SWIM
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Shelf life of some organic reagents.
I just picked up a kilo of DL-tryptophan and about a pound of pyrogallic acid.
The thing is, they've been around quiet a while. Like decades, and they have been opened.
The tryptophan is still a white, free flowing powder, and the pyrogallic acid is only slightly off white, but seriously caked.
Is it likely these materials are still any good?
I can do melting point tests on them, but don't have any more sophisticated way to test them short of making derivatives, so I'm hoping somebody might
have some Idea if there's much of a chance these things are still any good.
I knew it was a risk when I bought them, but the price made the gamble an attractive one.
I got some ancient sodium cyanide too, but I figure that should be fairly stable.
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j_sum1
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Wikipedia states that pyrogallic acid is sensitive towards atmospheric oxygen and assumes a brown coloration. So, based on that, if it is just
slightly off-white, you are probably ok. Sounds like it has absorbed some moisture though. You could grind it up to get it free-flowing again and
maybe dry it out a bit.
To test it you could make up a solution and then add an alkali solution. You should observe it going brown (reverting to the tannin) from exposure to
atmospheric oxygen. If you do this in a test tube then the discoloration should go from the top down.
Dunno about the tryptophan.
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woelen
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For many many chemicals the shelf life is limited, not due to its own decomposition, but due to reactions with something from the atmosphere (in most
cases oxygen or water, sometimes with carbon dioxide). Bottles, in which chemicals are sold, frequently are not that good. E.g. plastic bottles are a
little porous. Only slightly so, but over the years they can absorb moisture or oxygen from the air. Caps of bottles can be flimsy, or not closing
really air-tight. Another issue is that each time when a bottle is opened, fresh oxygen and moisture gets into it, which reacts with the chemical.
For this reason I repackage my sensitive chemicals in glass bottles with a thick and sturdy cap, preferably with a thick liner. I also take small
quantities from the main bottle which I use as working lot, while the main bottle only is opened when the small working bottle needs to be refilled.
As an example, I have a liter of ethylene diamine. This material is air-sensitive. It is oxidized to a brown tar-like material and it absorbs both
water and carbon dioxide from air. the liquid in my main stock bottle is nearly colorless (only very pale yellow), and I transferred 70 ml or so to a
small bottle, from which I do experiments. The contents of this small bottle has turned deep yellow over a year or so. It is opened frequently for
doing experiments (mostly for complex formation with transition metals). If I had done this with the main bottle, then that would have been much more
impure than it is now.
With the above kind of trick I can increase the shelf-life of my chemicals quite a lot.
Pyrogallic acid can also be kept indefinitely, provided it is protected from fresh air. As far as I know, the same is true for tryptophan.
The number of commercially available components which really decompose on storage by itself is very limited. I only know a few of them, most notably
H2O2 and hypochlorites.
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SWIM
National Hazard
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These materials are in glass fortunately.
I was hoping they might be okay, but will do an M.P. test and the other test recommended above.
I was concerned because I recently had an embarrassing incident where I sold somebody a bottle of Oxaloacetic acid which turned out to have degraded
to brown crud in the (Opaque)bottle even though it still had the original seal on it.
Nothing is worse than selling somebody bad chemicals, because it can lead to their wasting time and effort because of you.
Maybe a LOT of effort because they may think it was their mistake that the experiment failed and keep trying over and over before they realize it
wasn't their fault.
Thanks for the advice.
This was an unusual situation as I bought it not from a lab or university, but from a company that supplies props for film and television. They do
seem to be the real deal though.
I was quite taken aback to think that some of those props you see in lab scenes in movies are actually real reagents, and even hazardous ones.
The Idea of a bottle of sodium cyanide being used as set dressing where perhaps nobody there realizes it's authentic gives me the creeps. Say it's
next to an authentic bottle of acetic acid (they had those too) and they get broken
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