[OLIVIA]

what is the difference between preparing a phosphate buffer at pH 2.4 and adjusting the pH to 2.4 with phosphoric acid

[Borek]

Depends on your starting point. Could be no difference, could be different solution.

[OLIVIA]

if the starting point is just making a 5mM phosphate  solution and using the pH meter to obtain a pH of 2.4.  What is another way?

[Borek]

You may mix hydrogen and dihydrogen phosphates, you may use phosphoric acid and titrate to correct pH with NaOH, you may - although that'll be slightly unusual - start with sodium phosphate and titrate it with phosphoric acid.

[agrobert]

You want a buffer system with an initial pH that is close to your target pH because you are changing the molarity by adding concentrated phosphoric acid.  For example a 0.005 M KH₂PO₄ is prepared by adding 0.68 g of the salt to 1000-mL of water.  The initial pH will be ~4.6 and can be adjusted to pH = 3 by a few drops of concentrated phosphoric acid.  In general it shouldn't matter much but you wouldn't want to adjust your pH by adding too much concentrated phosphoric acid.

[AWK]

pH 2.4 means buffer is based on H3PO4/NaH2PO4 mixture

[OLIVIA]

so, if i go to zirchrom buffer wizard and prepare a pH 2.4 buffer.  I get the following recipie  2.5g H3PO4 + 1.3g Na2HPO4 / 1000ml = 0.025M.  When I calc it out, I don't get 25mM.  What am I doing wrong with my calc? 

[AWK]

so, if i go to zirchrom buffer wizard and prepare a pH 2.4 buffer.  I get the following recipie  2.5g H3PO4 + 1.3g Na2HPO4 / 1000ml = 0.025M.  When I calc it out, I don't get 25mM.  What am I doing wrong with my calc? 

NaH2PO4 ?
If you use Na2HPO4 then you should take into account a neutralization reaction
H3PO4 + Na2HP4 = 2NaH2PO4