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Author: Subject: Acetic acid/ sodium hydroxide
Hermes_Trismegistus
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smile.gif posted on 30-12-2003 at 01:13
Acetic acid/ sodium hydroxide


Pardon any foolishness on my part here, but like most science newbies I have more enthusiasm than experience

I've been pouring gallons of aqueous CH3COO- and 2Na+ and OH- down the drains because of a particular reaction I've been running that yields a great deal of it.

How might a man (or woman) seperate out all of the Na as NaOH. and would then there be enough H+ left over to consider the resultant mixture acetic acid?

mother always said "waste not, want not!" and being the dutiful son I am.....:D




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Mumbles
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[*] posted on 30-12-2003 at 10:40


I'm a bit confused on one point. Is the aqueous acetic acid and sodium hydroxide already mixed, or two seperate solutions?

Two seperate and you're set, but I doubt this is the case from the 3rd paragraph.

It is possible in a bit of a round about way. With those two compounds mixed, you'll get water and Sodium Acetate. A simple acid-base reaction. To get acetic acid, you could boil down the solution to crystalise the Sodium Acetate. Then, dehydrate as much as you can. Add an excess of sulfuric acid, and distill. There will be some water in it too.you'll most likely get the azeotrope, which boils at 70ish.

To get the sodium hydroxide back, convert to sodium metal or sodium oxide, then add to water, and voila you have sodium hydroxide. It'd probaby be cheaper, in terms of energy to just purchase sodium hydroxide.
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Saerynide
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[*] posted on 30-12-2003 at 15:05


Hardly seems worth wasting H2SO4 to make acetic acid when you can just buy vinegar, ditto with making Na to turn into NaOH. :P
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Mumbles
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[*] posted on 30-12-2003 at 18:59


Vinegar isn't the easiest thing to concentrate in the world. Water and Acetic Acid form an azeotrope. The azeotrope boils before water. You can't just boil the water and get acetic acid, and most of the acetic acid will boil off with the water. The only truely efficient way, that I know of, to concentrate vinegar is to distill with a fractionating column.

At 5%, you need to distill 5 gallons to get a little under a liter at 100% effiency.
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Al Koholic
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[*] posted on 30-12-2003 at 22:19


Dude, if you are just worried about keeping the post-drain environment clean then you are going to have to do more, especially if you don't want to "add more chemicals" to the mix. For one, the mixture you are putting down the sink SURELY contains acetone, bleach, and chloroform in addition to the mentioned ions. Well, maybe not so much on the bleach side but you get the picture...its highly contaminated. Secondly, I know that sodium acetate isn't all that toxic for the environment especially considering how diluted its getting if it is going into a city sewer system in which case it is probably being removed at a treatment plant.

If you are dead set on 'getting your clean on' then what I would recommend would be to start some sort of a waste jug...like 7 gallon size or so polypropylene. This is typically what I encountered in labs before. Next, now that you have collected all your waste into one area, you need to figure out what to do with it. If you are in a rural area I'm going to guess you have a burning barrel (I know all my neighbors do!!! hahaha). You can stock that with all kinds of rubbish/wood and boil off the water from the solution, evaportating the remaining acetone and chloroform as well. Hopefully for the environments sake, the chloroform doesn't escape the troposphere. Annnnyway, you will then have a nice cake of crystals which I would keep but I suppose you could burn. Convert it to something harmless like sodium carbonate I think. Anyone know what happens when you heat NaAc to decomp? I'd guess it'd give off CO or CO2, H2O and leave a mix of NaCO3 and NaOH. This is basically ashes from burning wood...

Either way you look at it though, the elements came from the earth, got messed with by someone else and put into a rearrangement. Then you got them in this form, rearranged them again and now you will be putting them back into the place they came from where they will eventually rearrange back to more stable forms. Bottom line is, why do you care about the environment because you surely aren't going to disturb it on any but human levels...
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[*] posted on 31-12-2003 at 10:04


Quote:

Bottom line is, why do you care about the environment because you surely aren't going to disturb it on any but human levels...

By definition, the things people care about are on human levels. You should be concerned about waste production if:

  1. You care about peoples' health and comfort.
  2. You care about the structure and diversity of life on Earth.
  3. You care about the elegance of your work.

