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Author Topic: determination of calcium in a sample  (Read 10252 times)

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Marija

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determination of calcium in a sample
« on: April 30, 2009, 08:41:32 AM »

Since I'm really bad at analytical chemistry and especially at practical work in laboratory I came across a problem regarding the determination of calcium in a sample of calcium salt. We precipitated calcium ions in a sample with ammonium oxalate and then we solubilized it in sulfuric acid. We titrated the oxalate in this solution with KMnO4.

The problem is that I have to write down some equations and I'm not quite sure I wrote down the right ones.
First I have to write down the equation for reaction between Ca2+ and ammonium oxalate. My idea: Ca2+ + (NH4)2C2O4  :rarrow: CaC2O4 + 2NH4+ (Am I missing something? We also added the indicator methyl red and NH3.)

The second is the equation for reaction of solubilization of calcium oxalate in sulfuric acid. I don't have any ideas what to write.  :-\

The last one is a reaction of oxalate in KMnO4 in acid environment. I'm sure for this one, so I won't bother you with it. ;D

I'd really appreciated if someone could write down these two equations, because it's important I get it right, 'cause I have to make some calculations afterwards and (of course) I need the right stoichiometry. I have to calculate how much calcium was in our sample.
Oh, and I apologize if my english isn't so good. I sometimes have problems, especially with chemical expressions.  ::)


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Arkcon

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Re: determination of calcium in a sample
« Reply #1 on: April 30, 2009, 08:53:40 AM »

The second is the equation for reaction of solubilization of calcium oxalate in sulfuric acid. I don't have any ideas what to write.  :-\

The last one is a reaction of oxalate in KMnO4 in acid environment. I'm sure for this one, so I won't bother you with it. ;D

OK, so if you're sure of the last one, you know the products, and the reactants.  And the reactants of the third one, contain the products of the second one.

Can you write, all three reactions, one above the other, without text in between them, and try to figure out what information you have, and what you need to figure out?
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Borek

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Re: determination of calcium in a sample
« Reply #2 on: April 30, 2009, 08:58:36 AM »

Ca2+ + (NH4)2C2O4  :rarrow: CaC2O4 + 2NH4+

Not bad, not good either.

Ammonium oxalate is completely dissociated. Go net ionic.
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Marija

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Re: determination of calcium in a sample
« Reply #3 on: April 30, 2009, 08:51:46 PM »

Yes Borek, I can see my mistake now. I had that in my mind, but I was just tiping too fast. :) You ment: Ca2+ + 2NH4+ + C2O42-  :rarrow: CaC2O4 + 2NH4+, right?

Hmm, OK Arkcon, I can try ...

The last one is: 5C2O42- + 2MnO4- + 16H+  :rarrow: 10CO2 + 2Mn2+ + 8H2O

So maybe the second one can be: CaC2O4 + H2SO4  :rarrow: C2O42- + CaSO4 + 2H+

Am I thinking in a right direction?
Calculations are not a problem, I'm having problems with these reaction and that's why I'm not sure about the stoichiometry. ::)
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Borek

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Re: determination of calcium in a sample
« Reply #4 on: April 30, 2009, 10:04:48 PM »

Yes Borek, I can see my mistake now. I had that in my mind, but I was just tiping too fast. :) You ment: Ca2+ + 2NH4+ + C2O42-  :rarrow: CaC2O4 + 2NH4+, right?

Close, but no banana yet. Remove spectators.

Quote
So maybe the second one can be: CaC2O4 + H2SO4  :rarrow: C2O42- + CaSO4 + 2H+

Is sulfuric acid dissociated, or not?
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Marija

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Re: determination of calcium in a sample
« Reply #5 on: April 30, 2009, 10:44:49 PM »

Quote
Close, but no banana yet. Remove spectators.
Oh, OK. Ca2+ + C2O42-  :rarrow:  CaC2O4
Quote
Is sulfuric acid dissociated, or not?
Damn, same mistake again. Of course it is. ::) Otherwise, the equation is OK?
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Borek

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Re: determination of calcium in a sample
« Reply #6 on: April 30, 2009, 10:49:15 PM »

Quote
Is sulfuric acid dissociated, or not?
Damn, same mistake again. Of course it is. ::) Otherwise, the equation is OK?

Not exactly. Oxalic acid is a weak one, so it gets protonated, especially in the presence of the excess strong acid.
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Marija

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Re: determination of calcium in a sample
« Reply #7 on: April 30, 2009, 11:14:29 PM »

Quote
Is sulfuric acid dissociated, or not?
Damn, same mistake again. Of course it is. ::) Otherwise, the equation is OK?

Not exactly. Oxalic acid is a weak one, so it gets protonated, especially in the presence of the excess strong acid.

Oh, OK. So I get: CaC2O4 + 2H+ + SO42-  :rarrow:  H2C2O4 + CaSO4 ? Or am I wrong once again? :-\
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Borek

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Re: determination of calcium in a sample
« Reply #8 on: May 01, 2009, 12:18:15 AM »

This one looks OK to me (even if some nitpickers might argue that it is still not exactly right... ;) ).
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Marija

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Re: determination of calcium in a sample
« Reply #9 on: May 01, 2009, 12:47:31 AM »

Well, thanks for helping.  ;D
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