Equilibrium solid-aqueous solution properties

A typical example is as follows. Benzoic acid, C6H5COOH, is a solid substance with only moderate solubility in water. The aqueous solutions conduct electric current and have the other properties of an acid listed in Section 11-2.1. We can describe this behavior with reaction (42) leading to the equilibrium relation (43) ... 

This permits provisional calculation of the compositional dependence of the equilibrium constant and determination of provisional values of the solid phase activity coefficients (discussed below). The equilibrium constant and activity coefficients are termed provisional because it is not possible to determine if stoichiometric saturation has been established without independent knowledge of the compositional dependence of the equilibrium constant, such as would be provided from independent thermodynamic measurements. Using the provisional activity coefficient data we may compare the observed solid solution-aqueous solution compositions with those calculated at equilibrium. Agreement between the calculated and observed values confirms, within the experimental data uncertainties, the establishment of equilibrium. The true solid solution thermodynamic properties are then defined to be equal to the provisional values. 

Most thermodynamic data for solid solutions derived from relatively low-temperature solubility (equilibration) studies have depended on the assumption that equilibrium was experimentally established. Thorstenson and Plummer (10) pointed out that if the experimental data are at equilibrium they are also at stoichiometric saturation. Therefore, through an application of the Gibbs-Duhem equation to the compositional dependence of the equilibrium constant, it is possible to determine independently if equilibrium has been established. No other compositional property of experimental solid solution-aqueous solution equilibria provides an independent test for equilibrium. If equilibrium is demonstrated, the thermodynamic properties of the solid solution are also... 

In application of this method to solubility data (8) in the KCl-KBr- O system at 25°C, it is found that equilibrium is in general not attained, though some mid-range compositions may be near equilibrium. As the highly soluble salts are expected to reach equilibrium most easily, considerable caution should be exercised before reaching the conclusion that equilibrium is established in other low-temperature solid solution-aqueous solution systems. It is not appropriate to derive thermodynamic properties of solid solutions from experimental distribution coefficients unless it can be demonstrated that equilibrium has been attained. 

Speciation is a dynamic process that depends not only on the ligand-metal concentration but on the properties of the aqueous solution in chemical equilibrium with the surrounding solid phase. As a consequence, the estimation of aqueous speciation of contaminant metals should take into account the ion association, pH, redox status, formation-dissolution of the solid phase, adsorption, and ion-exchange reactions. From the environmental point of view, a complexed metal in the subsurface behaves differently than the original compound, in terms of its solubility, retention, persistence, and transport. In general, a complexed metal is more soluble in a water solution, less retained on the solid phase, and more easily transported through the porous medium. 

Occasionally, determination of properties of the aqueous solution in equilibrium with the solid, such as pH, conductivity, or concentration of ionic species is also of interest—in particular, in the monitoring of cleaning and consolidation of archaeological artifacts. 

We will start our discussion by considering a special case, that is, the situation in which the molecules of a pure compound (gas, liquid, or solid) are partitioned so that its concentration reflects equilibrium between the pure material and aqueous solution. In this case, we refer to the equilibrium concentration (or the saturation concentration) in the aqueous phase as the water solubility or the aqueous solubility of the compound. This concentration will be denoted as Qf. This compound property, which has been determined experimentally for many compounds, tells us the maximum concentration of a given chemical that can be dissolved in pure water at a given temperature. In Section 5.2, we will discuss how the aqueous activity coefficient at saturation, y, , is related to aqueous solubility. We will also examine when we can use yf as the activity coefficient of a compound in diluted aqueous solution, y (which represents a more relevant situation in the environment). 

The charge density, Volta potential, etc., are calculated for the diffuse double layer formed by adsorption of a strong 1 1 electrolyte from aqueous solution onto solid particles. The experimental isotherm can be resolved into individual isotherms without the common monolayer assumption. That for the electrolyte permits relating Guggenheim-Adam surface excess, double layer properties, and equilibrium concentrations. The ratio u0/T2N declines from two at zero potential toward unity with rising potential. Unity is closely reached near kT/e = 10 for spheres of 1000 A. radius but is still about 1.3 for plates. In dispersions of Sterling FTG in aqueous sodium ff-naphthalene sulfonate a maximum potential of kT/e = 7 (170 mv.) is reached at 4 X 10 3M electrolyte. The results are useful in interpretation of the stability of the dispersions. 

