1.7.3 Extraction of Alkaloidal Base
The extraction of alkaloidal base may be accomplished by three different types of solvents that are discussed below, namely:
[A] Extraction with Water-Miscible Solvents A plethora of alkaloids and their respective salts are soluble in alcohols, such as: methanol, ethanol, isopropanol; therefore, these very solvents may also be employed for the extraction of the plant substances. The usual pretreatment of the crude drug with alkali may be avoided completely, because alcohol appears to affect dissolution of not only the alkaloidal salts but also the free bases found in the plant substances. It is, however, believed that alcohol predominantly exerts a hydrolyzing effect upon the alkaloidal tannates and other salts. In actual practice, neither pretreatment of the crude drug with an alkali nor acidification of the alcohol with a small amount of a mineral acid or an organic acid is required.
Note
1. The penetration and hence the subsequent extraction of the crude drug is almost complete with the help of four successive extractions with an alcohol. Further, the loss of solvent is comparatively less than the chlorinated solvents e.g., chloroform.
2. The extraction of total alkaloids with alcohol is highly recommended because of its maximum efficiency and economical viability.
[B] Extraction with Water-Immiscible Solvents In reality, the most widely used water-immiscible solvents for the extraction of alkaloids are: chloroform, diethyl ether (solvent ether) and isopropyl ether. However, a few other specific organic solvents, namely: ethylene chloride, carbon tetrachloride and benzene* may be employed with an evident advantage for certain specific alkaloids. Interestingly, chloroform is regarded as the choicest water-immiscible solvent for a broad-spectrum of alkaloids present in the plant kingdom and extracts them with varying degrees of ease.
Note: Chloroform is not suitable for the extraction of quaternary alkaloids e.g., tubocurarine.
[C] Extraction with Water The crude drug is subjected to extraction with water previously acidified with dilute solution of HCl, H2SO4 or CH3COOH, which is subsequently rendered alkaline, preferably with dilute NH4OH solution and finally extracted with a water-immiscible solvent as stated in [B] above.
Undoubtedly, water being an excellent and absolutely inexpensive polar solvent for the extraction of alkaloids, but if offers an enormous volume of disadvantages because it carries along with it a large number of other plant components, for instance: sugar, pigments (e.g., chlorophylls), starches, tannins, proteins etc., which ultimately puts across a collosal waste of time, energy and chemicals. Hence, its usage has been resulting to a bear minimum level.
In general, the alkaloids may be extracted by any of the following three well-defined and widely accepted processes, namely:
(a) Soxhlet Extraction Process
(b) Stas-Otto Process, and
(c) Kippenberger’s Process.
All these three processes shall now be discussed briefly in the sections that follows:
(a) Soxhlet Extraction Process: The soxhlet assembly is a continuous extractor which is generally suitable for the extraction of alkaloids from powdered plant materials with the help of organic solvents. In this instance, the powdered drug is usually moistened with dilute ammonia solution and then packed loosely in the thimble of the Soxhlet apparatus; and the organic solvent affords a deep penetration of the moist drug thereby allowing the greatest possible extraction of the alkaloids from the exposed surfaces of the cells and tissues of the crude drug. Once, the extraction is ascertained to have completed, the solvent is filtered and evaporated in a Rotary Thin-Film Evaporator and the residue is treated further for the isolation of individual alkaloids.
(b) Stas-Otto Process: The Stas-Otto process essentially consists of treating the powdered and sieved drug substance with 90–95% (v/v) ethanol, previously acidified with tartaric acid. The proportion of crude drug to solvent should be maintained as 1 Kg to 1 L. The alcohol is distilled off under vacuum and the resulting aqueous residue is treated with petroleum-ether (60-80°C) to remove the fatty components completely. If any alkaloid is removed by the petroleum ether, it must be recovered by treating it with dilute mineral acid. Thus, the resulting aqueous extract is mixed with the main bulk of aqueous extract. The combined aqueous extract is filtered and evaporated to dryness preferably in a Rotary Thin-Film Evaporator under vacuum. The residue is extracted with absolute ethanol thereby dissolving the total alkaloids.
(c) Kippenberger’s Process: In Kippenberger’s process the powdered and sieved plant substance is first and foremost digested with solution of tannin (100 g) in glycerol (500 g) at a constant temperature of 40°C for a duration of 48 hours. The resulting mixture is further heated to 50°C so as to help in the complete coagnlation of proteinous substances, cooled to ambient temperature and finally filtered. The resulting filtrate is thoroughly shaken with petroleum ether to get rid of faulty materials (oils, fats and waxes), and the last traces of petroleum ether is removed from the extract by heating either on a water-bath (electric) or exposure to Infra-Red Lamp. The fat-free crude plant extract is subsequently acidified and shaken with chloroform, successively to remove the bulk of the alkaloids, namely, atropine, codeine, colchicine, narcotine, nicotine, papaverine, spartenine and thebaine.
The resulting residual extract may still contain narceine, curarine and morphine. However, narceine and morphine may be isolated by passing freshly generated CO2 directly into extract so as to convert the alkali hydroxide into their corresponding carbonate, which is then ultimately subjected to solvent extraction using a mixture of alcohol and chloroform. Finally, the third alkaloid, curarine, may be extracted by agitation with a mixture of equal volumes of ether and chloroform.
However, a combination of Kippenberger’s process and Stas-Otto process by its application to the final alcoholic extract obtained by the latter process is found to give better separation of alkaloids.
