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Author: Subject: Purification by recrystalization.
Hazard to Self

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[*] posted on 23-1-2006 at 01:54
Purification by recrystalization.

As the only source of potassium that I have is carbonate or nitrate, I neutralised some carbonate with HCl to make the chloride. As I wasn't sure of the purity, I thought I would purify it by recrystallization. Looking at the solubility of KCl, I found 56g/100mL at 100C and 28g/100mL at 20C.

So, am I right in assuming the procedure to be
1.Dissolve 56g in 100mL at boiling point
2.Filter any solids
3.Cool to zero C
4.Filter pure crystals.

If so, I will end up with about 28g or 50% recovered.

Is this correct or can I assume that the 100mL can hold lots of impurities and therefore reuse it. That is add another 28g of impure KCl, reboil, etc to recover another 28g to increase my recovery amount to 66%.

Alternatively, can I boil the 100mL down to 50mL and recover another 14g.

I did google and found a few pages explaining it but they appeared to be happy with 50% recovery.


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Post Harlot

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[*] posted on 23-1-2006 at 13:19

I would recommend diluting it to ~28% (per cent as grams per centum (centum = 100) water, not solution, so that 100"%" = 100g per 100ml, not 100% = pure salt), filtering solids, and then boiling to approx. half volume or until crystals begin to precipitate. Then cool, seperate and repeat.

Your biggest ally in purification is that KCl is the most abundant in solution. Less soluble materials seperate first and are filtered before crystallization, while more soluble materials remain behind in solution. Crystallizing by temperature difference only seperates less differentially soluble salts, such as NaCl, which has almost the same solubility at 0C as at 100C. (Which reminds me, cover the solution when boiling is complete so it doesn't lose water.) As you work the solution down, you'll increase the concentration of more soluble components until they also precipitate. However, with the yield sitting on filter paper, you can just rinse it with clean water to dissolve it out of your sample. Even if you dissolve equal amounts of KCl and the impurity, there's still more KCl in the sample, so you've reduced yield a bit, but you've increase purity too. Plus, the wash water can be added back into the solution and boiled down, so you don't lose any KCl anyways.

Hum.. speaking of which, I'm currently purifying CuSO4 (beautiful stuff), NiCl2, CrCl3 and CuCl2 (all hydrated to whatever they are). The CuSO4 solution is growing nicely, while the NiCl2 and CuCl2 solutions only recently started forming crystals. NiCl2 appears to have a tilted cubic (i.e., rhombic) crystal form, light bluish-green in color, while CuCl2 forms in fine, acicular crystals (annoying to deal with when they get too dense).


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