Final answer:
The duration of a titration of phosphoric acid with sodium hydroxide is as long as required to observe the second equivalence point, which occurs around a pH of 9.8.
Step-by-step explanation:
The duration of each titration of phosphoric acid with sodium hydroxide will be as long as required to observe the second equivalence point. Based on the provided titration curves, phosphoric acid (H3PO4) is a polyprotic acid with three potential equivalence points. However, the third equivalence point is not well defined since HPO4- (the ion after the second deprotonation) is a very weak acid. The second equivalence point occurs at a pH≈ 9.8, which is when the titration would typically be considered complete as the third equivalence point near pH 13 is hard to detect precisely.