Leben
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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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Furboffle
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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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subsecret
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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.
Fear is what you get when caution wasn't enough.
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DraconicAcid
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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).
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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forgottenpassword
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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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Leben
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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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DraconicAcid
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| 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.
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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