Intermediate Topic 505 of 1102 Kinetics & Equilibrium
Buffer Equilibria
Conjugate pairs maintain nearly constant pH by shifting equilibrium after small additions.
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In-depth explanation of Buffer Equilibria
A buffer consists of a weak acid and its conjugate base, or a weak base and its conjugate acid, present in comparable amounts. When a small amount of strong acid is added, it reacts with the base component, and when a small amount of strong base is added, it reacts with the acid component, so the pH changes very little. The pH of the buffer is given by the Henderson-Hasselbalch equation, pH = pKa + log([A-]/[HA]), which shows that pH depends on the ratio of conjugate base to acid rather than their absolute amounts. Buffers work best when the two components are present in similar concentrations, that is, when pH is close to pKa. They are essential in biological systems and analytical work.
Going deeper: Buffer Equilibria
Key ideas: Use pH = pKa + log([A-]/[HA]), and the moles ratio works as well as concentrations since the volume cancels. Effective range is pKa plus or minus 1, with ratios from 0.1 to 10. Buffer capacity is greatest at ratio 1 and rises with total concentration. Example: acetate buffer pKa 4.76 works from about pH 3.8 to 5.8. Adding strong acid converts A- to HA mole for mole.
Common mistakes and connections: Students apply the equation after adding strong acid without first adjusting moles by stoichiometry, or think buffers resist all changes. They fail when the buffer capacity is exceeded. Another error is picking a buffer whose pKa is far from the desired pH. Dilution barely changes the pH but lowers capacity. Related to the common ion effect and Le Chatelier.
Visualization of Buffer Equilibria
The bicarbonate buffer has pKa 6.1 and the Henderson-Hasselbalch relation pH = pKa + log([A-]/[HA]); a 20 to 1 ratio gives blood pH 7.4. The interactive lesson shows this as a live animation or chart.
Worked example and use case
Blood is buffered mainly by the bicarbonate system, which keeps the pH of plasma close to 7.4.
Frequently asked questions about Buffer Equilibria
What is Buffer Equilibria?
Conjugate pairs maintain nearly constant pH by shifting equilibrium after small additions.
Is Buffer Equilibria a beginner, intermediate or advanced topic?
Buffer Equilibria is a intermediate topic in Kinetics & Equilibrium. It builds on the vocabulary of beginner topics in this area.
What should I learn before Buffer Equilibria?
Study these earlier topics in Kinetics & Equilibrium first: Zeolites, Reaction Coordinate Diagram, Equilibrium in Biology.
What comes after Buffer Equilibria?
The next topics in Kinetics & Equilibrium are Heterogeneous Equilibrium Vapor and Partition Coefficient.
Can I practise Buffer Equilibria with quizzes?
Yes. The interactive lesson includes a self-check question on Buffer Equilibria, and the Kinetics & Equilibrium quiz draws on every topic in this area.
Where Buffer Equilibria fits in Kinetics & Equilibrium
Buffer Equilibria is topic 86 of 91 in Kinetics & Equilibrium.