Tuesday, January 22, 2013

Crystalization -- Purifying organic chemicals

In lab last week we purified some organic chemicals using a technique known as crystallization  It's fairly simple and something you could easily do at your home or in your DIY lab.

There is a seven step process to recrystallizing a substance and removing impurities.
1. Choose a solvent
2. Dissolve the solute
3. De-color the solution
4. Filter suspended solids
5. Crystallize the solute
6. Collect and wash the crystals
7. Drying

1. Choosing a solvent.
The way crystalization works is, you place some impurity into a solution that will dissolve it. However, you don't want it to dissolve it very well at room temperature. it needs to be insoluble at room temperature, but very soluble at boiling point. For example, benzoic acid, when placed in water at 10 Celsius, only  2.1 grams will dissolve in 1 liter of water. When the water is brought to boiling, however, it will dissolve 68 grams in one liter.  
 - The essence is, find a solvent that will dissolve your solid while boiling, but is very poor at dissolving it while cool. 
- Some solutes (the solid you are trying to dissolve) are way too soluble in one solvent and not soluble enough in another solvent. To dissolve this problem, you simply heat up and dissolve as much solute as you can in the solution that is very soluble. Then you use the solvent that is not very soluble, and mix it together so that the over-all solution is now not very soluble. 

2. So, once you've picked the proper solvent for your crystallization, you need to dissolve your substance in that solvent. There are two ways this can be accomplished. 
You want to make sure you don't add too much solute because then re-crystallization will not give you the maximum possible yield. So, to make sure you use "just enough" solvent, you can either pick a desired amount of solvent and then heat it to a boil, then continue to add solute until it stops being dissolved.
- Another way to do this is to pick the amount of solute you need to purify, and place it in a beaker and heat the beaker, then slowly add boiling solvent until finally the last bit of solute is dissolved.
 - - This would be my preferred method because you will typically know how much solute you need to purify  before you start.
- Another way is to find the solubility product for each solute and solvent mixture at boiling and at another known temperature. Calculate the amount you can dissolve in that and use that theoretical amount as the amount you measure out for both the solute and the solvent. Then it can be adjusted by a few drops after it's all mixed together.

3. De-color the solution. -- you can use activated charcoal to remove colored impurities from your solution if they exist. To do this, make sure you have a good straining method to remove the charcoal from you solution when it is decolored. Decoloring should be done while the solution is still warm.

4. Using a filter (anything as simple as a coffee filter should work in most cases), you can take out any solids that were not able to be dissolved. These solids may be impurities or they may be undissolved particles of your solute (because you didn't have enough solvent)

5. To crystallize the solution, you simply let it cool. As it cools it should start creating crystals, but in the case that it doesn't do this by itself, this is called "supersaturation" and typically any disturbance will rapidly start the crystallization process.
You don't want your crystals to form too fast because if they do, they may re-trap some of the impurities that you were trying to remove. So, to prevent your solution from becoming super-saturated, you can simply add a very small seed of the solute (something for the crystals to adhere to) and it will move along from there.
If you don't have anything to re-seed it with, sometimes just scratching the inside of the glass will cause crystallization to occur (or even just tapping the glass against something lightly).

6. You want to dump out your leftover liquid (because it contains the impurities). Therefore, if you boiled out all of the liquid, you will have to re-do the entire experiment. You want that liquid to carry away the impurities when it leaves. So, you dump the liquid, then add more liquid to it (preferrably cold so that it doesn't dissolve all of your crystals), and then you dump it again. This is called Washing.

7. Finally, remove your crystals and let them dry. You can dry them by heating them slightly or by placing a vacuum hose over your glassware. This will decrease the vapor pressure and allow the liquid to evaporate at a higher rate.

I tried to be as non-technical as I possibly could I challenge you to pick a step and try to explain it in a more non-technical way. The only thing I can think of is to not use the words: Solute, Solvent, and Solution. What can you think of?

Lastly, can anyone think of a good "at home" recrystallization that could be added for practice? I know sugar and water are too soluble. I'm wondering how soluble flour, or sugar are in alcohol or acetone. I may try a few things in the kitchen this weekend and let you know how it goes.

No comments:

Post a Comment