Thermodynamic aqueous species

Chemistry and thermodynamics of europium and some of its simpler inorganic compounds and aqueous species. J. A. Rand, Chem. Rev., 1985, 85, 555 (343). 

Helgeson, H.C. and Kirkham, D.H. (1976) Theoretical prediction of the thermodynamic behavior of aqueous electrolytes at high pressure and temperatures. Ill Equation of state for aqueous species at infinite dilution. Am. J. Sci., 276, 97-240. 

Are the equilibrium constants for the important reactions in the thermodynamic dataset sufficiently accurate The collection of thermodynamic data is subject to error in the experiment, chemical analysis, and interpretation of the experimental results. Error margins, however, are seldom reported and never seem to appear in data compilations. Compiled data, furthermore, have generally been extrapolated from the temperature of measurement to that of interest (e.g., Helgeson, 1969). The stabilities of many aqueous species have been determined only at room temperature, for example, and mineral solubilities many times are measured at high temperatures where reactions approach equilibrium most rapidly. Evaluating the stabilities and sometimes even the stoichiometries of complex species is especially difficult and prone to inaccuracy. 

The modeler first encounters basis swapping in setting up a model, when it may be necessary to swap the basis to constrain the calculation. The thermodynamic dataset contains reactions written in terms of a preset basis that includes water and certain aqueous species (Na+, Ca++, K+, Cl-, HCOJ, SO4-, H+, and so on) normally encountered in a chemical analysis. Some of the members of the original basis are likely to be appropriate for a calculation. When a mineral appears at equilibrium or a gas at known fugacity appears as a constraint, however, the modeler needs to swap the mineral or gas in question into the basis in place of one of these species. 

Advective transport, or simply advection, refers to movement of chemical mass within a flowing fluid or gas. For our purposes, it is most commonly migration of aqueous species along with groundwater. In constructing a transport model, we prefer to consider how much of the thermodynamic components - the total masses of the basis entries Aw, A(, A, and Am - move, rather than track migration of the free masses of each individual aqueous species. 

We employ the LLNL thermodynamic data for aqueous species, as before, omitting the PbC03 ion pair, which in the dataset is erroneously stable by several orders of magnitude. The reactions comprising the surface complexation model, including those for which equilibrium constants have only been estimated, are stored in dataset FeOH+.dat . 

Johnson, J. W., E. H. Oelkers and H. C. Helgeson, 1991, Supcrt92 a software package for calculating the standard molal thermodynamic properties of minerals, gases, aqueous species, and reactions from 1 to 5000 bars and 0° to 1000 °C. Earth Sciences Department, Lawrence Livermore Laboratory. 

References (20, 22, 23, 24, 29, and 74) comprise the series of Technical Notes 270 from the Chemical Thermodynamics Data Center at the National Bureau of Standards. These give selected values of enthalpies and Gibbs energies of formation and of entropies and heat capacities of pure compounds and of aqueous species in their standard states at 25 °C. They include all inorganic compounds of one and two carbon atoms per molecule. 

Table 8.22 Partial molal thermodynamic properties of organic aqueous species at 25 HKF model, after Shock and Helgeson (1990). 

Discrete values of the HKF model parameters for various organic aqueous species are listed in table 8.22. Table 8.23 lists standard partial molal thermodynamic properties and HKF model parameters for aqueous metal complexes of monovalent organic acid ligands, after Shock and Koretsky (1995). 

Shock E. L. and Helgeson H. C. (1988). Calculations of the thermodynamic and transport properties of aqueous species at hight pressures and temperatures Correlation algorithms for ionic species and equations of state predictions to 5Kb and 1000°C. Geochim. Cosmochim. Acta, 52 2009-2036. 

Presently, thermodynamic data bases for environmental chemistry are far from being complete. We believe that many built-in data bases of geochemical codes that include an impressive number of data for aqueous species and solid phases for most elements may easily produce incorrect results if used without criticism. Indeed, one of the main lessons learnt during our update exercise is that completeness and reliability of the data are mutually exclusive. On the other hand, reducing the data base to a small number of best thermodynamic data severely limits its field of applicability. Thus, in order to model specific systems of fundamental relevance for radioactive waste disposal we were forced to make compromises and had to include estimated constants. 

