1.7.4 Purification of Alkaloidal Extract
The main bulk of the crude alkaloidal extract is invariably subjected to further purification by means of either anyone or combination of the following methods:
(a) Extraction with Acid Solution The extraction of the alkaloid from the bulk of the crude alkaloid solution in immiscible organic solvent is invariably carried out by shaking with an acid solution. In usual practice, the use of HCl is restricted when chloroform remains as the solvent because of the fact that quite a few alkaloidal hydrochlorides are distinctly soluble in the latter. However, dilute H2SO4 is always preferred over HCl for general use in the extraction of alkaloids. Subsequently, the acid solution is rendered alkaline with dilute NH4OH solution to liberate the alkaloids which is then extracted with an organic solvent. The solvent is removed under reduced pressure and the traces of moisture is removed with anhydrous sodium sulphate.
Note: The following two precautions may be observed, namely
(i) To avoid the formation of stubborn and troublesome emulsions a solution of gumtragacanth is often added to the aqueous-phase. In case, it still persists the two phases may be got separated by centrifugation, and
(ii) To discard the presence of foreign interfering extractive components present in plant substances, such as: pigments, resins, waxes, oils and fats, the use of a 2.5-5% (w/v) solution of lead acetate is made to the alkaloidal extract which precipitates them effectively. The excess of lead present in the filtrate is removed by either passing H2S gas through the Kipp’s Apparatus or by adding sodium phosphate.
(b) Precipitation of Alkaloid with Precipitating Reagent The usual precipitation of the alkaloid as a complex compound is accomplished by the addition of a suitable precipitating reagent. The resulting alkaloidal complex is further purified by filtration, recrystallization and ultimately decomposed to obtain the desired free alkaloid(s).
Example
(i) Tannic-acid Complex: It is normally decomposed by treatment with freshly prepared Pb(OH)2 or Pb(CO3)2.
(ii) Precipitates obtained with HgCl2, AuCl3, PtCl4, Mayer’s Reagent: These precipitates are decomposed by passing a stream of H2S gas through its suspension.
(iii) Precipitates with Double Salts: The double salt obtained with Dragendorff’s Regent is quickly boiled with 5% (w/v) BaCO3 solution.
(iv) Precipitates with Nitrogenous Acids: The precipitates obtained with nitrogenous acids like picric acid and picrolonic acid are normally decomposed by treatment with either NH4OH or NaOH.
(v) Reineckate Complex: The complex obtained from alkaloid with Reinecke Salt, NH4 [Cr(NH3)2 (SCN)4], is normally decomposed by treating its solution in a mixture of acetone and water (1:1) with a silver sulphate solution. It is pertinent to mention here that the free liberated alkaloid from the complexes stated above, (i) through (v), may be further extracted for its final recovery with an appropriate organic solvent, such as: chloroform.
(c) The purification of alkaloids may also be accomplished by the formation of its crystallised alkaloidal salt by the addition of an appropriate mineral or organic acid, such as: hydrochloric, hydrobromic, perchloric, sulphuric, oxalic and tartaric acids.
(d) Various known separation techniques, namely: partition, ion-exchange and column chromatography are invariably used for the purification of a host of alkaloids.
Besides, various physical parameters like: specific rotation, melting point, solubility are frequently used as a definite criteria of ascertaining the purity of alkaloids.
