The Home Scientist 022 - Testing for Alkaloids with Dragendorff Reagent

Uploaded by TheHomeScientist on 10.04.2010

I'm Robert Bruce Thompson and this is the Home Scientist video series
Before I get started I want to thank my friend Kip Kay for creating the new video intro
In this segment we'll take a look at another aspect of forensic toxicology
using Dragendorff Reagent to detect alkaloids
Dragendorff Reagent was widely used in the past and is still used today as a screening reagent
for alkaloids,
particularly for visualizing TLC or thin-layer chromatography plates.
It's quite sensitive, yielding positive results with many alkaloids at concentrations as low
as a few parts per million.
It's very specific in the sense that it yields a positive test with most alkaloids,
but reacts with very few non-alkaloids.
It's a screening test rather than a confirmatory test because it cannot be used to identify
the specific alkaloid present.
Once the presence of an alkaloid is established,
other tests are used to identify the specific alkaloid.
Dragendorff Reagent is a solution of bismuth sub-nitrate and potassium iodide.
If you don't happen to have any bismuth sub- nitrate around the lab, you can substitute
ordinary Pepto-Bismol tablets.
There are complete instructions in the sidebar for making up Dragendorff Reagent by either method.
We'll test for the presence of an alkaloid called quinine,
which is present in tonic water.
US law limits quinine in tonic water to
83 mg/L, or 83 ppm,
so we'll be using a relatively dilute alkaloid solution as our starting point.
You can also test alkaloids from other sources, such as the opium alkaloids
present in poppy seeds or the various
alkaloids present in many plants.
Alkaloids are organic bases that are generally extremely insoluble in water,
but you can treat solid plant matter with dilute hydrochloric acid to extract any alkaloids
present as water-soluble alkaloid hydrochlorides.
If you do such extractions, be extraordinarily careful and wear full protective gear.
Many alkaloids are incredibly toxic.
For example, although monkshood is often grown as an ornamental plant,
just one monkshood plant may contain enough aconitine to kill a hundred or more people.
Okay, here is the Dragendorff Reagent I'll be using. It's made up with bismuth sub-nitrate
rather than Pepto-Bismol tablets,
but I did verify that the solution made up with Pepto-Bismol tablets in fact works.
It's not quite the clear golden color of the actual stuff.
It's more a yellow-brown color,
but it indeed does work as expected.
I started by making a serial dilution. The test tube here on the right
contains undiluted tonic water, approximately 80 mg/L
or 80 parts per million
and then I did a serial dilution by doubling or halving rather
so that we have a test tube with eighty parts per million quinine,
forty parts per million quinine,
twenty parts per million,
ten parts per million,
five parts per million, and finally a test tube filled with 10 mL of
ordinary distilled water.
So, let's get started.
We'll start on the high end.
to see what kind of result we get. I'm going to add 1 mL
of the Dragendorff Reagent to each of these test tubes.
Okay, I've added 1 mL of
the Dragendorff Reagent to the far right test tube, the one with the most concentrated
quinine solution at eighty parts per million
and as you can see, or you may be able to see on the video
we have a rather opaque precipitate
of a very pretty
reddish brownish orange color.
So, I'm going to have 1 mL to the next test tube ...
Okay, and we can see the precipitate actually forming
So, at 40 mg/L we get
a very rapid and opaque precipitate.
Let's move down and add 1 mL
of Dragendorff Reagent to
the tube that contains 20 mg/L
of quinine.
and I don't know if it's visible on the video but we're getting a distinct precipitate.
All right. Let's add 1 mL of Dragendorff Reagent to the tube
with 10 mg/L.
and we can see the golden yellow color of the
Dragendorff Reagent itself, but there's no immediately obvious precipitate.
So we'll allow that to develop for a few minutes.
And in the meantime ...
we'll add 1 mL of Dragendorff Reagent
to the tube at
5 parts per million quinine.
And once again
we see no evidence of a precipitate.
I don't know if it's visible on the video or not, but the
second tube from the left, the 5 ppm tube
has a slightly more golden and less red color than the 10 ppm tube,
although the 10 ppm tube really isn't showing any sign of a precipitate as yet.
And finally, of course, we have our control tube
which has 10 mL of plain water in it.
So, we'll add 1 mL of Dragendorff Reagent for a reference.
All right, it's now been a couple of minutes
and we'll check the status of our various dilutions.
At 80 mg/L of quinine, the Dragendorff reagent is showing an intense precipitate. I don't know if it's visible on the video,
but I can just see some of it starting to settle out on the bottom of the test tube.
At 40 mg/L, the tube appears very similar.
40 mg/L of quinine causes
an intense precipitate, which
hasn't really started to flocculate yet. It's still sitting suspended in the tube
with just what looks like a few grains
of precipitate have settled to the bottom of the tube.
At 20 mg/L,
the tube is opaque.
Just looking, I can just very slightly see
some detail through the tube but it's generally opaque,
so we have a significant precipitate occurring here at
20 mg/L.
At 10,
the tube appears mostly clear with just some cloudiness.
Not an obvious precipitate, just a cloudy appearance.
And at 5 mg/L of quinine,
there is no cloudiness even in the tube.
However the color is significantly different. I don't know if it's noticeable on the video
from this tube at 10 mg/L
of quinine.
And also when we compare it
to our control test tube, which has just water in it,
we can see that the
water solution with a milliliter of Dragendorff Reagent
in it is a golden yellow color,
whereas even at 5 ppm quinine
there is a definite positive here.
The color of the solution has changed to an orangish-yellow rather than the golden yellow
of the diluted Dragendorff's Reagent.
So at this point we can say that at 5 ppm of quinine
Dragendorff's Reagent shows a positive test.
This has been just a quick demonstration of alkaloid testing using one of many
precipitation reagents with only one alkaloid.
In later videos, we'll look at identifying specific alkaloids using several methods,
including crystallization tests and color-spot tests like those we used for presumptive drug testing.
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