[yiyo]

I would like to ask the details about Two Solvent Recrystallization...

1. Why do we need to use two solvents in recrystallization? How can we define and choose those two solvents?

2. Is it the first step of this technique add the first solvent in heating? And then add the second solvent dropwise into the solution?

3. What is the purpose of adding the second solvent dropwise, instead of in one-portion?

[Babcock_Hall]

It is a forum rule that you must show an attempt before we can help.  Let me ask you a question that might get you started.  What are some properties of the idea recrystallization solvent?

[yiyo]

It is a forum rule that you must show an attempt before we can help.  Let me ask you a question that might get you started.  What are some properties of the idea recrystallization solvent?

Actually, this is the first time I come to this website and ask question... so I don't know about the rules....
The ideal recrystallization solvent should be quite volatile so it is easy to be removed from the crystal. It should not react with the substance that needs to be crystallized. The substance should easy to be melted in hot solvent, but the substance's solubility is low in cold solvent. It should have a boiling point that is lower than the melting point of the crystal(but I don't know why). It should be non-toxic and non-flammable.

[Archer]

Could you give us some idea why you are asking these questions?

Is this part of an assignment or have you made something you need to purifiy using mixed solvent recrystallisation?

[Vidya]

it is basically based on the solubility like solvent 1 is selected in which the compound is completely soluble at its boiling point.This will give a saturated or super saturated solution and now solvent two is added in this dropwise so that slow crystallization of the solid takes place
dropwise will ensure slow crystallization and now you must check why slow crystallization is important?

[yiyo]

it is basically based on the solubility like solvent 1 is selected in which the compound is completely soluble at its boiling point.This will give a saturated or super saturated solution and now solvent two is added in this dropwise so that slow crystallization of the solid takes place
dropwise will ensure slow crystallization and now you must check why slow crystallization is important?

But why can't we just use one hot solvent to dissolve the substance, and then cool it in the environment slowly to form crystals?

[yiyo]

Could you give us some idea why you are asking these questions?

Is this part of an assignment or have you made something you need to purifiy using mixed solvent recrystallisation?

Actually, I have a lab that is related to this topic. But I haven't studied about the Two Solvent Recrystallization... so I have some questions about it.

[Vidya]

here one more  basic requirement is the removal of the rest of the impurities so when you are using one at high temp then may be some undesirable impurities will also go which can be removed by the second solvent.

[yiyo]

here one more  basic requirement is the removal of the rest of the impurities so when you are using one at high temp then may be some undesirable impurities will also go which can be removed by the second solvent.

So both of the solvents will help removing the impurities? The first solvent is used to dissolve all the substances, including the impurities. And the second one is added to form crystals while cooling?

[Archer]

It should have a boiling point that is lower than the melting point of the crystal(but I don't know why). It should be non-toxic and non-flammable.

If your substance has a melting point lower than the boiling point of your solvent then your compound may precipitate as a liquid known as "oiling out" before it solidifies.

[Babcock_Hall]

The substance should easy to be melted in hot solvent, but the substance's solubility is low in cold solvent.

This is the key to your question.  Basically it could be restated as there must be a large change in solubility as a function of temperature.  What do these two properties together bring about with respect to the recovery of material in a recrystallization?  It may be helpful to make up some numbers.  Suppose that 10 grams of solute is soluble in hot solvent and 1 gram is soluble in cold solvent.  What is the maximum recovery under this set of assumptions?

[yiyo]

The substance should easy to be melted in hot solvent, but the substance's solubility is low in cold solvent.

This is the key to your question.  Basically it could be restated as there must be a large change in solubility as a function of temperature.  What do these two properties together bring about with respect to the recovery of material in a recrystallization?  It may be helpful to make up some numbers.  Suppose that 10 grams of solute is soluble in hot solvent and 1 gram is soluble in cold solvent.  What is the maximum recovery under this set of assumptions?

the solute will be easy to be recrystallized when the whole solution mixture is cooling.... the maximum recovery should be (10/11)*100%=90.9%... am i correct?

[Babcock_Hall]

Close; the recovery is 9 g/10 g = 90%.  The remaining 1 gram stayed in solution.  Now let's take another made-up example.  Suppose that your solute is soluble in two solvents A and B:

12 g per 100 mL in hot solvent A
8 g per 100 mL in cold solvent A
1 g per 100 mL in hot solvent B
0.5 g per 100 mL in cold solvent B
10 g per 100 mL in hot solvent (A + B)
1 g per 100 mL in cold solvent (A + B)

where (A + B) represents some mixture of the two solvents.  Which of the three possible solvents is the best choice for a recrystallization?

[yiyo]

Can I use the percentage recovery to determine which solvent is suitable?
If yes, then % recovery when using solvent A = (4/12)*100% = 33.3%
% recovery when using solvent B = (0.5/1)*100% = 50%
% recovery when using solvent (A+B) = (9/10)*100% = 90%
Therefore, solvent (A+B) should be the most suitable one.
Because both of them only dissolve small amount of the solid when worked independently? So we need to mix them to dissolve the solid?

[Babcock_Hall]

I would put it differently.  I would say that the solute is too soluble in A and not soluble enough in B.  However, the mixture of A and B had the desirable property that the solute was soluble when hot but not cold.  How could you use two solvents in practice?  Let us assume that you will start with one solvent and add the other one later.