Buffer Solutions

Seite 1

A buffer is a mixture of a weak acid and its conjugate base or a weak base and its conjugate acid. Buffer solution undergoes a minimal change of pH when small amounts of acid (H3O+) or base (OH-) are added.

When an acid is added to a buffer solution, the base or conjugate base part of the buffer acts to neutralise it by accepting a proton/H+

When a base is added to the buffer solution, then the acid or conjugate acid part of the buffer neutralises i tby donating a proton/H+

Thus, when small amounts of H3O+ or OH- ions are added, they react with the species in the buffer solution and do not contribute to a large change in pH, as would be expected if they were addded to pure water.

Buffers are important both in nature (e.g. blood system) and in pH sensitive industrial processes. The pH of blood, for example, is normally 7.4, and if it falls more than 0.4 from this value (as a result of disease, both of which generate acidic conditions) then you could die. You could also die if your blood plasma pH risies to 7.8, as could happen during the early stages of recovery fromsevere burns. To survive, your blood stream

Seite 2

Excersise:

1) Calculate the pH of a buffer solution made by mixing equal volumes of 0.024 molL-1 aqueous ammonia and 0.015 molL-1 ammonium chloride. pKa(NH4+) = 9.25 [Ans: 9.45]

2) Calculate the pH of a buffer solution containing 0.025 molL-1 ethanoic acid and 0.010 molL-1 sodium ethanoate. pKa (ethanoic acid) = 4.75

This is an example of when the concentration of acid and conjugate base in the buffer are known. This is the easiest calculation as values can be directly substituted in the the equations below:

Use [H3O+] = Ka x [HA]/[A-] OR pH = pKa - log [HA]/[A-]

Equation: CH3COOH (aq) + H2O (l) <=> CH3COO- (aq) + H3O+

Method 1 - Long Way

Ka expression:

Ka = [CH3COO-][H3O+]/[CH3COOH]

Rearrangeing gives:

[H3O+] = Ka x [HCOOH]/[HCOO-]

[H3O+] = 1.82 x 10^-4 x 0.15 / 0.05 = 5.46 x 10^4

pH =