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Author: Subject: What is it about basification that causes layer separation?
Leben
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[*] posted on 31-12-2014 at 21:16
What is it about basification that causes layer separation?


Why does adding sodium hydroxide cause layer separation among water and solvents that it would usually be miscible in?

It seems this is a reoccurring thing in many research studies, pre-extraction. Does raising the PH decrease the miscibility of some solvents in water? Or am I misinformed?
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[*] posted on 31-12-2014 at 22:32


what I remember from biochemistry is basically, say you have pure water and you add oil to it. obviously they won't mix. but with just liquid water the molecules are able to freely rotate. when you add whatever obligatory oil, the water molecules on the surface in contact with the oil will sort of rigidly align themselves, as well as the oil molecules in the layer of oil in contact with the water. they do this to further reject each other to maintain that separation. in biochem especially when dealing with proteins, its common when something that should precipitate out but for some reason doesn't, you add salt to force precipitate it. when you dissolve a salt or NaOH, the molecules will realign themselves to maintain solution of what is soluble, in turn, say a molecule that should precipitate has regions that have partial charges that are attracted to the water. the realignment will potentiate the rejection of the insoluble substance, this is effect is coupled with the partial charges of the water now being occupied by the soluble substance not allowing those partial charged regions to "bind" to the water.

I've seen this effect that you speak of with separated alcohol/water mixtures. I assume it follows this principle to some effect. though with alcohol in water you would only separate a portion of the alcohol. a good deal should stay soluble.
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[*] posted on 31-12-2014 at 23:02


The salt is more strongly bound to the water than the organic compound, so the organic compound is pushed out. Doing this with organic acids forms their salts, which move into the aqueous layer which can then be re-acidified to get the acid again, removing it from the ether, ethyl acetate, or whatever you're extracting with.



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[*] posted on 1-1-2015 at 01:53


Basically, the base you're adding is an ionic compound, so you're salting out the organic species. A mixed solvent of an ionic compound and water is a poorer match for an organic compound than water alone, just like a mixture of ethanol and water is a better solvent for organic compounds than pure water (and a worse solvent for ionic compounds).



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[*] posted on 1-1-2015 at 07:45


As mentioned, it is not basification as such, rather it is a salting-out effect: where the salt just happens to be a base. The reason why the salt just happens to be a base so often is that bases such as sodium hydroxide and potassium carbonate are cheap and readily available, whereas equally effective neutral salts such as potassium fluoride are comparatively expensive, and hence they are not used (nor spoken of) so often, although they may be used with similar effects.
Separation of alcohols is often salt-specific; methanol can be separated from water by addition of K2CO3, but NaOH will not separate the two because methanol will dissolve NaOH. Ethanol can be separated from water with K2CO3 (I think) or KF etc. Isopropanol can be separated from water with NaOH efficiently, but other salts such as NaCl will dry it to a lesser extent. Once one gets to n-butanol and higher, then the alcohol itself is sparingly soluble in water, so salting out is of little concern, but the same principle applies towards drying residual water.
As for ketones, they reach immiscibility even more quickly than alcohols: acetone may be salted out of water with NaOH or other salts; and so on.
Water soluble ethers such as THF may similarly be salted out of water by using salts, but using a basic salt is often a useful means of purifying a compound that is dissolved in the THF, so there is sometimes a practical advantage to using a base to salt out.

[Edited on 1-1-2015 by forgottenpassword]
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[*] posted on 7-1-2015 at 05:14


Hmmm this is what I thought, but I did just read somewhere that adding an organic-water miscible solvent to water with ions dissolved in it, is an effective way to get the ions to salt out. This is almost completely opposite to what everyone here is saying.

I did think the water would have a higher affinity for the ions than for the salt.

Quote: Originally posted by forgottenpassword  

As for ketones, they reach immiscibility even more quickly than alcohols: acetone may be salted out of water with NaOH or other salts; and so on.
Water soluble ethers such as THF may similarly be salted out of water by using salts, but using a basic salt is often a useful means of purifying a compound that is dissolved in the THF, so there is sometimes a practical advantage to using a base to salt out.

[Edited on 1-1-2015 by forgottenpassword]


So do you imagine I could get a pretty good separation of a water-acetone or water-butanone mixture by saturating the water? With butanone I think this wouldn't be too difficult seeing as it's only partially miscible with water at 200g/L, but acetone is entirely miscible.
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[*] posted on 7-1-2015 at 09:35


Quote: Originally posted by Leben  
Hmmm this is what I thought, but I did just read somewhere that adding an organic-water miscible solvent to water with ions dissolved in it, is an effective way to get the ions to salt out. This is almost completely opposite to what everyone here is saying.


Depends on the solvent and the ions. Sometimes adding an organic solvent is a good way to make an ionic compound precipitate. Sometimes it's the other way around.




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