Question

Citric acid has three pKa's: 3.13, 4.76 and 6.39. If you add sufficient solid NaOH to 100.0 mL of 0.5 M citric acid to bring the pH to 4.00 (assume all the NaOH dissolves without altering the total volume), what are the concentrations of the major species present in the solution? How many additional miles of NaOH would you need to add to bring the solution to a pH or 5? Which species are present in the solution at that point?

Answer 1

Citric acid three ionizable hydrogen. pH 4.00 is near to pKa 1 and pKa 2,

Using Henderson-Hasselbalch equation:

pH = pKa + log [base] / [acid] = pKa + log [HCit^2-] / [H₂Cit^-]

pKa = 4.76

4 = 4.76 + log [HCit2-] / [H2Cit-]

4 - 4.76 = log [HCit2-] / [H2Cit-]

log [HCit2-] / [H2Cit-] = - 0.76

[HCit2-] / [H2Cit-] = 0.17

Total volume is given i.e., 100ml,

let volume of H₂Cit^- = x ml, then volume of HCit^2- = 100 - x ml

V_HCit^2-/ V_H2Cit^-= 100 - x / x = 0.17

x = 85.4

Therefore, volume of H₂Cit^- = 85.4 ml and volume of HCit^2- = 100 - 85.4 = 14.6 ml

Species present at pH 5, are HCit^2- and Cit^3-.