The Home Scientist 022 - Testing for Alkaloids with Dragendorff Reagent

Using Dragendorff Reagent to detect the presence of alkaloids at extremely low concentrations.

Make up Dragendorff Reagent as follows:

1. Dissolve 0.4 g of bismuth subnitrate in about 10 mL of concentrated hydrochloric acid.

2. Dissolve about 5.0 g of potassium iodide in about 50 mL of distilled or deionized water.

3. Pour the bismuth subnitrate solution into the potassium iodide solution.

4. Make up the solution to 100 mL with distilled or deionized water and transfer to a storage bottle labeled Dragendorff Reagent.

If you're unable to obtain bismuth subnitrate, you can use the following alternative procedure to make up a usable solution of Dragendorff's Reagent from Pepto-Bismol tablets or the generic equivalent. These tablets contain 262 mg of bismuth subsalicylate per tablet, which is equivalent in bismuth mass to about 212 mg of bismuth subnitrate. We can therefore substitute two Pepto-Bismol tablets for the 0.4 g of bismuth subnitrate. This alternative method isn't pretty, but it does use readily available materials and it does work.

1. Transfer two Pepto-Bismol tablets and about 20 mL of water to a beaker.

2. Swirl the beaker until the tablets have broken up into powder. Bismuth subsalicylate and the binders used in the tablets are both very insoluble in water, so don't be concerned if it looks as though none of the powder has dissolved.

3. Add about 10 mL of concentrated hydrochloric acid to the beaker. Swirl the beaker occasionally until foaming ceases. At this point, the liquid appears chalky white.

4. Allow the contents of the beaker to settle. Most of the solid matter precipitates, but enough remains suspended to give the liquid a cloudy white appearance.

5. Filter or carefully decant the liquid into a second beaker to remove as much as possible of the undissolved solids.

6. Dissolve about 7.0 g of potassium iodide in a few mL of water and transfer that solution to the beaker that contains the bismuth solution. The solution immediately assumes a yellow-brown, cloudy appearance.

7. Bring up the volume in the beaker to about 100 mL with distilled water, allow the solid material to settle, and then pour off the clear yellow-brown solution into a storage bottle labeled Dragendorff Reagent.