A2-Level Acids and Bases
Buffer solutions are a mixture of a weak acid and its conjugate base.
They are used to minimise a change in pH of a solution.
An equilibrium is established between the weak acid and conjugate base.
The weak acid and conjugate base are in excess, meaning that the position of the equilibrium established between them will not be sensitive to changes in their concentrations, but it will be very sensitive to changes in the concentration of H⁺(aq) ions.
When H⁺(aq) ion concentration increases, the position of equilibrium moves to oppose the change. The concentration of HA increases by the same amount as the concentration of A⁻ decreases.
Buffer solutions are a mixture of a weak acid and its conjugate base. The position of the equilibrium that exists between the two can move, minimising a change in the pH of a solution when H⁺ ions are added or removed.
The above description is supposed to be a little blunt and confusing! If any words in the above definition are confusing or new to you, please read the relevant sections in ‘Acids and Bases’ before you continue with buffer solutions. The main reason students find buffer solutions difficult is because they don’t yet have a full grasp on the above topics before they attempt to learn about buffers. Once the above are well understood, buffers solutions and calculations involving them become much more straightforward.
A weak acid will always be in equilibrium between itself, its conjugate base and the H⁺ ions in a solution.
The purpose of a buffer is to keep the pH of a solution as constant as possible, even if H⁺ ions are added or removed.
From basic equilibrium theory, we know that if the concentration of the HA or H⁺ and A⁻ changes, the position of equilibrium will move to oppose the change. If the concentration of H⁺ decreases, the position of equilibrium will move to favour the forward reaction and produce more H⁺ to oppose the change. The problem here is that the concentration of HA will rapidly decrease, and as the concentration of A⁻ increases, the equilibrium will be ‘re-adjusted’ again; it’s not just the H⁺ ion concentration that is being kept constant by the equilibrium, but also the A⁻ (and, to an extent, the HA).
To get around this problem, a large excess of HA and A⁻ is used. This means that changing the position of equilibrium will not radically alter the concentration of either the HA and A⁻, so the concentration of H⁺ ions becomes the focus for the position of equilibrium.
To make a buffer solution, a weak acid (HA) and its conjugate base (A⁻) is needed. When they are mixed together an equilibrium is established, and any changes to the concentration of H⁺ ions is minimised because the position of equilibrium moves to oppose the change.
Note – the pH change is only minimised, there is still a slight change as the change in the concentrations of HA and A⁻ when the position of equilibrium moves causes a new position of equilibrium to be established. Because the original amounts of HA and A⁻ are in excess however, this change is very slight.
There are two scenarios:
More H⁺ ions added
Here, the position of equilibrium will want to oppose the change and remove the added H⁺ ions to restore the concentration back to the level it was before the H⁺ ions were added.
It does this by moving the position of equilibrium to favour the backward reaction. Excess A- reacts with the ‘added’ H⁺ ions and forms HA. The added H⁺ ions are removed and the pH stays (nearly) the same. However, the amount of HA and A⁻ now changes – the amount of A⁻ will decrease and the amount of HA will increase.
H+ ions are removed
This is not strictly speaking ever possible. In reality, a base or alkali is added which reacts with the H⁺ ions present in solution, therefore the concentration of H⁺ ions decreases.
In this situation, the position of equilibrium will move to increase the concentration of H⁺ ions back to the original level, before the base or alkali reacted with them. To do this, more HA will dissociate to produce H⁺ ions and A- ions; the amount of HA in the solution will decrease and the amount of A⁻ will increase.