phosphate?

Question

Although notated as PO4^(3-), the phosphorous-containing species actually present in an experiment as HPO4^ (2-) why is this?

Answer 1

Phosphoric acid is a triprotic acid. Therefore for its dissociation you have

H3PO4 + H2O <=> H2PO4(-) + H3O(+)
H2PO4(-) + H2O <=> HPO4(-2) + H3O(+)
HPO4(-2) + H2O <=> PO4(-3) + H3O(+)

with equilibrium constants Ka1, Ka2, Ka3

As you can understand it is a weak acid and therefore if you dissolve a phosphate salt in water it will not really remain PO4(-3) but you will have

PO4(-3) + H2O <=> HPO4(-2) + OH(-)
HPO4(-2) + H2O <=> H2PO4(-) + OH(-)
HPO4(-) + H2O <=> H3PO4 + OH(-)

with equilibrium constants Kb1=Kw/Ka3, Kb2=Kw/Kb2 and Kb3=Kw/Ka1

Since you have equilibrium reactions, all species will be present in theory, but practically speaking the species that will be present in significant quantities will depend on the point of the equilibrium.
In this case it depends a lot on the pH. The more alkaline your conditions the more dominant the highly dissociated species will be. Obviously under your experimental conditions the dominant is HPO4(-2).

Maybe it is an important note for your experimental conditions (having the HPO4(-2)) or maybe it's just an exaggeration

If you could be more specific I guess we could be more helpful.

Answer 2

Phosphoric acid H3PO4 ionizes into 2H^(1+) and HPO4^(2-) ions