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CaptainPike
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drying with sodium metal
Good Afternoon,
I'm new to this (and I don't just mean the forum) and sometimes a little slow on the uptake. And even a little sensitive, so go easy.
The expression, "dried over sodium", intrigues me, especially when you're talking about oxidizable solvents. Take for example ethanol, please (that's
a joke) I have some ethanol here which I have approached the theoretical azeotrope and applied some (homemade) anhydrous magnesium sulfate. After
several days this desiccant is free-flowing and not sticky looking. I think the ethanol is quite dry. Is it safe to add some sodium metal shavings,
to ensure this aridity?
I mean, that's what you do, right? My concern is, what if the sodium metal did find some water in there? Couldn't I have myself a little mini-China
syndrome? Could it ignite the ethanol?
That's my question. Any help I might receive is appreciated. Thank you.
– Phil
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Sublimatus
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I don't believe this will work. Sodium and ethanol react in the following way:
2 CH<sub>3</sub>CH<sub>2</sub>OH + 2 Na → 2 CH<sub>3</sub>CH<sub>2</sub>ONa +
H<sub>2</sub>
The result, sodium ethoxide, is a salt and a strong base that reacts with water to return ethanol and NaOH.
Drying with sodium metal is usually reserved for something like diethyl ether that has no reactive groups that the sodium can interact with.
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kristofvagyok
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Yes, we dry solvents with alkali metals.
But there is a few things to note.
I usually use sodium+benzophenone for drying THF/dioxane/other ethers as seen here:
This is a good method for making abs. solvents, because if every water is gone, even traces, the color will turn blue because of the benzophenone.
For alcohols we usually do not use any drying, because what we buy is pure enough, but back to your problem, sodium is only added in that case, if the
main part of the water is removed by other methods.
What do I mean? If you add the sodium if there is 1,5% of water in the alcohol, than you will need a lot sodium to get rid of this amount of water
what would be really expensive and also do not forget the exothermic reaction between the water/sodium and ethanol/sodium. So you would end up with a
boiling concentrated lye solution what would eat as much sodium, as it gets.
What is recommended? Remove the water with some solid salt, like sodium-sulfate or magnesium-sulfate, filter it out and add a few small piece of
sodium, let it dissolve (sodium-ethoxide forms) and distill the water free solvent.
Also good to note that in the chemical industry water free abs. ethanol is made by adding cylohexane or benzene, azeotropically removing the water,
than separate the two solvents from each other.
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CaptainPike
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Thank you both for very useful answers. And I love the visual– that's part of the attraction for me, chemistry is art in motion. Glassware is our
medium of choice.
For once, I have pretty much done the right thing :-), that never happens. So thanks again.
_P
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DJF90
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Drying over sodium does not work well for alcohols. The sodium hydroxide formed from the sodium and the water present in the sample is a stronger base
than sodium ethoxide and so the equilibrium NaOH + EtOH <=> NaOEt + H2O lies to the right, i.e. your ethanol is still "wet".
Super dry alcohols (MeOH, EtOH) are obtained by distillation over Magnesium. The respective equations are Mg + 2ROH => Mg(OR)2 and
most importantly Mg(OR)2 + H2O => MgO + 2ROH i.e. the water ends up chemically bound as the insoluble MgO.
This is all detailed in Vogel which is available from the forum Library, if you'd care to look.
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turd
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You're doing it wrong. THF (but not diethyl ether) is dried over K so you can look at funny silver bubbles.
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kristofvagyok
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Quote: Originally posted by turd  |
You're doing it wrong. THF (but not diethyl ether) is dried over K so you can look at funny silver bubbles. |
If we would have that much K, than I would be happy, but since we need to make abs. dioxane/THF quite frequently in larger quantities we use sodium
and benzophenone. It is just as good as potassium, just much more cheaper. Trust me, it works perfectly.
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DJF90
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turd is messing; Na-ketyl works fine but with potassium its much more aesthetically pleasing. An alternative to benzophenone is 9-fluorenone, which
supposedly has a nice green ketyl. I'm sure there are others out there that have different colours, too.
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CaptainPike
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Super dry alcohols (MeOH, EtOH) are obtained by distillation over Magnesium.
I messed up the quote feature there pretty good – I don't think it's compatible with Dragon NaturallySpeaking, but anyway, am I to understand– you
mean that I should've done the final distillation over, magnesium metal? Elemental Magnesium? That is interesting, I'm not sure I understand the
balanced equation. Like, where did the original hydroxyl come from?
And I have Vogel's book, but I find that pretty slow going, it's cool in that it reminds me of Conan Arthur Doyle's Sherlock Holmes mysteries, the way
he writes. I'm going back and reading the SOLVENT EXTRACTION, and then the next part about DRYING SOLVENTS… It's already helped me out.
But thanks, to all again
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FreeMirage
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You're better off with some 3A molecular sieves when it comes to drying ethanol. Much simpler than messing with magnesium and just as effective.
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DJF90
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| Quote: Originally posted by FreeMirage | | You're better off with some 3A molecular sieves when it comes to drying ethanol. Much simpler than messing with magnesium and just as effective.
