Sciencemadness Discussion Board - drying with sodium metal

drying with sodium metal

CaptainPike - 22-8-2013 at 13:57

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

Sublimatus - 22-8-2013 at 14:32

I don't believe this will work. Sodium and ethanol react in the following way:

2 CH3CH2OH + 2 Na → 2 CH3CH2ONa + H2

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.

kristofvagyok - 22-8-2013 at 14:32

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.

CaptainPike - 22-8-2013 at 18:45

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

DJF90 - 23-8-2013 at 11:02

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.

turd - 23-8-2013 at 13:20

Quote: Originally posted by kristofvagyok

I usually use sodium+benzophenone for drying THF/dioxane/other ethers.

You're doing it wrong. THF (but not diethyl ether) is dried over K so you can look at funny silver bubbles.

kristofvagyok - 23-8-2013 at 13:36

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.

DJF90 - 24-8-2013 at 04:10

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.

CaptainPike - 26-8-2013 at 16:50

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

FreeMirage - 30-9-2013 at 05:20

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.

DJF90 - 30-9-2013 at 12:07

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.

You've got a reference for that?

kristofvagyok - 30-9-2013 at 14:06

Quote: Originally posted by DJF90

You've got a reference for that?

Drying of Organic Solvents: Quantitative Evaluation of the Efficiency of Several Desiccants - J. Org. Chem., 2010, 75 (24), pp 8351–8354

testimento - 30-9-2013 at 16:24

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.

FreeMirage - 30-9-2013 at 20:33

Quote: Originally posted by kristofvagyok

Quote: Originally posted by DJF90

You've got a reference for that?

Drying of Organic Solvents: Quantitative Evaluation of the Efficiency of Several Desiccants - J. Org. Chem., 2010, 75 (24), pp 8351–8354

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.

hive3 - 1-10-2013 at 06:30

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.

Ascaridole - 1-10-2013 at 17:33

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.

testimento - 4-10-2013 at 15:29

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?

Sublimatus - 4-10-2013 at 16:06

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.

DJF90 - 5-10-2013 at 02:01

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.

CaptainPike - 5-10-2013 at 09:07

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%?

testimento - 5-10-2013 at 16:21

"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]

FreeMirage - 5-10-2013 at 21:52

Sieves are so cheap... I don't know why anyone would bother with a time consuming distillation.

frenchemist1 - 7-10-2013 at 12:12

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.

WPK0129 - 8-10-2013 at 13:40

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.)

testimento - 9-10-2013 at 22:25

Quote: Originally posted by FreeMirage

Sieves are so cheap... I don't know why anyone would bother with a time consuming distillation.

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.

WPK0129 - 11-10-2013 at 13:57

Your remote, totalitarian country will let you get sodium metal?

Pyro - 11-10-2013 at 14:13

sodium is surprisingly easy to get if you look properly, ebay has quite a few offers for element collectors, even some Chinese companies will sell you a mere kg or so for a reasonable price. also it can be bought online in East-Europe

ElectroWin - 11-10-2013 at 16:32

Quote: Originally posted by testimento

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.

sulfuric acid is so useful that its production is used as a measure of a nation's industrial health. such a pity to be poor dude. does your mom cook and clean house?

sorry, i digress

why not dry with anhydrous CaO?

[Edited on 2013-10-12 by ElectroWin]

PeeWee2000 - 11-10-2013 at 16:48

Indeed sodium is quite easy to get if you look in the right place dont forget its a DEA special surveillance item so dont buy too much at once

FreeMirage - 12-10-2013 at 00:00

Quote: Originally posted by testimento

Quote: Originally posted by FreeMirage

Sieves are so cheap... I don't know why anyone would bother with a time consuming distillation.

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.

You can buy them online for $15-20 a pound and they're re-usable if you're allowed to own an oven in said totalitarian country.

Sodium on the other hand has way better uses than drying solvents and it's a one time use material. Also, it's not even as good at drying as a column of sieves.

Hell, I've got a ton of sieves laying around from some alkoxide reactions I was trying out, I'd sell you a pound shipped for $20 (as long as you aren't in some crazy remote part of the world where shipping packages involves a rogue parachutist).

Sodium

cal - 12-10-2013 at 00:45

Quote: Originally posted by PeeWee2000

Indeed sodium is quite easy to get if you look in the right place dont forget its a DEA special surveillance item so dont buy too much at once

Purchased through a Gold refinning supply house there is no watch at all. I have bought 5 lbs at a time.

Ascaridole - 27-10-2013 at 11:13

Just to explain the Magnesium metal method to CaptainPike, the Mg metal reacts with residual water when the mixture is refluxed. Initially you will form Mg(OH)2 + H2 gas and eventually MgO as the water becomes more and more scarce. Eventually the Mg starts to react with the ethanol to form Mg(OEt)2 + H2 gas. The magnesium ethoxide acts as a "getter" to eliminate the last few bits of residual water.

According to purification of Laboratory Chemicals the magnesium method is only done on ethanol at 99% purity. The reason for this (my opinion at least) is that the amount of magnesium needed to pull the 95.5% azeotrope to 99.99% is unreasonable. You will be wasting perfectly good magnesium on removing water that can be easily removed by reflux with CaO or 3A sieves. Also use of the magnesium method to dry ethanol is reserved for highly moisture sensitive reactions such as making alkoxides for reactions very sensitive to water. In those cases the purified ethanol is usually handled via air free techniques.

In industry 200 proof ethanol used for biochemistry is produced by distillation under high vacuum, a very nice and clean product free from benzene and other contaminants can be obtained however it probably is more like 99.9% pure due to absorption of moisture from the air during filling and transport.

testimento - 28-10-2013 at 19:03

Quote: Originally posted by FreeMirage

Hell, I've got a ton of sieves laying around from some alkoxide reactions I was trying out, I'd sell you a pound shipped for $20 (as long as you aren't in some crazy remote part of the world where shipping packages involves a rogue parachutist).

I once tried to look for sieves. One supplier listed them, but it was the same supplier who called the state force after me when I inquired for glass bottles. And hell, I've even got the documents from this left. I'd probably be raided down by special task unit if you'd happen to send those sieves for me. And yes, I live in country where supplying stuff via parachute is cheaper than using the state postal services...

Anyway, drying with NaOH is enough for me so far for ether.