EDTA Titration of an antacid tablet

[ReidMerrill]

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!

 

[Borek]

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.

 

[ReidMerrill]

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.

 

[Borek]

In pH 10 you determine sum of Mg/Ca. For just calcium you need pH of 12.

 

[ReidMerrill]

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!

 

[Borek]

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.

 

[ReidMerrill]

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?

 

[Borek]

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.

 

[ReidMerrill]

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!