Acid-Base Titrations
Introduced| English | Meaning |
|---|---|
| titration curve/taɪˈtreɪʃn kɜːv/ | A graph showing how pH changes as a titrant is added to a solution. |
| equivalence point/ɪˈkwɪvələns pɔɪnt/ | The point in a titration at which the reacting substances are present in the proportions required by the balanced equation. |
The story a curve tells
- Slowly add base to an acid and track the pH.
- The plot starts low, creeps up, then leaps at one point.
- That steep jump marks the moment of exact neutralization.
- The curve's shape reveals the acid's strength.
Reading the titration curve
- The titration curve 滴定曲线 plots pH against added titrant.
- It rises slowly, then jumps sharply at the equivalence point.
- The steep part is where the reaction just completes.
A titration curve has a steep jump near the equivalence point
A titration curve rises steeply near the equivalence point.
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The steep jump marks the equivalence point.
Where they exactly match
- At the equivalence point 等当点, the moles of acid and base are equal.
- For a strong acid with a strong base, the pH there is 7.
- For a weak acid with a strong base, the pH there is above 7.
At the equivalence point, the moles of acid and base are...
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Equivalence means the moles exactly match.
For a weak acid titrated with a strong base, the equivalence-point pH is...
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The leftover conjugate base makes the solution basic.
For a strong acid titrated with a strong base, the equivalence pH is...
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Strong-with-strong gives a neutral salt, so pH is 7.
The halfway clue
- At the half-equivalence point, half the weak acid is neutralized.
- There, $\text{pH} = \text{p}K_a$ — a handy shortcut.
- This is also where the buffer is strongest.
The titration curve
Track pH as titrant is added; the steep vertical jump marks the equivalence point.
At the half-equivalence point of a weak acid titration, pH equals...
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Half-neutralized, $\text{pH} = \text{p}K_a$.
Titrating a weak acid with a strong base — is the pH at equivalence exactly 7?
- No: the leftover conjugate base makes it basic.
- The equivalence pH is above 7.
Choose an indicator that changes colour near the ____ point.
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The endpoint should match the equivalence point.
The equivalence-point pH is not always 7 — it is 7 only for strong-with-strong; a weak acid gives a basic equivalence point. The half-equivalence point ($\text{pH} = \text{p}K_a$) is different from the equivalence point. And pick an indicator that changes colour near the equivalence pH.
A titration curve plots pH against added titrant, jumping steeply at the equivalence point where moles match. That pH is 7 for strong-strong but above 7 for a weak acid with strong base. At the half-equivalence point, $\text{pH} = \text{p}K_a$.