Question

Describe how you would prepare each of the following:

100.0 mL of a buffer with pH 7.4 using species from the ionizations of phosphoric acid.
100.0 mL of a buffer with pH 7.4 using species from the ionizations of carbonic acid.
Will these buffers be more effective against added acid or added base?

Answer 1

By means of the Henderson-Hasselbach equation the relationship of concentrations is calculated:

For phosphoric acid:

[HPO4-2] / [H2PO4-] = 10 ^ pH-pKa = 10 ^ 7.4-7.21 = 1.55

Assuming the 0.1M buffer concentration:

[HPO4-2] + [H2PO4-] = 0.1 M

it clears:

[HPO4-2] = 0.1 - [H2PO4-]

it is replaced and cleared:

0.1 - [H2PO4-] / [H2PO4-] = 1.55

[H2PO4-] = 0.039 M

[HPO4-2] = 0.061 M

for 100 mL the moles must be added:

n H2PO4- = 0.039 mol / L * 0.1 L = 0.0039 mol

n HPO4-2 = 0.0061 mol

For carbonic acid:

[HCO3-] / [H2CO3] = 10 ^ 7.4 - 6.4 = 10

the concentration of the 0.1 M buffer is assumed and the result is:

[HCO3-] = 0.0991 M

[H2CO3] = 0.009 M

for 100 mL:

n HCO3- = 0.00991 mol

n H2CO3 = 0.0009 mol

These buffers have high pKa, therefore they are more effective for acids than for bases.