5. Acid-Base Titrations

A titration allows us to determine the concentration or quantity of a component in a solution by dropwise addition of a standard solution of known concentration (the titrant) to a known volume of the solution to be analyzed. The end point of the titration (equivalence point EP/ stoichiometric point SP) should be able to be determined by a suitable method, often by adding an indicator that changes color close to the stoichiometric point. Plotting the pH of the solution, using a pH meter and a glass electrode, in function of the volume of titrant added to the solution is an other possibility (titration curve).

• Titration of a strong acid (HCl) with a strong base (NaOH)

Example: Titration of 25.00 mL 0.100 mol/L HCl(aq) with 0.100 mol/L NaOH(aq) Reaction: 1 HCl(aq) + 1 NaOH(aq) H2O + NaCl(aq) At the start we have only HCl in the erlenmeyer flask. This strong acid is completely ionized. The pH can be calculated. Upon adding strong base, this will react with the strong acid to form a neutral salt. The pH will increase with decreasing concentration of the strong acid (for two reasons: there is less acid due to the reaction and the volume is increasing due to adding solution of strong base out of the burette). When the equivalence point EP is reached, the only compound that is present in the erlenmeyer flask is sodium chloride NaCl: a neutral salt. If we add more strong base from the burette, the pH will increase slightly.

 

• Titration of a weak acid (HOAc) with a strong base (NaOH)

Example: Titration of 25.00 mL 0.100 mol/L HOAc(aq) with 0.100 mol/L NaOH(aq) Reaction: 1 HOAc(aq) + 1 NaOH(aq) H2O + NaOAc(aq) At the start we have only HOAc in the erlenmeyer flask. This weak acid is partially ionized. The pH can be calculated. Upon adding strong base, this will react with the weak acid to form a salt NaOAc, containing the conjugate base of the weak acid. So, the weak acid (HOAc) and the OAc--ion from the salt form a buffer solution. The pH can be calculated. When the equivalence point EP is reached, the only compound that is present in the erlenmeyer flask is sodium acetate NaOAc: a basic salt. If we add more strong base from the burette, the pH will increase slightly.

• Titration of a weak triprotic acid (H₃PO₄) with a strong base (NaOH)

Example: Titration of 10.00 mL 0.100 mol/L H3PO4(aq) with 0.100 mol/L NaOH(aq) Reactions: 1 H3PO4(aq) + 1 NaOH(aq) H2O + NaH2PO4(aq) 1 NaH2PO4(aq) + 1 NaOH(aq) H2O + Na2HPO4(aq) 1 Na2HPO4(aq) + 1 NaOH(aq) H2O + Na3PO4(aq) At the start we have only H3PO4 in the erlenmeyer flask. This weak acid is partially ionized. The pH can be calculated. Upon adding strong base, this will react with the weak acid H3PO4 to form NaH2PO4, containing the conjugate base of the weak acid. So we have a buffer solution. The pH can be calculated. When the first equivalence point EP is reached, the only compound that is present in the erlenmeyer flask is NaH2PO4: an ampholyte. If we add more strong base from the burette, the second reaction will occur. NaH2PO4 is converted into Na2HPO4. Both compounds form a buffer. When the second equivalence point EP is reached, the only compound that is present in the erlenmeyer flask is Na2HPO4: an ampholyte. If we add more strong base from the burette, the third reaction will occur. Na2HPO4 is converted into Na3PO4. Both compounds form a buffer. When the third equivalence point EP is reached, there is no clear inflection point. The phosphate anion can hydrolyse according to: PO43–(aq) + H2O(l) HPO42–(aq) + OH–(aq) The corresponding equilibrium constant (2.1 x 10-2) is not that small. This means that the equilibrium position of this reaction is not sufficiently shifted to the left and that at EP3 a HPO42–/PO43–-buffer is still present.

 

Summary

Acid-Base Titrations

In general, there is a pH-inflection at every equivalence point EP.

Upon adding the appropiate acid-base indicator the EP will be visible.

The unknown concentration of the solution can be calculated.