The pH of an aqueous solution provides a description of the concentration of H₃O+ ions (or water molecules that have accepted an extra proton) in the solution. The pH is calculated as the negative logarithm (base 10) of the activity of H₃O⁺ ions in solution. A pH near 7 indicates that the solution is nearly neutral, in the sense that this would be the pH of pure water without any acid or base added. A pH value lower than 7 indicates that the solution is at least somewhat acidic, while a pH above 7 indicates the solution is basic.

The equation describing the pH of a solution is: