dicyanin
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Dragendorff reagent preparation
Does anyone have some experience on the preparation of this reagent? Apparently there are several variants but from what I read on wikipedia (not very
trustworthy I know) the tartaric acid containing version is preferred.
I usually do more in depth research, it will teach me a lesson not to be lazy, but here goes. I followed one of the first hits on yandex and it gave
me this:
http://rcmishra.blogspot.com/2007/11/dragendorff-reagent.htm...
| Quote: |
2nd Method:
Solution A: 1.7 g basic bismuth nitrate and 20 g tartaric acid in 80 ml of water.
Solution B: 16 g potassium iodide in 40 ml of water.
Stock solution: (stable several weaks in a refrigerator): mix equal volumes of solutions A and B
Procedure: Spray with a solution of 10 g tartaric acid, 50 ml water & and 5 ml stock solution. |
Assuming "basic bismuth nitrate" is bismuth subnitrate aka bismuth oxynitrate, I dissolved 20g of tartaric acid in 80ml water and added 1.7g bismuth
oxynitrate to that.
Problem is, it doesn't dissolve. There is no "solution A", it's a suspension A.
It's been standing here for a few weeks and if I don't shake it, the solid bismuth oxynitrate just stays on the bottom of the flask.
[Edited on 4-6-2021 by dicyanin]
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njl
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That's really odd, I would assume the mixed oxide/nitrate wouldn't be soluble but with added acid I can't see why it doesn't dissolve. What's the pH
of the solution?
edit: Also, "Dragendorff reagent" is a badass name.
[Edited on 6-4-2021 by njl]
Reflux condenser?? I barely know her!
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dicyanin
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pH = ~2 (measured with pH paper)
Maybe I can try the other method, using 25% acetic acid, but I doubt it will dissolve in it.
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njl
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How confident are you in the purity of the bismuth compound? I would check its reaction with other acids before continuing. If it won't dissolve by
itself in AcOH/HCl/HNO3 I would guess that's your problem. Also, looking around I keep seeing "basic bismuth nitrate" referenced in this prep but the
equations/stoichiometry given almost always just use bismuth 3 nitrate. Now chemistry is finicky and there are as many exceptions as there are rules,
but if the active reagent is KBiI4 then I don't see what difference the staring material makes. BEFORE anyone starts yelling, yes I know inorganic
chemistry preparations aren't that simple and there will be side effects from switching precursors.
Reflux condenser?? I barely know her!
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dicyanin
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I bought the bismuth subnitrate from a reputable supplier, so I'm pretty confident about the purity. I have done some digging and have found the
source of the error, a recent (2020) Estonian review paper on the Dragendorff reagent. And the whole wikipedia article on the Dragendorff reagent has
been based on this article, which led to the error being spread.
If you look at the wiki entry for "regular" (acidic) bismuth nitrate you see they mention the preparation of the Dragendorff reagent.
I also checked my Merck Index: bismuth(III)nitrate usually occurs as the pentahydrate which are lustrous, hygroscopic crystals ; acid reaction ; odor
of nitric acid ; soluble in dilute acids but decomposed by water forming insoluble subnitrate. Used for the precipitation of alkaloids.
But if you follow the link to their entry on the Dragendorff reagent the whole page suddenly speaks about "basic bismuth(III) nitrate" and "bismuth subnitrate". If you
look at the page before 2020 it was a stub but already had the erroneous statement "basic bismuth(III) nitrate" (which should be acidic bismuth(III)
nitrate). In 2020 the Estonian paper was added, which also took over this error. Which is rather grievous for a peer-reviewed work imho.
This is the paper in question: Dragendorff's reagent: Historical perspectives and current status of a versatile reagent introduced over 150 years ago at the University of Dorpat,
Tartu, Estonia
You can download the full PDF there for free. The review paper focuses on the life of professor Dragendorff, the importance of his discovery for the
advance of forensic science, that he opposed the Russification of his university etc... But on the preparation of the reagent itself, the paper speaks
only about bismuth subnitrate, which is wrong. I don't think they tried to prepare the reagent themselves.
| Quote: |
DR is prepared by dissolving bismuth subnitrate in diluted acid (acetic or tartaric acid, very rarely hydrochloric or sulfuric acid) and mixing it
with a strong solution of potassium iodide. A low pH is mandatory for this reagent.
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You can't dissolve bismuth subnitrate in diluted acid. You have to use regular "acidic" bismuth(III)nitrate. Big difference. Maybe I am hammering too
hard on this point, but you have to get your bismuth compound into solution in order to prepare the active reagent KBiI4.
I've tried dissolving bismuth subnitrate in both dilute and concentrated (azeotropic 68%) nitric acid, doesn't work. It does dissolve in aqua regia
with ease. So that could probably be the way to go in order to prepare the Dragendorff reagent "solution A" if the excess of mixed nitric and
hydrochloric acids do not interfere with the preparation of the final reagent.
And I agree, "Dragendorff reagent" is a pretty esthetic name 
[Edited on 13-6-2021 by dicyanin]
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dicyanin
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It just occured to me, maybe the source of error is the name "basic bismuth(III)nitrate", which in this case basic doesn't refer to its pH, but
meaning "regular", "as opposed to the other nitrate". Which led to confusion for the use of the basic (pH-wise) nitrate?
[Edited on 13-6-2021 by dicyanin]
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