Sciencemadness Discussion Board

Calcium Acetate Preparation Errr

smaerd - 15-6-2011 at 14:52

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!

m1tanker78 - 15-6-2011 at 15:14

Why don't you use calcium hydroxide instead? I've never made calcium acetate so maybe I'm missing something (solubility, etc).

Tank

entropy51 - 15-6-2011 at 16:08

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?

smaerd - 15-6-2011 at 16:42

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.

m1tanker78 - 15-6-2011 at 17:06

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

smaerd - 15-6-2011 at 17:36

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).

#maverick# - 15-6-2011 at 19:00

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

ldanielrosa - 15-6-2011 at 23:21

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.

Random - 16-6-2011 at 06:24

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]

LanthanumK - 16-6-2011 at 07:03

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.

not_important - 16-6-2011 at 10:24

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.

#maverick# - 16-6-2011 at 11:54

i tried with reagent grade and got a clear probably a barium sulfate impurity

m1tanker78 - 16-6-2011 at 13:20

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

LanthanumK - 16-6-2011 at 13:28

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.

m1tanker78 - 16-6-2011 at 14:12

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]

m1tanker78 - 17-6-2011 at 08:42

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

LanthanumK - 17-6-2011 at 12:03

Does it have an excess of acetic acid?

S.C. Wack - 17-6-2011 at 16:41

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.

m1tanker78 - 18-6-2011 at 07:14

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

S.C. Wack - 18-6-2011 at 11:55

Quote: Originally posted by m1tanker78  

S.C.: The label on the vinegar jug states that it's distilled and "diluted with water to 5% acidity".


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.

m1tanker78 - 18-6-2011 at 15:59

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.