That being said, the bulk of your contaminants (acetate, sodium, and hydroxide) are relatively harmless. Acetone too. If you're using an excess of hypochlorite, you can release most of the left over chlorine as gas and use it for something else. The dissolved chloroform is somewhat worse. CHCl3 is a common pollutant and many techniques exist to eliminate it, search for "wastewater remediation". You could probably separate it from the solution using activated charcoal. Most of the other products could be destroyed with Fenton type chemistry, but it would be hard to separate them from the acetate.
How much chloroform do you need, anyway? The best solution is simply to recycle your solvent. All these pollutants are common, a little bit down the drain won't hurt much.
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[*] posted on 31-12-2003 at 12:09


Quote:

I want to know whether I should try to precipitate out the hydroxide through simple concentration, or distill off the acetate, or what?

Yes, I know it ionizes to form the hydroxide ion to a somewhat extensive extent, however it is nowhere near as much as say trisodium phosphate, the pKa is'nt terribly extensive. Acetic acid is a weak acid and sodium hydroxide a strong base, neutralization between the two is essentially complete and although you might have hydroxide ion and sodium ion in soluion you will not be able to precipiate out the sodium hydroxide from this solution. If you really want it might I suggest a ion exchange resin...

If you want acetic acid from the mixture Mumbles has already given the direction in which to go.




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Hermes_Trismegistus
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[*] posted on 1-1-2004 at 17:22


Quote:
Originally posted by Mumbles
.

It is possible in a bit of a round about way. With those two compounds mixed, you'll get water and Sodium Acetate. A simple acid-base reaction. To get acetic acid, you could boil down the solution to crystalise the Sodium Acetate. Then, dehydrate as much as you can. Add an excess of sulfuric acid, and distill. There will be some water in it too.you'll most likely get the azeotrope, which boils at 70ish.



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[*] posted on 3-1-2004 at 08:00


I don't think acetic acid forms an azeotrope with water.
It's just a thought, but if you convert it to the calcium salt and pyrolyse it you get acetone which you may find useful.
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[*] posted on 3-1-2004 at 13:35


Acetic Acid forms an azeotrope with water at something like 97 or 98%. It boils at 76.6 degrees. Some references say there isn't, but some say there is. I don't know which is true. I think there is one personally. Otherwise why wouldn't you be able to just heat off the water of vinegar to get glacial acetic acid? If this is possible, I've been missing out for a long time.
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[*] posted on 3-1-2004 at 16:03


Quote:
Originally posted by Hermes_Trismegistus
Quote:
Originally posted by Mumbles
.

It is possible in a bit of a round about way. With those two compounds mixed, you'll get water and Sodium Acetate. A simple acid-base reaction. To get acetic acid, you could boil down the solution to crystalise the Sodium Acetate. Then, dehydrate as much as you can. Add an excess of sulfuric acid, and distill. There will be some water in it too.you'll most likely get the azeotrope, which boils at 70ish.


Of course, with all this boiling you are consuming quite a bit of energy. You might reduce the amount of discarded Sodium Acetate (which is about as harmless a chemical as you can find), but you're burning a large amount of energy to do it. No environmental benefit overall.
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[*] posted on 3-1-2004 at 16:04


If that's true then adding a few percent of water to the acid drops the boiling point by about 40 degrees; that's a big change. For what it's worth the CRC handbook explicitly says there isn't an azeotrope.

OTOH getting the stuff to 97% would do for most purposes and you could always freeze it to remove it from the last few percent of water.
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[*] posted on 3-1-2004 at 20:35
acetic acid - water azeotrope


My 49th ed. (1969) of CRC says that acetic acid and water form a low boiling azeotrope (76.6 deg C) that consists of 97% water. Therefore, the acetic acid cannot be separated from the water as pure acetic acid by fractional distillation, IIRC. I believe this is the same problem that you face with trying to get pure ethanol from ethanol-water mixtures. If this were not true you could obtain the pure acetic acid from vinegar (5% acetic acid) as Mumbles has concluded. A search on Google quickly showed that 30% acid or so could likely be obtained from vinegar by reverse osmosis. But this is not practical for the amateur chemist either due to the specialized membranes and high pressure equipment required.
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[*] posted on 4-1-2004 at 07:50


If acetic acid forms an azeotrope as you say then you will be able to get it pure by simple distillation. (not like alcohol)
Take vinegar (about 95 % water) and distill it. The first thing to come off will be the azeotrope with 97% water. That is a higher water content than the initial mixture. This means that the concentration of acid in the still pot will rise. After all the water has been distilled out as the azeotrope (and taking a fair bit of the acid with it) all that will be left is acetic acid; this can be distilled in turn. OK there is other trash in vinegar but I can't see it making any significant difference.
The reason this works (unlike alcohol) is that you start off with a mixture which loses the unwanted component into the vapour phase. In the same way I could prepare pure water by distilling wine; boil off the alcohol (OK it takes some of the water with it) then distill the water.

