Question

Lets make a buffer at pH = 9. Look up an appropriate acid / base, assuming the ionic strength equals 0.1 M. Determine the concentrations of the acid and the base needed to make 100 mL of 0.1 M total concentration of buffer at pH = 9. Show all calculations and report the mass of the acid and basic form needed to make this buffer.

Answer 1

To prepare a buffer of pH = 9 we use the Henderson-Hasselbalch equation.

pH = pKa + log{[CO3^2-]/[HCO3^-]}

The Ka value comes from the equilibrium:

NaHCO3 = Na+ (aq) + HCO3^-(aq)

HCO3^-(aq) + H2O(l) <=> H3O^+(aq) + CO3^2-(aq)

Ka = [H3O^+][CO3^2-]/[HCO3^-] = 4.7 x 10^-11
pKa = -logKa = 10.3

9.00 = 10.3 + log{[CO3^2-]/[HCO3^-]}
log{[CO3^2-]/[HCO3^-]} = -1.3

{[CO3^2-]/[HCO3^-]} = 10^-1.3

[CO3^2-]/[HCO3^-] = 0.050
[CO3^2-] = 0.050[HCO3^-]

Thus to prepare buffer at pH = 9 use sodium carbonate and sodium bicarbonate in following ratio:

[CO3^2-]/[HCO3^-] = 0.050