Speciation Equilibrium

Equilibrium calculations for electrolyte solutions include speciation equilibrium, vapor-liquid equilibrium, solid-liquid equilibrium, and liquid-liquid equilibrium. As an example of the first three types of equilibria, we will consider the ternary H2O-NH3-CO2 system. 

Chemical and phase equilibria are attained when the chemical potential of the reactants equals that of the products. This criterion for equilibrium can be expressed by the mass action law as 

AG° is the change in standard state Gibbs energy by the process, calculated from the standard state chemical potentials that can be found tabulated. R is the gas constant, T is the temperature in Kelvin, a, is the activity of species i, and Vj is the stoichiometric coefficient of component i in the process under consideration. Components on the left-hand side in the process equation are given negative stoichiometric coefficients. Usually, only the values of the chemical potentials at 298.15K are found in tables. It is then necessary to calculate the value of AG at the temperature of interest. This can be done by using the Gibbs-Helmholtz equation  

The variation of AW , the change in standard state enthalpy by the process, with temperature can be calculated from the heat capacity of the species involved in the process.  

The term speciation is used to describe the reactions that take place when an electrolyte is dissolved in water. Water dissociates, sour gases hydrolyze, some ions dissociate, and other ions associate until thermodynamic equilibrium is attained. The liquid phase of the ternary H2O-NH3-CO2 system contains at least the following nine species HjO, NH3(aq), COjiaq), H , OH, NH4, HCOj, COj , and NHjCOO. (aq) indicates that the species is in aqueous solution to avoid ambiguity. In order to adequately model this system, interaction parameters for the interaction between each pair of species need to be determined thus, speciation calculations are performed simultaneously with the parameter estimation, and the calculated amount of each species is compared with experimental data. Some models also require ternary parameters and consequently an additional amount of data to determine these parameters. 

---

The solution species were characterised in terms of ionic molar mobility, diffusivity, mobility, and hydrated ion radius prior to speciation. Equilibrium pH of the backffound solution was estimated as a function of gas type and operatingpressure. 

Allison, J. D., Brown, D. S. Novo-Gradac, K. J. 1990. M1NTEQA2 metal speciation equilibrium model for surface and groundwater, version 3.00. Center for Exposure Assessment Modelling, U.S. EPA, Athens, Georgia. 

In Figure 2 the solubility and speciation of plutonium have been calculated, using stability data for the hydroxy and carbonate complexes in Table III and standard potentials from Table IV, for the waters indicted in Figure 2. Here, the various carbonate concentrations would correspond to an open system in equilibrium with air (b) and closed systems with a total carbonate concentration of 30 mg/liter (c,e) and 485 mg/liter (d,f), respectively. The two redox potentials would roughly correspond to water in equilibrium wit air (a-d cf 50) and systems buffered by an Fe(III)(s)/Fe(II)(s)-equilibrium (e,f), respectively. Thus, the natural span of carbonate concentrations and redox conditions is illustrated. 

Eh-pH diagram for the speciation of plutonium in equilibrium with Pu02 in water (10). 

Although the speciation of some minor elements has been determined directly by experimental means (e.g., ion selective electrodes, polarography, electron spin resonance) most of our thinking about speciation is based on equilibrium calculations. Garrels and Thompson... 

Equilibrium complexation constants for Cu reactions with natural organic matter and the details of Cu speciation are bound to remain somewhat uncertain, since the composition of the complexing molecules varies from site to site. What is not in dispute is that the fraction of dissolved copper present as free aquo Cu is probably very small in any natural water. In extremely pristine waters, hydroxide and carbonate complexes may dominate, but organic complexes usually dominate in waters containing more than a few tenths of a mg/L organic carbon. 

The distribution of metals between dissolved and particulate phases in aquatic systems is governed by a competition between precipitation and adsorption (and transport as particles) versus dissolution and formation of soluble complexes (and transport in the solution phase). A great deal is known about the thermodynamics of these reactions, and in many cases it is possible to explain or predict semi-quantita-tively the equilibrium speciation of a metal in an environmental system. Predictions of complete speciation of the metal are often limited by inadequate information on chemical composition, equilibrium constants, and reaction rates. 

