EDTA Complexometric Titration failed why?

[keags0]

In my attempt at standardizing an EDTA solution, the blank solution titration was successful, but the standardization titration failed... here's the procedure of my experiment with some details:

We first prepared a titration blank by using 35mL distilled water, 5mL of pH10 buffer, 5 drops of Eriochrome Black T indicator, which made a black solution. We then added 15 drops of 0.03M magnesium chloride to the solution, which turned it red. We titrated this using an unknown concentration of EDTA solution.

The next part of the experiment involved using 0.2-0.3g of calcium carbonate (dissolved with 1 mL 3M HCl and 4 mL DI water). We mixed this solution with the indicator solution prepared in the titration of the blank. We titrated this solution using EDTA, expecting somewhere between 30-50 mL to be needed. We went over 80 mL in 3 consecutive trials, and never ended up reaching the endpoint of the titration.

Any ideas as to where our experiment could have gone wrong?

Thanks!

 

[Borek]

Was the calcium carbonate dissolved? As far as I can tell there was not enough HCl for the reaction.

You should try to titrate much smaller sample of calcium, just to check. Could be EDTA concentration was too low. 0.01M solutions are routinely used (compare www.titrations.info/EDTA-titration-solutions), that would be not enough even to titrate half of the calcium used with 80 mL of titrant.

 

[keags0]

Yes, the calcium carbonate was completely dissolved. Also - the instructor told us the EDTA concentration was somewhere around 0.05M once the lab was over.

What was odd was that the class that had the lab the day before me had no problems titrating, whereas in my lab not a single person could complete the titration successfully.

I'm guessing it has nothing to do with the EDTA solution itself, or anything used in the indicator solution since the blank titration went as predicted... but thanks for your input. Any other ideas?

 

[Borek]

If CaCO₃ dissolved completely, concentration of HCl was was not 3M. Check the stoichiometry.

If concentration of HCl was not 3M, you can't be sure there was enough buffer to neutralize excess HCl, so you have no idea if pH was suitable for titration.