pH is the measure of the acidity or basicity of a solution. It is formally a measure of the activity of dissolved hydrogen ions (H+), but for very dilute solutions, the molarity (molar concentration) of H+ may be used as a substitute with little loss of accuracy. In solution, hydrogen ions occur as a number of cations including hydronium ions (H3O+). In pure water at 25°C, the concentration of H+ equals the concentration of hydroxide ions (OH-). This is defined as "neutral" and correspo… (More on Ph)