equivalence point
/ ee-KWIV-uh-luns point /
Imagine two teams tugging a rope: the exact instant the pull is perfectly balanced — neither side ahead — is special and precise, even if no flag waves to announce it. The equivalence point of a titration is that balanced instant for the chemical reaction.
Formally, the equivalence point is the moment when the amount of titrant added is exactly enough to react completely with all of the analyte, according to the reaction's stoichiometry — no more, no less. It is a theoretical, ideal point fixed by the chemistry, not by what you happen to observe.
It matters because the equivalence point is the true target of every titration; the volume of titrant at this point is what the calculation really wants. The practical snag is that you cannot see it directly, so you rely on a visible end point chosen to fall as near to it as possible.
Titrating 0.0010 mol of hydrochloric acid with 0.10 mol/L sodium hydroxide, the equivalence point falls at exactly 10.0 mL of base, because that is the volume holding 0.0010 mol of hydroxide — one-to-one with the acid.
The exact, stoichiometric instant the reaction is complete.
The equivalence point is ideal and invisible; the end point is real and observed. The difference between the volumes at these two points is the source of the titration error, which is why indicator choice is so important.