LJUNGSKILE: a program for assessing uncertainties in speciation calculations

Speciation calculations are often the base upon which further and more important conclusions are drawn, e.g., solubilities and sorption estimates used for retention of hazardous materials. Since speciation calculations are based on experimentally determined stability constants of the relevant chemical reactions, the measurement and experimental uncertainty in these constants will affect the reliability of the simulation output. The present knowledge of the thermodynamic data relevant for predicting the behaviour of a complex chemical system is quite heterogeneous. In order to predict the impact of these uncertainties on the reliability of a simulation output requires sophisticated modelling codes. In this paper, we will present a computer program, LJUNGSKILE, which utilises the thermodynamic equilibrium code PHREEQC to statistically calculate uncertainties in speciation based on uncertainties in stability constants. A short example is included.     

Thermodynamics of saturated lanthanide nitrate-water systems

The thermodynamic properties of saturated aqueous lanthanide nitrate solutions were determined using recently published critically evaluated solubility and activity data. The variation of the thermodynamic functions and congruent melting points as a function of atomic number are interpreted in terms of changes in inner sphere coordination number in both the solid hexahydrates and in the aquo ions, and in terms of the double-double effect.Inconsistencies in experimental solubility data are generally caused by uncertainties in solid phase composition which is shown to be due to the very small Gibbs energies accompanying transitions from stable to metastable systems differing in the number of hydrating water molecules.Presentation to First International Symposium on Solubility Phenomena, University of Western Ontario, London, Ontario, August 21–23, 1984.     

A retraceable method to assess uncertainties in solubility estimations exemplified by a few americium solids

An important safety function in ensuring nuclear waste safety is the possible formation of solubility limiting phases that incorporate actinides and other critical nuclides. For a credible demonstration of such a safety function, it is essential that uncertainties related to the choice of models, calculation cases, and parameter values are evaluated. In this paper, a method is outlined to evaluate thermodynamic data uncertainty and to analyse situations in the presence of uncertainty. Americium(III) solubility in an Äspö groundwater is taken as an illustrative example. Methods include a sensitivity analysis to single out critical parameters and a Monte Carlo analysis to provide an empirical probability distribution of americium solubility taking into account relevant uncertainty contributions. The analysis shows that the solubility products of ²⁴¹Am(III) phases are the most sensitive parameters. Aqueous ²⁴¹Am(III) species, on the contrary, are of less importance while the stability of several other complexes incorporating major element ions have considerable impact.     

Thorium and americium solubilities in cement pore water containing superplasticiser compared with thermodynamic calculations

The solubility of thorium and americium in pore water squeezed from a cement paste was investigated by a batch method from oversaturation. The cement paste was prepared by mixing ordinary Portland cement with deionised water; in some cases the deionised water contained a polycarboxylic acid–base type superplasticiser. Following solidification, pore water was squeezed from the cement paste and collected for use in the solubility experiments. The aim of these was to investigate whether there was any effect of superplasticiser on the solubility of thorium and americium in the squeezed cement pore waters. The obtained solubility values in the two squeezed pore waters (with and without superplasticiser present) were similar. Thermodynamic calculations were performed with the thermodynamic database developed by Japan Atomic Energy Agency and compared with the experimental data to verify their applicability. These results showed that the superplasticiser used in the present study after mixing with the cement paste did not have a significant effect on solubility of thorium and americium, and the thermodynamic calculations were applicable in the present system. Size distribution of colloidal species of thorium and americium was also investigated.     

Spectrophotometric study of the solubility and the protolytic properties of 1-(2-pyridylazo)-2-naphthol in different ethanol–water solutions

The solubility and the protolytic constants of 1-(2-pyridylazo)-2-naphthol (PAN) have been accurately determined at temperatures between 20.0 and 30.0°C in ethanol–water solutions with ethanol concentrations ranging from 10.0% to 95.0% (v/v). The measurement of the protolytic constants is based on the spectrophotometric determination of the concentrations of the corresponding conjugate acid–base pairs in ethanol–water solutions containing PAN and hydrochloric acid or sodium hydroxide. The traditionally used pH measurements in such studies in non-aqueous solutions are thus avoided. The solubility and the protolytic constants of PAN in pure water have been calculated by extrapolating the ethanol–water results to pure water where PAN is practically insoluble. The determination of the thermodynamic data mentioned above is necessary for elucidating the interactions of PAN with various metal ions in solutions and in cation-exchange membranes (e.g., Nafion^®) used as PAN-based optodes.     

