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Sodium phosphate

Sodium phosphate, monobasic is NaH2PO4.
Sodium phosphate, dibasic is Na2HPO4.

The formula weights (FW) depend on the hydration state of the chemical.
For example, what I have in the lab is sodium phosphate, monobasic,
monohydrate (NaH2PO4.H20) and the FW is 138.

On the otherhand, I have two forms of sodium phosphate, dibasic:
The anhydrous form is simply Na2HPO4 and the FW is 142.
The 7-hydrate form is Na2HPO4.7H2O and the FW is 268.
Note that being hydrated does not mean being wet. These are all dry in
the bottle.

BTW, remember that the pH will be affected by which you add to your
aquarium water. Sodium phosphate, monobasic will lower the pH while the
dibasic form will raise the pH.

Mixing precalculated amounts of monobasic and dibasic sodium phosphates
has long been used as the method of choice for preparing phosphate
buffer. For example, for a buffer solution with a pH of 6, the molar
ratio of monobasic to dibasic sodium phosphate is 87.7 to 12.3%. For pH
7, it is 39.0 to 61.0%. One can prepare 0.2M solutions of both. To make
a buffer with a certain pH, one would simply mix the two in the correct
ratio, verify the pH with a pH meter, adjust if necessary (with
phosphoric acid or NaOH) and then dilute to the desired phosphate

Hope this helps and good luck with the school project.

Shiao Y. Wang
University of Southern Mississippi
sywang at whale_st.usm.edu