Buffer solutions: ratio and capacity are different
Learn how a buffer responds, why it is finite, and which calculation comes before the equilibrium approximation.
A buffer limits change
A suitable weak-acid/conjugate-base system can limit pH changes after small additions of acid or base. It cannot maintain the same pH indefinitely. The amount added can exceed what the available components can accommodate.
Ratio is not the whole story
Under suitable assumptions, the Henderson–Hasselbalch relationship connects the base-to-acid ratio with pH. That ratio alone does not establish buffer capacity. Two mixtures with the same ratio but different total concentrations need not respond equally to the same addition.
The distinction is practical: similar initial pH does not mean similar resistance to a subsequent change.
Do stoichiometry first
When strong acid or base is added, account for its reaction with the buffer components before using the new ratio in an equilibrium approximation. Ignoring that initial consumption uses the wrong composition. If one component is exhausted, the original buffer approximation may no longer apply.
Record concentration, volume and temperature, and keep calculated and measured pH in separate entries. Consider calibration and assumptions when interpreting a difference.
