[GreenHorn]

Hi guys, just wondering... I was having this problem with titrating (50ml burette) a 0.01moldm3

Sodium salt of EDTA into a 50ml solution of 0.01moldm3 Calcium Chloride. The pH was kept

constant at 10 using 3 drops of ammonium chloride and Eriochrome Black T indicator solution

was added. The solution was supposed to turn from red to blue but wouldnt! After a while I

tried halving the volume of Calcium Chloride solution to 25ml solution. This worked, the solution

then turned from red to blue, but i don't know why only after i halfed the volume of CaCl...

Any ideas? I have to explain why in an evaluation. Ta in advance.

[Borek]

Think about reaction stoichiometry.

[GreenHorn]

Call me stupid but i just don't see it?

[Borek]

At what molar ratio do the EDTA and Ca²⁺ react?

How much of both do you use?

When does the color change?

[GreenHorn]

Ok for one of the results the titre was 20ml for EDTA and it reacted with 25cm3 of the CaCL in the conical flask, the moles react 1 to 1.

The colour changed after 20ml of EDTA was titrated into the water sample. This worked.

Previously, i could not obtain a result as the CaCl solution would not turn colour. But i was using 50ml of CaCl in the conical flask instead of 25ml. No matter how much EDTA i added the solution would not turn colour.

[Borek]

For the color change you have to add small excess of EDTA so that all Ca²⁺ is complexed. Calculate if your combination of volumes/concentrations/burette size allows for that if you use 50 mL of calcium solution.