Hexavalent
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Calcium Sulfate not Precipitating?
I recently required some high-purity calcium sulfate and knew that my plaster o' Paris wasn't going to cut it, so, in my attempt to make some purer
stuff I dissolved a heaped teaspoon worth of calcium chloride, that I got from those absorbent crystal packs, in about 150ml room-temperature
distilled water. I then did the same using sodium sulfate, but used about 75ml of water instead. I dumped the sulfate solution into the chloride
solution, expecting precipitation of calcium sulfate and sodium chloride remaining in solution. However, nothing happened. I did not use properly
weighed out stoichiometric equivalents, I just wanted a few hundred miligrams of high purity CaSO4 there and then.
CaCl2 + Na2SO4 ---> 2NaCl + CaSO4 was what I expected, or specifically;
[Ca2+] (aq) + 2[Cl-](aq) + 2[Na+](aq) + [SO42-](aq) ---> CaSO4 (s) + 2[Na+](aq) + 2[Cl-](aq)
I left it for an hour and still nothing, not even the water going slightly turbid. I repeated the experiment, checked thrice that I was actually
using the correct reagents and nothing happened again.
What happened here?
"Success is going from failure to failure without loss of enthusiasm." Winston Churchill
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DrNoiZeZ
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The CaSO4 is insolluble in water so even if you use non stoichiometric reagents the precipitation must occurs and at once. If dont or it wasnt CaCl2
or wasnt Na2SO4, there is no other explanation that I can figure out. Maybe the absorbent was not CaCl2. Hope to help.
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Hexavalent
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Possibly, thanks.
http://www.kontrolukltd.com/products/products.php?section=1&...
It is the top product on this page that I purchased, and the bottom rear of the bag looks like this the first attachment.
The second attachment shows how, to conserve shelf space, I transferred the product to a tape and teflon-tape sealed glass bottle.
The third and fourth images show what the product itself looks like, it comes in the form of small, perhaps 5mm^2 in size, flakes.
   
The sodium sulfate I used came from a 3-year old chemistry set, and the small vial had been opened about 3 months previously, and was the decahydrate
form.
[Edited on 19-4-2012 by Hexavalent]
"Success is going from failure to failure without loss of enthusiasm." Winston Churchill
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barley81
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Are you sure that the sodium sulfate is really sodium sulfate? Try testing it with a soluble barium salt. Or, just make some with baking soda and
sulfuric acid and you won't have a problem. The website says that the desiccant is calcium chloride and I doubt it has much else in it.
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Hexavalent
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Thanks barley.
I have no barium salts ATM, but will synthesise my own sodium sulfate and try again.
"Success is going from failure to failure without loss of enthusiasm." Winston Churchill
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S.C. Wack
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Try baking soda instead of the sulfate. There is also the solubility of the suspected CaCl2 in alcohol. Crystallization from aq. solution works OK for
it.
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