點解Titration時要加excess dilute Sulphuric acid?

during the titration, you add a catalyst.
excess dilute sulphuric acid quench the reaction(or poison the catalyst), stopping the reaction to go further, thus the rate at time T can be measured.
(If you don't stop the reaction, how can you measure the concentration of the
hydrogen peroxide remained?)

use excess to ensure the reaction is quenched.

now that the reaction is stopped, the concentration of H2O2 is supposed to
be constant within a period of time. You use standard potassium manganate to
titrate against H2O2, when the standard potassium manganate changes colour,
the concentration of H2O2 can be determined.

standard potassium manganate is used to determine the concenration of H2O2,
you may use Fe2+ to determine as well. potassium manganate may react with
HCL, so HCL is not used.