Dear Oluwatosin Olagbaju ,

Unfortunately this is not the way to make a phosphate buffer. When you want to make a for example sodium phosphate buffer you need a sodium hydrogen phosphate (Na2HPO4) and a sodium dihydrogen phosphate (NaH2PO4).

See for example:

http://cshprotocols.cshlp.org/content/2006/1/pdb.tab20

Sodium hydrogen phosphate (Na2HPO4) can occur in various hydrated forms (Na2HPO4*12H2O, Na2HPO4*7H2O and Na2HPO4*2H2O). In one report even more hydrated forms are reported. See for example:

Ghule, A., Bhongale, C., & Chang, H. (2003). Monitoring dehydration and condensation processes of Na2HPO4· 12H2O using thermo-Raman spectroscopy. Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, 59(7), 1529-1539.

You have now two forms of sodium hydrogen phosphate but they are and remain sodium hydrogen phosphate. As said ‘real’ phosphate buffer makes use of principle of preparing a buffer solution: a mixture of a weak acid and its conjugate base, or vice versa. In this case sodium hydrogen phosphate (Na2HPO4) and a sodium dihydrogen phosphate (NaH2PO4). For more background and a reminder on the basics behind buffers, see for example:

https://en.wikipedia.org/wiki/Buffer_solution

Throughout the years many discussions and questions regarding phosphate buffers are asked, with also matters like how to correct for the various hydrated forms of the salts of choice (or what happens to be available). See for example:

https://www.researchgate.net/post/Phosphate_buffer_calculation

https://www.researchgate.net/post/How_to_prepare_Phosphate_buffer_solution_pH_74

https://www.researchgate.net/post/How_to_prepare_a_0_2_M_phosphate_buffer_Na2HPO4-NaH2PO4_pH_64

Hope this helps and good luck!

Also be aware that buffers change their ionic strength with pH. You may have to compensate for this; e.g., by adding NaCl.

You could make a buffer by mixing variable ratios of 10 mM solutions of an acidic and a basic component, say, NaH2PO4 and Na2HPO4. For example, use 0, 1, 2,..., 10 mL of the dihydrogen phosphate and fill up to a total of 10 mL with the monohydrogen phosphate. These solutions would have various pH-values near neutral.

Just changing the fully dissociated counter-ions (Na and K, respectively) will not affect the pH. However, if you want to produce a physiological solution, you'll need to add both Na and K in the ratio these occur in blood. Phosphate buffered saline (PBS) and Krebs-Henseleit-Buffer are examples for such solutions.

Or you make a 10mM NaH2PO4 solution, aliquot it into appropriate volumes and titrate the pH of each aliquot with NaOH to the desired pH-value

Just make sure your pH-electrode is healthy and the pH-meter is well calibrated.