Question

Phosphoric acid (H3PO4) is a triprotic acid with the following pKa values: pKa1 = 2.15 pKa2 = 7.20 pKa3 = 12.35

a). 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)

b). 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?

c). You accidently grabbed the 10x concentrated phosphate buffered saline (or 10xPBS) instead of the 1xPBS 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 1xPBS buffer. Why?

Answer 1

if you need pH = 6.8

then choose pKA2 = 7.2 value

which is --> HPO4- and H2PO4- ionization

that is, we need XHPO4 and Y2H2PO4 salts

so choose:

KH2PO4 and Na2HPO4

for..

pH = pKa + log(base/acid)

6.8 = 7.21 + log(ratio)

ratio = 10^(6.8-7.21) = 0.38904

base/acid = 0.38904

base = 0.38904*acid

and

base + acid = 0.045

0.38904*acid + acid = 0.045

acid = 0.045 / (1.38904) )= 0.032396 mol of acid NaH2PO4

mass KH2PO4 = mol*MW = 0.032396 *136.086=  0.032396 *136.086g of KH2PO4

for base

base = 0.38904*acid = 0.38904*0.032396 = 0.012603 mol

mass of Na2HPO4 = 0.012603 *141.959 = 1.789109 g of Na2HPO4

b)

concentrated is due to the amount of mass, you could always follow the ratio, since ratio is simply division, but the more you addd, the stronger and powerful it will get

c)

you require much more acid/bases to neutralize the base/acid respectively... the ratio is kept in the solution but the amount of acid/bases is larger... therefore more meterial is required