Which ion concentration is directly measured by pH?

pH is a measure of the hydrogen ion concentration in a solution. It is defined mathematically as the negative base 10 logarithm of the hydrogen ion concentration: pH = -log[H⁺]. Therefore, when we determine the pH of a solution, we are directly measuring how many hydrogen ions are present.

The importance of hydrogen ions comes from their role in defining the acidity or basicity of a solution. A lower pH indicates a higher concentration of hydrogen ions, which corresponds to a more acidic solution. Conversely, a higher pH indicates a lower concentration of hydrogen ions, leading to a more basic or alkaline solution.

While hydroxide ions, sodium ions, and chloride ions are important in various chemical reactions and properties of solutions, they do not define pH directly. Instead, the concentration of hydroxide ions can be indirectly related to pH through the ion product constant of water, but the pH itself is specifically a measure of hydrogen ion concentration.