A2-Level Acids and Bases
To find the pH of an alkaline solution, the concentration of H⁺(aq) ions in solution can be found using the ionic product of water, Kw.
Water dissociates to release H⁺(aq) ions and can be considered a (very) weak acid mathematically.
The ionic product of water, Kw, is a constant that links the concentration of OH⁻(aq) ions in solution to the concentration of H⁺(aq) ions in solution
At 298K, Kw has a fixed value (1 x 10 ).
Ionic Product of water, Kw
So far, we have just linked pH to H⁺ ion concentration in a solution. This is fine for calculating the pH of an acid solution, as the concentration of acid (together with Ka for a weak acid) can be used to find the pH of the solution.
If we only have an alkali, however, there is no H⁺ ion concentration to use for a pH calculation – this concentration needs to be found.
In water, a very small percentage of molecules dissociate to release H⁺ and OH⁻ ions. It’s only a small percentage that dissociate but because the number of water molecules present in a solution is so high, an actual concentration of H⁺ ions exists. There is an equilibrium between the water, the hydroxide ions and protons present.
Here, the water is acting as a weak acid and, even though the position of equilibrium massively favours the backward reaction, a Ka expression can still be applied.
As already mentioned, the actual number of water molecules dissociating is so small the overall concentration of water molecules remains effectively the same – this means we can consider it to be ‘constant’.
The Ka of a weak acid is also constant (at a given temperature), this means Ka and [H O] can be treated as two constants that are never going to change, these are multiplied together to give a new ‘combined’ constant, Kw. This constant is called the ionic product of water and has a value of 1x10 at 298K.
This gives us a new expression:
When dealing with alkaline solutions, the concentration of the alkali can be used to find the concentration of H⁺ ions present, which can then be used to find the pH.