Removing impurities via recrystallization

1. scienceboy2000

7
I have read online that suppose I am trying to purify compound_A, which contains an impurity, lets call that compound_B, that the solvent that I choose for recrystallization must either:

1) dissolve compound_A but not compound_B, so that compound_B can be filtered out after dissolving compound_A in the solvent

or

2) Both compound_A and compound_B must both be completely soluble in the solvent (at least when its hot) so that when crystallization starts to occur, compound_B remains in the solvent.

I have a question. I do my own experiments and I am trying to recrystallize a compound, lets call it compound*. I know that compound* is impure. The solvent Im using is water. Now, the hot water dissolves both compound* and its impurity. So according to what I read online I would expect the impurity remain in the solvent while crystallization occurs (i.e. while the solution is cooling to room temperature). However, after leaving the solution for several hours, there is no more solvent. That is, everything has been crystallized. Does this mean that the resulting crystals are just as impure as my original compound? Does it contain the original impurity that I was trying to get rid of?

All help would be appreciated.

Staff: Mentor

Yes, if you dried it out nothing has changed. Correct procedure calls for getting rid of solvent containing mostly impurities (and as small amount of the compound you are purifying as possible). Finding correct solvent and correct conditions is not always easy.

3. scienceboy2000

7
I guess I could use a multi-solvent system. My compound is extemely soluble in water and so is its impurity, even at room temperature, so water is not a good solvent for this. But my compound is completely insoluble in acetone while its impurity is soluble in acetone. So I could dissolve my compound in the smallest amount of water possible, creating a saturated solution, then slowly add acetone to force it to precipiate out. My question is, do I add acetone until precipitation starts, then leave it to its own devices? Or do I add acetone until precipitation completely stops?

Also, my acetone will most definitely not be anhydrous. Let's assume that there's a 5% concentration of water in the acetone. Will this effect my plan to force precipitation via adding acetone?

Last edited: Jan 4, 2014

Staff: Mentor

Why don't you just wash the solid with the acetone?