The application of UNIFAC to the solid-liquid equilibrium of sohds, such as naphthalene and anthracene, in nonaqueous mixed solvents provided quite accurate results [11]. Unfortunately, the accuracy of UNIFAC regarding the solubility of solids in aqueous solutions is low [7-9]. Large deviations from the experimental activity coefficients at infinite dilution and the experimental octanol/water partition coefficients have been reported [8,9] when the classical old version of UNIFAC interaction parameters [4] was used. To improve the prediction of the activity coefficients at infinite dilution and of the octanol/water partition coefficients of environmentally significant substances, special ad hoc sets of parameters were introduced [7-9]. The reason is that the UNIFAC parameters were determined mostly using the equihbrium properties of mixtures composed of low molecular weight molecules. Also, the UNIFAC method cannot be applied to the phase equilibrium in systems containing... 

Cd(OH)2 is precipitated from aqueous Cd + by addition of bases. It is colorless and soluble in acids and aqueous NH4CI, but only slightly in NaOH solutions. Cd(OH)2 is a base stronger than Zn(OH)2l its solubility product is 10 and log K for the equilibrium Cd + + OH = CdOH+ is 6.38. Cd(OH)2 begins to undergo thermal decomposition at about 150°C (at 200°C it is complete). Several basic salts are known and may be prepared from alkaline Cd + solutions or by heating CdO with solutions of Cd + salts at 200 °C. Hydroxide halides have been carefully investigated both in solution and in solid state.Physical properties of Cd(OH)2 are reported in Table 8. 

Chapter 9 Energy, Enthalpy, and Thermochemistry Chapter 12 Quantum Mechanics and Atomic Theory Chapter 13 Bonding General Concepts Chapter 14 Covalent Bonding Orbitals Chapter 10 Spontaneity, Entropy, and Free Energy Chapter 11 Electrochemistry Chapter 6 Chemical Equilibrium Chapter 7 Acids and Bases Chapter 8 Applications of Aqueous Equilibria Chapter 15 Chemical Kinetics Chapter 16 Liquids and Solids Chapter 17 Properties of Solutions... 

In this chapter, we shall survey some of the outstanding properties of pure water in the gaseous, liquid, and solid phases. We shall discuss only equilibrium thermodynamic quantities. Properties of aqueous solutions are deferred to Chapters 3 and 4. 

The acid-base properties of the ion HF2 pose a special problem. This species is well defined both in the solid state and in solution. Estimates based on the lattice energies of the salts KHF2 and RbHF2 show that the process HF + F" HF7 in the gas phase is exothermic to the extent of 58 5 kcal moP " and the equilibrium constant for the same process in aqueous solution has the value 8 at There is no doubt that the... 

Because of the great abundance and importance of water and its remarkable effectiveness as a solvent, many miliar chemical reactions occur in aqueous solutions. Most of these reactions involve ions, often along with neutral molecules or undissolved solids. Ionic reactions usually come rapidly to equilibrium, and the properties of the solution depend on the concentrations of the species present at equilibrium. For all these reasons, ionic equilibria deserve careful study. 

Development of conditional (or variable) surface charges involves chemical reactions in the interfacial layer. Therefore, the individual material features, (i.e., the chemical properties of both the potentially charged sohd material and the dissolved species) have to be considered. When a chemically reactive surface group is exposed to an aqueous solution, the surface may become charged due to a surface reaction (e.g., dissociation, association, complexation) if the aqueous solution contains the other reactant as a dissolved species. The charging process on variable-charge sites is determined by not only the quality and quantity of active sites but also the composition of aqueous solution. An elechical double layer develops around particles due to the distribution of ionic species between the solid-liquid interfacial layer and the equilibrium Uquid phase. 

Fig. 2.37. Phase diagram for Ca0-Na20 Si02-(Al203)-H20 system in equilibrium with quartz at 400°C and 400 bars. Plagioclase solid solution can be represented by the albite and anorthite fields, whereas epidote is represented by clinozoisite. Note that the clinozoisite field is adjacent to the anorthite field, suggesting that fluids with high Ca/(H+) might equilibrate with excess anorthite by replacing it with epidote. The location of the albite-anorthite-epidote equilibrium point is a function of epidote and plagioclase composition and depends on the model used for calculation of the thermodynamic properties of aqueous cations (Berndt et al., 1989).

---