Phosphoric Acid & Sodium Phosphates
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.
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:
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]