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Wize University Chemistry Textbook > Buffers and Titrations

Titration Curves

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Introduction to Titrations

Acid-base titrations are neutralization reactions between a titrant (solution in the buret) and analyte (solution in the flask).

Wize Concept
Titrations always involve an acid reacting with a base!

Photo by Rice University / CC BY


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Titrant

  • Solution in the buret
  • We control the amount we add of the titrant to the flask
  • We know its volume, concentration, and identity Example: 1M 100mL NaOH
  • It is usually a strong acid or base


Analyte

  • Solution in the flask
  • It is the unknown (we don't know its concentration, identity, or if it is weak or strong)
  • We do know it's volume (can easily measure its volume in the flask) and its pH (we can simply use a pH meter to measure this in the flask)
  • The other way to know if there's a pH change in the flask is if we see a visual change in color
  • Indicators help to indicate this pH change by changing the color inside the flask
  • When the color changes, we call this an end-point.


Wize Concept
We do titrations (where we add small amounts of titrant to the analyte) to determine the unknown concentration of an acid or a base!


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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:
NaOH
What is the volume for the equivalence point?
25mL
What is the volume for the half equivalence point?
12.5mL

Wize Concept
@ Half-Equivalence Point: [HA] = [A-] and moles of acid=moles of conjugate base
  • Exactly half of the initial amount of weak species has been reacted by the addition of a strong base or acid
  • Remember according to the Henderson Hassalbalch equation: pH=pKa−log⁡ [conj base][conj acid]pH=pKa-\log\frac{\ \left[conj\ base\right]}{\left[conj\ acid\right]}
  • When the [HA]=[A-] how are pH and pKa related?
  • log1=0 so pH=pKa of analyte

@ Equivalence Point: [HA] = [OH-] added and moles of acid=moles of base
  • Enough titrant has been added to completely neutralize ALL of the unknown
  • In other words, the acid and base have completely reacted together so there is no more acid or base left.
Finally, the pH @ Equivalence Point (pH=7, pH <7, or pH>7) is determined by the acidity/basicity of the salt produced
  • Strong acid and strong base → pH @ equivalence point will be neutral (=7)
  • Strong acid and weak base → pH @ equivalence point will be acidic (<7)
  • Weak acid and strong base → pH @ equivalence point will be basic (>7)




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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.
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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
CH3COOH(aq)+H2O(ℓ)⇌CH3COO−(aq)+H3O+(aq)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.
CH3COOH(aq)+OH−(aq)→CH3COO−(aq)+H2O(ℓ)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.
CH3COO−(aq)+H2O(ℓ)⇌CH3COOH(aq)+OH−(aq)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-]


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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
NH3(aq)+H2O(ℓ)⇌NH4+(aq)+OH−(aq)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
NH3(aq)+H3O+(aq)→NH4+(aq)+H2O(ℓ)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+]
NH4+(aq)+H2O(ℓ)⇌NH3(aq)+H3O+(aq)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



Practice: Equivalence Point

For which type of titration will the pH be basic at the equivalence point?