# How to prepare phosphate buffer solution from mono- and disodium phosphate with a specific pH? [closed]

I want to prepare an aqueous solution of a mobile phase of $\pu{0.04M}$ Brij-35. The mobile phase has to be adjusted to pH $7.4$ with $\pu{0.5M}$ phosphate buffer which will be prepared with $\ce{NaH2PO4}$ and $\ce{Na2HPO4}$ solutions.

Can anyone help me to prepare a phosphate buffer solution of $\pu{0.05M}$ using $\ce{NaH2PO4.2H2O}$ and $\ce{Na2HPO4.7H2O}$ with pH $7.4$?

## closed as off-topic by Mithoron, andselisk♦, Todd Minehardt, bon, airhuffOct 16 '17 at 17:52

This question appears to be off-topic. The users who voted to close gave this specific reason:

If this question can be reworded to fit the rules in the help center, please edit the question.

• @fl.pf. The shorter $\ce{NaH2PO4.2H2O}$ works perfectly fine, mhchem handles quite a lot of shorthand. (I am not saying that that would have been my reason to further edit, but it might help you save some time for future edits. I simply wanted to fix the title in this case.) – Martin - マーチン Oct 16 '17 at 11:31

One possibility is to use the Henderson-Hasselbalch equation $(1)$.
$$\mathrm{pH} = \mathrm pK_\mathrm a + \lg\frac{[\ce{HPO4^2-}]}{[\ce{H2PO4-}]}\tag{1}$$
Plugging the $\mathrm pK_\mathrm{a2}$ value of phosphoric acid ($7.198$) and the desired $\mathrm{pH}$ value will give you a ratio of hydrogenphosphate and dihydrogenphosphate concentrations which you can solve to the desired total concentration $c_\text{tot}$ by simple algebra.
However, SigmaAldrich also maintains a nice buffer reference centre that includes ready-calculated masses for different $\mathrm{pH}$ values. All that remains is to make sure your final concentration is the desired one.