Chemical Tests and Reactions

Chemical tests and reactions are qualitative procedures that detect the classes of constituents present in a crude drug—alkaloids, flavonoids, tannins, saponins, glycosides, steroids, and others—through characteristic colour changes, precipitates, or froth. Together with analytical or physico-chemical parameters such as extractive values, ash values, and loss on drying, they provide a chemical layer of identity and purity testing that complements morphological methods.

Definition

Chemical tests and reactions in crude-drug evaluation are qualitative colour, precipitation, and froth reactions used to detect classes of secondary metabolites, together with physico-chemical determinations (ash values, extractive values, moisture content) that characterize a drug's purity and consistency.

Scope

The entry covers qualitative phytochemical screening reactions for major constituent classes and the standard physico-chemical determinations used to characterize crude-drug purity, framed as quality-control methods rather than clinical guidance.

Core questions

• Which classes of constituents can be detected to support the drug's chemical identity?
• Do physico-chemical parameters such as ash and extractive values fall within specified limits?
• Does the chemical profile indicate adulteration, exhaustion, or contamination?

Key concepts

• Phytochemical screening for constituent classes
• Colour, precipitation, and froth tests
• Reagent-based detection (e.g., alkaloid and flavonoid tests)
• Total, acid-insoluble, and water-soluble ash values
• Water- and alcohol-soluble extractive values
• Loss on drying and moisture content
• Limit tests for impurities

Mechanisms

Each class of constituent reacts with selective reagents to give a recognizable colour or precipitate—for example, alkaloids form precipitates with reagents such as Mayer's or Dragendorff's, while saponins produce persistent froth on shaking. These qualitative reactions indicate which constituent classes are present without quantifying them. Physico-chemical parameters add a complementary purity dimension: ash values reflect inorganic and earthy matter, acid-insoluble ash flags siliceous adulteration, extractive values estimate the amount of soluble matter and can reveal exhausted or substituted material, and loss on drying controls moisture that affects stability. These determinations are codified in pharmacopoeial monographs (evans-2009, who-2011-qc, harborne-1998).

Clinical relevance

Chemical screening and physico-chemical limits contribute to confirming that a herbal material contains its expected constituent classes and meets purity specifications, which is part of documenting product quality. This entry describes analytical methods and is not a basis for individual treatment decisions.

Evidence & guidelines

Pharmacognosy and phytochemistry texts and WHO quality-control guidance set out qualitative screening reactions and the physico-chemical determinations (ash, extractive, moisture, limit tests) used as standard identity and purity controls for herbal materials (evans-2009, harborne-1998, who-2011-qc, kunle-2012).

History

Qualitative chemical tests for plant constituents developed alongside nineteenth- and twentieth-century phytochemistry, and physico-chemical determinations such as ash and extractive values were incorporated into pharmacopoeial monographs as objective purity parameters; Harborne's mid-century phytochemical methods helped standardize the screening approach (harborne-1998, evans-2009).

Seminal works

• harborne-1998
• evans-2009

Frequently asked questions

What do chemical screening tests actually tell you?

They indicate which classes of constituents—such as alkaloids, flavonoids, tannins, or saponins—are present through characteristic colour or precipitate reactions, but they are qualitative and do not quantify the constituents.

What is the purpose of ash and extractive values?

Ash values estimate inorganic and earthy matter and can reveal adulteration, while extractive values estimate soluble matter and can detect exhausted or substituted material; both serve as objective purity parameters in pharmacopoeial monographs.