I have this experiment, but I haven't done it yet. It's a gravimetric analysis one where we want to determine the percentage of calcium in a calcium diet supplement tablet.
The procedure goes as follows:
1. Weight one Caltrate tablet accurately into a clean 100ml conical flask
2. Add 50.0mL of the 2M acetic acid and allow the reaction to proceed until the tablet is digested (approx. 30 minutes)
3. Using a Buchner funnel, filter the mixture carefully, rinsing the flask and paper with 2M acetic acid.
4. Examine and record the residue on the filter paper and discard.
5. To the filtrate, add the 2M H2SO4 10mL aliquots until no further precipitate forms. Allow to stand to effect the complete reaction.
6. Weigh accurately a clean, dry filter paper.
7. Filter the precipitate, rinsing the flask with distilled water.
8. Transfer the filter paper with the CaSO4.2H2O to a previously weighed evaporating basin.
9. Dry to constant mass using oven at 100C.
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Is the acetic acid used to dissolve all the Ca ions by forming Calcium acetate?

I was a little confused when I read up that CaSO4 was only slightly soluble. I know that all compounds are not completely soluble or insoluble anyway, but the fact that CaSO4 is mentioned specially as slightly soluble probably means it is very true for this case. Does that mean that not all the Ca ions from the solution are precipitated? Wouldn't that produce a lowered/inaccurate result? (ie. why would we want to produce a precipitate that's not insoluble?) Not sure whether anyone knows...

You see, I came up with these successive reactions for this experiment, I'm pretty sure they're all right, but they just don't connect:

First, CH3COOH(aq) + CaCO3 (s) ----> H2CO3(aq) + Ca(CH3COO)2
which then becomes: CH3COOH(aq) + CaCO3 (s) ----> H2O(l) + CO2(g) + Ca(CH3COO)2 (aq)
Then there is: H2SO4(aq) + CaCO3(s) ---> CaSO4(s) + CO2(g) + H2O(l)
or: H2SO4(aq) + CaCO3(aq) ----> CaSO4(aq) + H2O(l) + CO2 (g)
(Both of these are valid, yes?)

My dilemma is this: H2O(l) and CO2(g) and Ca(CH3COO)2 (aq) form first. The CO2 probably escapes into the air (if not, correct me ), so you're left with H2O and Ca(CH3COO)2 in solution. We next add Sulphuric acid, H2SO4. There is no CaCO3 for it to react with! Instead, there is Ca(CH3COO)2, so the SO4 ions would react with the Ca ions to form the precipitate, CaSO4, is that right?
So if I'm right at all, this reaction: H2SO4(aq) + CaCO3(aq) ----> CaSO4(aq) + H2O(l) + CO2 (g) (or the other form, whichever will do) never occurs. Instead, we should probably just write the ionic equation: Ca(2+) (aq) + SO4(2-) (aq) ----> CaSO4 (s) ... Am I right? I need clarification...


Also, I came up with these possible 'sources of error' with the fact that CaSO4 was partially soluble in mind:
-For the experiment, we assume that all the Ca ions are precipitated as CaSO4 (since CaSO4 is slightly solube). (?)
-Assume no other precipitates form with the SO4 ions apart from the Ca ions.
-Assume all CaCO3 dissolves
-Assume that all water is removed from the precipitate
-Assume there are no impurities on the surface of the precipitate (?)
-Loss of precipitate during filtering, transfers or if precipitate's attached to any of the apparatus.
Ok, I'm not entirely sure all of them/ any of them are valid, can anyone correct me on them? Also, if you know of any other typical gravimetric analysis errors, or questions that might be asked about one of these experiments, that would be pretty helpful

I shall be doing this tomorrow. Really urgent, so any insight at all today would be really good! This is about as close we can get to the actual assessment.

Thanx a heap in advance!

kp2171

youve done a nice job preparing for this lab

yes, the acetic acid is being used to help solubilize the Ca2+, specifically by consuming the CO3 component. the first filtration then discards inert and insoluble binders and such.

second, yes, low solubility does not mean no solubility, so you may not precip. all the calcium. yes this might mean a lower than real number. its ok. its great you see this. many times the "insoluble" products we precipitate out are actually of limited solubility, and some is lost to the filtrate.

last, your analysis of experimental errors is thoughtful.

outside of systematic errors (such as an improperly calibrated balance, an improper weight for a vial/crucible/filter paper) youve done a nice job of covering the major bases.

nice to see someone post a question here who has really thought out the problem and tried it. good job.

Aerlinn

Thanx I'm an inquisitive person ^^
I was wondering whether you could offer any insight to this? (See above)
I reckon working out the reactions (what's happening) is pretty important to understanding your experiment

kp2171

well, the only issue with the first part was that the sulfuric acid was not reacting with calcium carbonate, as you have already consumed the carbonate with the acetic acid and solubilized the calcium ion.

so then you did address it correctly later, that you are actually ignoring the acetate, adding in sulfate and precipitating out the product.

go to sleep. you know what you are doing. =)

Aerlinn

Hahah ^_^ Well, I hope you're sure I'm right, 'cause most of the people in my class appear to have come up with this equation: H2SO4(aq) + CaCO3(s) ---> CaSO4(s) + CO2(g) + H2O(l) as the reaction happening in this gravimetric analysis reaction. And our teacher did say to make sure we knew acid reactions... So are the people wrong, or have they not considered what I've considered? In other words, if the question said: 'Write a reaction for this experiment', would writing H2SO4(aq) + CaCO3(s) ---> CaSO4(s) + CO2(g) + H2O(l) be wrong?

Oooh, just had a slight brainwave. H2SO4(aq) + CaCO3(s) ---> CaSO4(s) + CO2(g) + H2O(l) could just be the OVERALL reaction, whilst my two was what actually happened... Clarification needed...

Aerlinn

kk, just quickly...
My suggested improvements for some errors:
-For the experiment, we assume that all the Ca ions are precipitated as CaSO4 (since CaSO4 is slightly soluble).
To make sure, add many aliquots of H2SO4?
-Assume no other precipitates form with the SO4 ions apart from the Ca ions.
I ain't too sure how to improve this, 'cause in another Grav. analysis thingy we did, we lowered the pH by adding HCl (it involved getting a ppt of BaSO4) to make sure no other ions formed a ppt with the Ba(2+). Don't know if there's a similar thing that can be done for this one, or maybe something else... help =S
-Assume all CaCO3 dissolves
Make sure it all does by taking care to make sure reaction goes to completion, add acetic acid to dissolve all the little bits?
-Assume that all water is removed from the precipitate
Make sure it happens by weighing to constant mass, except with the CaSO4.2H2O ppt, I was informed you can't drive away all the water 'cause there is that H20 in the crystals...?
-Assume there are no impurities on the surface of the precipitate
Hm, if you notice impurities, how would you get rid of em? *confuzzled*
-Loss of precipitate during filtering, transfers or if precipitate's attached to any of the apparatus.
Make sure you don't lose any?

Ok, so with my rough attempt at writing out some improvements, does anybody have any possible suggestions about them? ie. if I'm wrong, what you suggest should be done...