Rucete ✏ Lehninger Principles of Biochemistry In a Nutshell
2.2 Ionization of Water, Weak Acids, and Weak Bases
This chapter explains how water ionizes slightly to form hydrogen and hydroxide ions, how weak acids and bases behave in water, and how these properties underlie the concept of pH and buffering in biological systems.
Ionization of Water and Its Equilibrium
• Water molecules can reversibly ionize to yield hydrogen ions (protons) and hydroxide ions, with the reaction governed by an equilibrium constant.
• In solution, free protons do not exist; they are rapidly hydrated to form hydronium ions (H₃O⁺) through hydrogen bonding.
• The ionization of water is measured by its electrical conductivity, and the movement of H⁺ and OH⁻ in water is exceptionally fast due to “proton hopping.”
• The degree of ionization is very small: at 25°C, only about two water molecules in a billion are ionized at any instant.
• The product of the equilibrium concentrations of H⁺ and OH⁻ is constant (the ion product of water, Kw = 1.0 × 10⁻¹⁴ at 25°C).
pH Scale and Measurement
• pH is defined as the negative logarithm of the hydrogen ion concentration: pH = –log[H⁺].
• In pure water at 25°C, [H⁺] = [OH⁻] = 1.0 × 10⁻⁷ M, so pH = 7 (neutral).
• The pH scale is logarithmic: a change of 1 pH unit corresponds to a tenfold change in [H⁺].
• pH can be measured accurately using a glass electrode and pH meter, and is a critical parameter for biochemical and medical applications.
• The pH of blood is tightly regulated; deviations can lead to acidosis (low pH) or alkalosis (high pH), both of which can be life-threatening.
Weak Acids, Bases, and Equilibrium Constants
• Strong acids and bases ionize completely in water, but weak acids and bases only partially ionize.
• A weak acid (HA) and its conjugate base (A⁻) exist in equilibrium, and this relationship is defined by the acid dissociation constant (Ka).
• The strength of a weak acid is expressed by its Ka or by pKa = –log Ka. Lower pKa values indicate stronger acids.
• Titration curves graphically show the pH changes as a weak acid is neutralized by a strong base and reveal the acid’s pKa.
• At the midpoint of titration, the concentrations of acid and conjugate base are equal, and the pH equals the pKa.
Titration Curves and Buffering
• The titration curve of a weak acid shows the region of effective buffering, typically between 10% and 90% neutralization.
• A buffer consists of a weak acid and its conjugate base and resists changes in pH when small amounts of acid or base are added.
• Buffering is crucial for maintaining pH in biological systems, including the cytosol and blood plasma.
In a Nutshell
Pure water ionizes slightly, yielding equal concentrations of hydrogen and hydroxide ions, described by an equilibrium constant. The pH scale expresses [H⁺] in a simple logarithmic form. Weak acids and bases only partially ionize, and their acid dissociation constants (Ka or pKa) quantify their strength. The midpoint of a titration curve marks where pH equals the acid’s pKa. Buffers—mixtures of weak acids and their conjugate bases—are essential for stabilizing pH in biological systems.