correct procedure to prepare a phoshate buffer at pH=7

[jcg200]

Disovle 3.4 g KH2PO4 (potassium phosphate monobasic) in 1 L water, pH of the solution is around pH=4.5. Then use KOH solution to adjust pH to pH=7. then you get a phosphate buffer that buffers around pH=7.

Is that a correct procedure to prepare buffer solution?

[tom jupille]

Or, you could simply do it by weight. Here's an on-line calculator for sodium phosphate:
http://www.egr.msu.edu/biofuelcell/tool ... phate.html

Just scale those by the relative molecular weights of the potassium and sodium salts:
monobasic: 136/120
dibasic: 174/142

No pH meter needed.

[mattmullaney]

Tom's approach is preferable to me as it is easier to duplicate mass determination(s) for salts than it is to titer a solution containing a single salt due to differences in pH meters and other variables in preparation.

[Markus Laeubli, Metrohm]

Dear Matt

The titration approsch is also no problem in this case. The second pKa of phosphoric acid is 7.2 therefore change in pH is slow and reaching the expected pH value is pretty straight forward. There is almost no danger of overshooting.

[mattmullaney]

Hi Markus,

I agree with you in that the titration approach works well when properly executed--with the caveat being the care with which multiple analysts calibrate/measure pH/take care of the pH electrodes. I've observed less troubles with weighing the buffer ingredients than with titration in method transfer exercises (this is just my own observation set where I have been). Of course, if everyone is on the same page with regard to measuring pH and the meters/electrodes are treated properly, either buffer preparation method will afford the proper final result.

I've also been in places where, when isocratic eluents containing organic/aqueous (buffer) solutions are pre-mixed off-line of a HPLC instrument, that the solutions were Weighed before mixing (instead of, say, using graduated cylinders) in an effort to ensure a higher degree of uniformity between different analysts and labs. Nothing like using a pycnometer!

All in the effort to maintain consistency...