Henderson–Hasselbalch equation

The Henderson–Hasselbalch equation estimates a buffer’s pH from an acid’s pKa and the ratio of conjugate base to acid. It follows from the acid dissociation equilibrium.

Henderson–Hasselbalch approximation: A simplified form of the Henderson–Hasselbalch equation that assumes activities can be approximated by concentrations. This assumption explains why the equation works well in dilute solutions but can fail at high ionic strength.
pH: The negative base-ten logarithm of hydrogen-ion activity in a solution. The equation predicts this measure of acidity from buffer composition.
Blood gas analysis: Laboratory measurement of blood pH and dissolved gases, especially carbon dioxide and oxygen. Clinical interpretation often uses the equation to relate bicarbonate concentration and carbon dioxide to blood pH.
Full equilibrium calculation: A solution calculation that solves equilibrium, charge-balance, and mass-balance equations together. Unlike the Henderson–Hasselbalch estimate, it handles cases where simplifying assumptions fail.
Acid dissociation constant: An equilibrium constant that quantifies the dissociation of an acid in solution. Its negative logarithm, pKa, sets the equation’s reference point for acid strength.
Buffer solution: A solution that resists changes in pH when small amounts of acid or base are added. The equation is commonly used to estimate the pH of such solutions.
Bicarbonate buffer system: The carbon dioxide, carbonic acid, and bicarbonate equilibrium that buffers extracellular fluids. Its pH is commonly estimated using bicarbonate and dissolved carbon dioxide as the conjugate pair.
Strong acid: An acid that dissociates nearly completely in water under ordinary dilute conditions. A weak-acid conjugate pair is needed for the familiar buffer form of the equation.
pKa: The negative base-ten logarithm of an acid dissociation constant. At pH equal to pKa, the conjugate base and acid have equal activities.
Buffer capacity: A measure of how much strong acid or base a solution can absorb before its pH changes substantially. The equation gives pH but does not by itself measure a buffer’s resistance to added acid or base.

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