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Every article or text I've read on crystallization states that it works by taking advantage primarily of differences in solubilities, but I've also been told that it can purify compounds with equivalent solubilities if one is present in a greater quantity.

For instance, if there is a solution of 90% Compound A, and 10% Compound B, both with equal solubilities, a crystallization would supposedly purify it. But how? I've been told that, if say, 20% of the product was left behind in the solution, then it would consist of equal ratio of Compound A and Compound B, causing the formed crystals to be almost pure Compound A (saying a 1:1 ratio of A:B gets left behind). I was told that this ratio that gets left behind in the solution depends on the relative solubilities of the compounds, and not the quantity that which they were present.

Is this accurate? How does this work exactly? I've read dozens of articles on crystallization and none of them seem to explain this sort of instance well, yet it seems very widely used in preparative chemistry.

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    $\begingroup$ I don't see what is there to explain. I seems you don't understand what solubility is or something else? $\endgroup$
    – Mithoron
    Mar 2, 2018 at 17:34
  • $\begingroup$ @Mithoron Intuitively if both have equal solubility why would they not both precipitate out equally? Why would Compound A precipitate out selectively? Wouldn't intuition dictate that the precipitate contain 90% of A and 10% of B, if the solubilities equal? I'm told this isn't the case, and that goes against intuition... Do you see what I mean? $\endgroup$
    – John
    Mar 2, 2018 at 17:46
  • $\begingroup$ Well then of course you don't understand solubility - only surplus over solubility value crystallises, not all of compound. $\endgroup$
    – Mithoron
    Mar 2, 2018 at 17:56
  • $\begingroup$ @Mithron Clearly I don't. Usually solubilities are affected by other compounds dissolved in solutions, at least in instances of salting out and such. If B is soluble at 1g/L in a solution, and A is the same, and 1g of each is added to said solution, would they both dissolve? Intuition about dissolved compounds affecting solubility would suggest that only 1g max of the 2 compounds (total) could be dissolved.Or would 2g total?Or is this an incorrect understanding of solubility? I'm basing this off my understanding of how salting out works, decreasing solubility of one compound by adding another. $\endgroup$
    – John
    Mar 2, 2018 at 18:15
  • $\begingroup$ chemistry.stackexchange.com/questions/72700/… $\endgroup$
    – Mithoron
    Mar 2, 2018 at 18:22

1 Answer 1

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Such recrystallization procedures are often used in organic chemistry where A and B would both be dissolved in hot solvent, but a large amount of A would precipitate as the solution cools. The rub here is that as A precipitates it will also adsorb some of the B. So to purify A further, often a second, or third, crystallization may be necessary.

So imagine that 20 g of each of A and B are soluble in some quantity of the cold liquid. You start out with 100 g total of solute, so 90 grams of A and 10 grams of B dissolve in the hot solution. So all of the B would be soluble in the given quantity of cold solution, but only 20 grams of A. So 70 grams of A should precipitate.

Notice that in order for this method to work all of A and B must dissolve in the hot solution. The idea is that crystals of compound A prefer to have A in the crystals rather than B. But if A has been coating grains of B, then if the whole solid particle is not dissolved, a lot of B will still be in the solid matter which didn't dissolve.

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  • $\begingroup$ So, to put this in mechanical terms, would you say that because A is present in a much larger quantity, it's formed crystals will have greater surface area, and thus crystalize faster than the less present compounds? In a sense, would it be the surface area that facilitates the fractionation of the ratios, so to speak? I'm basically just trying to understand the variable which causes the ratios to shift from solution to crystal. $\endgroup$
    – John
    Mar 2, 2018 at 17:55
  • $\begingroup$ No that is the wrong notion. Let's say that 70 g of B are soluble in the hot solvent, but only 20 grams of B in the cold solvent. So I get the solvent hot, dissolve 10 grams of B. How much B will crystallize out when the solution cools? $\endgroup$
    – MaxW
    Mar 2, 2018 at 18:12
  • $\begingroup$ 50g. My confusion seems to relate to how compounds affect the solubility of each other. In your example in your original response, why would 30g total stay dissolved in solution? (20g A 10g B)... if the solution can only hold 20g of either of those compounds, how can 30 stay dissolved? Rather, if 20g of B is already dissolved in the solution, and A has an equal solubility, how would any additional A be able to become dissolved, without precipitating B? My understanding is that solubilities depend on the solvent having nothing else dissolved in it - Based on my understanding of "salting out" $\endgroup$
    – John
    Mar 2, 2018 at 18:23
  • $\begingroup$ HUH? I only dissolved 10 g of B in the hot solvent. How can I be getting 50 grams of ppt in the cold??? $\endgroup$
    – MaxW
    Mar 2, 2018 at 18:34
  • $\begingroup$ In the original comment the notion is that 20 g of A and 20 g of B are soluble in the cold solvent. Since there are only 10 g of B, all of the B is soluble in the cold solvent. $\endgroup$
    – MaxW
    Mar 2, 2018 at 18:36

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