Phosphoric acid (H3PO4) is a triprotic acid with the following pKa values: pKa1
ID: 1049910 • Letter: P
Question
Phosphoric acid (H3PO4) is a triprotic acid with the following pKa values: pKa1 = 2.15 pKa2 = 7.20 pKa3 = 12.35 You wish to prepare a 1.000 L of a 0.0450 M phosphate buffer at pH 6.8. Which of the following salts do you use (PICK 2) and what mass of each salt will you add to the mixture? H3PO4 (M.W.= 97.995) KH2PO4 (M.W.= 136.086) Na2HPO4 (M.W.=141.959) AlPO4 (M.W.=121.953) A concentrated phosphate buffer known as "phosphate buffered saline" (or PBS) is made with the salts you choose from above in addition to NaCl and KCl. The 10x concentrated solution has a pH ~ 6.8 and the 1x concentrated (i.e., diluted with water) solution is ~7.1. Why? You accidently grabbed the 10x concentrated phosphate buffered saline (or 10xPBS) instead of the 1 times PBS solution and start to adjust the pH of your buffer with NaOH to reach your desired buffer pH. You notice it is taking much more base than with your usual 1 times PBS buffer. Why?Explanation / Answer
a)
if you want pH = 6.8 approx.. the best fit is the 2nd ionization point
pKa2 = 7.20
apply
pH = pKa + log(Na2HPO4 / NaH2PO4)
solve for 1 l
6.8 = 7.21 + log(Na2HPO4 / NaH2PO4)
10^(6.8-7.21) = 0.3890
Na2HPO4 = 0.3890* NaH2PO4
NaH2PO4 + NaH2PO4 = 0.045
substitute
0.3890* NaH2PO4 + NaH2PO4 = 0.045
1.3890NaH2PO4 = 0.045
KH2PO4 = 0.045/1.3890 = 0.032397 mol
Na2HPO4 = 0.3890* NaH2PO4 = 0.3890*0.032397 = 0.012602 mol
mass of KH2PO4 = mol*MW = 0.032397 *136 = 4.4059 g
mass of Na2HPO4 = 0.012602 *141.96 = 1.7889g
so..
b)
this is mainly due to the activity/inic force of the acids/conjguate ions interactng between each other
c)
the ratios are the same, but actual amounts of acid/base are larger, so you require more base/acid respectively to react with the buffer