EDTA Titration of an antacid tablet

In summary, the lab partner and I think that the magnesium in the antacid tablet is throwing off our titration results and we are looking for a way to account for it. We were told that a pH of 8 is needed to bind calcium and 10 is needed for magnesium, but our results seem to be indicating that this is not the case. If we can find a way to account for the magnesium, our titration results may be more accurate.
  • #1
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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  • #2
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.
 
  • #3
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.
 
  • #4
In pH 10 you determine sum of Mg/Ca. For just calcium you need pH of 12.
 
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  • #5
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!
 
  • #6
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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  • #7
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?
 
  • #8
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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  • #9
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!
 

1. What is EDTA titration and why is it used to test antacid tablets?

EDTA titration is a chemical process used to determine the concentration of a metal ion in a solution. It is commonly used to test antacid tablets because they contain metal ions, such as calcium, which react with EDTA and can be measured to determine the amount of active ingredient in the tablet.

2. How is the EDTA titration process carried out for an antacid tablet?

The antacid tablet is first dissolved in a known volume of acid. Then, a specific amount of EDTA solution is added to the acid solution until the metal ions in the tablet have completely reacted. The amount of EDTA used can then be used to calculate the concentration of metal ions in the tablet.

3. What is the purpose of using a back titration in EDTA titration of an antacid tablet?

A back titration is used when the amount of metal ions in the antacid tablet is too small to be accurately measured directly. By adding an excess of EDTA and then back-titrating with a known concentration of metal ions, the amount of metal ions in the tablet can be determined more accurately.

4. What is the significance of the endpoint in EDTA titration of an antacid tablet?

The endpoint is the point at which all the metal ions in the tablet have reacted with the EDTA. It is important to accurately determine the endpoint in order to calculate the concentration of metal ions in the tablet. This is usually indicated by a color change in the solution, which can be detected using an indicator.

5. What are some sources of error in EDTA titration of an antacid tablet?

Sources of error in EDTA titration of an antacid tablet can include inaccurate measurement of solutions, incomplete dissolution of the tablet, and human error in identifying the endpoint. It is important to carefully follow the procedure and use precise measurements to minimize these sources of error.

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