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You've got a reference for that?
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kristofvagyok
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Drying of Organic Solvents: Quantitative Evaluation of the Efficiency of Several Desiccants - J. Org. Chem., 2010, 75 (24), pp 8351–8354
I have a blog where I post my pictures from my work: http://labphoto.tumblr.com/
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testimento
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Just a side note, are other alkali metals, like potassium, calcium and especially lithium appliciable for most solvent drying operations? One can
scavenge OTC lithium from batteries, therefore I'd thought it'd be a good idea to dry for ex. ether with it, instead of non-otc or severe chemical
engineering - requiring sodium.
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FreeMirage
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Thanks for that, I assumed this was common knowledge. Also, according to that paper 20% m/v 3A sieves is in fact twice as effective as drying over
magnesium. Didn't know it was THAT good.
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hive3
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Purification of laboratory chemicals, D. D. Perrin has been recommended before. It can be had in an older edition for less than $20. This is an
incredibly time saving book, given the often conflicting advise on the forums. I wish I had it before I bought my first chemicals.
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Ascaridole
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According to Purification of Laboratory Chemicals Ethanol is taken from its 95% azeotrope to 99% with Calcium Oxide and then taken to absolute ethanol
by reflux over magnesium turnings as everyone has recommended. The 3A molecular sieves are also recommended as a quick drying method.
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testimento
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Just a question about ether purity and grignards, is 99.8% stated ether with added stabilizers(meaning that the water residue is less than 0.2%) dry
enough for chlorine based grignard reactions?
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Sublimatus
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Probably.
I've run Grignard reactions with ether that was only dried by distillation and then standing over NaOH. Ether treated in this way isn't exactly
anhydrous and the reaction worked fine.
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DJF90
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| Quote: Originally posted by testimento | | Just a question about ether purity and grignards, is 99.8% stated ether with added stabilizers(meaning that the water residue is less than 0.2%) dry
enough for chlorine based grignard reactions? |
What kind of chloride are you talking? If a chlorobenzene, you can probably forget it unless you're using more active magnesium.
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CaptainPike
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I love how intellectuals squabble, LOL.
Suffice it to say, there is no, "cut and dried", way of bringing ethanol from 96% up through those last, delightful percentage points. Depends on
what it'll be used for and how much we're preparing. Is money no object, or is time of the essence? These things, really, will direct the manner of
the production of pure alcohol, more than any one particular and esoteric means.
It is also cool to see the manner in which chemists flex their muscles – you guys are the coolest! I can just imagine sitting around with some of
you, down in some dungeon, scribbling on the table with a couple of pitchers of something more like what's left in the still pot after the first
azeotrope, INSISTING that his way is the perfect way, and so forth.
So, let me get this straight. Refluxing my significantly dry ethanol over magnesium turnings will substantially purify my product? How important is
it that my ethanol is above the 95/5 azeotrope? I mean what would happen if I was at 93.5%?
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testimento
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"At pressures less than atmospheric pressure, the composition of the ethanol-water azeotrope shifts to more ethanol-rich mixtures, and at pressures
less than 70 torr (9.333 kPa), there is no azeotrope, and it is possible to distill absolute ethanol from an ethanol-water mixture."
-Wikipedia
Diethyl Ether can be effectively dried with powdered KOH/NaOH, which shows up even greater efficiency in drying than sodium hydride. This is dictated
in an old article concerning about solvents drying. Introducing these elements, including their bare elemental forms, to ethanol/methanol will cause
ethoxide/methoxide formation aside of residual sodium hydroxide, although if the losses from the reaction are not critical, one will acquire anhydrous
ethanol this way, since sodium&sodium hydroxide will definitely eat the water off and absolute ethanol can be distilled(NaEtOH MP 260C).
Should the MgSO4, CaSO4 or CaCl2 be effective on drying ethanol? IMO the easiest way, if one has vacuum pump and a regulator, is to vacuum distill it
at 7-9mbar to yield anhydrous, absolute ethanol. This will always give absolute product with virtual zero losses.
[Edited on 6-10-2013 by testimento]
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FreeMirage
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Sieves are so cheap... I don't know why anyone would bother with a time consuming distillation.
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frenchemist1
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I have done a grignard with common diethyl ether, without purification or drying, it will works easily. A small cristal of iodine, dry glassware
assembled while hot, protected from moisture with silicagel tubes, it will work conveniently.
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WPK0129
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Some food for thought: how pure do you need it? Could you get away with a 95/5 EtOH/H2O mixture and then clean out byproducts from the
reaction with H2O? It could save you some headache - or shift headache from one step to another, depending on your reaction and workup.
Back in grad school, we would use sodium-benzophenone distillation for drying solvents to be used in radical polymerizations, where even a tiny amount
of water could kill your active radical species. (Not that ethanol wouldn't do the same, but... you get the point.)
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testimento
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Sorry to tell, but most people don't live in university labs. If you ever happen to cross your feet in a remote, totalitarian country as mine is,
you'd sell your mother to get some sulfuric acid.
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