Chemical Forums
Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: wuutoshi on April 21, 2014, 07:18:11 AM
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We are titrating Cu2+ ions (in 0.01M solution of CuSO4 with Chelaton III (sodium salt of EDTA). Metallochromic indicator is murexide which forms weaker yellow complex with Cu2+. After Chelaton has chelated all Cu2+, the solution turns blueish - purplish that corresponds to the free form of murexide.
When approx. 10% of Chelaton remains to add before equivalence point is reached, we are asked to add few drops of NH3(aq). What is the reason of doing this?
My thought about this is that the complex formed with ammonia makes Cu22+ more accessible and thus assures swifter transition to equivalence point (?)
//Hunter2: Ah, it makes sense. Thank you.
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It should keep the pH on alkaline side.
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@wuutoshi
you should post in order
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Please read forum rules
//Hunter2: Ah, it makes sense. Thank you.
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By titrating with chelaton you are lowering pH over time of the analysis, the reaction is
Cu2+ + EDTA :rarrow: [CuEDTA] + 2H+
The different pH might alter the results so thats why you add the NH3, to neutralize the acidic pH