Acid-base Extraction - Theory

Theory

The fundamental theory behind this technique is that salts, which are ionic, tend to be water-soluble while neutral molecules tend not to be.

The addition of an acid to a mixture of an organic base and acid will result in the acid remaining uncharged, while the base will be protonated. If the organic acid, such as a carboxylic acid, is sufficiently strong, its self-ionization can be suppressed by the added acid.

Conversely, the addition of a base to a mixture of an organic acid and base will result in the base remaining uncharged, while the acid is deprotonated to give the corresponding salt. Once again, the self-ionization of a strong base is suppressed by the added base.

The acid-base extraction procedure can also be used to separate very weak acids from stronger acids and very weak bases from stronger bases as long as the difference of their pK_a (or pK_b) constants is large enough. Examples are:

• Very weak acids with phenolic OH groups like phenol, 2-naphthol, or 4-hydroxyindole (pK_a around 10) from stronger acids like benzoic acid or sorbic acid (pK_a around 4–5)
• Very weak bases like caffeine or 4-nitroaniline (pK_b around 13–14) from stronger bases like mescaline or dimethyltryptamine (pK_b around 3–4)

Usually the pH is adjusted to a value roughly between the pK_a (or pK_b) constants of the compounds to be separated. Weak acids like citric acid, phosphoric acid, or diluted sulfuric acid are used for moderately acidic pH values and hydrochloric acid or more concentrated sulfuric acid is used for strongly acidic pH values. Similarly, weak bases like ammonia or sodium bicarbonate (NaHCO₃) are used for moderately basic pH values while stronger bases like potassium carbonate (K₂CO₃) or sodium hydroxide (NaOH) are used for strongly alkaline conditions.