Chemists need a logarithmic scale because hydrogen ion concentrations vary greatly. Typical aqueous chemical solutions span fourteen full orders of magnitude in ion concentration. They range from roughly 1.0 M in strong acids to 1.0 x 10^-14 M in strong bases.
Expressing these values as simple integers from 0 to 14 avoids cumbersome scientific notation. Because the scale is logarithmic, a decrease of one pH unit means a tenfold increase in hydrogen ions. At 25 degrees Celsius, pure water undergoes a specific process called autoionization.
This process produces equal concentrations of newly formed hydrogen and hydroxide ions. Both ion concentrations measure exactly 1.0 x 10^-7 M, establishing the neutral pH point at 7.0. The neutral point shifts slightly based on the overall temperature of the water.
At a normal human physiological temperature of 37 degrees Celsius, neutral pH is approximately 6.8. A complementary scale called pOH measures the hydroxide ion concentration instead. The relationship pH + pOH = 14 holds strictly true at 25 degrees Celsius.
This simple equation enables straightforward interconversion of acidity and alkalinity from a single measurement. The pH values can theoretically extend below 0 or above 14 in highly concentrated solutions. However, standard glass electrode measurements become less reliable in these extreme conditions. This inaccuracy happens due to specific measurement issues called the acid error and alkaline error.