Buffer

When ammonia gas is added to water, it forms a solution of a weak base whose equilibrium can be represented as follows:

To make a buffer solution out of this system, we can add many more ammonium ions to the solution in the form of ammonium chloride, which is a strong electrolyte and dissociates completely. Ignoring the chloride ions that come along with the ammonium chloride because they do not affect the acidity at all, we will then have:

If we add hydrogen ions to this buffer solution, they will be neutralized by reacting with the hydroxide ions to form water. According to LeChâtelier's principle, this removal of hydroxide ions will shift the equilibrium to the right, producing more ammonium ions, which does not affect the pH (but ammonium ions are slightly acidic). If we add hydroxide ions to the ammonia buffer solution, they will shift the equilibrium to the left, which uses up the added hydroxide ions and forms more whole NH₃molecules, which does not affect the pH.