β0 = 0.41, β1 = 0.7), and Cd(OH)42−−K+ (β0 = 0.44, β1 = 1.44) obtained in this study, show that our low-ionic strength solubility data are also consistent with Cd(OH)2(c) solubility data obtained in solutions as concentrated as 10M in NaOH or KOH and 7M in Na(OH, ClO4).

An aqueous thermodynamic model for Ca2+−SO 3 2− ion interactions and the solubility product of crystalline CaSO3·1/2H2O

The solubility of powellite [CaMoO4(c)] was studied in aqueous Na2MoO4, CaCl2 and Ca(NO3)2 solutions ranging in concentrations from 1×10−4M to 1.0M and over equilibration times extending to 36 days. Our experimental data were interpreted using the aqueous ion-interaction model of Pitzer and coworkers. The Ca2+−MoO42− ion-interactions were found to be analogous to Ca2+−SO42−. The use of Ca2+−MoO42− ion-interactions parameters (β(0)=0.2, β(1) = 3.1973 and β(2)) and a logKsp of −7.93 gave excellent predictions of all of the experimental data. Commonion ternary interaction parameters such as MoO42−−Cl− or MoO42−−NO3− were not required.