Dragendorff's reagent

Dragendorff's reagent is a color reagent to detect alkaloids in a test sample. Alkaloids, if present in the solution of sample, will react with Dragendorff's reagent and produce an orange or orange-red precipitate.[1] This reagent was invented by the German pharmacologist, Johann Georg Dragendorff (1836–1898) at the University of Dorpat.[2]

Reaction between alkaloid extract from Capparis spinosa L and Dragendorff’s reagent

Composition

Dragendroff's reagent is a solution of potassium bismuth iodide prepared from basic bismuth(III) nitrate (Bi(NO3)3), tartaric acid, and potassium iodide (KI).[2][3]

Preparation

Dragendorff's reagent is prepared by mixing a concentrated solution of potassium iodide with a solution of bismuth subnitrate in a diluted acid (acetic acid or tartaric acid, hydrochloric acid or sulfuric acid is rarely being used) as a low pH is mandatory for this reagent.[2]

The formation is as follows:

The black precipitate of bismuth iodide is formed from the reaction of bismuth ion and potassium iodide.

Bi3+
 + 3 KI → BiI
3 + 3 K+

Then, the reaction between bismuth ion and excess potassium iodide will produce a soluble complex of potassium tetraiodobismuthate which has an orange color.

Bi3+
 + KI → K(BiI
4)

Reaction

Most of the alkaloids have a tertiary amine group, so the explanation for color reaction could be the following:

R
3N + HX → [R
3NH]+
 + X−
(X−
 = anions of acid)

Then the insoluble complex salt is formed from the reaction between ammonium salt and potassium tetraiodobismuthate.

[R
3NH]+
X−
 + K[BiI
4] → [R
3NH]+
[BiI
4]−
 + KX

This ion pair has different colors: yellow, orange, red, and brown, which depend on the nature of alkaloids.[2]

References
  1. Ghani A. (1998). Medicinal Plants of Bangladesh (1st ed.). Dhaka, Bangladesh: The Asiatic Society of Bangladesh. p. 505.
  2. Raal, A.; Meos, A.; Hinrikus, T.; Heinämäki, J.; Romāne, E.; Gudienė, V.; Jak⋅tas, V.; Koshovyi, O.; Kovaleva, A.; Fursenco, C.; Chiru, T. (2020-07-01). "Dragendorff's reagent: Historical perspectives and current status of a versatile reagent introduced over 150 years ago at the University of Dorpat, Tartu, Estonia". Die Pharmazie. 75 (7): 299–306. doi:10.1691/ph.2020.0438 (inactive 2021-01-19). PMID 32635970.CS1 maint: DOI inactive as of January 2021 (link)
  3. Khatun A, Rahman M, Jahan S (2014). "Preliminary phytochemical and pharmacological screening of Murraya exotica Linn. leaves extract". Oriental Pharmacy and Experimental Medicine. 14 (3): 223–229. doi:10.1007/s13596-014-0150-x. S2CID 16481315.


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