[LabPoodle]

Hey everybody,

We did an EDTA titration lab with copper sulfate and zinc sulfate, and I got some really confusing results. For moles of EDTA used, I'm getting results that are far larger than the moles of the metal salt that was used. This feels wrong; shouldn't the moles of EDTA be equal to the moles of the metal salt? I've read that the stoichiometric ratio should be 1:1. I'm getting stoichiometric ratios as low as 4.8 (probably 5) and as high as 5.9 (probably six), with one outlier  at 9.05 (WHAT?!?!?!)

Is this experimental error? Is there something else going on that I don't know about? I've been trying to figure this out for so long!

[Hunter2]

Which method. Which indicator is used, Which  pH is adjusted.
Accuracy of volume of sample and titrate.

[LabPoodle]

Which method. Which indicator is used, Which  pH is adjusted.
Accuracy of volume of sample and titrate.

A weighed sample of the metal salt was dissolved in  mL of DI water, and then a few drops of murexide were added. The EDTA, which was in a burette, was then added, and the volume required was recorded.

I should note that for zinc sulfate, we attempted to titrate the 50 mL, and it simply did not titrate. Our TA told us to titrate just 5 mL, so we did that and it still didn't change color even after the addition of 50 mL of 0.1 M EDTA. He then told us to use a total of 12-15 drops of murexide, so we used added 10 for a total of 13 drops; we were then able to titrate. So this is a possible source of experimental error.

[Hunter2]

I do not see the adjustment of pH.


http://www.titrations.info/EDTA-titration-zinc

[LabPoodle]

I do not see the adjustment of pH.


http://www.titrations.info/EDTA-titration-zinc

I don't understand what "adjustment of pH" means. Are you saying I was supposed to fine-tune the pH in some way? I described all of the steps in the process.

[Hunter2]

Yes in the procedure there is missing to adjust the pH with an ammonia buffer to pH 10. Zinc sulfate is acidic.

[LabPoodle]

Yes in the procedure there is missing to adjust the pH with an ammonia buffer to pH 10. Zinc sulfate is acidic.

Now I'm even more confused... what's the purpose of this? Does pH affect the stability constant of the complex? I carefully reviewed the instructions we were given, and it never mentioned anything about adjusting the pH to be more basic.

 I emailed my TA and he said that it's okay if my data makes no sense as long as I make it clear that I understand the concepts, but I still want to understand what went wrong out of personal curiosity, lol.

[Hunter2]

The best working window of EDTA is pH 9 to 12, preferred 10.

It has something to do to get the complex. In acidic solution the complex is not stable or existing depending of the metal to analyze.

[Borek]

The best working window of EDTA is pH 9 to 12, preferred 10.

Not exactly, depends on the cation.

In general it is about selecting pH at which the side reactions are the weakest - these mostly include protonation of the EDTA and reactions of the cation with OH^-.

And for example for Zn²⁺/Ca²⁺ optimal pH is around 10, but for Bi³⁺ it lies somewhere in the 2-3 range.