Making buffer solutions

Buffer solutions contain relatively high concentrations of a weak acid and its conjugate base.

When small amounts of acid or base are added to the solution, buffers react with the added acid or base and resist large changes in pH.

Many biochemical reactions require specific buffers to work properly.

Therefore, making buffer solutions is an important laboratory skill in biochemistry.

For a brief review of how buffers work, let’s look at a generic buffer that consists of a weak acid, HA, and its conjugate base, A minus.

If an acid, H plus, is added to the buffer solution, the conjugate base A minus will react with it.

We can write the balanced equation for this acid-base neutralization reaction as A minus plus H plus gives HA.

If a base, OH minus, is added to the buffer solution, the weak acid HA will react with it.

We can write the balanced equation for this acid-base neutralization reaction as OH minus plus HA gives H2O plus A minus.

Therefore, a buffer resists changes in pH because it contains both an acid, HA, to neutralize added OH minus ions and a base, A minus, to neutralize added H plus ions.

There are two ways to make a buffer solution.

The first way to make a buffer solution starts with an aqueous solution of a weak acid, HA.

Since weak acids have a low percent ionization, in aqueous solution, most of the weak acid will stay as HA, and only a small percentage of HA will turn into A minus.

So, to represent an aqueous solution of a weak acid HA, in our particulate diagram, we start out with four HA molecules, and zero A minus anions.

Water molecules are left out of the particulate diagram for clarity.

Next, we add a solid salt, NaA.

When NaA dissolves in water, NaA turns into the Na plus ion and the A minus anion.

So if we add four particles of NaA to the aqueous solution of HA, they will dissociate and form four Na plus ions and four A minus anions.

Buffers work the best when the concentrations of the weak acid, HA, and its conjugate base, A minus are equal.

Since we have four HA molecules and four A minus anions in solution, we have a good buffer solution.

There are also four sodium ions present in the solution; however, sodium ions don’t react with water.

Since the sodium ions don’t react with water, they don’t affect the pH.

The second way to make a buffer solution also starts with an aqueous solution of a weak acid, HA.

However, this time, instead of adding a salt, we will add a strong base.

This time let’s start with 8 HA molecules in the particulate diagram.

Our next step is to add an aqueous solution of sodium hydroxide.

Sodium hydroxide is a strong base that dissociates into sodium ions, Na plus, and hydroxide ions, OH minus, when dissolved in water.

OH minus reacts with HA in an acid-base neutralization reaction to form H2O and A minus.

So, in the particulate diagram, if we pour in a solution that contains four particles of OH minus, the four OH minus ions will react with four HA molecules to form four A minus ions.