What is pH? The pH is the Latin abbreviation for "Pondus hydrogenii" (Pondus = pressure, hydrogenium = hydrogen), which measures the activity of hydrogen ions in a substance. This activity is directly related to the acidic, neutral and alkaline aqueous solution. Water is chemically neutral, but it is not free of ions, even if it is chemically pure. Trace amounts of water are chemically dissociated: Strictly speaking, hydrogen nuclei do not exist in a free state until hydrated.