[katay]

hiii

i titrated different types of milk with EDTA to try and find the concentration of Ca2+ ions in each. I used Eriochrome Black T as the indicator and from what I understand the indicator forms a weak complex with the calcium ions (giving red colour to indicator) but as the EDTA is added, the calcium cations are released and complex instead with the EDTA because it forms a more stable complex. This leaves the free indicator which is blue. The colour change shows the end point?

However, whilst looking on the internet and writing up this investigation, I have become confused about the involvement of magnesium ions in these reactions. I did not add any extra magnesium at any point and I wondered whether the indicator has to bind to the magnesium in the milk to give a sharp end point and generally what happens.

Im now thoroughly confused! any help would be much appreciated!

Thanks!

[oxygenfluid]

Hello katay.

the calcium cations are released and complex instead with the EDTA because it forms a more stable complex

you gave the answer to your 1st question, so yes, it marks the end point.

About the Mg2+, he will bind to EDTA too, so the end point you got its for both ions. One way you can do its precipitate all the Mg2+ so you can tritate only the Ca2+.

I hope this will help you in your study.

[Borek]

What was pH of the solution during titration?

[katay]

thankyou for the replies! 

I used an ammonia buffer so the pH was about 10 i think.

[Borek]

So you have sum of magnesium and calcium. For calcium alone you should go up to pH 12 at least to precipitate Mg(OH)₂.

[katay]

thankyou!

the last things I wanted to know are:

does the calcium give a sharp enough end point with the indicator? or since there are magnesium ions in the milk, will they be involved in complexing with the indicator instead?

the problem is i am writing up my investigation which is only about the concentration of calcium ions in solution and i dont want to start writing about the magnesium complexing with the indicator if i just need to focus on the calcium :s

thanks for all your *delete me*

[Borek]

does the calcium give a sharp enough end point with the indicator?

If alone, yes.

or since there are magnesium ions in the milk, will they be involved in complexing with the indicator instead?

Both ions react with indicator, just like EDTA reacts with both ions. As I already told you you have determined sum of Ca/Mg.

[katay]

ok thankyou!!