Acid-Base Titrations (8.5)

Q: What is the primary difference between the analyte and the titrant in a titration?

The analyte is the solution with an unknown concentration, while the titrant is the solution with a known concentration added to it.

Q: How is the equivalence point defined in a titration?

The equivalence point occurs when the number of moles of titrant added is stoichiometrically equal to the number of moles of analyte originally present.

Q: What is the difference between the equivalence point and the end point?

The equivalence point is the theoretical point of stoichiometric equality, while the end point is the physical point where an indicator changes color.

Q: What is the pH at the equivalence point of a strong acid-strong base titration at \(25^\circ C\)?

The equivalence point is always exactly \(pH = 7.00\).

Q: Why does the pH curve for a weak acid titration level off before the equivalence point?

This is the buffer region , created because the addition of a strong base converts some weak acid into its conjugate base.

Q: What unique relationship exists at the half-equivalence point of a weak acid-strong base titration?

At the half-equivalence point, \([HA] = [A^-]\) and \(pH = pK_a\).

Q: Why is the pH at the equivalence point of a weak acid-strong base titration greater than 7?

The equivalence point is basic (\(pH > 7\)) because the weak acid is entirely converted into its conjugate base , which then reacts with water to produce \(OH^-\).

Q: What is the pH at the equivalence point of a weak base-strong acid titration?

The pH will be less than 7 (acidic).

Q: How can you identify a polyprotic acid from its titration curve?

The curve will show multiple equivalence points (multiple steep climbs), one for each ionizable hydrogen atom.

Q: What should a particulate diagram show at the half-equivalence point of a weak acid titration?

It should show an equal number of weak acid molecules (\(HA\)) and conjugate base ions (\(A^-\)).

AP Chemistry · AP (Advanced Placement) · 6 min read · Updated: Sep 7, 2026

Introduction to Acid-Base Titrations

Welcome to one of the most practical and visual chapters in AP Chemistry! Acid-base titration is a laboratory technique used to determine the unknown concentration of a solution (the analyte) by reacting it with a solution of known concentration (the titrant). Think of it like a chemical "tug-of-war" where we add exactly enough of one side to perfectly cancel out the other. By the end of these notes, you will be able to interpret titration curves, identify key points like the equivalence point, and perform stoichiometry calculations just like a pro.

1. The Fundamentals of Titration

Before we dive into the math and graphs, let's get the vocabulary straight. In a typical titration, a buret is used to slowly add the titrant to a flask containing the analyte.

The Equivalence Point: This is the "magic moment" in a titration. It occurs when the number of moles of titrant added is stoichiometrically equal to the number of moles of analyte originally in the flask. For a monoprotic acid and base, this is simply when \(moles \ of \ H_3O^+ = moles \ of \ OH^-\).

The End Point: This is the point where the indicator changes color. In a perfect world, the end point and the equivalence point happen at the same time. We choose indicators that change color as close to the equivalence point pH as possible.

Did you know? Even though we often use indicators, AP Chemistry students also use pH meters to create titration curves—graphs showing pH on the y-axis and the volume of titrant added on the x-axis.

Key Takeaway: The equivalence point is defined by moles, not volume or concentration alone. It is the point where the reaction is complete according to the balanced equation.

2. Strong Acid-Strong Base Titrations

When you titrate a strong acid (like \(HCl\)) with a strong base (like \(NaOH\)), the reaction goes to completion:
\(H_3O^+(aq) + OH^-(aq) \rightarrow 2H_2O(l)\)

The Curve Shape: The pH starts very low (for an acid analyte). It stays low for a long time and then shoots up almost vertically through \(pH = 7\).
The Equivalence Point: For any strong acid-strong base titration at \(25^\circ C\), the equivalence point is always exactly \(pH = 7.00\). This is because the resulting salt and water do not have any acidic or basic properties.

Common Mistake: Don't assume the equivalence point is always 7! This is only true for strong-strong titrations.

3. Weak Acid-Strong Base Titrations

This is a favorite topic on the AP Exam. When we titrate a weak acid (like \(CH_3COOH\)) with a strong base (like \(NaOH\)), the curve looks a bit different.

The Buffer Region: Unlike strong acids, weak acid curves show a "leveling off" early on. This is because as we add strong base, we create the conjugate base of the weak acid, forming a buffer in the flask. (Cross-reference: See Unit 8.4 for more on buffers).
The Half-Equivalence Point: This is a "gold mine" for information! At the volume exactly halfway to the equivalence point, the concentration of the weak acid \( [HA] \) equals the concentration of its conjugate base \( [A^-] \). At this specific point, \(pH = pK_a\).
The Equivalence Point: The pH at the equivalence point will be greater than 7 (basic). This is because all the weak acid has been converted into its conjugate base, which then reacts with water to produce \(OH^-\) ions.

Memory Trick: "The Stronger one wins the pH." If you have a Strong base and a Weak acid, the equivalence point will be on the Strong side (Basic, \(pH > 7\)).

4. Weak Base-Strong Acid Titrations

This is simply the reverse of the previous section. We start with a high pH (the weak base) and add a strong acid.

