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I have compound A contaminated with impurity B. Recrystallization in water would not yield pure compound A since impurity B is less soluble in it. The only solvent in which substances A and B have opposite solubility is glycerol. Compound A is soluble in it while impurity B is insoluble.

I would propose my first step to be heating the glycerol to minimize the amount of solvent needed to fully dilute compound A. I would then filter the solution to have my purified compound A in glycerol solution (impurity B would remain in filter).

As far as I have heard though, glycerol is not as easy to boil as water, being similar to cooking oil. Unfortunately, I don't have a vacuum distillation apparatus to extract the glycerol without caramelizing it.

I was thinking of slowly add anhydrous ethanol to (purified compound A + glycerol), since compound A is insoluble in it. I would expect multi-solvent recrystallization to occur as I add ethanol and lower the mixture temperature.

Would it work and does it seem to be the easiest method of purification at home?

For the curious: $A = \ce{KNO3}, B = \ce{K2SO4}$.
No, I don't want the $\ce{KNO3}$ to make a bomb, but to make a sugar rocket for model rocketry.

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  • $\begingroup$ That's a lot of thinking for making sugar rocket... $\endgroup$
    – Mithoron
    Nov 20, 2017 at 0:56
  • $\begingroup$ @Mithoron I have heard it might work well even when contaminated with 7% K2SO4, what I have now. But I would like to test with pure KNO3 to see if I can extract some extra juice from it. $\endgroup$
    – Adam Smith
    Nov 20, 2017 at 7:11
  • $\begingroup$ Well actually you can make a bomb with only sugar... so don't worry about your question. $\endgroup$
    – ParaH2
    Nov 20, 2017 at 20:06
  • $\begingroup$ As an inverse to recrystallisation; you could add varying quantities of water to the same mass of impure solid and experiment with varying temperatures. The goal here is not to dissolve everything, but to dissolve up more A and leave B behind. i. e. A warm reslurry. When you get your best result, filter B off warm. $\endgroup$
    – Beerhunter
    Nov 21, 2017 at 19:02
  • $\begingroup$ @Beerhunter Best result would be at 100C, and it has a mass ratio of 91% between KNO3 and K2SO4. My KNO3 is already at 93% in the mixture. I would actually lose purity. Unless you are supposing the K2SO4 solubility would be reduced by the KNO3 presence, and not the opposite. Could you precise further? $\endgroup$
    – Adam Smith
    Nov 21, 2017 at 22:50

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