pH = −log₁₀[H⁺]
pH = −log₁₀[H⁺] quantifies the acidity or basicity of aqueous solutions on a scale typically from 0 to 14 (though values outside this range are possible for very concentrated solutions). Strong acids dissociate completely: pH = −log(C) for monoprotic acids. Strong bases: pOH = −log(C), then pH = 14 − pOH. Weak acids require the equilibrium expression: [H⁺] = √(Ka × C) for the simple case. Buffers use the Henderson-Hasselbalch equation. pH affects virtually every chemical and biological process: enzyme activity (most enzymes have narrow optimal pH ranges), drug bioavailability (ionization state changes absorption), corrosion rates, soil chemistry, and aquatic ecosystem health. pH measurement is done with glass electrodes (accuracy ±0.01) or indicator dyes (accuracy ±0.5).