Dragendorff's reagent is a color

reagent In chemistry, a reagent ( ) or analytical reagent is a substance or compound added to a system to cause a chemical reaction, or test if one occurs. The terms ''reactant'' and ''reagent'' are often used interchangeably, but reactant specifies a ...

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

Preparation

Dragendorff's reagent is prepared by mixing a concentrated solution of potassium iodide with a solution of bismuth subnitrate in a diluted

acid In computer science, ACID ( atomicity, consistency, isolation, durability) is a set of properties of database transactions intended to guarantee data validity despite errors, power failures, and other mishaps. In the context of databases, a sequ ...

(

acetic acid Acetic acid , systematically named ethanoic acid , is an acidic, colourless liquid and organic compound with the chemical formula (also written as , , or ). Vinegar is at least 4% acetic acid by volume, making acetic acid the main component ...

or tartaric acid, hydrochloric acid or

sulfuric acid Sulfuric acid (American spelling and the preferred IUPAC name) or sulphuric acid ( Commonwealth spelling), known in antiquity as oil of vitriol, is a mineral acid composed of the elements sulfur, oxygen and hydrogen, with the molecular formu ...

is rarely being used) as a low pH is mandatory for this reagent. The formation is as follows: The black precipitate of bismuth iodide is formed from the reaction of bismuth ion and potassium iodide. : + 3 → + 3 Then, the reaction between bismuth ion and excess potassium iodide will produce a soluble complex of potassium tetraiodobismuthate which has an orange color. : + → Many compositions degrade over time and are sensitive to light, so for long-term storage it is often prepared as two separate solutions to be mixed before use, one containing bismuth subnitrate and acid with the other containing potassium iodide. The most common composition is as follows: Part A: 0.85g bismuth subnitrate, 40mL water, and 10mL glacial acetic acid. Part B: 8g potassium iodide and 20mL water. There are many different compositions in literature for the combined reagent. Some combine the entire volume of both solutions without dilution, but most involve diluting equal parts of these solutions with acid and water. One common composition is 5mL each of part A & B with 20mL of glacial acetic acid and 70-100mL of water.

Reaction

Most of the alkaloids have a tertiary amine group, so the explanation for color reaction could be the following: : + → + :( = anions of acid) Then the insoluble complex salt is formed from the reaction between ammonium salt and potassium tetraiodobismuthate. : + → + This ion pair has different colors: yellow, orange, red, and brown, which depend on the nature of alkaloids.

References

Analytical reagents {{chemistry-stub