LAB 11

Determination of Buffer Capacity

Purpose

Students will determine the buffer capacity of several acetic acid / acetate buffer

solutions using a pH probe.

Buffers

A buffered solution is a solution that resists a change in pH.

In order to have a buffer, two components are essential:

a weak acid a weak base

These two must have a common ion:called a conjugate acid-base pair

Buffer Solution

CH3COOH

CH3COOH

CH3COOHCH3COO-

CH3COO-

CH3COO-

Your buffer solution will have an equimolar

concentration of acetic acid and the acetate ion.

Na+

Na+

Na+

Buffer Example

Acetic Acid / Acetate Ion: CH3COOH CH3COO- + H+

Weak acid neutralizes OH-:OH- + CH3COOH CH3COO- + H2O

Weak base neutralizes H+:H+ + CH3COO- CH3COOH

Thus, a change in pH is resisted.

Equilibrium Expression and theHenderson-Hasselbalch Equation

CH3COOH CH3COO- + H+

Ka =

pH = pKa + log

3

3

H CH COO

CH COOH

3

3

CH COO

CH COOH

Things to remember:

Buffers are most effective when the pH of the buffered system is equal to the pKa of the conjugate acid.

Buffers are only effective when the pH is within 1 unit from the pKa. pH = pKa ± 1

When this pH is exceeded, we have exceeded the buffer capacity.

Buffer Capacity

The moles of acid or base that can be absorbed by a buffered system without changing the pH by more than one unit is called buffer capacity.

Cb =

The concentrations of all your buffers for this lab have a 1:1 ratio.

We will titrate to a change in pH of exactly 1 unit.

NaOH NaOH

buffer,L

M VpH V

Safety ConcernsReagents:

• Acetic Acid (1 N)• Acetate Buffers• Sodium Hydroxide (0.1 N) / Potassium Hydroxide (0.1 N)

Eye Contact:• Irritation, tearing, redness, pain, impaired vision, severe

burns and irreversible eye injury.Skin Contact:

• Severe skin irritation, soreness, redness, destruction of skin (penetrating ulcers) . May cause sensitization and / or allergic reaction.

Inhalation:• May cause coughing, serious burns, pneumonitis,

pulmonary edema, and coma.Ingestion:

• Toxic. Corrosive to mucous membranes. May cause perforation of the esophagus and stomach, abdominal pain, nausea, vomiting, diarrhea, general gastro-intestinal upset.

Solutions and Waste

Only solutions with a pH between 6 and 8 can go down the drain.

All other solutions need to go in the acid/base waste container in the fume hood.

Next Week: Skill Evaluations

Study for the quiz.You may be evaluated on any of the posted

skills.Read through pages 281 – 300 in your lab

manual.Remember your goggles and lab manual!