Water as Solvent for Electrolytes

Still further expressions have been suggested for the description and prediction of the solubility of non-electrolytes in water with emphasis on drugs and environmental pollutants, e.g. very recently (Sprunger et al. 2007,2009 Jain and Yalkowsky 2010). It is not the purpose of this section to present the best available solubility expressions, but to show the properties of water as a solvent for such solutes. The emerging picture, well known for many years, is that the solubility is governed by the hydrophobic-hydrophilic balance of the solute. This is dominated by the former, except for small 

For the purpose of this section electrolytes (subscriptE) are considered as neutral combinations of ions (Chap. 2) that are capable of ionic dissociation in water. It is recognized that some electrolytes are gaseous (e.g. HCl) or liquid (H2SO4) at the temperature range considered here (0-100 °C), also that they may be only partly dissociated (NH4+OH ) in dilute solutions or associate to ion pairs in more concentrated ones (Mg +S04 ), or that they are surface active (Ci2H25S03 Na+) or are polymeric (-[CH(CH3)C02 Na+] -). However, typical electrolytes dealt with in this section are salts, such as Na+CC, which are practically completely dissociated in dilute solutions and consist of small ions. 

The enthalpies of hydration of the ions transferring from the ideal gas state into their standard state in the aqueous solution, AhydW°i, have been tabulated for 

15 K (Marcus 1997), derived for individual ions by using an extra-thermody namic assumption, see Sect. 2.3. However, the sums of such values for complete salts, Ahyd °E = l]Ahyd-ff°i are independent from such an assumption, since they are based on measurable values, and have estimated errors of about 6 kJ mol.  

The solubilities of salts in water are, of course, related to the Gibbs energy changes rather than to the enthalpies. The solubility product of a sparingly soluble salt, sp. the product of the ion concentrations in the saturated solution, is related to the Gibbs energy changes by 

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