Alkaloids are precipitated from a neutral or slightly acid solution by a number of metallic salts. These precipitation reactions may be used for the detection of the presence of alkaloids in solution. Some of the metallic salts also give colour reactions with proteins and hence results need to be interpreted with caution. Following are some of the common chemical tests used to detect alkaloids.

1. A solution of the alkaloid treated with dragendroff’s reagent (solution of potassium bismuth iodide) gives an orange red precipitate.
2. A solution of the alkaloid when treated with Mayer’s reagent (potassiomercuric iodide solution) gives a cream-coloured precipitate.
3. A solution of the alkaloid when treated with Wagner’s reagent (solution of iodine in potassium iodide) gives a reddish-brown precipitate.
4. A solution of the alkaloid when treated with Hager’s reagent (a saturated solution of picric acid) gives an yellow precipitate.
5. A solution of the alkaloid with solution of tannic acid gives a buff-coloured precipitate.
6. Murexide test: A solution of the alkaloid is mixed with a very small quantity of potassium chlorate and a drop of hydrochloric acid. It is evaporated to dryness and a drop of ammonia is added. A purple colour is formed with purine alkaloids like caffeine, which do not precipitate most other alkaloidal reagents.

Alkaloid form precipitating complexes with phosphomolybdic acid, phosphotungstic acid, and chlorauric acid and chlorplatinic acids and these may be used for their identification.