pH

pH is a numerical measure that describes whether a solution (a mixture of a substance dissolved in water) is acidic or alkaline (basic). It is based on the concentration of hydrogen ions (H⁺) present in the solution. When these ions are abundant, pH is low and the solution is acidic; when they are scarce, pH is high and the solution is alkaline.

The standard pH scale ranges from 0 to 14, with 7 considered neutral at a temperature of approximately 25 degrees Celsius. Pure water at this temperature has a pH close to 7. Highly concentrated solutions can sometimes fall outside the 0–14 range.

Calculation

pH is calculated using the base-10 logarithm of the hydrogen ion concentration. This means that a change of one pH unit represents a tenfold change in the concentration of H⁺ ions. Therefore, a solution with a pH of 4 contains ten times more H⁺ ions than a solution with a pH of 5.

Measurement

Two common methods are used. The first is an indicator—a substance that changes colour when pH changes, such as litmus paper or pH paper. The second is a pH meter, an electronic device that measures the small voltage difference between two electrodes and displays a numerical reading. A pH meter is calibrated using solutions of known pH.

Significance

pH affects many chemical and biological reactions. Human blood is maintained within a very narrow slightly alkaline range; deviation from this range causes health problems. In soil, pH determines the availability of nutrients to plants. pH monitoring is also critical in water treatment, food manufacturing, and wastewater treatment.