bdgackle
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separation by crystalization
Given an aqueous solution of sodium nitrate and potassium nitrate, I've seen procedures for separating the two that involve repeated heating and
cooling, taking advantage of the differences in solubility to selectively crystallize one or the other. I've also seen the claim that the two
substances can be completely separated in this manner by repeated application of this technique. I've seen this specifically described in Caveman
Chemistry, and in several places on the web describing the extraction of potassium nitrate from soil.
I have a couple of questions about this procedure. First, how is repeated crystalization helpful? It seems that you could evaporate water until one
substance starts precipitating, and get a pure sample of that substance, but only until you had a saturated solution of BOTH, then any additional
evaporation would precipitate both. How does one exploit this property to continue the separation?
Second, where can I find more info on the quantitative side of solubility? Basic textbooks always describe things quantitatively, but I am having
trouble finding more. I get the feeling I'm missing vocabulary I need to search for this information. I'm willing to do the legwork, but if someone
can supply me with some search terms that would be very helpful.
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12AX7
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Quote: Originally posted by bdgackle  |
I have a couple of questions about this procedure. First, how is repeated crystalization helpful? It seems that you could evaporate water until one
substance starts precipitating, and get a pure sample of that substance, but only until you had a saturated solution of BOTH, then any additional
evaporation would precipitate both. How does one exploit this property to continue the separation? |
Two things: one, in the first case, the substance crystallized isn't entirely pure. Obviously there's some adhering solution that you can never quite
wash away, but there are also impurities entrained or dissolved in the crystals. Indeed, many crystalline systems have a solid solubility range, for
instance Al(3+) and Cr(3+) ions are similar enough that regular alum can be grown on top of a crystal of chrome alum and vice versa, making a peculiar
crystal with layered colors.
The other is that, some systems will always precipitate the less soluble material; others will alternate, and some will precipitate both so finely
that all you get is an inseperable mush. To abuse terminology a bit, these might be called "eutectic solutions". (The abuse is because 'eutectic'
refers to a system of constant composition, not one where a component (i.e., the solvent) is varying. The system still follows a eutectic line,
though.)
Tim
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hissingnoise
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The difference in solubilities of those salts is more marked at low temperatures; they converge as temperature increases.
Cooling solutions to 0*C will precipitate most of the KNO3. . .
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watson.fawkes
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| Quote: Originally posted by bdgackle | | First, how is repeated crystalization helpful? It seems that you could evaporate water until one substance starts precipitating, and get a pure
sample of that substance, but only until you had a saturated solution of BOTH, then any additional evaporation would precipitate both. How does one
exploit this property to continue the separation? |
You want the paper referenced in this post, titled "How to Design Fractional Crystallization Processes".
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