chem help please?

Question

why does the solution decolorize on standard after the equivalence point has been reached?

Can someone please help me answer this question? thanks

Answer 1

I do not exactly know what kind of indicator you use. Hence you need to be very specific. However, oftentimes people do not always relate issues to the fact that there are O2, CO2, etc in the air.
For acid-base titration, when the end point is at a pH higher than 5~6, especially at a higher pH with high [OH-], CO2 can readily dissolve into the solution to react:
CO2 + OH- ==> HCO3-
hence pH may be reduced to 5~6 range.
For redox titration, there would be two cases. First, the reaction near the endpoint may be quite slow, hence a TRUE end point may or may not reached. Second, O2 may dissolve in to the solution to further oxidize.