Buffer solutions

Buffer solutions

A single drop of dilute HCl is added to water.

The water is stirred…

and the final solution has a pH of about 2.

We repeat the process using a drop of sodium hydroxide solution.

This solution has a pH of about 12.

We have seen that a single drop of acid or base added to water can change the pH by 5 pH units – that’s a huge change in H3O+ concentration.

Buffer solutions are solutions which resist changes in pH caused by addition of small amounts of acid or base.

We can make buffer solutions by mixing a weak acid (or base) with its conjugate base (or acid).

Ethanoic acid is added to a solution of sodium ethanoate containing a few drops of universal indicator solution.

The final mixture has a pH of about 3.

A large squirt of NaOH is added.

No change in pH is noticed.

We add more NaOH…

… and more NaOH. Still little change in pH.

Remember that one drop of NaOH was enough to change the pH of water by 5 units, yet here we have added several mL of NaOH and not changed the pH significantly.

Adding significant amounts of HCl does not change the pH significantly either.

The CH3COOH/CH3COO- mixture absorbs excess OH- or H3O+ as follows:

You’ll be asked to write equations like these in the exam.

CH3COOH(aq) + OH-(aq) CH3COO-(aq) + H2O(l)

CH3COO-(aq) + H3O+(aq) CH3COOH(aq) + H2O(l)

We can turn ammonium chloride solution into a buffer by adding ammonia solution:

An ammonia/ammonium buffer has a pH of about 9.5.

The pH is not changed by addition of moderate amounts of acid…

… or base.

Can you write the equations to show what happens when acid or base is added to an ammonia/ammonium buffer?

NH3(aq) + H3O+(aq) NH4+(aq) + H2O(l)

NH4+(aq) + OH-(aq) NH3(aq) + H2O(l)

In these examples we took salt solutions and added the appropriate acid or base to make the buffers.

Buffer solutions can also be made starting from just the acid (or base) and adding sufficient NaOH (or HCl) to react with half the acid (or base), resulting in a mixture of unreacted acid (or base) and its salt.

Buffer solutions containing equal amounts (moles) of the acid and its conjugate base are the most effective at absorbing moderate amounts of acid or base added.

When the amounts of acid and conjugate base are exactly equal the pH of the solution is equal to the pKa of the acid.