How does the starting pH of a titration curve help identify the strength of the acid analyte?

A strong acid will have a very low starting pH (typically around 1 for 0.1 M), whereas a weak acid will have a higher starting pH (typically 3-5) because it only partially dissociates in solution.

What is the difference between the equivalence point and the endpoint of a titration?

The equivalence point is the theoretical point where moles of titrant equal moles of analyte. The endpoint is the physical point where an indicator changes color; ideally, the endpoint should occur as close to the equivalence point as possible.

Compare the pH at the equivalence point for a Strong Acid/Strong Base titration versus a Weak Acid/Strong Base titration.

In a SA/SB titration, the pH is 7.0 because the salt does not hydrolyze. In a WA/SB titration, the pH is > 7.0 because the conjugate base of the weak acid reacts with water to produce hydroxide ions.

Why is it a mistake to assume the equivalence point always occurs at pH 7.0?

This only occurs for strong acid-strong base titrations. If a weak species is involved, the resulting conjugate acid or base will undergo hydrolysis, shifting the pH away from neutral.

What error occurs if an indicator is chosen whose range is outside the vertical section of the curve?

The indicator will change color either too early or too late, leading to a significant difference between the measured endpoint and the true equivalence point, resulting in inaccurate concentration calculations.

Why is the 'half-equivalence point' often confused with the 'equivalence point' in calculations?

Students may forget that the half-equivalence point is used to find $pK_a$, while the full equivalence point volume is used for stoichiometric molarity calculations. They represent different chemical states of the system.

Define the 'Buffer Region' on a titration curve.

The buffer region is the relatively flat portion of the curve for a weak acid/base titration where the solution contains significant amounts of both the weak species and its conjugate, resisting large changes in pH.

What is the mathematical relationship at the half-equivalence point for a weak acid titration?

At this point, $pH = pK_a$. This occurs because the concentrations of the weak acid and its conjugate base are equal, making the log term in the Henderson-Hasselbalch equation zero.

How is the equivalence point identified on a titration curve graph?

It is the point of inflection located exactly in the middle of the steepest vertical section of the pH curve.

Why does a weak acid titration curve show an initial 'jump' in pH before leveling off?

The initial jump occurs as the first bit of strong base added neutralizes some weak acid, immediately creating conjugate base and establishing a buffer system that then resists further pH changes.

Titration Curves & Equivalence Points | College Board AP Chemistry

Revision Notes

College Board

Chemistry

Unit 8: Acids & Bases

Titration Curves & Equivalence Points

Summary

A titration curve is a graphical representation of the pH change in a solution as a titrant is added. It provides critical data regarding the stoichiometry of the reaction, the strength of the acids and bases involved, and the specific pH at which neutralization is complete.

1. Definition & Core Concepts

Titration Curve: A plot of the pH of the analyte solution versus the volume of the titrant added from a burette. It typically follows an S-shaped (sigmoidal) path, reflecting the logarithmic nature of the pH scale.
Analyte and Titrant: The analyte is the solution of unknown concentration in the flask, while the titrant is the solution of known concentration added to it. The curve tracks how the analyte's chemical environment evolves as it reacts with the titrant.
Equivalence Point: The theoretical point in a titration where the number of moles of titrant added is stoichiometrically equal to the number of moles of analyte originally present. It is identified as the midpoint of the steepest vertical section of the curve.

A standard sigmoidal titration curve showing pH on the y-axis and volume of titrant on the x-axis, with the equivalence point marked at the center of the vertical inflection.

2. Underlying Principles of Curve Shape

Strong Acid-Strong Base: The curve starts at a very low pH (around 1) and remains relatively flat until very close to the equivalence point. At the equivalence point, the pH is exactly 7.0 because the resulting salt does not undergo hydrolysis.
Weak Acid-Strong Base: The curve starts at a higher pH (around 3) and shows an initial rapid rise followed by a leveling off known as the buffer region. The equivalence point occurs at a pH greater than 7 due to the production of a basic conjugate base.
Weak Base-Strong Acid: The curve starts at a high pH (around 11) and drops into a buffer region. The equivalence point occurs at a pH less than 7 because the resulting conjugate acid lowers the pH of the solution.

3. The Half-Equivalence Point

4. Key Distinctions in Titration Types

Titration Type	Starting pH	pH at Equivalence	Vertical Section Range
Strong Acid + Strong Base	Very Low (~1)	7.0	Large (pH 3 to 11)
Weak Acid + Strong Base	Moderate (~3-5)	> 7.0	Medium (pH 7 to 11)
Strong Acid + Weak Base	Moderate (~9-11)	< 7.0	Medium (pH 3 to 7)
Weak Acid + Weak Base	Moderate (~3-5)	~ 7.0	Very Small / None

Weak-Weak Titrations: These are rarely performed because the vertical section is too small to accurately identify an equivalence point using standard indicators or pH meters.

5. Polyprotic Acid Curves

6. Exam Strategy & Tips