Check all of the following statements which are TRUE for a titration.

Select one or more:

a. A suitable primary standard must be available.

b. The concentration of the solution in the buret (the titrant) must be known before the titration is begun.

c. Titrations can only be performed for acid + base reactions.

d. The size of the aliquot added should be reduced as the end-point is approached.

e. It must be possible to use phenolphthalein as the indicator.

f. The reaction must be rapid and irreversible.