Ionic equilibria between solids solutions

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.)... 

If we make the assumption that there is complete dissociation of a slightly soluble ionic compound into its component ions, then equilibrium exists between solid solute and aqueous ions. Thus, for example, for a saturated solution of lead(II) sulfate in water, we have... 

The equilibrium constant for the solubility equilibrium between an ionic solid and its dissolved ions is called the solubility product, Ksp, of the solute. For example, the solubility product for bismuth sulfide, Bi2S3, is defined as... 

The solubility of an ionic solute, Sca, may be expressed in terms of its solubility product, The equilibrium between a pure solid salt, Cv+Av and its saturated solution in a solvent where it is completely dissociated to ions (generally having e > 40 see section 2.6) is governed by its standard molar Gibbs energy of dissolution... 

When equilibrium is reached, solubility product constants are used to describe saturated solutions of ionic compounds of relatively low solubility. When the ion concentration in solution reaches saturation, equilibrium between the solid and dissolved ions is established. 

Sorption coefficients quantitatively describe the extent to which an organic chemical is distributed at equilibrium between an environmental solid (i.e., soil, sediment, suspended sediment, wastewater solids) and the aqueous phase it is in contact with. Sorption coefficients depend on (1) the variety of interactions occurring between the solute and the solid and aqueous phases and (2) the effects of environmental and/or experimental variables such as organic matter quantity and type, clay mineral content and type, clay to organic matter ratio, particle size distribution and surface area of the sorbent, pH, ionic strength, suspended particulates or colloidal material, temperature, dissolved organic matter (DOM) concentration, solute and solid concentrations, and phase separation technique. 

EXAMPLE 2 Suppose HC1 (supplies H+) is added to a saturated solution of Mg(OH)2 in equilibrium with some undissolved solute. The H+ removes nearly all the OH- in solution to form water. This greatly decreases the [OH-] and more Mg(OH)2 dissolves so that the ion concentration product can again come to the value of Ksp for Mg(OH)2- If all the Mg(OH)2 dissolves, there is no longer an equilibrium between the ionic solid (it is all gone) and the solution Q will be less than Ksp. 

Bis(o-dimethylaminomethylphenyl)dichlorosilane reveals in solution an intramolecular dynamic coordination of one dimethylaminomethyl group to silicon with pentacoordi-nation122,123 (equation 69). The corresponding monochloro derivative shows in solution an equilibrium between a neutral hexacoordinated modification (which is also observed in the solid state) and an ionic pentacoordinated modification124 (equation 70). 

The value of Dy is determined by shaking or stirring a specified amount of solution to which the ion of interest, such as the radionuclide, had been added with a specified amount of ion-exchange medium. When the radionuclide distribution between solid and liquid phase reaches equilibrium, the concentration of the radionuclide in each phase is measured. The batch of ion-exchange medium must be saturated initially with the specified backing ion and the initial solution must contain the same ion at a specified concentration. For example, if the radionuclide is a cationic radionuclide such as and the system for comparison is a sodium salt, then the ion-exchange medium must be in the sodium form and the solution that contains must be at a specified sodium backing-ion concentration. The concentration of nonradioactive potassium ion must also be specified. Any other radionuclides from which the radionuclide of interest is to be separated must be equilibrated under identical conditions of known volume ratios, ionic concentration, temperature, and equilibration period. 

It is important to distinguish carefully between solubility and the solubility-product constant. The solubility of a substance is the quantity that dissolves to form a saturated solution. (Section 13.2) Solubility is often expressed as grams of solute per liter of Solution (g/L). Molar solubility is the number of moles of solute that dissolve in forming 1 L of saturated solution of the solute (mol/L). The solubility-product constant (K p) is the equilibrium constant for the equilibrium between an ionic solid and its saturated solution and is a unitless number. Thus, the magnitude of is a measure of how much of the solid dissolves to form a saturated solution. 

SOLUBILITY EQUILIBRIA (SECTION 17.4) The equilibrium between a solid compound and its ions in solution provides an example of heterogeneous equilibrium. The solubility-product constant (or simply the solubility product), K p, is an equilibrium constant that expresses quantitatively the extent to which the compound dissolves. The fCq, can be used to calculate the solubility of an ionic compound, and the solubility can be used to calculate K p. 

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