Titration Curves

Labeling a Titration Curve

A plot showing the pH of the solution as a function of the quantity of acid or base added is known as a titration curve.

Photo by Rice University / CC BY

The titrant (substance in the buret that is being added to the analyte in the flask slowly overtime) is: ________

What is the volume for the equivalence point? ________

What is the volume for the half equivalence point? ________

Titration Curves

1. Strong Acid – Strong Base (HCl(aq) and NaOH(aq))

Photo by Rice University / CC BY

• pH is low initially
• As base is added, the pH increases slowly.
• The pH rises steeply when the moles of OH- nearly equals the moles of H3O+
• pH ________ 7 at equivalence point
• The additional drop of base neutralizes the tiny excess acid and introduces a tiny excess of base
• Then, pH increases smoothly as more base is added.

2. Weak acid – Strong base Titration (CH3COOH(aq) with NaOH(aq))

Photo by Rice University / CC BY

• In a weak acid – strong base titration: pH ________ 7 at the equivalence point
• Past the equivalence point: pH determined by the [OH-] from the excess strong base. You need the total volume of the solution to determine [OH-]

Calculating pH at different points in the titration

At the start: solution of the weak acid, CH3COOH; use ICE table and Ka to determine pH

$CH_3 COOH(aq)+H_2 O(\ell)⇌CH_3 COO^- (aq)+H_3 O^+ (aq)$

Between the start and equivalence point: buffer region due to presence of both CH3COOH and CH3COO-; determine moles of acid left unreacted and moles of base produced. Use Henderson – Hasselbach equation to solve for pH.

$CH_3 COOH(aq)+OH^- (aq)\to CH_3 COO^- (aq)+H_2 O(\ell)$

At half equivalence point: pH = pKa

At equivalence point: all CH3COOH has been converted to CH3COO-; the pH is controlled by the acetate ion, and is thus basic.

$CH_3 COO^-(aq)+H_2 O(\ell)⇌CH_3 COOH (aq)+OH^- (aq)$

Past the equivalence point: pH determined by the [OH-] from the excess strong base. You need the total volume of the solution to determine [OH-]

3. Weak base –Srong acid Titration: (NH3 and HCl)

Draw the titration curve where HCl(aq) is the titrant and being added in gradually.

• In a weak base – strong acid titration: pH ________ 7 at the equivalence point
• Past the equivalence point: pH determined by the [H3O+] from the excess strong acid

Calculating pH at different points in the titration

At the start: solution of the weak base, NH3; use ICE table and Kb to determine pH

$NH_3(aq)+H_2O(\ell)⇌NH_4^+(aq)+OH^-(aq)$

Between the start and equivalence point: buffer region due to presence of both NH3 and NH4+ determine [NH3] left unreacted and [NH4+] produced; use Henderson-Hasselbach equation to determine pH

$NH_3 (aq)+H_3 O^+ (aq)\to NH_4^+ (aq)+H_2 O(\ell)$

At half equivalence point: pH = pKa of NH4+

At equivalence point: all NH3 has been converted to NH4+; use Ka of NH4+ to determine [H3O+]

$NH_4^+(aq)+H_2O(\ell)⇌NH_3(aq)+H_3O^+(aq)$$$

Past the equivalence point: pH determined by the [H3O+] from the excess strong acid