I'm just not sure acetic acid does form an azeotrope. I still think that the addition of a few percent of acid dropping the Bpt of the water by nearly 30 degrees is remarkable, so much so that I would expect to have seen a reference to it. (For what it's worth, I guessed the wrong way round for the proportions of acid / water before, it doesn't matter a lot). Also my CRC handbook (1991 student edition) says that the mixture is zeotropic.
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[*] posted on 4-1-2004 at 22:17
fractional distillation of vinegar


unionized,

I must agree that you are right about the azeotrope not preventing the aquisition of pure acetic acid in the pot. Thank you for the correction.

Assuming the azeotrope exists, I calculate that there should be about 10 g of pure acetic acid in the pot if one starts with 500 g of vinegar, neglecting consideration of the impurities that surely exist in vinegar, as you say.

As to whether the azeotrope exists or not would also seem to favor your arguments, especially since you have the later reference. If it does not, the yield would be closer to the theoretical maximum of 25 g. If I had my lab set up I would be trying this tommorrow!
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[*] posted on 5-1-2004 at 15:32


Are you sure you posted that azeotrope proportion correctly Magpie? Everytime I've seen it, it is 97% Acetic Acid and 3% water, not the other way around.

To get up to glacial from this point, refluxing over CaCl2 or MgSO4 is supposed to work. Those are both anhydrous of course. A tertiary azeotrope, like Benzene should work too.
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[*] posted on 5-1-2004 at 16:06


Does anybody have any evidence for the existance of the azeotrope?
I looked in the library at work and I couldn't find any (most recent CRC book explicitlty excludes it, Kirk Othmer doesn't mention it (but it goes on about the eutectic))
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[*] posted on 5-1-2004 at 20:00


<blockquote>quote:<hr>Add an excess of sulfuric acid, and distill. There will be some water in it too.you'll most likely get the azeotrope, which boils at 70ish<hr></blockquote>I expected someone mention: no azeotrope when distilled with conc sulfuric acid => PURE acetic acid (unless acetic acid is more eager for the water than sulfuric acid!!!!:o:O:P)



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[*] posted on 5-1-2004 at 20:01


<blockquote>quote:<hr>Add an excess of sulfuric acid, and distill. There will be some water in it too.you'll most likely get the azeotrope, which boils at 70ish<hr></blockquote>I expected someone mention: no azeotrope when distilled with conc sulfuric acid => PURE acetic acid (unless acetic acid is more eager for the water than sulfuric acid!!!!:o:O:P)



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[*] posted on 6-1-2004 at 10:23


Some more info

acetic acid purification
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[*] posted on 6-1-2004 at 11:07


Looks like there isn't an azeotrope if that website is right.
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[*] posted on 6-1-2004 at 15:29
azeotrope verification


Mumbles,

I checked my 49th ed CRC again - it says 97% water.

My 4th ed. of Perry's (1963) and my 10th ed. of Lange's (1967) do not list this azeotrope. In fact, the Lange's specifically says "No azeotrope."

Guaguanco's reference is most interesting. Preparation of glacial acetic acid from vinegar does not apear to be a trivial problem. My text on chemical engineering processes (Shreve's "The Chemical Process Industries", 2nd ed) does not list it, but infers that it is made directly from the oxidation of paraffins. Again - not likely practical for the amateur chemist.

If someone here can come up with a reasonably cheap and easy way to make glacial acetic acid from vinegar they would likely be awarded a Medal of Honor from Vulture (or at least honourable mention).
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[*] posted on 6-1-2004 at 15:56


Add lime
dry
add conc H2SO4
Distill (gently)

Or
Freeze out most of the water
distill
freeze out pure acid.
Both of these might be improved by fermenting a higher alcohol concentration in the first place. That way you have less water to get rid of.
The bacteria will run up to about 10% acid IIRC and you might be able to do better still by adding the lime to remove the acid as the bacteria produce it.
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[*] posted on 6-1-2004 at 16:42


Quote:
Originally posted by Magpie
Mumbles,

I checked my 49th ed CRC again - it says 97% water.


The 54th edition (1973-1974) says '97% water'
I trust my CRC. CRC good.:)

[Edited on 7-1-2004 by guaguanco]
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[*] posted on 8-1-2004 at 15:02


My (1991) edition still says it doesn't.

Old CRC good; new CRC better (because they have removed some of the mistakes).

It doesn't matter how many times you check it if it's wrong.
Has anyone out there got a still, a thermometer and some vinegar? That way we can get can experimental verification one way or the other.
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