Titration curves show how the pH of a solution changes as titrant is added, making them one of the most useful graphs in acid base chemistry. The curve shape reveals whether the acid and base are strong or weak, where buffering occurs, and how many equivalence points are present. By reading the steep jumps, flat regions, and midpoint values, students can connect a graph to the chemical species present in the flask.
These curves matter because they guide indicator choice, concentration calculations, and identification of unknown acids or bases.
In a strong acid strong base titration, the pH changes sharply near pH 7 at the equivalence point because the final solution contains mostly neutral salt and water. In a weak acid strong base titration, the curve has a clear buffer region and an equivalence point above pH 7 because the conjugate base makes the solution basic. At the half-equivalence point for a weak acid titration, pH = pKa, which lets chemists determine acid strength from the graph.
Polyprotic acids produce multiple buffer regions and multiple equivalence points because they donate protons in steps.
Understanding Chemistry: Titration Curve Shapes
A titration curve is really a record of which particles control the pH at each stage. Before much titrant has entered, the original acid or base has the main effect. As neutralisation proceeds, that original substance is used up.
Its conjugate partner builds up in the flask. Near an equivalence point, a very small added volume can remove most of the remaining acid or base. This causes the steep part of the graph.
The steepness depends on concentration and on how completely the reacting substances transfer protons. More dilute solutions usually give a less dramatic vertical rise because the pH is less sensitive to a small amount of added titrant.
The buffer region is often the most informative part of a weak acid curve. Here, both the weak acid and its conjugate base are present in useful amounts. Added hydronium is taken up by the conjugate base.
Added hydroxide is taken up by the weak acid. This slows the pH change. At the midpoint of this region, the two forms have equal concentrations.
That is why the pH gives the acid dissociation constant on the pH scale. Students should remember that this midpoint is measured by volume, not by the visual middle of a curved line. It occurs when half the starting weak acid has reacted.
Equivalence does not mean that every particle in the flask is neutral. It means the acid and base were mixed in the exact reacting amounts from the balanced equation. The salt left behind can still react with water.
For example, the conjugate base from a weak acid can remove a proton from water, making the solution alkaline. The conjugate acid from a weak base can donate a proton to water, making it acidic. This is why the pH at equivalence depends on the identity of the salt, not just on the amount of titrant added.
Temperature matters too. The familiar neutral value of seven applies specifically to water at twenty five degrees Celsius.
Polyprotic acids lose protons one at a time because each successive proton is usually harder to remove. A curve may show several rises, with a buffer region before each rise. The first proton is commonly the easiest to remove, so its buffer region is often clearer.
Separate equivalence points are visible only when the acid strengths for successive steps differ enough. If the values are too similar, the changes overlap and the curve can look like it has fewer stages.
In laboratory work, an indicator must change colour within the steep pH range around the endpoint. A pH probe gives more detail, but students should still check volumes carefully, stir continuously, and read the graph scale before deciding where an endpoint lies.
Key Facts
- Strong acid plus strong base equivalence point: pH = 7 at 25 °C.
- Weak acid plus strong base equivalence point: pH > 7 because A- reacts with water to form OH-.
- Weak base plus strong acid equivalence point: pH < 7 because BH+ reacts with water to form H3O+.
- At the half-equivalence point for a weak acid titration: pH = pKa.
- Henderson-Hasselbalch equation: pH = pKa + log([A-]/[HA]).
- For a polyprotic acid, the number of major equivalence points equals the number of ionizable protons if the Ka values are sufficiently different.
Vocabulary
- Titration curve
- A titration curve is a graph of pH versus volume of titrant added during a titration.
- Equivalence point
- The equivalence point is the point where stoichiometric amounts of acid and base have reacted.
- Half-equivalence point
- The half-equivalence point is the point where half of the original weak acid or weak base has been neutralized.
- Buffer region
- The buffer region is the relatively flat part of a titration curve where both a weak acid and its conjugate base, or a weak base and its conjugate acid, are present.
- Polyprotic acid
- A polyprotic acid is an acid that can donate more than one proton per molecule in separate steps.
Common Mistakes to Avoid
- Assuming every equivalence point has pH = 7 is wrong because weak acids and weak bases form conjugates that hydrolyze and shift the pH.
- Confusing the endpoint with the equivalence point is wrong because the endpoint is when the indicator changes color, while the equivalence point is based on stoichiometry.
- Using pH = pKa at the equivalence point is wrong because pH = pKa applies at the half-equivalence point in a weak acid or weak base titration.
- Ignoring the number of steep jumps in a polyprotic titration is wrong because each clear jump can represent a separate proton-transfer equivalence point.
Practice Questions
- 1 A 25.0 mL sample of 0.100 M HCl is titrated with 0.100 M NaOH. What volume of NaOH is required to reach the equivalence point, and what is the approximate pH there at 25 °C?
- 2 A weak acid HA is titrated with NaOH. The half-equivalence point occurs after 18.0 mL of NaOH has been added, and the pH is 4.76. What is the pKa of HA, and what volume of NaOH is needed for the equivalence point?
- 3 A titration curve begins at low pH, has a broad buffer region, and reaches its equivalence point above pH 7. Explain what type of acid and titrant are most likely involved and why the equivalence point is basic.