The pH of a solution indicates how acid or alkaline the solution is. A pH of less than 7 indicates that it is an acid, and a pH of more than 7 indicates that it is an alkali.
...the pH of sea water. [+ of]
Skin is naturally slightly acidic and has a pH of 5.5. [+ of]
The fluid that emerges from the vents is acidic (pH 3) and hot.