Buffers and Titrations: Conjugate Pairs, Endpoints, and Equivalence

A buffer can absorb a small addition of acid with a modest pH change. It has a limited capacity, however. A titration explores how composition and pH change as a measured amount of another reagent is added.

A buffer contains appreciable amounts of a weak acid and its conjugate base, or a weak base and its conjugate acid. It resists modest pH changes by consuming added acid or base. Titration determines an unknown amount using a measured reacting solution. Equivalence is stoichiometric completion, while an endpoint is the observed signal used to detect it.

How does a conjugate pair resist pH change?

In an acetic acid-acetate buffer, acetate reacts with added acid, and acetic acid reacts with added base. Both components must be present in useful amounts. Enough added acid can consume most of the acetate and exhaust the buffer’s capacity in that direction.

Under the usual buffer approximations, the Henderson-Hasselbalch equation is

\[ \mathrm{pH}\approx\mathrm{p}K_a+\log_{10}\frac{[\mathrm{A}^{-}]}{[\mathrm{HA}]}. \]

When the conjugate-base and acid amounts are equal in the same volume, their ratio is one and the logarithm is zero. The buffer pH is then approximately the acid’s pKa.

Worked example: use titration stoichiometry

A 25.00 mL sample of a monoprotic acid reaches equivalence after addition of 20.00 mL of 0.1000 M sodium hydroxide. Assume a one-to-one acid-base reaction.

\[ n_{\mathrm{OH^-}}=(0.1000\,\mathrm{mol/L})(0.02000\,\mathrm{L})=0.002000\,\mathrm{mol}. \]

The initial acid amount is the same. Divide by the original acid-sample volume:

\[ M_{\mathrm{acid}}=\frac{0.002000}{0.02500}=0.08000\,\mathrm{M}. \]

The combined final mixture volume is not the denominator for the original acid concentration.

Why are endpoint and equivalence different?

Equivalence describes the reaction’s stoichiometric relationship. An indicator endpoint is the color change you observe. Choose an indicator whose transition range falls within the steep part of the titration curve near equivalence.

The equivalence-point pH depends on the reacting acid and base. A weak acid titrated with a strong base commonly gives a basic equivalence solution because its conjugate base reacts with water. Equivalence does not automatically mean pH 7.

How should you approach a titration calculation?

1. Write the balanced acid-base reaction.
2. Convert the delivered titrant volume to moles.
3. Apply the stoichiometric ratio at equivalence.
4. Use the original sample volume for its original concentration.
5. State whether the observed endpoint is being used as an approximation to equivalence.

Can you apply the idea?

1. Which pair can form an acid buffer?

   Check your answer

   A weak acid and an appreciable amount of its conjugate base.

2. What happens when the base-to-acid ratio in the buffer equation is one?

   Check your answer

   The logarithm is zero, so pH is approximately pKa.

3. Can a buffer resist unlimited added acid?

   Check your answer

   No. Its reactive components can be depleted.

4. What is the difference between endpoint and equivalence?

   Check your answer

   Endpoint is an observed signal. Equivalence is the stoichiometric condition.

5. How many moles are in 10.00 mL of 0.2000 M titrant?

   Check your answer

   \(0.002000\,\mathrm{mol}\).

6. Is every equivalence-point solution neutral?

   Check your answer

   No. The acid-base properties of the species remaining at equivalence determine its pH.

Watch the idea explained

Buffers, the Acid Rain Slayer: Crash Course Chemistry #31 — CrashCourse.

This selected excerpt runs from 2:26 to 3:29. A buffer contains a weak acid and its conjugate base.

Open this video on YouTube.

Where does this fit?

Use the chemistry learning hub to choose a lesson or practice test. Connect this topic with acids, bases, and ph, electrochemical cells, net ionic equations, organic chemistry basics.

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