Thermodynamic Approach for Predicting Actinide and Rare Earth Concentrations in Leachates from Radioactive Waste Glasses

Studies aimed primarily at determining leach rates of different elements from doped glasses have resulted in computerized models for predicting leachate concentrations. However, leach rate related data should be limited to predicting the stability behavior of the glass matrix; the radionuclide release data based on these studies are empirical and are highly dependant on many variables and processes which have not been systematically evaluated and thus do not provide a reliable method of predicting leachate concentrations. A better approach is available for those elements that can readily form relatively insoluble solids during preparation of glass or glass/water interactions. This alternate approach relies on the experimental solubilities of pulverized doped glasses, in a wide range of well-controlled important variables such as pH and pe, and their comparisons at the given aqueous composition to predicted solubilities of known solid phases from the thermodynamic data. These comparisons are used to indirectly identify specific solubility-controlling solids in doped glass/water systems to determine scientifically defensible aqueous concentrations of different elements for any given groundwater composition, independent of glass dissolution kinetics and independent of time. This paper summarizes data available for the application of this alternate approach to reliably predict concentrations of thorium, uranium, neptunium, plutonium, and trivalent actinides and rare earth elements leachable from the doped glasses. The thermodynamic data, in addition to that reported in recent critical reviews, includes new data that were developed for the solubility products of Th₃(PO₄)₄(s) and the solid solutions of trivalent actinides and rare earth hydroxides. Thermodynamic interpretations of the doped glass solubility data show specifically that tetravalent actinide hydrous/crystalline oxides and solid solutions of trivalent actinides and rare earths hydroxides in non-phosphate glasses and Th₃(PO₄)₄(s) and MPO₄(s), where M denotes trivalent actinides or rare earths, in phosphate-containing glasses are the dominant solubility-controlling solids. Needed future research in this area is briefly outlined.     

Calcium carbonate solubility: a reappraisal of scale formation and inhibition

Considerable disparity exists in the published thermodynamic data for selected species in the Ca(2+) /CO(2)/H(2)O system near 25 degrees C and 1 atm pressure. Some authors doubt the significance of CaCO(3)(0)aq) complexes although there is experimental evidence of their occurrence. Evaluation of all the published experimental and estimated data for aqueous calcium carbonate species confirms that the consistent set of constants given by Plummer and Busenberg in 1982 is the best available, and suggests a formation constant log beta = 3.22 for CaCO(3)(0)(aq). This value was confirmed by additional experimental data and calculations using a specially developed computer program. The solubility s and solubility product K(s) are critically evaluated for each solid polymorph (amorphous CaCO(3), ikaite, vaterite, aragonite and calcite) using a hydrated ion pair model and we give coherent explanations for the calcium carbonate precipitation/dissolution process and the existence of supersaturated waters. The practical cases of scale formation and its inhibition by phosphonate-type compounds are discussed and explained with the same model, taking into account the CaCO(3)(0)(aq) species.     

Interactions of fluorinated gases with ionic liquids: solubility of CF4, C2F6, and C3F8 in trihexyltetradecylphosphonium bis(trifluoromethylsulfonyl)amide

The interactions between ionic liquids and totally fluorinated alkanes are investigated by associating gas solubility measurements with molecular simulation calculations. Experimental values for the solubility of perfluoromethane, perfluoroethane, and perfluoropropane in one ionic liquidtrihexyltetradecylphophonium bis(trifluoromethylsulfonyl)amide [P 6,6,6,14][Ntf 2]are reported between 303 and 343 K and close to atmospheric pressure. All mole fraction solubilities decrease with increasing temperature. From the variation of Henry's law constants with temperature, the thermodynamic functions of solvation were calculated. The precision of the experimental data, considered as the average absolute deviation of the Henry's law constants from appropriate smoothing equations, is always better than +/-3%. By the analysis of the differences between the solute-solvent radial distribution functions of perfluoromethane and perfluoropropane obtained by molecular simulation, it was possible to explain why solubility increases with the size of the perfluoroalkane. The trend of solubility is explained on the basis of the location of the solute with respect to the solvent ions as well as on the differences in the solute-solvent energies of interaction.     

Nickel Aqueous Speciation and Solubility at Ambient Conditions: A Thermodynamic Elegy

Summary.  This review reveals that, in contrast to the general opinion, the aqueous speciation of nickel is poorly known. Besides the fairly well established first hydrolysis constant, data are scarce and only poor estimates can be derived for higher Ni hydrolysis constants from a few solubility studies. The situation is even worse in the case of aqueous carbonate complexes. No reliable experimental study has been published so far and almost all numbers reported in thermodynamic databases are unacceptable estimates. In this review we scrutinise all these published estimates and propose expectation ranges of nickel carbonate complex stability through correlation with other known thermodynamic constants. Solubility constants for a few simple nickel solids are known or have been estimated from thermochemical data. However, none of these simple solids is of geochemical relevance at ambient conditions. Based on field evidence, classes of solids are identified which potentially govern nickel concentrations in ground and surface waters. Recent spectroscopic data indicate that magnesium clay minerals and layered double hydroxides are the most prominent candidates for nickel-bearing solids at ambient conditions. Corresponding author. E-mail: wolfgang.hummel@psi.ch Dedicated to the memory of Prof. Rolf Grauer Received January 14, 2003; accepted January 24, 2003 Published online May 15, 2003     

