Question

2. 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) 2

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

As pH of 6.8 is required, pKa2 of H3PO4 has to be condidered. Two salts :KH2PO4 and Na2HPO4. In this buffer KH2PO4 will act as an acid and Na2HPO4 will act as salt of the acid. pH of the solution can be calculated by usng Hinderson hasselbalch equation.

pH = pKa + log[salt/acid]

6.8 = 7.2 + log[Na2HPO4/KH2PO4]

or, [Na2HPO4/KH2PO4] = 0.398/1

calculate the decimal fraction of Na2HPO4 and KH2PO4

decimal fraction of [Na2HPO4 ] = 0.398 /1.398 = 0.285

Molarity of Na2HPO4 = 0.045M* 0.285 = 0.013 M = 0.013 moles/L

amount of Na2HPO4 = 0.013 moles * 141.959gm/mol =1.85 gm

decimal fraction of KH2PO4 = 1/1.398 =0.715

Molarity of kH2PO4 = 0.715*0.045M = 0.032 M = 0.032 moles/L

amount of KH2PO4 required = 0.032 moles *136.086 gm/mol = 4.34 gm

---------------------------------------------------------------------------

(b) Dilution does not have significant effect on the ph of the buffer as the both the acid and its salt is diluted to the equal amount. thats why 10x concentrated and 1x concentration does not differ much in pH.

-----------------------------------

(c) Dilution does not affect the pH of the buffer however it does affect the b+uffer capacity of the buffer solution. As a more concentrated solution has greater number of H+ ion, a greater amount of base will be required to achieve desired ph change.