Not all published and widely accepted thermodynamic values are reliable. Nordstrom and Archer (2003) provide a detailed review of the controversies, uncertainties, and problems related to thermodynamic data for arsenic and its compounds and aqueous species. Many of the data are contradictory and the methods that produce the data are sometimes questionable or have not been thoroughly documented. Too often, data in the literature have been passed from reference to reference without critical evaluations. Some of the data have high measurement errors, were produced under undefined or poorly defined laboratory conditions, and involved unrepresentative sampling (Matschullat 2000, 298 Nordstrom and Archer, 2003). Furthermore, other questionable data originate from obscure documents or are written in languages that many individuals cannot read and properly interpret. Therefore, thermodynamic results must be accepted with a certain amount of caution. The table in this appendix includes thermodynamic data from various sources, which provide users with some idea of their variability. Although sometimes unavoidable, users... 

Table C. 1 Thermodynamic data for arsenic, its compounds, and its aqueous and gaseous species at 1 bar pressure. Note that the units of G and Hi are 1000 times larger than Si and Cpt. 1 kcal = 4.184 kj. ° C = K —273.15. (See Appendix A for other unit conversions). Phases include amorphous (am), aqueous species (aq), gas, liquid (liq), and crystalline solids (xls). 

There is very little known about the neutral vanadate species, V04H3, because it is, at best, only a minor component in aqueous solution [35], The initial protonation of V04H2 at about pH 3 is accompanied by a second protonation that cannot be separated from the first. The result is the formation of a cationic species. Thermodynamic and spectroscopic evidence [33] suggests that formation of this compound is accompanied by incorporation of water to form the octahedral derivative, V02(H20)4+, commonly referred to as V02+. Theoretical calculations also support the assignment of tetrahedral coordination to the monoanion and octahedral geometry to the cationic form of vanadate [36],... 

The representation of chemical composition data in terms of free ionic and complex aqueous species is neither unique nor necessary to a thermodynamic description of the data (cf. Section 1.2). 

The formal similarity between adsorption and complexation reactions can be exploited to incorporate adsorbed species into the equilibrium speciation calculations described in Sections 2.4 and 3.1. To do this, a choice of adsorbent species components (SR r in Eq. 4.3) must be made and equilibrium constants for reactions with aqueous ions must be available. A model for computing adsorbed species activity coefficients must also be selected.8 Once these choices are made and the thermodynamic data are compiled, a speciation calculation proceeds by adding adsorbent species and adsorbed species (SR Mp(OH)yHxLq in Eq. 4.3) to the mole-balance equations for metals and ligands, and then following the steps described in Section 2.4 for aqueous species. For compatibility of the units of concentration, njw) in Eq. 4.2 is converted to an aqueous-phase concentration through division by the volume of aqueous solution. 

See, for example, Chap. 9 in K. Denbigh, The Principles of Chemical Equilibrium, Cambridge University Press, Cambridge, 1981. ThelUPAC recommendation for the symbol to represent rational activity coefficients is yx, which is not used in this book in order to make the distinction between solid solutions and aqueous solutions more evident. In strict chemical thermodynamics, however, all activity coefficients are based on the mole fraction scale, with the definition for aqueous species (Eq. 1.12) actually being a variant that reflects better the ionic nature of electrolyte solutions and the dominant contribution of liquid water to these mixtures. (See, for example, Chap. 2 inR. A. RobinsonandR. H. Stokes,Electrolyte Solutions, Butterworths, London, 1970.)... 

Shock, E. L., Oelkers, E. H., Johnson, J. W., Sveijensky, D. A. Helgeson, H.C. (1992). Calculation of the thermodynamic behavior of aqueous species at high pressures and temperatures effective electrostatic radii, dissociation constants, and standard partial molal properties to 1000 °C and 5 kb. Journal of the Chemical Society (London) Faraday Transactions, 88, 803—26. 

The actual reduction of aqueous species is limited by the potentials corresponding to the oxidation and reduction of water, respectively, given by E = 1.23 - 0.059 pH and E = —0.059 pH, as discussed in Section 2.3.4. However, thermodynamics and kinetics do not always coincide, and a more re-... 

Pokrovskii V. A. and Helgeson H. C. (1995) Thermodynamic properties of aqueous species and the solubilities of minerals at high pressures and temperatures the system AI2O3-HsO-NaCl. Am. J. Sci 295, 1255-1342. 

Sverjensky D. A. (1987) Calculation of the thermodynamic properties of aqueous species and the solubilities of minerals in supercritical electrolyte solutions. In Thermodynamic Modeling of Geological Materials Minerals, Fluids, and Melts, Reviews in Mineralogy (eds. I. S. E. Carmichael and H. P. Eugster). Mineralogical Society of America, Washington, DC, vol. 17, pp. 177-209. 

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