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Complexometric titration- yelp

hi guys,

i need some help. Basically im doing a coursework and using this titration technique to find the concentration of copper ions in a solution. To titrate- the procedure is to add an excess of ammonia to the solution before titrating it with EDTA. Im not able to explain why the Ammonia solution is necessary and what the equations for the different steps of the reaction are:

I know that after adding ammonia- this complex forms [Cu(NH3)4(H20)2]2+ which then goes to [Cu(EDTA)]2- ?

but the ammonia should be present in the (NH4+) form as ammonia solution so why should it do anything. This isnt really a problem though- i know i can find the exp if i need to.

But why do we need to use ammonia in the first place?

Thanks in advance.

Reply 1

ooze

I presume it's something to do with the colour change acting as an indicator, maybe. The first one is "deep blue" and Copper (II) EDTA is blue, I know that.

Reply 2

cpchem

I can't imagine it being anything other than the colour change, as RusholmeRuffian said.

I bloody hate titrations. Bane of my life.

Reply 3

Krystal

well along with ammonia you add the indicator murexide for the colour change- but it may be just to intensify the change i dont know. SO colour change is the best bet for explaining it then?

I put down something stupid like the need to form stable complexes with all the copper ions before titrating with EDTA. Because copper will be present as a complex with water which is ...er...unstable? but probably not right...?

Thankyou so much for helping out btw- much appreciated!!!