EDTA Titration of an antacid tablet

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    Edta Titration
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Discussion Overview

The discussion centers around the titration of an antacid tablet to determine the percent calcium by mass using EDTA and magnesium chloride. Participants explore issues related to the accuracy of their results, specifically the interference of magnesium in the titration process and the appropriate pH levels for accurate measurements.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Homework-related

Main Points Raised

  • One participant reports that their calculated percent calcium is 30% higher than expected and suspects that magnesium in the antacid is affecting the results.
  • Another participant suggests maintaining a high pH to precipitate magnesium hydroxide, which may help clarify the titration results.
  • There is a discussion about the pH levels required for titrating calcium and magnesium, with one participant stating that a pH of 12 is necessary to only complex calcium.
  • Some participants express confusion about the implications of their calculated moles of EDTA, questioning whether these correspond to both calcium and magnesium.
  • A later reply indicates that the procedure provided for titrating calcium is flawed, as it is the same as that for magnesium.

Areas of Agreement / Disagreement

Participants express differing views on the appropriate pH levels for accurate titration and whether the current procedure can yield reliable results. There is no consensus on how to isolate the moles of calcium from the total moles calculated.

Contextual Notes

Participants reference specific pH levels and procedures that may not be universally applicable, indicating potential limitations in their experimental setup. The discussion also highlights the reliance on specific literature that may not be widely accessible.

ReidMerrill
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In my analytical chemistry lab course my lab partner and I are trying to determine the percent calcium by mass of an antacid tablet by EDTA (mixed with 0.001M MgCl2) titration using an EBT inidcator.
We have ran into a problem which is we've got poor results which we refuse to accept. Long story short we calculated the percent calcium to be 30% higher than it actually is.
We think the magnesium in the antacid tablet is throwing things off. Since the endpoint we observe through color change is cause by magnesium replacing the calcium bound to the EBT the color change won't occur exactly when all of the calcium is titrated by the EDTA. Basically we think the magnesium in the tablet is causing the sample to 'titrate itself,' for lack of a better term.

We know how much magnesium and calcium actually is in the tablet so if that assumption is true is there a way we can factor that in?

Any help would be greatly appreciated.

Thanks!
 
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What is the procedure you use? In general, in the presence of Mg2+ the best approach is to keep the pH high enough to precipitate the Mg(OH)2.
 
Borek said:
What is the procedure you use? In general, in the presence of Mg2+ the best approach is to keep the pH high enough to precipitate the Mg(OH)2.
The antacid was dissolved in water at pH 10 in an ammonia-ammonium buffer solution. I can post the whole procedure if you'd like.
 
In pH 10 you determine sum of Mg/Ca. For just calcium you need pH of 12.
 
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Borek said:
In pH 10 you determine sum of Mg/Ca. For just calcium you need pH of 12.
Oh that changes things significantly. In the chart my professor gave us it says a pH of 8 is needed to bind calcium and 10 is needed for magnesium. So that must mean that the moles of EDTA I calculated from each of the volumes is equal to the number of moles of calcium AND magnesium?

Second dumb question: Is there a way to find moles of just calcium from that?Thanks!
 
ReidMerrill said:
the moles of EDTA I calculated from each of the volumes is equal to the number of moles of calcium AND magnesium?

Yes.

Second dumb question: Is there a way to find moles of just calcium from that?

No.
 
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Borek said:
Yes.
No.
Can you show me how or where you found that you need a pH of 12 to only complex calcium?
Is it a solubility issue?
 
See my post #2 in the thread.

And compare http://www.titrations.info/EDTA-titration-calcium

The information is from an analytical chemistry book by Minczewski & Marczenko, I doubt it was translated into English. But it is so basic thing every serious analytical chemistry book should have it.
 
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Borek said:
See my post #2 in the thread.

And compare http://www.titrations.info/EDTA-titration-calcium

The information is from an analytical chemistry book by Minczewski & Marczenko, I doubt it was translated into English. But it is so basic thing every serious analytical chemistry book should have it.
The procedure we were given to titrate calcium is the same as the procedure for titrating magnesium on that website so it was a flawed procedure from the beginning.

You've been immensely helpful. Thanks!
 

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