Buffers

A buffer is a solution that minimizes changes in pH when moderate amounts of acid or alkali are added.

But if large amounts of acid or alkali are added the system cannot adjust and pH rapidly changes.

Composition of buffers

Buffers consist of a mixture of a weak acid and one of its salts. (Or a weak base and its salt.)

Eg; Ethanoic acid

Sodium ethanoate

+

Ammonia

Ammonium nitrate

+

Buffers minimize changes in pH through Le Chatelier’s Principle.

CH3CO2H ⇌CH3CO2- + H+

If acid is added [H+] increases.

So eqm shifts LHS to remove it.

CH3CO2H ⇌ CH3CO2- + H+

Reducing [H+]

Minimizing pH changes.

CH3CO2H ⇌CH3CO2- + H+

If alkali is added [H+] decreases.

So eqm shifts RHS to regenerate H+.

CH3CO2H ⇌ CH3CO2- + H+

Increasing [H+]

Minimizing pH changes.

OH- + H+ → H2O

Uses of buffers

• Eg; Baby lotion• Skin has a natural pH of c5.5• Ammonia raises skin pH and causes nappy

rash.• Ammonia is made by bacteria from urea.

• CO(NH2)2 + H2O → 2NH3 + CO2

• These bacteria flourish at pH 7 – 9, but do not multiply at pH 6.

• Baby lotion is therefore buffered at pH 6 to ensure that minimal ammonia is formed.

Soaps and detergents in shampoos are alkaline and could irritate the skin and eyes, so they are buffered at cpH5.5.

The buffer consists of;

Citric acid

Sodium hydroxide is added to form sodium citrate, creating the buffer.

Check out the ingredients!

CO2 + H2O → H2CO3

The rate is slow, but is speeded up by the enzyme carbonic anydrase in red blood cells.

Carbonic acid is a weak acid and dissociates;

H2CO3 H⇌ + + HCO3–

Buffers and blood

Aerobic respiration produces carbon dioxide, which reacts with water to form carbonic acid;

If blood pH alters by c+/- 0.5 a coma results.

So blood must be buffered,

Haemoglobin can buffer, but only when deoxygenated…

Hb- + H+ HHb⇌

HbO2 Hb⇌ - + O2

So when blood acidity rises, due to increased respiration in the tissues…

Oxyhaemoglobin gives up its

oxygen to buffer.

The opposite happens in the lungs.

Calculating the pH of buffers.

• HA ⇌ H+ + A-

• Ka = .[H+] [A-] / [HA]• Rearranging;

• Ka [HA] / [A-] = .[H+] • Take logs;

• logKa + log([HA]/[A-]) = log [H+]

• pKa + log([A-]/[HA]) = pH

General equation for an acid buffer

pH = pKa + log ( [salt] / [acid] )

pH= pKw+ pKb + log ( [base] / [salt] )

For a base buffer:

• 1) How much sodium ethanoate must be dissolved in 1 decimetre of 0.01 mol/dm3 ethanoic acid to give a buffer of pH 5?

• pKa = 4.75 Mr = 82

• pH = pKa + log([salt]/[acid])• 5 = 4.75 + log ([salt] / 0.01)• 5 – 4.75 = log [salt] – log 0.01• 0.25 = log [salt] + 2• -1.75 = log [salt] • 10-1.75 = [salt] = 1.78 x 10-2 mol/dm3

• So 82 x 1.78 x 10-2 = 1.46g must be added.

• 2) What would be the change in pH if 1 ml of 1M sodium hydroxide was added to the buffer in the previous question?

• 1ml = 1x .001 = .001 mols NaOH

• CH3CO2H + NaOH ⇌CH3CO2- Na++ HOH

• The reacting ratii are all equimolar so work out the new amounts of acid/salt.

• So CH3CO2H decreases by 0.001mol.• CH3CO2

- Na+ increases by 0.001mol.• [CH3CO2H} = 0.01 – 0.001= 0.009• [CH3CO2

- Na+] = 0.0178 + 0.001 = 0.0188

• Substitute;

• pH = pKa + log ([salt]/[acid])

• pH = 4.75 + log (0.0188 / 0.009)

• pH = 5.07

• ΔpH = 5 – 5.07 = 0.07