Is it quantitatively acceptable to titrate all KHC8H4O4 samples with the NaOH solution to the same dark red endpoint ?Explain.

thank for helping~

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If you can be absolutely certain that the dark red end point is the same every single time, yes it is acceptable if the NaOH is standardized the same way. That way the standardized NaOH is being titrated to the same end point as the samples. However, in practice, it is not acceptable because the same dark red end point is not possible, even with the same set of eyes. The eyes are very good at distinguishing between colorless and the slight pink/red of the end point, but not very good at titrating to some end point several drops later past the traditional end point.
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