nlegaux - 5-1-2015 at 13:36
The past few days I have tried twice to synthesize and grow crystals of Rochelle's salt. My procedure is as follows:
- Create 80ml of saturated Potassium Bitartrate solution (I used Cream of Tartar because the MSDS claims it is 99.5% pure) in a 100ml beaker
- Add small amounts of Sodium Carbonate (created by decomposing Sodium Bicarbonate) until gas is no longer evolved
- Boil solution down to less than 20ml and allow to sit overnight
The first time I attempted the synthesis, no crystals formed. I let it sit 2 days, and at the end of the second day mold began to grow in the
solution, so I disposed of it. The second time I attempted the synthesis, large dendritic crystals grew up the sides of the beaker. By the end of the
next day, a mass of small needle-like crystals had grown throughout the solution. I still have the solution/crystals, but they do not seem consistent
with what I have read about Rochelle's salt/Potassium Sodium Tartrate. Any ideas on what I may be doing wrong and/or any improvements to my procedure
you can think of?
Sorry if this would fit better in the beginnings section...
Thank you,
nlegaux
Pasrules - 5-1-2015 at 13:40
Just a simple trick. when you try for a third time use the crystal from your second run as a seed to encourage crystal formation.
And be very gentle the moment you let it cool, just sit it on the bench and let it do it's thing.
[Edited on 5-1-2015 by Pasrules]
DJF90 - 6-1-2015 at 00:17
The data I found states that potassium bitartrate has an aqueous solubility of 6 mg/mL. Given the vastly improved solubility of Rochelle salt, you may
be much better off slurrying the bitartrate with a stoichiometric amount of e.g. 2M hydroxide solution. Given the acid-base reaction, the solids
should dissolve over time to give a full solution of rochelle salt. Any undissolved matter can be filtered off before concentration and
crystallisation. It may be advantageous to use a stronger caustic solution (less to boil away later...) but you'd have to play around with it
yourself.