Question

# 3.Phosphate-buffered saline (PBS) is a buffer solution commonly used in biological research. Most formulations contain a...

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

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

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] = 100.20

[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.

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