Topic 4.6 Notes – Introduction to Titration

1. What a Titration Is

A titration is a controlled reaction used to determine the amount or concentration of an analyte (unknown solution) by reacting it with a titrant (known concentration).

For this to work, the reaction must:

• Be specific (only reacts with what you care about)
• Go to completion (no partial reaction at equivalence)

Key Terms You Must Know

• Analyte: unknown solution, usually in the flask
• Titrant: known solution, delivered from a burette
• Burette: long, graduated tube that measures volume very precisely
• Molarity (M): $$M = \frac{\text{moles}}{\text{liters of solution}}$$

Everything flows through this idea:

Titrations are mole problems disguised as lab procedures.

If you can track moles using the balanced equation, you can solve it.

2. Types of Titrations

AP Chemistry emphasizes acid-base titrations, but you should recognize the broader categories.

Acid-Base Titrations (Most Tested)

Based on Brønsted-Lowry theory:

• Acid = proton donor
• Base = proton acceptor

Typical reaction: $$\ce{HNO3(aq) + KOH(aq) -> KNO3(aq) + H2O(l)}$$

At equivalence, the acid and base have reacted in the correct stoichiometric ratio.

Common scenarios you’ll see later in Unit 4:

• Strong acid + strong base
• Weak acid + strong base
• Weak base + strong acid

Other Types (Know What They Are)

• Redox titrations → based on electron transfer
• Precipitation titrations → form insoluble solid
• Complexation titrations → form complex ions (often metal ions)

You don’t calculate these differently at this level. The same mole logic applies.

3. Equivalence Point vs Endpoint

Students mix these up constantly.

Equivalence Point

This is the stoichiometric completion point.

• Moles of titrant added = moles of analyte reacted
• Determined using the balanced equation
• Reaction is complete

For a 1:1 reaction:

$$M_{1}V_{1} = M_{2}V_{2}$$

If the equation shows a 1:2 ratio:

$$M_{1}V_{1} = 2M_{2}V_{2}$$

The coefficients matter. The AP loves giving reactions that are not 1:1 to see if you’re awake.

Endpoint

This is the observable signal that tells you to stop.

• Usually a color change (indicator)
• Happens experimentally
• Should be very close to equivalence

Here’s the distinction clearly:

Good indicator choice makes them almost coincide.

4. Identifying the Equivalence Point in Calculations

This is what shows up on quizzes and MCQs.

The Process

1. Write the balanced equation

   Example: $$\ce{H2SO4(aq) + 2NaOH(aq) -> Na2SO4(aq) + 2H2O(l)}$$

2. Calculate moles of titrant added

   Suppose 0.150 M NaOH and 0.0200 L used:

   $$\text{moles NaOH} = 0.150 \times 0.0200 = 0.00300$$

3. Use mole ratio

   2 mol NaOH react with 1 mol H₂SO₄:

   $$\text{moles H2SO4} = \frac{0.00300}{2} = 0.00150$$

4. Solve for unknown concentration

   If analyte volume was 0.0250 L:

   $$M = \frac{0.00150}{0.0250} = 0.0600\,M$$

Notice: Volumes are not equal at equivalence. Moles are.

5. Titration Curves and What They Show

A titration curve graphs pH vs volume of titrant added. The example below shows a strong acid titrated with a strong base.

Strong acid-strong base titration curve

Three regions matter:

1. Initial region
   At the start of the curve, the pH is determined only by the analyte in the flask.

2. Steep vertical region
   This sharp rise contains the equivalence point. The midpoint of the vertical jump, labeled near pH 7 here, is the equivalence point.

3. After equivalence
   Past the vertical jump, the pH is controlled by excess titrant.

Important patterns:

• Strong acid + strong base → equivalence at pH = 7
• Weak acid + strong base → equivalence > 7
• Weak base + strong acid → equivalence < 7

Even if you haven’t mastered weak acid math yet, you should recognize the direction of the shift.