3.3.4 Further comments on acid-base extraction

• The pH of the aqueous phase should not be made too low or too high. Extreme pH's may cause undesired chemical reactions. Only a change to or from a salt form is required. In particular, the pK$_{\textrm{a}}$ or pK$_{\textrm{b}}$ of the substance being converted determines the target pH.
• More complex mixtures than the one in Section 3.3.3 can be separated by subsequent pH manipulations.
• Some organic bases (phenethylamines, for example) do not crystallize well (or at all) which may require conversion to the salt form after extraction. The procaine in Section 3.3.3 is also an example.
• Some materials have multiple crystal forms (polymorphism). Each crystal form will have a different IR spectrum; therefore, in these cases, it is important to crystallize from the same solvent used to obtain standard or library spectra.