酸鹼萃取：修订间差异

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酸鹼萃取的基礎理論是應用了[[鹽]]是[[離子化合物]]的一種，因此可溶於水，而大部分中性的物質則不溶於水這一點。

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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 acid and base 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 [[acid dissociation constant|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.

Usually, the mixture is dissolved in a suitable solvent such as [[dichloromethane]] or [[ether]], and poured into a [[separating funnel]]. An aqueous solution of the acid or base is added, and the pH of the aqueous phase is adjusted to bring the compound of interest into its required form. After shaking and allowing for phase separation, the phase containing the compound of interest is collected. The procedure is then repeated with this phase at the opposite pH range. The order of the step is not important and the process can be repeated to increase the separation. However, it is often convenient to have the compound dissolved the organic phase after the last step, so that [[evaporation]] of the solvent yields the product.

The procedure works only for acids and bases with a large difference in solubility between their charged and their uncharged form. The procedure does not work for:

* [[Zwitterion]]s with acidic and basic [[functional group]]s in the same [[molecule]], e.g. [[glycine]] which tend to be water soluble at most pH.

* Very lipophilic amines that do not easily dissolve in the aqueous phase in their charged form, e.g [[triphenylamine]] and [[trihexylamine]].

* Very lipophilic acids that do not easily dissolve in the aqueous phase in their charged form, e.g [[fatty acid]]s.

* Lower amines amines like [[ammonia]], [[methylamine]], or [[triethanolamine]] which are miscible or significantly soluble in water at most pH.

* Hydrophilic acids like [[acetic acid]], [[citric acid]], and most [[inorganic acid]]s like [[sulfuric acid]] or [[phosphoric acid]].

Alternatives to acid-base extraction including:

* filtering the mixture through a plug of [[silica gel]] or [[alumina]] — charged salts tend to remain strongly adsorbed to the silica gel or alumina

* [[ion exchange chromatography]] can separate acids, bases, or mixtures of strong and weak acids and bases by their varying affinities to the column medium at different pH.

* [[Chromatography]], a more powerful but more complex procedure to separate compounds

* [[Extraction]]

* [[Multiphasic liquid]]

* [[Separating funnel]]

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* [http://web.centre.edu/muzyka/organic/lab/24_extraction.htm 酸鹼萃取]

[[en:Acid-base extraction]]

[[Category:酸鹼化學]]