The Curve Shape: The pH starts high, drops into a buffer region, and then falls through the equivalence point.
The Half-Equivalence Point: At this point, \( [B] = [HB^+] \) and \(pOH = pK_b\). You can also find \(pH\) here and use it to find \(pK_a\) of the conjugate acid.
The Equivalence Point: The pH will be less than 7 (acidic) because the conjugate acid produced during the reaction lowers the pH.

Key Takeaway: At the half-equivalence point, the pH is most resistant to change, and the identity of the weak substance can be determined using \(pK_a\) or \(pK_b\).

5. Titrations of Polyprotic Acids

A polyprotic acid (like \(H_2CO_3\) or \(H_3PO_4\)) has more than one ionizable hydrogen. If you titrate a polyprotic acid with a strong base, the curve will have multiple "humps" or equivalence points—one for each hydrogen that is removed.

Visualizing the Curve: If an acid is diprotic (\(H_2A\)), you will see two steep climbs and two plateaus.
First Equivalence Point: All \(H_2A\) has been converted to \(HA^-\).
Second Equivalence Point: All \(HA^-\) has been converted to \(A^{2-}\).

Note: The AP Exam does not require you to calculate the exact pH at every single point of a polyprotic curve, but you must be able to identify the equivalence points and half-equivalence points visually.

6. Particulate Representations

The AP Exam often asks you to look at "beaker drawings" (particulate diagrams) and match them to points on a titration curve. Here is what to look for:

1. Before any titrant is added: You should see only the analyte particles (e.g., weak acid molecules \(HA\) and a few \(H_3O^+\)).
2. At the half-equivalence point: You should see an equal number of \(HA\) molecules and \(A^-\) ions.
3. At the equivalence point: You should see no original \(HA\) molecules left. They have all been converted to \(A^-\) and \(H_2O\).
4. Past the equivalence point: You will see excess titrant ions (like \(OH^-\)) floating in the solution.

7. Step-by-Step Stoichiometry Calculations

When the task verb is Calculate, follow these steps to find the unknown concentration or volume at the equivalence point:

Step 1: Write the balanced equation. (Usually a 1:1 ratio for AP problems, but always check!)
Step 2: Calculate moles of the "known" substance. \(moles = Molarity \times Volume \ (in \ Liters)\).
Step 3: Use the mole ratio from the balanced equation to find the moles of the "unknown" substance.
Step 4: Solve for the final requirement. If you need Molarity, divide the moles by the volume of the analyte. If you need Volume, divide the moles by the Molarity.

Quick Review Box:
- Strong Acid + Strong Base: Equivalence Point \(pH = 7\).
- Weak Acid + Strong Base: Equivalence Point \(pH > 7\); Half-Equivalence \(pH = pK_a\).
- Weak Base + Strong Acid: Equivalence Point \(pH < 7\); Half-Equivalence \(pOH = pK_b\).
- Equivalence Point: \(moles \ of \ H^+ = moles \ of \ OH^-\).

Final Encouragement: Titration curves might look like a lot of data at once, but they are just maps! If you can find the equivalence point (the center of the steepest vertical section) and the half-equivalence point (half the volume of the equivalence point), you can solve almost any problem in this chapter.

Quick check

Can you answer these now?

Open each question to check the key ideas from this chapter.

What is the primary difference between the analyte and the titrant in a titration?

The analyte is the solution with an unknown concentration, while the titrant is the solution with a known concentration added to it.

How is the equivalence point defined in a titration?

The equivalence point occurs when the number of moles of titrant added is stoichiometrically equal to the number of moles of analyte originally present.

What is the difference between the equivalence point and the end point?

The equivalence point is the theoretical point of stoichiometric equality, while the end point is the physical point where an indicator changes color.

What is the pH at the equivalence point of a strong acid-strong base titration at \(25^\circ C\)?

The equivalence point is always exactly \(pH = 7.00\).

Why does the pH curve for a weak acid titration level off before the equivalence point?

This is the buffer region, created because the addition of a strong base converts some weak acid into its conjugate base.

What unique relationship exists at the half-equivalence point of a weak acid-strong base titration?

At the half-equivalence point, \([HA] = [A^-]\) and \(pH = pK_a\).

Why is the pH at the equivalence point of a weak acid-strong base titration greater than 7?

The equivalence point is basic (\(pH > 7\)) because the weak acid is entirely converted into its conjugate base, which then reacts with water to produce \(OH^-\).

What is the pH at the equivalence point of a weak base-strong acid titration?

The pH will be less than 7 (acidic).

How can you identify a polyprotic acid from its titration curve?

The curve will show multiple equivalence points (multiple steep climbs), one for each ionizable hydrogen atom.

What should a particulate diagram show at the half-equivalence point of a weak acid titration?

It should show an equal number of weak acid molecules (\(HA\)) and conjugate base ions (\(A^-\)).

What are the four basic steps for titration stoichiometry calculations?

1. Write the balanced equation; 2. Calculate moles of known; 3. Use mole ratios; 4. Solve for the unknown (Molarity or Volume).

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