Sciencemadness Discussion Board

separation by crystalization

bdgackle - 12-4-2010 at 15:54

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.

12AX7 - 12-4-2010 at 21:30

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

hissingnoise - 13-4-2010 at 04:18

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. . .

watson.fawkes - 13-4-2010 at 04:22

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".