pH

pH is a measure of how acidic or basic (alkaline) a solution is. It is defined as the negative logarithm of the hydrogen ion activity: pH = -log10(aH+). In dilute aqueous solutions, a good approximation is pH ≈ -log10([H+]). The common pH scale runs from 0 to 14 at 25°C, with values below 7 indicating acidity, above 7 indicating basicity, and 7 indicating neutrality. Each unit represents a tenfold change in hydrogen ion activity.

pH is measured using either electronic or chemical methods. Electronic measurements use a pH meter with a reference and a glass electrode, requiring calibration with standard buffers (typical values around pH 4, 7, and 10). Chemical indicators are dyes that change color over a pH range, providing a rough estimate. Temperature affects pH and hydrogen ion activity, so pH values can shift with temperature and ion strength.

pH and pOH are related by pH + pOH = 14 at 25°C, reflecting the balance of hydrogen and hydroxide ions. Solutions often rely on buffers to resist pH changes; buffering capacity depends on the amounts and strengths of weak acids and bases present. Accurate pH control is important because many chemical reactions, enzyme activities, solubilities, and biological processes are pH-dependent.

Applications span science and industry, including chemistry, biology, medicine, environmental monitoring, agriculture, food and beverage production, and water quality assessment. Understanding pH is fundamental to predicting reaction behavior, designing experiments, and maintaining the stability of products and ecosystems.