Sciencemadness Discussion Board

Potassium Sodium Tartrate Not Crystallizing

gdflp - 17-2-2014 at 09:50

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?

blogfast25 - 17-2-2014 at 12:47

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?

gdflp - 17-2-2014 at 13:22

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?

blogfast25 - 17-2-2014 at 14:06

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]

gdflp - 17-2-2014 at 14:42

I don't think I can evaporate more water, it's a little thicker than jello right now.

UnintentionalChaos - 17-2-2014 at 15:00

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]

Metacelsus - 17-2-2014 at 15:26

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)?

Paddywhacker - 17-2-2014 at 20:35

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.

blogfast25 - 18-2-2014 at 11:05

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.