Removing impurities via recrystallization

  • Thread starter Thread starter scienceboy2000
  • Start date Start date
  • Tags Tags
    Impurities
Click For Summary

Discussion Overview

The discussion revolves around the process of recrystallization for purifying a compound, specifically addressing the choice of solvent and the behavior of impurities during the crystallization process. Participants explore theoretical and practical aspects of recrystallization, including the solubility of compounds in different solvents and the effectiveness of various methods to achieve purity.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Experimental/applied

Main Points Raised

  • One participant notes that for effective recrystallization, the solvent must either dissolve the target compound while leaving impurities behind or allow both to dissolve initially but keep impurities in solution during cooling.
  • The same participant questions whether their crystallization process was successful, given that both the target compound and impurity dissolved in water, leading to complete crystallization without separation of impurities.
  • Another participant emphasizes that if the crystallization process resulted in a dry product, it likely contains impurities, suggesting that the correct procedure involves removing solvent with impurities before crystallization.
  • A different participant proposes using a multi-solvent system, suggesting a method where a saturated solution of the target compound in water is created before adding acetone to induce precipitation, and questions the optimal approach for adding acetone.
  • This participant also raises a concern about the presence of water in acetone and its potential impact on the precipitation process.
  • Another participant suggests washing the solid with acetone as an alternative method to remove impurities.

Areas of Agreement / Disagreement

Participants express differing views on the effectiveness of the recrystallization methods discussed, with no consensus on the best approach to achieve purity. The discussion remains unresolved regarding the optimal solvent system and the impact of impurities on the final product.

Contextual Notes

Participants mention various assumptions about solubility and the behavior of solvents, but these are not fully explored or resolved within the discussion. The effectiveness of the proposed methods is contingent on specific conditions that are not entirely clarified.

scienceboy2000
Messages
7
Reaction score
0
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.
 
Chemistry news on Phys.org
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.
 
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:
Why don't you just wash the solid with the acetone?
 

Similar threads

Potassium alum: how to prevent recrystallization in water?
  • · Replies 1 ·
Replies
1
Views
2K
Simple question about dissolving a compound
  • · Replies 4 ·
Replies
4
Views
3K
Recrystallizing but not getting any crystals
  • · Replies 2 ·
Replies
2
Views
6K
Cleaning a small batch of HDPE flakes obtained from motor oil containers
  • · Replies 2 ·
Replies
2
Views
3K
Can ammonium nitrate be safely used as a water pigment in dioramas?
  • · Replies 10 ·
Replies
10
Views
3K
Recrystallizing out of methanol
  • · Replies 3 ·
Replies
3
Views
4K
Why Isn't My Quaternary Amine and Ester Compound Soluble in Organic Solvents?
  • · Replies 17 ·
Replies
17
Views
6K
Recrystallization in the case of two solvents
  • · Replies 1 ·
Replies
1
Views
3K
Terminology? (heated to reflux, washed, etc)
  • · Replies 2 ·
Replies
2
Views
12K
What Do Flame Tests Reveal About Sodium and Chloride?
  • · Replies 6 ·
Replies
6
Views
3K