I used 5% H2SO4 in the extraction of alkaloids (using IEC). However, the yield was incredibly low (about 0.2%w/w). What acid+concentration would provide optimum yield of alkaloids? Mineral acids or organic acids?
You can used to extract in order to have alkaloids organic acids replaced with inorganic. Either use 0.5% sulfuric acid to extract crude alkaloids .Alkaloids are produced by a large variety of organisms including bacteria, fungi, plants, and animals. They can be purified from crude extracts of these organisms by acid-base extraction
You can use acetic glacial acid (CH3COOH) 10% or hydrochloric acid (HCl) 3%. However, the problem with acetic glacial acid when compared with hydrochloric acid is the low yielding obtained in the extraction with acetic glacial acid. I recommend to use hydrochloric acid 3% for acid extraction and the ammonium hydroxide concentrated for base extraction. For aporphine and tetrahydroprotoberberine alkaloids the acid-base extraction could be performed using hydrochloric acid (HCl) 3% and ammonium hydroxide concentrated in the end of the extraction.
The extraction of alkaloids such as an organic acid or inorganic acid is associated the amount of the alkaloid that the plant produces. Usually the plants that produce alkaloids have in its composition less than 1.0% of the crude alkaloids. So, the extraction of these compounds will be always low. Plants of the genus Cinchona possess about 1.5% of the crude alkaloids in this composition and this depends of the part of the plant to be investigated, such as leaves, roots, etc.
It depends on the molecular nature of your alkaloids. If you have acid-labile or otherwise reactive moieties in your structure, mineral acids are less advisable, especially HCl (phosphoric acid might be an option). I always had good success with citric or tartaric acids, especially with macrocyclic or peptide alkaloids. Subsequent neutralization is best done with NaHCO3 or an ion exchange resin. I would strongly advise against NH3, unless you know your structure and there is no danger of artifact formation.
The acids listed above in earlier posts will work. Do you care about any other compounds in the aqueous extract (water soluble neutral compounds)? Will you be making the solution basic to extract the free-base alkaloids into organic solvent?
If so, you may consider the use of acetic acid or formic acid, and ammonium hydroxide as the base. ammonium formate and ammonium acetate are volatile salts and can be removed under vacuum.
Taking into account that many natural alkaloids are known to be soluble in ethyl acetate ─ a low toxicity medium polarity solvent ─ a solution of acetic acid in ethyl acetate may, possibly, be also effective for the acid extraction of the considered alkaloids.
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