Prediction of Stability and Solubility Constants

Brown and Sylva (1987) developed one of the best theories for the prediction of stability and solubility constants (Langmuir, 1997 Moriyamaef /., 2005). The unified theory of metal ion complexation (UTMIC), as it relates to hydrolysis species and phases, was described in Chapter 2. A variation of Eq. (2.86) is given in the following for the stability constant of the hydrolysis species 

The UTMIC formulation did not account for the tetrad behaviour of the lanthanide series since the values of j and 2 were the same for all the lanthanide ions, and the only change in stability was related to the change in the ionic radius. Kawabe and Masuda (2001) utilised refined spin-pairing energy theory to describe the tetrad effect. The equation they utilised was 

Hydrolysis of Metal Ions, First Edition. Paul L. Brown and Christian Ekberg. 

The stability of complexes of the d and d transition metal ions (e.g. Cu and Mn ) is greater than would be expected (using Eq. (16.1)) on the basis of their respective ionic radius, whereas those of the d and d transition metal ions (e.g. NP and Cr ) are less stable than would be expected. Brown and Sylva (1987) accounted for these differences by utilising an ionic radius for a lower coordination number for the d and d metal Ions and a larger coordination number for the d and d ions (this reduces and increases the ionic radius, respectively). The reduced ionic radius for the d and d ions was justified on the basis that these ions undergo the Jahn-Teller effect and, as such, exhibit pseudo-four-coordinate behaviour. However, it is difficult to justify a larger ionic radius (higher coordination number) for the d and d ions, and moreover, the d and d ions are in reality six coordinate. To account for this behaviour, Eq. (16.3) has been modified to 

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I 16 Prediction of Stability and Solubility Constants redefined according to... 

An overview of the utilization of non-aqueous solvents in macromolecular applications has been presented. A variety of organic solvents have been used to characterize and understand the parameters necessary for their applications to biotechnology. Although no single parameter is predictive of a final utility, the critical parameters that continue to surface in these studies are the dielectric constant, hydrophobicity, dipole moment, viscosity and solubility factor of the solvent. Furthermore, the pH memory, molecular memory and water content of the solvent play important roles in stabilizing these molecules. Not all of... 

Solubility products measured in pure systems may not represent soil conditions very well. The impurities in solids affect their aqueous solubility soil minerals are characteristically impure. Nonetheless, predictions of soil solution concentrations usually assume the solubility products of pure minerals apply. The water molecule is ignored in stability constant and solubility product equations. The concentration of water is assumed to be unity because water is present in great excess and does not change significantly during the reaction. This assumption is good in all but the most concentrated aqueous solutions and in dry soils. 

It has long been known that the solubilities of sulfide ore minerals in hydrothermal fluids results from the complexation of metals such as Cu, Zn and Fe by Cl and HS ligands (Seward and Barnes 1997). Complexation of heavy metals such as As, Pb and Cd by mineral surfaces controls the mobility of such metals in the environment. Geochemists need to have a reliable thermodynamic data set to predict mineral solubilities and metal sorption reactions. Such data are found by fitting measured solubilities and sorption isotherms to a set of stability constants for aqueous and surface complexes. However, fits to experimental data are often non-unique and depend on the speciation model an independent way to determine the nature of metal complexes in aqueous solutions and on mineral surfaces is needed. 

The pH-stability profile is a plot of reaction rate constant for drug degradation versus pH and may help to predict if some of the drug will decompose in the GI tract. The stability of erythromycin is pH-dependent. In acidic medium, erythromycin decomposition occurs rapidly, whereas at neutral or alkaline pH the drug is relatively stable. Consequently, erythromycin tablets are enteric coated to protect against acid degradation in the stomach. In addition, less soluble erythromycin salts that are more stable in the stomach have been prepared. 

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