Removing impurities via recrystallization

[scienceboy2000]

I have read online that suppose I am trying to purify compound_A, which contains an impurity, let's 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 it`s 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, let`s call it compound*. I know that compound* is impure. The solvent I am 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.

 

[Borek]

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.

 

[scienceboy2000]

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?

 

[Borek]

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

 

[LudusRex]

I can confirm that recrystallization is a common technique used for purifying compounds. The principle behind this process is that different substances have different solubility in a given solvent at different temperatures. By carefully selecting the appropriate solvent and controlling the temperature, we can selectively dissolve the desired compound and leave behind impurities.

In your case, it seems that the solvent you have chosen (water) is able to dissolve both your compound* and its impurity at high temperatures. However, as the solution cools down, the solubility of both compounds decreases and they start to crystallize. This process should ideally result in the impurity remaining in the solution while the desired compound* forms crystals.

However, it is possible that during the process of cooling and crystallization, some of the impurity may have become trapped within the crystals of compound*. This could be due to a number of factors, such as the solubility of the impurity increasing at lower temperatures or the crystals forming too quickly, not allowing enough time for the impurity to separate out.

To answer your question, it is possible that the resulting crystals may still contain some impurity. The level of purity will depend on the effectiveness of the recrystallization process and the nature of the impurity itself. It is important to carefully monitor the process and analyze the purity of the resulting crystals to ensure that the desired compound has been successfully purified.

In conclusion, recrystallization can be an effective method for removing impurities from compounds, but it is important to carefully consider the choice of solvent and the conditions of the process to ensure the best results. I would also recommend consulting with a more experienced chemist or conducting further research to optimize your recrystallization technique.