The definition of the pH of a solution as the negative logarithm of the hydrogen ion concentration is valid only for an ideal solution.

In practice, the pH measurement considers the negative logarithm of the hydrogen ion activity rather than concentration.

So, the pH can be more accurately redefined as the negative logarithm of the product of the hydrogen ion concentration and its activity coefficient.

In the case of pure water at 25 degrees C, the extremely low ionic strength suggests that the activity coefficients of the ions are close to one.

However, adding a salt like potassium chloride to the water increases the ionic strength of the solution, thus decreasing the activity coefficients.

It follows here that the hydrogen ion activity is slightly increased, corresponding to a decrease in the pH of the solution.

Notably, the addition of potassium chloride causes a negligible change in pH but a significant increase in the hydrogen ion concentration.