Understanding the Concept:
Anthraquinone glycosides are an important class of naturally occurring glycosides found in several medicinal plants such as Senna, Aloe, Cascara, and Rhubarb. These glycosides are widely known for their laxative and purgative properties.
The identification of anthraquinone glycosides is commonly carried out using Borntrager's test and Modified Borntrager's test. These tests are based on the liberation of anthraquinone aglycones followed by the development of a characteristic color in an alkaline medium.
Modified Borntrager's test is especially useful for detecting C-glycosides, which do not readily hydrolyze under ordinary conditions and therefore require additional treatment before testing.
Step 1: Understand the principle of Modified Borntrager's test.
In this test, the drug is first treated with ferric chloride and hydrochloric acid to facilitate oxidation and hydrolysis.
The liberated anthraquinone derivatives are then extracted using an organic solvent such as benzene or chloroform.
The organic layer is subsequently shaken with dilute ammonia solution.
Step 2: Observe the characteristic color reaction.
If anthraquinone glycosides are present, the ammoniacal layer develops a pink, cherry-red, or rose-red coloration.
This color develops because anthraquinone derivatives dissolve in the alkaline ammoniacal layer and produce a characteristic colored complex.
Therefore, the appearance of a pink-colored ammoniacal layer serves as a positive indication for anthraquinone glycosides.
Step 3: Examine Option (A): Salkowski Test.
Salkowski's test is primarily used for the identification of steroids and triterpenoids.
It involves treatment with concentrated sulfuric acid and produces characteristic color changes associated with sterol compounds.
Since it is unrelated to anthraquinone glycosides, this option is incorrect.
Step 4: Examine Option (B): Hager Test.
Hager's test is used for the detection of alkaloids.
The test employs saturated picric acid solution and produces a yellow precipitate when alkaloids are present.
Therefore, it is not applicable to anthraquinone glycosides.
Step 5: Examine Option (C): Legal Test.
Legal's test is used for the identification of cardiac glycosides, particularly cardenolides containing an unsaturated lactone ring.
The test produces a characteristic color reaction but has no relation to anthraquinone glycosides.
Hence, this option is also incorrect.
Step 6: Identify the correct test.
Among all the given options, only Modified Borntrager's test specifically detects anthraquinone glycosides and produces a pink color in the ammoniacal layer.
Conclusion:
Modified Borntrager's test is the standard pharmacognostic test used for detecting anthraquinone glycosides and produces a pink coloration in the ammoniacal layer. Therefore, the correct answer is Option (D).