jamit
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recrystallization -- general techniques of purifying hygroscopic chemicals
Buying reagent grade chemicals for a home lab can be expensive. Thus I have resorted to purifying OTC. Most OTC are technical grade and sufficient
for most home chemistry use... but I enjoy purifying my chemicals to the highest grade possible , so I've been recrystallizing various OTC and its synthesis (i.e. copper sulfate, potassium nitrate, potassium
chloride, ammonium nitrate, copper chloride, copper nitrate, etc.)
There are various challenges to recrystallization... some are rather straight forward and easy like copper sulfate and potassium nitrate... but others
are rather difficult because they are hygroscopic or deliquescent. For example, drying copper nitrate is very difficult as it absorbs water so
quickly. I've dried copper nitrate in a desiccator until dry but once I put them in a bottle, a month later they are wet again (maybe I need a better
container!)
I've also learned that adding a drop or two of nitric acid to a solution of copper nitrate helps in producing a more transparent crystal when you
evaporate them... although it also leaves behind a strong nitrogen oxide/dioxide odor.
Does anyone have any practical suggestions and things you have learned (i.e. technique) on recrystalling hygroscopic chemicals like copper nitrate,
nickel nitrate, etc ... they all tend to be "nitrate" compounds?
[Edited on 13-11-2014 by jamit]
[Edited on 13-11-2014 by jamit]
[Edited on 13-11-2014 by jamit]
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Dan Vizine
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I don't understand your question. The preferred way of recrystallizing metal nitrates involves aqueous media. The water of crystallization won't
effect the exclusion of impurities from the growing crystalline lattice. Removing water is done after the compound is pure. At home, one really nice
way of removing water can be azeotroping it off with toluene. It's very effective. That could be done after you first heat it to get the easily
removed water out.
"All Your Children Are Poor Unfortunate Victims of Lies You Believe, a Plague Upon Your Ignorance that Keeps the Youth from the Truth They
Deserve"...F. Zappa
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macckone
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I second the use of a non-polar solvent.
Adding a little acid is also fairly common for things that easily
change to an oxide, cupric nitrate for example.
You can't get cupric nitrate down to anhydrous, btw. At
least to my knowledge you can't.
To make anhydrous curpic nitrate you need to start with
copper and dinitrogen tetroxide
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DraconicAcid
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I don't think the OP is asking about making anhydrous copper(II) nitrate, but just keeping the hydrated crystals from turning into puddles (I've had
crystals do that to me).
Please remember: "Filtrate" is not a verb.
Write up your lab reports the way your instructor wants them, not the way your ex-instructor wants them.
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DJF90
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I've recrystallised a deliquescent N-oxide before, and that was a real pain. The buchner funnel was placed in a bag of high flowing nitogen whilst
under vacuum in order to minimise/exclude aerial contact (see pic). Digging the product off was the hardest part, but doable with a little bit of
ingenuity. Theres 175 g on that funnel in the picture.
[Edited on 14-11-2014 by DJF90]
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Magpie
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I made copper nitrate by dissolving copper wire in nitric acid. I gave up trying to dry it due to extreme deliquescence. So to quantify the copper
molarity I performed a quantitative analysis. This was a practical solution for my intended use.
The single most important condition for a successful synthesis is good mixing - Nicodem
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blogfast25
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Which 'N-oxide', DJF90?
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Dan Vizine
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Quote: Originally posted by jamit  | ....For example, drying copper nitrate is very difficult as it absorbs water so quickly. I've dried copper nitrate in a desiccator until dry but once
I put them in a bottle, a month later they are wet again (maybe I need a better container!).......
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There's a perfect example of the relentless march of diffusion. Closed, threaded bottles of the highest quality still won't be enough to keep the
moisture out unless the mating surfaces close to a perfection that won't allow water molecules to slip by (in short, never). Outside of a sealed glass
ampoule, you can't control moisture, you can only manage it. It will always defeat your seals, you just accept that and minimize it.
That means that [most] "pressure" joints, like all caps on bottles, are inherently poor. A better option has tight-fitting surfaces with a film of
grease. This is an approximation of a closed ampoule with a tiny area of low viscosity glass.
The best quick alternative is to funnel the material into a vessel with a ground glass joint and apply the greased stopper or cap or whatever. You can
purchase storage vials or even inexpensive round-bottom flasks for this purpose. This is effective in storing most things, even, for instance, alkali
metals.
It goes without saying, that your best friend is low humidity days for the manipulations, unless you have N2 or Ar, and then you can do whatever you
want to the extent that you can afford it.
[Edited on 14-11-2014 by Dan Vizine]
"All Your Children Are Poor Unfortunate Victims of Lies You Believe, a Plague Upon Your Ignorance that Keeps the Youth from the Truth They
Deserve"...F. Zappa
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jamit
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Dan Vizine -- thank you for that advice. I've dried copper nitrate during winter when the humidity is low and put itin a desiccator with silica gel
and it can get pretty dry... I then put it into a tightly sealed bottle for storage but then come summer and somehow moisture gets into the bottle and
you can see the crystals slowly clumping together as it absorbs water -- it's no longer free flowing.
What I find even more frustrating is that when i try to dried it to free flowing by putting it back into the desiccator... and it dries, but then I
put some of the dried crystals into distilled water, the solution is not transparent but opaque indicating that it has impurities which it didn't have
before. I've been trying to "master" making dry copper nitrate and keeping it dry but it has been a crazy project.... as a last resort, I guess I'll
just have to ampule it.
I'm tempted to buy a bottle of reagent grade copper nitrate to see what it looks like and whether it behaves like my own personal copper nitrate which
I've synthesized from copper carbonate and nitric acid.
Also, one more thing: I noticed that copper nitrate is best produced from dissolving copper in nitric acid with a slight excess of nitric acid...
rather than via a copper carbonate with nitric acid. strange? copper nitrate made by the copper and nitric acid produces cleaner crystals. But why?
Has anyone experienced this also?
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blogfast25
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| Quote: Originally posted by jamit |
Also, one more thing: I noticed that copper nitrate is best produced from dissolving copper in nitric acid with a slight excess of nitric acid...
rather than via a copper carbonate with nitric acid. strange? copper nitrate made by the copper and nitric acid produces cleaner crystals. But why?
Has anyone experienced this also?
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A tentative explanation is purity of the copper carbonate. As this is invariably precipitated copper carbonate, washing out the last bits of
whatever carbonate may have been used to cause the copper to precipitate can be difficult. Redissolving the copper carbonate in nitric acid may then
result in sulphate (e.g.) or sodium (e.g.) impurities. Perhaps these cause the problem? Recrystallisation should probably solve that problem.
To produce 'clean' precipitates, precipitate from fairly dilute solution, using a fairly dilute carbonate solution. Allow precipitate to digest
overnight (with stirring is possible).
Filter on a Buchner funnel and suck 'dry', then repeat this with several smallish aliquots of tap water and finally with two rinses of deionised or
distilled water.
[Edited on 15-11-2014 by blogfast25]
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jamit
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Thanks blogfast25, your suggestion on using dilute solution makes sense. I try that out.
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