Lemon juice has a pH of about 2. Pure water has a pH of 7. Bleach has a pH of about 13. The pH scale measures the acidity or alkalinity of a solution. It is logarithmic, meaning each unit represents a tenfold change in hydrogen ion concentration. A pH of 3 is ten times more acidic than a pH of 4. The scale runs from 0 to 14, though values outside that range are possible for concentrated solutions.

The pH is defined as the negative logarithm of the hydrogen ion concentration. In water, hydrogen ions exist as hydronium ions, H3O+. The concentration of hydronium ions determines the pH. A low pH means high hydronium concentration. A high pH means low hydronium concentration and high hydroxide concentration. The product of the two concentrations is constant at a given temperature, equal to 1 times 10 to the minus 14 at 25 degrees Celsius.

pH is measured with indicators, pH meters, or titration. Indicators change color over a specific pH range. Litmus paper turns red in acid and blue in base. Phenolphthalein is colorless in acid and pink in base. pH meters use a glass electrode that responds to hydrogen ion activity. They are more precise than indicators.

pH matters in biology, industry, and the environment. Enzymes work best at specific pH. Blood pH is tightly regulated between 7.35 and 7.45. Soil pH affects crop growth. Water pH affects aquatic life. Industrial processes require precise pH control. The scale is simple. The consequences are not.