Phosphoric Acid & Sodium Phosphates

Problem

What is the pH of the following four solutions:

0.2 M H3PO4 phosphoric acid
0.2 M NaH2PO4 sodium dihydrogen phosphate
0.2 M Na2HPO4 disodium hydrogen phosphate
0.2 M Na3PO4 trisodium phosphate

Note: NaH2PO4 and Na2HPO4 are an acid/base conjugate pair. They will make an excellent buffer.

Answer

In all four cases we start with pure water (button New) and use the reaction tool (button Reac). Then select “H3PO4” and enter 200 mmol/L:

input of H3PO4 into reaction tool

Just after Start the resulting pH will be displayed. Repeating this procedure for the other three cases leads to the following results (for 25):

0.2 molar H3PO4 pH = 1.46 ( I = 0.04 M )
0.2 molar NaH2PO4: pH = 4.40 ( I = 0.20 M )
0.2 molar Na2HPO4: pH = 9.23 ( I = 0.53 M )
0.2 molar Na3PO4: pH = 12.57 ( I = 1.08 M )

Note the high ionic strength up to I = 1.08 M for Na3PO4. Thus, activity corrections are obligatory (which are always performed in aqion).

More about phosphoric acid – as a polyprotic acid – is here.

[last modified: 2016-03-06]