The Solubility of the Chloromethanes in Water

Summary.  There are more than 150 papers that report on the solubility of the four chloromethanes in water. Volume 62 of the Solubility Data Series compiles and evaluates the papers concerned with the liquid–liquid solubility of the chloromethanes with water. A similar evaluation has not been carried out for the vapor–liquid solubility at a partial pressure up to the saturation vapor pressure of the four chloromethanes. This paper uses the liquid–liquid evaluated solubility values to calculate vapor–liquid Henry’s constants. They are compared with a compilation of Henry’s constants with good agreements between the calculated constants and the median of the compiled constants. It is believed that the calculated constants are a reliable set of values to use for the vapor–liquid solubility of the chloromethanes in water. E-mail: hlclever@worldnet.att.net Received October 11, 2002; accepted October 22, 2002 Published online April 24, 2003 RID="a" ID="a" Dedicated to Prof. Dr. Heinz Gamsj?ger on the occasion of his 70^th birthday     

Conductivity of Sodium Chloride in Water + 1,4-Dioxane Mixtures from 5 to 35°C II. Concentrated Solution

Conductivity data of sodium chloride in binary mixtures of water and 1,4-dioxanefrom 5 to 35°C were measured, covering an electrolyte concentration range upto the limit of solubility in the solvent mixtures and up to 5 mol-dm^–3 inpure water. Data analysis is based on the mean spherical approximation (MSA).Comparison is made with the data representation by the empirical Casteel-Amisequation. The association constants of the MSA are compared with those fromchemical model calculations at low concentrations (lcCM).     

The theoretical basis of mercurimetry in non-aqueous solutions

The methods for thermodynamic calculations of the equilibria in solutions of mercury salts and complexes are presented. The calculations of equilibrium constants in non-aqueous solvents are based on the transfer activity coefficients of mercury ions from water to the non-aqueous solvent. The dismutation and precipitation reaction constants are calculated, and the redox potentials of mercury systems are measured. Examples of analytical use of the thermodynamic functions of mercury salt solvation are given in the text.     

Mutual Solubilities of Water and Alkanes

Summary.  The evaluation of mutual solubility data for systems water with n-alkanes, isoalkanes, and cycloalkanes along the three phase line is reported and a formula for the prediction of solubility of alkanes in water is developed. Then a cubic equation of state with an added term, which accounts for hydrogen bonding is used for correlation of liquid–liquid equilibrium data and for prediction of solubility of water in hydrocarbons using alkane in water solubility data. Comparison of the predicted and experimental solubilities is performed using all accessible experimental data. With this approach it is possible to predict the solubilities of water in alkanes with good accuracy over the temperature range up to about 20 K below critical temperature. Solubility of alkanes in water can also be calculated using experimental data for solubility of water in alkanes but results of these calculations are more sensitive to experimental errors of the data. Corresponding author: E-mail: macz@ichf.edu.pl Received August 5, 2002; accepted (revised) September 13, 2002 Published online March 13, 2003 RID="a" ID="a" Dedicated to Prof. Dr. H. Gamsj?ger on the occasion of his 70^th birthday anniversary     

Determination of thermodynamic acidity constants and limiting ionic mobilities of weak electrolytes by capillary electrophoresis using a new free software AnglerFish

Thermodynamic acidity constants (acid or acid-base dissociation constants, sometimes called also as ionization constants) and limiting ionic mobilities (both of them at defined temperature, usually 25°C) are the fundamental physicochemical characteristics of a weak electrolyte, that is, weak acid or weak base or ampholyte. We introduce a novel method for determining the data of a weak electrolyte by the nonlinear regression of effective electrophoretic mobility versus buffer composition dependence when measured in a set of BGEs with various pH. To correct the experimental data for zero ionic strength we use the extended Debye-Hückel model and Onsager-Fuoss law with no simplifications. Contrary to contemporary approaches, the nonlinear regression is performed on limiting mobility data calculated by PeakMaster's correction engine, not on the raw experimental mobility data. Therefore, there is no requirement to perform all measurements at a constant ionic strength of the set of BGEs. We devised the computer program AnglerFish that performs the necessary calculations in a user-friendly fashion. All thermodynamic pKa values and limiting electrophoretic mobilities for arbitrarily charged substances having any number of ionic forms are calculated by one fit. The user input consists of the buffer composition of the set of BGEs and experimentally measured effective mobilities of the inspected weak electrolyte.     

