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Topic: recrystallization technique  (Read 5432 times)

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briteyellowness

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recrystallization technique
« on: September 10, 2005, 11:01:49 PM »
hi, we're recrystallizing phthalic acid and my lab guide says i need to cool it in an ice bath and if the crystals have formed a solid mass, break it up.  ... which i don't get.  what's wrong with forming crystals?  i thought that's what recrystallization is all about, what's the point of breaking it up?  later is says that we're going to collect the purified crystals through  a hirsch funnel thingie, if that helps with why it's broken up.. thanks if you can help.
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Offline Organic_lover*

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Re: recrystallization technique
« Reply #1 on: January 21, 2012, 08:12:23 PM »
Hey briteyellowness.

I think that you need to break the solid mass formed because when crystals are formed they can "bond" to other species present in the reactional mixture, like impurities and reagents (in excess or some molecules of them that haven't react).

The procedure of use a hirsch funnel it's to remove this species through several washes. The crystals will be obtained with a better purity. :)
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Offline Arkcon

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Re: recrystallization technique
« Reply #2 on: January 21, 2012, 08:44:58 PM »
I figure that batch of crystals from 5 and a half years ago is long past taken care of. ;D
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Offline orgopete

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Re: recrystallization technique
« Reply #3 on: January 21, 2012, 11:01:44 PM »
If I remember correctly, quite a lot of phthalic acid can dissolve in hot water and most of it can be recovered upon cooling. If you are doing this on a small scale, for example in a test tube, the entire mass of crystallized phthalic acid will be lodged in the test tube.

The instructions to break it up is for those people whose phthalic acid is lodged in the bottom of their test tube. If your sample differed, for example by using more water (my advice), then you may be able to collect it easily without breaking it up. I suggest using more water rather than trying to dissolve the phthalic acid in the minimum amount of water. This simply speeds the experiment up. The entire recrystallization can probably be completed in about 5-10 minutes if you don't use the minimum amount of water.
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Offline zaphraud

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Re: recrystallization technique
« Reply #4 on: January 28, 2012, 12:56:50 AM »

If you break up a large crystal in a saturated solution into several small crystals, the available surface area for crystallization increases. The result is that the remaining crystallization happens faster due to more available surface area.
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