Question

Give a recipe for preparing a carbonate/bicarbonate buffer solution having a pH of 7.5.

For carbonic acid,  H2CO3,  pKa1 = 6.37 and pKa2 = 10.25.

Answer 1

Preparation of buffer CO3^2-/HCO3- of pH 7.5

Using Hendersen-Hasselbalck equation,

pH = pKa + log([CO3^2-]/[HCO3-])

with pKa2,

7.5 = 10.25 + log([CO3^2-]/[HCO3-])

[CO3^2-] = 0.0018[HCO3-]

let us say we are preparing 1.0 M of 1 L buffer solution with pH 7.5

then,

[HCO3-] + [CO3^2-] = 1 M x 1 L = 1 mol

[HCO3-] + 0.0018[HCO3-] = 1 mol

[HCO3-] = 1 mol/1.0018 = 0.998 mol

[CO3^2-] = 1 - 0.998 = 0.002 mol

mass of NaHCO3 to be taken = 0.998 mol x 84.01 g/mol = 83.842 g

mass of Na2CO3 to be taken = 0.002 mol x 106 g/mol = 0.212 g

So we mix 83.842 g of NaHCO3 and 0.212 g of Na2CO3 in 1 L water to make buffer solution of pH 7.5.