Buffers

A solution that contains significant amounts of a weak acid and its conjugate base(or a weak base and its conjugate acid) is known as a buffer solution. Because of their composition, buffers resist changes in pH when a strong acid or base is added to them. In other words, if you add a strong acid or base to a buffer solution, the buffer solution's pH won't change very much at all because of how much weak acid/base is within it.

Let's take a simple buffer with acetic acid as the very weak acid. The base is technically the acetate ion but usually in buffer solutions, the conjugate base is in the form of a salt. However, sodium won't play a factor in the buffer reaction.

CH₃CO₂H(aq) + H₂O(l) ⇌ H₃O⁺(aq) + CH₃CO₂^-(aq)

If you add a strong base to this solution, the hydronium ion will neutralize it so the concentration of hydronium(H₃O⁺) will decrease. By Le Chatelier's principle, since the concentration of a right-side species decreased, the equilibrium will shift right so more acetate ions will form and some of the lost hydronium will be restored(but not all). Similarly, if you add a strong acid, the strong acid will fully dissociate so you'll end up adding hydronium, a product of this reaction. This means the equilibrium will shift left. A chart summarizing these details is below.

A solution that contains significant amounts of a weak acid and its conjugate base(or a weak base and its conjugate acid) is known as a buffer solution. Because of their composition, buffers resist changes in pH when a strong acid or base is added to them. In other words, if you add a strong acid or base to a buffer solution, the buffer solution's pH won't change very much at all because of how much weak acid/base is within it.

Let's take a simple buffer with acetic acid as the very weak acid. The base is technically the acetate ion but usually in buffer solutions, the conjugate base is in the form of a salt. However, sodium won't play a factor in the buffer reaction.

CH₃CO₂H(aq) + H₂O(l) ⇌ H₃O⁺(aq) + CH₃CO₂^-(aq)

If you add a strong base to this solution, the hydronium ion will neutralize it so the concentration of hydronium(H₃O⁺) will decrease. By Le Chatelier's principle, since the concentration of a right-side species decreased, the equilibrium will shift right so more acetate ions will form and some of the lost hydronium will be restored(but not all). Similarly, if you add a strong acid, the strong acid will fully dissociate so you'll end up adding hydronium, a product of this reaction. This means the equilibrium will shift left. A chart summarizing these details is below.

The buffer capacity of a solution is just how much strong acid or base the solution can take before its pH changes significantly. Usually, the standard for this is 1 unit but its up to whoever is measuring to decide.

To find the pH of a buffer solution, you can use an equation known as the Henderson-Hasselbach(HH) equation. In terms of pH, the equation is:

In terms of pOH, the equation is:

Recall that p(anything) = -log(anything) for these equations.