Chapter 21 - Buffers and Neutralisation

Define buffer solution and explain how it works

A system that minimises pH changes when small amounts of an acid or base are added. Its two components are a weak acid and its conjugate base.

Weak acid (HA) - removes added alkali

Conjugate base (A-) - removes added acid

When acids/alkalis are added to a buffer, the two components of the buffer will react and eventually be used up, once this happens, the solution loses its buffering ability.

Describe 2 methods for preparing a buffer solution

1 - Add one of its salts (the conjugate base)

2 - Add alkali to generate the conjugate base in situ

Give an example of how buffers are used in the body and what would happen if pH slip is out of the normal range

Blood plasma needs to be maintained at a pH between 7.35 and 7.45. This pH is controlled by a mixture of buffers with that carbonic acid-hydrogencarbonate buffer system being the most important.

If pH falls below this range, people can develop acidosis causing fatigue, shortness of breath and even shock/death

If pH rises above this range, people can develop alkalosis causing muscle spasms, light-headedness and nausea

How does the carbonic acid-hydrogencarbonate buffer system work

On addition of acid:

• [H+] increases

• H+ ions react with conjugate base, HCO3-

• Equilibrium shifts left, removing H+ ions

On addition of alkali:

• [OH-] increases

• Small concentration of H+ ions react with the OH- ions

• H2CO3 dissociates

• Equilibrium shifts right to restore H+ ions

How do you use a pH meter

• Using a pipette, add a measured volume of acid to a conical flask

• Place the electrode of the pH meter in the flask

• Add aqueous base to the burette and acid to the conical flask

• Perform a titration, ensuring to swirl the flask

• Record the volume of base added and pH at each point

• Go 1cm³ at a time, then more slowly when the pH reading changes less rapidly, then 1cm³ at a time again until pH has little change

Explain the 3 main sections of the pH titration curve

• At first, the acid is in excess so pH increases very slowly as as base is added

• The vertical section is where pH increases rapidly with addition of a very small volume of base as the acid is used up more quickly

• After the vertical section, pH will rise very slowly again as the base is now in excess

Define the equivalence point

The volume of one solution that exactly reacts with the volume of the other solution

This is the centre of the vertical section of the curve

What is an acid-base indicator

A weak acid, HA, that has a distinctively different colour from its conjugate bas, A-

At the end point of the titration, there are equal amount of acid and base so the colour will be a mix of both

E.g. In methyl orange: weak acid = red

conjugate base = yellow

end point = orange

How does the indicator colour change when adding a base to acid or acid to base

Adding base to acid:

Weak acid dissociates shifting equilibrium to the right so more conjugate base is made. Colour changes to the colour of the base

Adding acid to base:

H+ ions react with conjugate base shifting equilibrium to the left so more acid is made. Colour changed to the colour of the acid

Within what range to most indicators change colour

Usually over a range of about two pH units

How do you choose an indicator

• Should choose an indicator in which the end point and the equivalence point coincide as much as possible

Which indicators are suitable for:

1. Strong acid x Strong base

2. Strong acid x Weak base

3. Weak acid x Strong base

4. Weak acid x Weak base

1. Phenolphthalein , Methyl Orange, Litmus

2. Methyl Orange

3. Phenolphthalein

4. None