As pure water dissociates (breaks up into H+ ions and OH– ions), the relative concentrations of hydrogen ion (H+) and hydroxide ion (OH–) remain equal. However, if additional H+ ions or OH– ions are added to the solution, it can cause an imbalance in the relative concentrations of the two ions. The measure of the concentration of H+ (or H3O+) ions in solution is called the pH. The pOH of a substance is a measure of the concentration of OH– ions in solution.
The pH scale ranges from zero to 14, with values under 7 considered acidic and values over 7 considered basic. Acidic solutions have a higher concentration of H+ (or H3O+) ions, relative to the concentration of OH– ions. An acid is any substance that lowers pH by increasing the levels of hydrogen ions. Basic solutions have a lower concentration of H+ (or H3O+) ions, relative to the concentration of OH– ions. A base is any substance that increases pH by removing hydrogen ions from a solution. The relationship between H+ ion concentration and pH is logarithmic (base 10). The pH value itself is the value of the exponent, , when the concentration is expressed as . In other words, in a substance that has a pH of 5, the concentration of H+ ions is . The total of the exponents must equal 14, so in a substance with a pH of 5, the pOH must be 9. When the two values are compared, and , the concentration of H+ is greater than that of OH– by four orders of magnitude.The following equation relates pH to the concentration of H+: