June 03, 2020, 06:39:44 AM
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Topic: Lack of colour change in titration for standardising EDTA (Eriochrome Black T)  (Read 188 times)

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Offline TheSimpliFire

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I am currently following this method to standardise my [itex]0.01\ce M[/itex] EDTA solution. As instructed, I have added

- [itex]10\ce{cm^3}[/itex] of the buffer solution ([itex]7\ce g[/itex] of [itex]\ce{NH4Cl}[/itex] with [itex]57\ce{cm^3}[/itex] conc. ammonia) and

- [itex]1\ce{cm^3}[/itex] eriochrome black T solution ([itex]0.2\ce g[/itex] indicator with [itex]15\ce{cm^3}[/itex] conc. ammonia and [itex]5\ce{cm^3}[/itex] ethanol)

to [itex]10\ce{cm^3}[/itex] of EDTA which made a blue-black solution.

However, during the titration with [itex]0.025\ce M[/itex] [itex]\ce{MgCl2}\,(\ce{aq})[/itex], there was no colour change (stayed blue-black) except for some red bubbles on top of the solution. This is clearly not in accordance with the supposed reaction given in Figure [itex]1[/itex] of the link as the colour changes from blue to pink.

The only thing I can think of is that in the method, [itex]0.05\ce M[/itex] EDTA solution was used instead of [itex]0.01\ce{M}[/itex]. Would this have caused the lack of colour change, or something else?

Offline Borek

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Way too much indicator IMHO. Try a single drop (or even a drop of a less concentrated solution).
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Offline TheSimpliFire

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1 drop worked, thanks!

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