Meylan S, Odzak N, Behra R, Sigg L (2004) Speciation of copper and zinc in natural freshwater comparison of voltammetric measurements, diffusive gradients in thin Aims (DGT) and chemical equilibrium models. An Chim Acta 510 91... 

Van Luik, A.E. and Jurinak, J.J., Equilibrium chemistry of heavy metals in concentrated electrolyte solution, in Chemical Modeling in Aqueous Systems Speciation, Sorption, Solubility and Kinetics, Jenne, E.A., Ed., ACS Symp. Series 93, American Chemical Society, Washington, 1979, pp. 683-710. 

Wigley, T.M.L., WATSPEC A computer program for determining the equilibrium speciation of aqueous solutions, Brit. Geomorph. Res. Group Tech. Bull., 20, 48, 1977. 

This paper discusses (1) soil and groundwater and (2) aquatic equilibrium and ranking models. The second category deals with the chemical speciation in soil and groundwater, and with the environmental rating of waste sites, in cases where detailed modeling is not desirable. 

An evaluation of the fate of trace metals in surface and sub-surface waters requires more detailed consideration of complexation, adsorption, coagulation, oxidation-reduction, and biological interactions. These processes can affect metals, solubility, toxicity, availability, physical transport, and corrosion potential. As a result of a need to describe the complex interactions involved in these situations, various models have been developed to address a number of specific situations. These are called equilibrium or speciation models because the user is provided (model output) with the distribution of various species. 

Since carbonate and high pH are unique characteristics of arid and semi-arid soils, we will first examine solution speciation and the equilibrium reactions of the C02-H20 system. We will then examine the solution speciation of Ca and Mg, Zn, Cu, Ni, Cd, Pb, Cr(III) and Cr(IV), Hg, and Se. 

Fig. 17. Speciation of [Ru(ri6-bip)Cl(en)]+ (10) [5pM] in blood plasma, cytoplasm, and nucleus at equilibrium, based on the equilibrium constants of aquation, pKa, [Cl-], and pH of the environments. Data from Ref. (78).

Sibley TH, Morgan JJ (1975) Equilibrium speciation of trace metals in freshwater-seawater mixtures. In Hutchinson HC (ed) Proceedings of international conference on heavy metals in the environment, University of Toronto, Toronto, Ontario pp 310-338... 

The fluoride ion is the only inorganic ligand to form a complete substitution series, Be(H20)4 flFJ(2 1 (n = 1-4), though there is considerable variation in the equilibrium constants that have been reported. The most reliable values are probably those of Anttila et al. (117) who used both glass and fluoride-ion selective electrodes and also took account of the competing hydrolysis reactions. They did not, however, make measurements in the conditions where BeF2 would have been formed. A speciation diagram based on reported equilibrium constants is shown in Fig. 12. It can be seen that the fluoride ion competed effectively with hydroxide at pH values up to 8, when Be(OH)2 precipitates. 

Fig. 12.2. Redox-pH diagram for the Fe-S-H20 system at 100 °C, showing speciation of sulfur (dashed line) and the stability fields of iron minerals (solid lines). Diagram is drawn assuming sulfur and iron species activities, respectively, of 10-3 and 10-4. Broken line at bottom of diagram is the water stability limit at 100 atm total pressure. At pH 4, there are two oxidation states (points A and B) in equilibrium with pyrite under these conditions.

As a final example, we consider a fluid of known fluoride concentration whose calcium content is set by equilibrium with fluorite (CaF2). The speciation of fluorine provides for two solutions to this problem. In dilute solutions, in which the free ion I dominates, the reaction,... 

Parker, D.R., W. A. Norvell and R. L. Chaney, 1995, GEOCHEM-PC, a chemical speciation program for IBM and compatible personal computers. In R. H. Loep-pert, A. P. Schwab and S. Goldberg (eds.), Chemical Equilibrium and Reaction Models. Soil Science Society of America Special Publication 42,253-269. 

One of the best resources for looking at the way equilibrium constants K vary with ionic strength is the Web-based resource Joint Expert Speciation System (JESS) available at http //jess.murdoch.edu.au/jess/jess home.htm. Notice the way that values of any equilibrium constant (Ka, Kw, etc.) changes markedly with ionic strength I. 

---