Solubility: Experimental Procedures and Examples

As discussed in Chapters 6 and 7, there is no “best solvent” for all compounds and, after selecting a solvent, conditions in the system can be changed which will affect the properties of the compound. If a compound is uncharged it will, in general, be more soluble in solvents having lower dielectric constants, although, if the compound has polarizable groups or a dipole, then, protic solvents or solvents possessing a dipole will be better solvents. Solvents having higher dielectric constants, protic solvents and solvents which possess a dipole will be better solvents for compounds which have charges. Those compounds which are uncharged can be treated according to Hildebrand theory to obtain estimates of the solubility in different solvents. A solubility theory has not been developed for charged compounds which can be applied without imposing empirical relationships to specific types or classes of compounds. The difficulty in being able to develop a generalized theory for the solubility of charged compounds is that the solute-solvent and solvent-solvent interactions are much more important in protic and dipolar solvents and these interactions change with changes in concentration. It is with this in mind that the solubilities of several compounds in different types of solvents will be discussed.