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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 H2O) and use the reaction tool (button Reac). Then select “H3PO4” and enter 200 mmol/L:

Click on Start; the resulting pH is displayed immediately: pH = 1.46. 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).