gdflp
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Potassium Sodium Tartrate Not Crystallizing
I made some potassium sodium tartrate using potassium bitartrate and sodium carbonate. I filtered the solution while it was hot and then let it cool
and evaporate, but now a few weeks later I have a light brown gel at the bottom of my beaker. Is there any special thing I need to do to crystallize
it, and why is it brown?
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blogfast25
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Quote: Originally posted by gdflp  | | I made some potassium sodium tartrate using potassium bitartrate and sodium carbonate. I filtered the solution while it was hot and then let it cool
and evaporate, but now a few weeks later I have a light brown gel at the bottom of my beaker. Is there any special thing I need to do to crystallize
it, and why is it brown? |
Do you know what quantities you used? Did you check solubility of the salt?
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gdflp
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I used about 20g of potassium bitartrate. The salt was fully dissolved when I first did the synthesis. Should I redissolve it and try cool it to
initiate crystallization?
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blogfast25
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| Quote: Originally posted by gdflp | | I used about 20g of potassium bitartrate. The salt was fully dissolved when I first did the synthesis. Should I redissolve it and try cool it to
initiate crystallization? |
I meant the solubility of the end product. One preferred trick in synthesis is to control the product concentration and end temperature of the
solution so that on cooling the solubility limit of the product is exceeded and it then crystallises out.
Of course that doesn't work if your product has a flat solubility vs. temperature curve.
See also:
http://en.wikipedia.org/wiki/Solubility_table
But it appears that La Rochelle's salt has a very high RT solubility (63 g/100 g water, Wiki), so that means you need to slowly evaporate water,
before crystals will start to form. Maybe you just need to evaporate more water...
[Edited on 17-2-2014 by blogfast25]
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gdflp
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I don't think I can evaporate more water, it's a little thicker than jello right now.
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UnintentionalChaos
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I couldn't get homemade rochelle salt to crystallize either, using cream of tartar and bicarbonate quite a long time ago. If you used baking grade
potassium bitartrate, it is probably produced by fermentation as a byproduct of making wine and grape juice and has other (harmless
when used for cooking) organic gunk in it that makes the solution brownish. This may also inhibit crystallization. I'm under the impression that
Rochelle salt is also prone to supersaturation, so seeding is probably a good idea.
You can purchase pure crystalline tartaric acid through a place that would sell beer/wine supplies. It is used to adjust the acidity of the starting
grape juice. If it's not acidic enough , competing yeast/bacteria can grow and ruin the batch. It also has considerable buffering capacity.
[Edited on 2-17-14 by UnintentionalChaos]
Department of Redundancy Department - Now with paperwork!
'In organic synthesis, we call decomposition products "crap", however this is not a IUPAC approved nomenclature.' -Nicodem
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Metacelsus
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The brown gunk is probably impurities in the starting bitartrate. You could try recrystallizing it first.
Also, is it possible that microorganisms "ate" the tartrate (if you left it out for a couple weeks)?
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Paddywhacker
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Salts don't always crystallize when you would expect. A few years ago I made up a series of tartrate and citrate salts and most of them evaporated
down to glassy concentrates.
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blogfast25
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| Quote: Originally posted by Paddywhacker | | Salts don't always crystallize when you would expect. A few years ago I made up a series of tartrate and citrate salts and most of them evaporated
down to glassy concentrates. |
True. It's quite common with some stuff.
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