Question

3.Phosphate-buffered saline (PBS) is a buffer solution commonly used in biological research. Most formulations contain a mixture of KCl and NaCl buffered to a pH of 7.40 using phosphate-containing compounds.

a.The following reagents are available in your lab. Choose the correct reagents and calculate the volumes necessary to prepare exactly 5.0 L of a phosphate buffer with a pH of 7.40.

                                               0.100 M H3PO4 0.150 M Na2HPO4

                                               0.200 M KH2PO4 0.120 M Na3PO4

b.Commercial PBS solutions are typically prepared by dissolving solid reagents in water and then adjusting the pH to 7.40 using HCl. The formulation shown below is for the preparation of 10.0 L of a PBS solution. What is the pH of this solution and how many moles of HCl must be added to

adjust the pH to 7.40?

Component	Mass (g)
NaCl	80.0669
KCl	2.0129
Na2HPO4	14.1960
KH2PO4	2.4496

Answer 1

a). We know that,

Ka of H₃PO₄ = 6.2X10^-8

pKa = - logKa

= - log(6.2X10^-8 )

= 7.20

pH of buffer = 7.4

pH = pKa + log[salt] / [acid]

7.40 = 7.20 + log[salt] / [acid]

log[salt] / [acid] = 0.20

[salt] / [acid] = 10^0.20

[salt] / [acid] = 1.584

For 0.100 M H₃PO₄ 0.150 M Na₂HPO₄

[Na₂HPO₄] / [H₃PO₄] = 0.150 / 0.100

= 1.5

For 0.200 M KH₂PO₄ 0.120 M Na₃PO₄

[Na₃PO₄] / [KH₂PO₄] = 0.120 / 0.200

= 0.6 1.58

So, 0.200 M KH₂PO₄ 0.120 M Na₃PO₄ are necessary to prepare exactly 5.0 L of a phosphate buffer with a pH of 7.40.