The alkaloids are a group of basic, secondary plant substance, which usually possess a N-containing hetero-cycle. Their basic character is due to this feature. The alkaloids are mostly colorless, crystalline and non-volatile solids. They are insoluble in water, but soluble in organic solvents like ethanol, ether, benzene, chloroform etc.
There are a number of reagents which will precipitate most or all of the alkaloids and these are of value in proving the presence or absence of the class, although the results are often so similar that they cannot be used for the identification of the individual members. Among the most important of these alkaloidal group reagents are:
Tannic acid, which precipitates most alkaloids, as well as some other similar substances, as white or yellowish, flocculent compounds. They are often soluble in excess of the precipitant or in other acids.
Picric acid, which from not too dilute solutions precipitates yellow compounds, often crystalline in form.
Phosphomolybdic acid precipitates the alkaloids and similar nitrogenous compounds in the form of yellowish or brownish-yellow solids. These can be filtered from the solution and the alkaloid set free from them by the alkalis and. their carbonates.
Phosphotungstic acid acts like the phosphomolybdic in most cases.
Mercuric potassium iodide precipitates most alkaloids from solutions of their sulfuric or hydrochloric acid salts as white or yellow compounds.
Iodine in potassium iodide forms brown precipitates with alkaloidal solutions.
Mercuric chloride, platinum chloride, or gold chloride, from not too dilute solutions, throw down white or yellowish precipitates. With dilute solutions they may only form a turbidity.
In order to identify a compound as an alkaloid it must not only give the general tests for the class but be characterized by some special reaction or reactions. In the special reactions that follow it may be assumed, unless it is otherwise stated, that the alkaloid under discussion will, in the main, give the general reactions.
The qualitative chemical tests used for detection of alkaloids are depend on the character of alkaloids to give precipitate as salts of organic acids or with compound of heavy metals like Hg, Au, Pt, etc.
• Test by Dragendorff reagent(Potassium-bismuth-iodide solution) : – Dragendorff reagent consists of (Nitrooxy)oxobismuthine (BiNO4xH2O), tartaric acid and KI and it is used for the DC detection of alkaloids and – more general – tertiary amines (and also some secondary amines as well). The Tertiary amine is protonated due to the tartaric acid and an ion pair is formed that consists of BiI4- and HNR3+. In dependence of the nature of the tertiary amine (or alkaloid) this ion pair has a yellow, orange, red, brown color. Secondary amines will create less colored spots.
Alkaloids give reddish-brown precipitate with this reagent.
• Test by Mayer reagent (Potassium-mercuric-iodide solution): –
Used for the detection of alkaloids in natural products.
Mayer’s reagent is freshly prepared by dissolving a mixture of mercuric chloride (1.36 g) and of potassium iodide (5.00 g) in water (100.0 ml).
Most alkaloids are precipitated from neutral or slightly acidic solution by Mayer’s reagent (potassiomercuric iodide solution) to give a cream colored precipitate.
Alkaloids gives cream color precipitate with this reagent.
• Test by Wagner reagent (iodine-potassium-iodide solution): –
An aqueous solution of iodine and potassium iodide; used for microchemical analysis of alkaloids. Also known as Wagner’s solution. Iodo-potassium iodide (reagent for alkaloids) that prepared by Dissolve 2 g of iodine and6 g of KI in 100 ml of water.
Alkaloids give Brown color precipitate with this reagent.
• Test by Hager reagent (Saturated solution of picric acid): –
Saturated solution of picric acid by dissolving 1g of picric acid in 100ml of water. To 1mL of the extract, add 2-3 drops of the Hager’s reagent is added.
Alkaloids give yellow color precipitate with this reagent.
• Test by Tannic acid: –
The reagent is formed from the extract of the tannin group by dissolving 10g of tannic acid in 10ml of ethanol and dilute with water to 100ml. To1mL of the extract, 2-3 drops of the Tannic acid solution reagent is added.
Alkaloids gives buff color precipitate with this acid
• Test by Picrolonic acid: –
C10H8N4O5. Reagent for the isolation of amines as crystalline ?-complexes. Picrolonic acid. It is an analytical reagent used for the estimation of Ca, Mg, Pb, Sr and Th. It is also used for the analysis of alkaloids, tryptophan and phenylalanine.
Alkaloids give yellow color precipitate with this acid.