Question

How do you determine the third endpoint and pKa3 of H3PO4?

Answer 1

Typically:

third endpoint:

there are several ways:

- identify when the third point of pH changes drastically, this point is special because it will be the least change of pH of all other points

- The volume should be 3x that volume of the FIRST endpoint

for pKa3:

either

analytically:

pKa3 = -log(Ka3)

so

Ka3 =[H+][PO4-3]/[HPO4-]

or

via

half point of 3rd point of titration

the

pH = pKa3 + log(PO4-3 / HPO4-2)

if we know that in half point

(PO4-3 / HPO4-2) = 1

so

pH = pKA3