CO2 solubility in aqueous solutions of N-methyldiethanolamine+piperazine by electrolyte NRTL model

Accurate modeling of the solubility behavior of CO₂ in the aqueous alkanolamine solutions is important to design and optimization of equipment and process. In this work, the thermodynamics of C_O2 in aqueous solution of N-methyldiethanolamine (MDEA) and piperazine (PZ) is studied by the electrolyte non-random two liquids (NRTL) model. The chemical equilibrium constants are calculated from the free Gibbs energy of formation, and the Henry’s constants of CO₂ in MDEA and PZ are regressed to revise the value in the pure water. New experimental data from literatures are added to the regression process. Therefore, this model should provide a comprehensive thermodynamic representation for the quaternary system with broader ranges and more accurate predictions than previous work. Model results are compared to the experimental vapor-liquid equilibrium (VLE), speciation and heat of absorption data, which show that the model can predict the experimental data with reasonable accuracy.     

Thermodynamic study in aqueous solutions of weakly soluble ionic compounds

Any investigation for a better knowledge of precipitation/dissolution problems necessitates the availability of all the beta formation constants of the uncharged soluble species (ion-pair). Several difficulties dealing with solubility measurements are briefly reviewed, especially related to phase structure variations, time-lag or supersaturation phenomena. Thanks to some thermodynamic considerations, the evolution of the uncharged soluble species with hydration and solid phase modifications can give a new explanation about the observed dispersion in literature values for some weakly soluble ionic compounds. When not given elsewhere, the evaluation of thermodynamic data of interest (formation constants, solubility product, etc.) is made possible according to given methods.     

Thermodynamic Model for Amorphous Pd(OH)2 Solubility in the Aqueous Na+–K+–H+–OH?–Cl?–ClO 4 ? –H2O System at 25?°C: A Critical Review

Only a limited number of experimental investigations have been conducted to determine hydrolysis constants of Pd(II) or the solubility product of Pd(II) hydroxide, and the reported values differ considerably from each other. No comprehensive reliable thermodynamic model is available for the Pd?COH and Pd?CCl systems. To obtain such a model, thermodynamic data for palladium compounds and complexes with chloride and hydroxide were critically evaluated using the SIT model. These evaluations, in most cases, involved reinterpretation of the original data reported in various publications to produce values of equilibrium constants for various reactions that are consistent with all of available reliable literature data. Final recommended values for solubility products and complexes of Pd(II) with hydroxide and chloride, along with the values for SIT ion interaction parameters, are tabulated.     

Occurrence of the radionuclides in groundwater of crystalline hard rock regions of central Tamil Nadu,India

A study was conducted to understand the occurrence of the radionuclides in groundwater of crystalline hard rock region. Samples were collected to analyze major cations, anions, U, ²²²Rn and stable isotopes of oxygen, hydrogen. It was inferred that few samples have U and ²²²Rn concentrations higher than the permissible limit of drinking water standard. High degree of weathering of granitic rocks and long contact time of groundwater with the aquifer matrix could be the reason for enhanced U and ²²²Rn levels in groundwater. The association of U with SO₄ also proves that there exists anthropogenic influence in groundwater composition.     

Application of automated docking to the binding of naphthalenes to βCD in water: correlation with spectrofluorimetric data

Automated semi-rigid docking has been explored as an alternative approach for the theoretical study of the inclusion complexes with cyclodextrins. To this purpose we have chosen as a model for the binding to βCD some naphthalene derivatives (naphthalene, 2-ethylnaphthalene, 2-acetylnaphthalene, 1-naphthyl acetate, 2-naphthyl acetate and 1-naphthol). For comparison purposes, the binding constants in water and the associated thermodynamic parameters have been obtained under the same experimental conditions by steady-state fluorescence spectroscopy. The calculations of the automated docking regarding the topology of the guest inside the cavity produce a cluster of structures that qualitatively agrees with fluorescence results and literature data. However, the predicted values of the free energy of binding are lower than the experimental ones by ca. −10 kJ mol⁻¹, and very close to the experimental enthalpy of binding deduced from the temperature dependence of the association constants. The differences are ascribed mainly to the assumption of rigidity of the CD into the auto-docking scheme.     

Uncertainties in chemical modelling of solubility, speciation and sorption

Chemical analyses become more and more expensive to perform, and more and more research is based on numerical simulation. However, all numerical models are subject to uncertainties, either in the implementation of the model towards reality or simply that the desired input data are not explicitly known without uncertainties. These uncertainties will affect the predictability of any model and thus it is of vital interest that the effect of these uncertainties is known. Especially if the result of the simulations is a topic upon which serious decisions are going to be made. In this paper, we outline a simple and rather straight forward approach to the effect of uncertainties on such simple chemical calculations as solubility of a solid phase in a given water and the chemical speciation of a solution. In addition, we also touch upon the much more complicated matter of uncertainties in sorption modelling??a subject which will be enlightened in much greater detail in an upcoming NEA publication in the matter.