Solubility product with ionic strength

Simplified expression for the variation of the solubility product with ionic strength... 

If the ionic strength of the saturated salt solution is sufficiently low (i.e., the solubihty is sufficiently low), it may be practical to evaluate the solubility product with Eq. 12.5.26 and an estimate of y from the Debye-Hiickel hmiting law (see Prob. 12.19). The most accurate method of measuring a solubility product, however, is through the standard cell potential of an appropriate galvanic cell (Sec. 14.3.3). 

The Stijf-Davis index attempts to overcome the shortcomings of the LSI with respect to waters with high total dissolved solids and the impact of "common ion" effects on the scale formation driving force. Like the LSI, the Stiff-Davis index has its basis in the concept of saturation level. The solubility product used to predict the pH at saturation (pHs) for a water is empirically modified in the Stiff-Davis index. The Stiff-Davis index will predict that water is less scale forming than the LSI calculated for the same water chemistry and conditions. The deviation between the indices increases with ionic strength. Interpretation of the index is by the same scale as for the LSI. 

It is important to note that the solubility product relation applies with sufficient accuracy for purposes of quantitative analysis only to saturated solutions of slightly soluble electrolytes and with small additions of other salts. In the presence of moderate concentrations of salts, the ionic concentration, and therefore the ionic strength of the solution, will increase. This will, in general, lower the activity coefficients of both ions, and consequently the ionic concentrations (and therefore the solubility) must increase in order to maintain the solubility product constant. This effect, which is most marked when the added electrolyte does not possess an ion in common with the sparingly soluble salt, is termed the salt effect. 

Metal hydroxides (e.g., Fe, Mn, Al) can also be a problem (Rauten-bach and Albrecht, Membrane Processes, Wiley, New York, 1989). A chemical analysis of the feed solution composition along with consideration of solubility products allows one to determine the significance of precipitation. Solubility products can be affected by temperature, pH, and ionic strength. Seasonal temperature variations must be considered. Concentrations of silica need to be < 120 mg/L in the feed. 

Dissolution test data will be required in all cases (and for all strengths of product) for development and routine control and should be based on the most suitable discriminatory conditions. The method should discriminate between acceptable and unacceptable batches based on in vivo performance. Wherever possible Ph Eur test methods should be used (or alternatives justified). Test media and other conditions (e.g., flow through rate or rate of rotation) should be stated and justified. Aqueous media should be used where possible and sink conditions should be maintained. A small amount of surfactant may be added where necessary to control surface tension or for active ingredients of very low solubility. Buffer solutions should be used to span the physiologically relevant range—the current advice is over pH 1 6.8 or perhaps up to pH 8 if necessary. Ionic strength of media should be reported. The test procedure should employ six dosage forms (individually) with the mean data and a measure of variability reported. 

In Figure 10.1 the time course of thermodynamically and kinetically controlled processes catalysed by biocatalysts are compared. The product yield at the maximum or end point is influenced by pH, temperature, ionic strength, and the solubility of the product. In the kinetically controlled process (but not in the thermodynamically controlled process) the maximum yield also depends on the properties of the enzyme (see next sections). In both processes the enzyme properties determine the time required to reach the desired end point. The conditions under which maximum product yields are obtained do not generally coincide with the conditions where the enzyme has its optimal kinetic properties or stability. The primary objective is to obtain maximum yields. For this aim it is not sufficient to know the kinetic properties of the enzyme as functions of various parameters. It is also necessary to know how the thermodynamically or the kinetically controlled maximum is influenced by pH, temperature and ionic strength, and how this may be influenced by the immobilization of the biocatalysts on different supports. 

Of the cations (counterions) associated with polar groups, sodium and potassium impart water solubility, whereas calcium, barium, and magnesium promote oil solubility. Ammonium and substituted ammonium ions provide both water and oil solubility. Triethanolammonium is a commercially important example. Salts (anionic surfactants) of these ions are often used in emulsification. Higher ionic strength of the medium depresses surfactant solubility. To compensate for the loss of solubility, shorter hydrophobes are used for application in high ionic-strength media. The U.S. shipment of anionic surfactants in 1993 amounted to 49% of total surfactant production. 

Due to their polyanionic nature, ASOs are extremely water-soluble under neutral and basic conditions. Consequently, dmg-product concentrations are often only limited by an increase in solution viscosity at very high concentrations (e.g. >300 mg/mL) [3]. Not surprisingly, extremes of pH and ionic strength influence the apparent solubility. In acidic environments such as the stomach, inter-nucleotide linkages are partially neutralized by protonation. With this consequent decrease in their polyanionic status there is a marked decrease in oligonucleotide solubility, an event that can be easily reversed by raising the pH. 

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