smaerd
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Calcium Acetate Preparation Errr
Alright so I know I'm dealing with a weak acid and a weak base so there is some kind of yucky equilibrium hapening here, and to top it off CaCO3's
solubility in water is minimal to be stated lightly.
So here is what I have done:
8.5g of CaCO3 was added to 220mL's of household white 5% by wieght(I believe) concentration vinegar. I stirred vigorously for some time. The solution
was still very milky. Applied to a boiling water bath for 10 minutes, which helped some(removing CO2/carbonic acid from solution). Now I added 80mL
more of acetic acid/vinegar, and stirred. Slight evolution of CO2 but nothing major(shifting the equilibrium further I hope). Though the solution is
still milky.
I wish I could calculate this out, but in general chemsitry we never learned how to do titrations with weak acid and weak base.
So should I vacuum-filter to remove the resulting ppt, and evaporate the liquid and call it quits, or is there a better way to go about this?
Also I checked google for preparations using CaCO3 and found none, as well as the forum.
Thanks for your time as always scimadness!
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m1tanker78
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Why don't you use calcium hydroxide instead? I've never made calcium acetate so maybe I'm missing something (solubility, etc).
Tank
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entropy51
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I've made calcium acetate using essentially the same procedure as yours. My notes don't mention any problem dissolving the CaCO3, which was very fine
powder. The yield was essentially theoretical.
Might your CaCO3 contain an insoluble impurity?
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smaerd
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Thank you for looking into this  . The CaCO3 I have was obtained at a pottery
store, so the purity is speculative. It very well could be an insoluble calcium salt impurity. Which would explain it's exceptional resiliance to form
calcium acetate and go into solution. hmmm.
Looks like the best route of action is to filter dry and wiegh the contaminant, then add more CaCO3 to make up for the loss keeping in mind the % of
impurity.
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m1tanker78
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It's possible that there is some calcium sulfate mixed in. That's the only Ca salt (off the top of my head) that would be less soluble, maybe
downright inert in dilute vinegar. Barium seems like a good candidate for impurity in pottery CaCO3. If you have any antacid tablets (Tums), you could
do a comparison as these should be fairly pure CaCO3.
It seems that even with good agitation the reaction would be slow (hours?). Maybe it just needs more time...
Tank
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smaerd
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I filtered it off an it left a grey/brown precipitate on the filter paper. could be a mixture of things like dirt and CaSO4, and maybe some unreacted
CaCO3. I'm blindly leaning towards something undesirable. It was probably a 7-8 hour period that the agitation was going on including the 10 minutes
of boiling.
Thanks for all of the advice. It works perfectly for the metathesis reaction I did with it(with adjusted molar ratios).
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#maverick#
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I have some calcium carbonate of reagent grade on hand. I'll try this procedure next time I'm in the lab and see if I get the same result
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ldanielrosa
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I did this with calcium hydroxide because I thought that acetic acid wouldn't attack the carbonate very well, and heating it would just evaporate the
acetic acid. Even so, the hydroxide had some impurities which turned the solution light orange after concentration.
The other thing to keep in mind is _don't_ cook it to dryness. I lost some this way. I redissolved what I had left, filtered and recrystallized,
then mixed with isopropanol to get most of the water out. I'll let you know how it finishes out, it's in the desiccator now.
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Random
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When I was making Calcium Acetate, I mixed lime that contained a lot of CaCO3 and saw some insoluble stuff, but I just filtered it and then evaporated
the clear solution on the stove.
[Edited on 16-6-2011 by Random]
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LanthanumK
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When I created calcium acetate, there was an insoluble residue as well. It just seems that there are not enough H(+) ions left in the solution to
attack the rest of the CaCO3.
hibernating...
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not_important
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Pottery CaCO3 may contain small amounts of CaSO4 and/or calcium silicates and silica.
