Why is the pH of the first equivalence point, of a titration of polyprotic acid, not equal to7?

Question

A titration of phosphoric acid with sodium hydroxide. The first equivalence point has a pH of about 5. 

 

My thoughts are that because it is only the first equivalence point, only one H+ has been pulled away from the polyprotic acid, therefore the acid hasn't been fully neutralized leaving the solution more acidic.

Am I on the right track or am I missing something?

Answer 1

That is correct.

 

You can also use equation to explain:

 

phosphoric acid is a weak triprotic acid (pKa1 = 2.16, pKa2 = 7.16 and pKa3 = 12.3). 

 

The first equivalent point: 

 

                      H3PO4  +  NaOH  ------- NaH2PO4   +   H2O

 

The pH of the resulting solution:    pH = (pKa1  +  pKa2)/2  = 4.66.

 

The pH of the first equivalent point is close to the above pH.