[MargaaMarius]

We want to determine the amount of phosphorus in a water sample using titration. What should we use as indicator and titrant?

[Borek]

First of all: total phosphorus, or just phosphates?

I am afraid this is not trivial. The only way I know of is to oxidize phosphorus to phosphates with permanganate, precipitate phosphates as ammonium phosphomolybdate, then doing back titration - dissolving phosphomolybdate in known amount of NaOH and titrating excess with HCl. It requires separation of the solid phosphomolybdate which is also an important step in gravimetric determination of phosphates, so once you have it you can as well try to dry the precipitate out and weight it.