Chemical Forums
Chemistry Forums for Students => High School Chemistry Forum => Topic started by: TheSimpliFire on February 05, 2020, 03:19:45 PM
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I am currently following this method (https://www.canterbury.ac.nz/media/documents/science-outreach/magnesium_calcium.pdf) 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?
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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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1 drop worked, thanks!