2.5: Buffers

Buffer

A solution that resists changes in ppH when small to moderate amounts of strong acid or base are added

What does a buffer contain?

A weak acid (HA) and its conjugate base (A-)

How does a buffer respond when H+ is added?

The conjugate base reacts with OH- to form the conjugate base and water: HA + OH- → A- +H2O

How does a buffer respond when OH- is added?

The weak acid reacts with OH⁻ to form the conjugate base and water: HA + OH⁻ → A⁻ + H₂O.

Why does a buffer resist changes in pH?

The weak acid and conjugate base use up added H⁺ or OH⁻, preventing a large change in pH.

What happens HCl is added to pure water?

The pH drops drastically because there is no buffer to absorb the added H⁺

What happens when NaOH is added to pure water?

The pH rises drastically because there is no buffer to absorb the added OH⁻.

What is the phosphate buffer pair?

H₂PO₄⁻ / HPO₄²⁻

What is the pKa of the phosphate buffer pair discussed?

7.20

When does a buffer work most efficiently?

When the pH is close to the buffer’s pKa

What happens when pH = pKa?

The concentrations of the weak acid and conjugate base are equal: [HA] = [A⁻]

What happens when pH < pKa?

The acid form (HA) predominates

What happens when pH> pKa?

The conjugate base form (A-) predominates

What is the effective buffering range?

Approximately 1 pH unit above and below the pKa

What is buffering capacity?

The amount of acid or base a buffer can absorb before its pH changes significantly

What gives a buffer a high buffering capacity?

Having greater amounts/concentrations of both the weak acid and conjugate base

What gives a buffer a low buffering capacity?

Having very low concentrations of both the weak acid and conjugate base

How do you choose a buffer for an experiment?

Choose one with a pKa close to the desired pH, generally within about ±1 pH unit.

What other factors should be considered when choosing a buffer?

It should not interfere with the reaction, have suitable ionic strength, or cause precipitation

Why are buffers important in biology?

Many biological reactions require a relatively narrow pH range to occur properly

What is an important buffer system inside cells?

The phosphate buffer system (H₂PO₄⁻/HPO₄²⁻).

What buffer system is important in blood?

The carbonic acid/bicarbonate system (H₂CO₃/HCO₃⁻).

What is the pH of human blood mentioned in the section?

About 7.4

What buffer is commonly used in laboratory experiment?

TRIS

What is the main purpose of a buffer?

To keep pH relatively stable by using a weak acid and its conjugate base to absorb added H+ or OH-