The alkaloids are a class of vegetable compounds which are chemically similar to ammonia. Like this, the free alkaloids are of a basic nature and they form salts by uniting with acids without the setting free of hydrogen. Only a few of the alkaloids are liquid, most of them, as well as their salts, being crystallize-able solids. Their salts are, as a rule, much more easily soluble in water than the alkaloids themselves They are decomposed by alkalis, the alkaloid being set free, just as ammonia is freed from its salts by the same reagents. When the solution is concentrated this process often results in their precipitation, as the alkaloid is less soluble than its salts. There is a great difference in the comparative solu-bility of the alkaloids in many of the organic liquids like chloroform, benzene, ether, petroleum ether, amyl alcohol, etc., and the methods devised for their separation are based upon this. They all have a bitter taste, as do their salts. The separation and identification of the alkaloids is not as simple an operation as that of the metallic compounds. It should be borne in mind that a very small amount of impurity may con-ceal or to a great degree modify the reactions.
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.
In the reactions of the alkaloids and in others
where it often becomes necessary to identify minute quantities of material
it is convenient, instead of test-tubes, to make use of small, rather flat
watch crystals. The results of the treatment of a drop of solution with
one of the reagent are plainly perceptible if the watch-glass stands upon
a suitably colored paper, and the precipitate can be examined with the
microscope, placing the watch-glass upon the stage. If it is desired to
make the test upon a solid compound the drops of solution on the watch-glass
may be evaporated to dryness on the steam-bath. At first rather large amounts
of the alkaloids may be used, but after some skill has been gained this
should be reduced to very minute quantities in order to show the sensitiveness
of the reagents.