When making the acetate you should actually boil the mixture to drive off CO2. You can keep adding more vinegar and CaCO3 to increase the
concentration of product until the solution is saturated - about 300 g/L at 100 C. An excess of CaCO3 isn't really going to be a problem as filtration
takes care of it. Calcium acetate, like CaSO4, decreases in solubility as the temperature is increased, so allowing the reaction mix to cool with
occasional stirring should get all the acetate into solution. The relatively small change in solubility vs temperature combined with the negative
solubility slope makes it a bugger to purify by crystallisation; concentration of the solution to get the solid is likely the best way.
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#maverick#
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i tried with reagent grade and got a clear probably a barium sulfate impurity
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m1tanker78
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I neutralized some 5% vinegar with Ca hydroxide and came up with a water clear solution along with some insoluble fluff which settled quickly and
completely. I may have gone too far with adding the lime which would explain the stuff that settled out. The clear solution shows dead neutral pH.
Tank
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LanthanumK
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That sounds like excess Ca(OH)2 dissolved in the water. Ca(C2H3O2)2, as the salt of a strong base and a weak acid, should be basic.
hibernating...
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m1tanker78
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The acetate salt is far more soluble than the hydroxide of calcium. The carbonate is almost insoluble. The result should be precipitated hydroxide
and/or carbonate, which agrees with my observation.
If this is correct then it should be OK to use an excess of lime/carbonate and filter off the excess (I think someone mentioned that further
upthread).
EDIT: Yep, not_important brought this up.
Tank
[Edited on 6-16-2011 by m1tanker78]
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m1tanker78
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I filtered and concentrated the acetate liquor and then gently heated the solution for 2 or 3 hours to drive off moisture. I expected it would take
longer because I only heated to around 65 degrees C. I suppose I could have kept it on the heat longer but decided to quit and snap a pic.
Calcium Acetate (unknown hydrate):
The concentrated liquor and [especially] the above waxy solid smell strikingly similar to Elmer's glue (Polyvinyl Acetate). Hmmm...
Tank
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LanthanumK
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Does it have an excess of acetic acid?
hibernating...
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S.C. Wack
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Heating for some time below 150C should help with the smell. In the solid, not your house.
Acidifying a solution of such a result with H2SO4, heating to boiling, and adding KMnO4 to permanent purple may prove insightful, but I assume the
vinegar process produces some things besides acetic acid not so easily oxidized. How well do they filter vinegar before bottling it?
IME PVA in water smells ickier than Elmer's. But it makes wonderful very quick-setting cheap finishable cement for drywall patching, etc. when mixed
with 3 volumes of CaSO4, then water; some fiberglass makes it fairly strong.
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m1tanker78
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Lanth: Although I can't be 100% sure, I don't see how any acetic acid could have remained. I used an excess of Ca hydroxide and left it stirring
overnight (by accident).
S.C.: The label on the vinegar jug states that it's distilled and "diluted with water to 5% acidity". I have a feeling they use RO water for diluting.
I never thought about mixing PVAc in with plaster for drywall. Thanks for the tip!
I took some of the dilute liquor and allowed it evaporate at ambient temperature rather than heating it. The result was a combination of 'cauliflower'
and 'glass wool' crystallization...
Now I'm wondering if a borax solution added to the liquor would produce a slimy polymer. Probably not but I'll try it anyway... 
Tank
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S.C. Wack
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Do you see the word "distilled" on cider, malt, and wine vinegars? The word refers to the alcohol. When vinegar is distilled, it no longer meets the
legal definition of vinegar in most of the world. Vinegar is filtered enough to remove vinegar eels, and the producers are under no obligation to do
more.
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m1tanker78
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| Quote: | | When vinegar is distilled, it no longer meets the legal definition of vinegar in most of the world. |
So what you're saying is that my jug of vinegar is mislabled? Sorry, I may have misunderstood. I gather you mean that it should say "dilute acetic
acid" and not "white vinegar"?
What would be the end point in the workup you mentioned before? I understand that acetic acid is displaced by H2SO4. I don't have any KMnO4 but am
curious about the product.
Thanks.
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