Measuring acidity using a pH meter (left), showing a pH of 3.20 for the solution in the beaker. pH is the measure of the amount of hydrogen ions in a solution, and is used to measure its alkalinity or acidity. The probe of the pH meter, dipped in the solution, contains electrodes used to measure the electrical potential of the hydrogen ions. This is directly related to the pH. The temperature dial is adjusted to the room temperature, allowing the pH meter to compensate for the temperature. This pH is acidic. The solution contains aluminium ions and bromocresol green, an indicator that displays a yellow colour in acidic solutions.