In my next lab, I'll be doing a complexometric titration of a copper solution with EDTA and a murexide indicator. The lab guide advises to keep the pH level nearly neutral by adding a diluted $\ce{NH3}$ solution. Why is that necessary?

I understand that during the formation of the copper complex $\ce{[Cu(EDTA)]^{2-}}$, the solution becomes more acidic. Do we need to add the basic solution in order to make sure the murexide does not change color due to the decreasing pH level? Or is it simply so that we can revert the color change (of the formation of the copper complex) and continue the titration until the equivalence point is reached (and all copper ions are in complex form)?

  • $\begingroup$ The colour of murexide in aqueous solution does indeed depend on the pH: blue in alkaline, red in acidic solution. $\endgroup$ – Klaus-Dieter Warzecha Mar 16 '15 at 18:55

Your last guess is exact. This is a classical method, the equivalent point is seen by our eyes, so in our interest is for the complex to be formed so we can recognize the colour, and we should add $\ce{NH3}$ so the colour won't change immediately after the equivalent point.

  • $\begingroup$ I am still having problems understanding exactly how the ammonia changes the color of the solution. Is it because the murexide acts simultaneously as a pH indicator and an indicator for the metal complex? Adding ammonia does not change the copper complexes that have already been formed, right? $\endgroup$ – ahemmetter Mar 16 '15 at 19:27
  • 1
    $\begingroup$ Yes, it only keeps it that way for a while so we can recognize the colour easily, so the acid doesn't dissociate and change the colour immediately. $\endgroup$ – Ndrina Limani Mar 16 '15 at 19:45

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