General Chemistry
Buffer Solution
Resists pH changes — weak acid + conjugate base maintains constant pH
A buffer solution resists changes in pH when small amounts of acid or base are added. Composition: weak acid + its conjugate base (or weak base + conjugate acid). Mechanism: weak acid neutralizes added base; conjugate base neutralizes added acid. Henderson-Hasselbalch equation: pH = pKa + log([A⁻]/[HA]). Critical in biology — blood (carbonic acid/bicarbonate), buffers in cells, lab work, industry. Buffer capacity highest near pKa; ratio [A⁻]/[HA] determines pH; absolute concentration determines capacity.
- CompositionWeak acid + conjugate base (or weak base + conjugate acid)
- Henderson-HasselbalchpH = pKa + log([A⁻]/[HA])
- Best at pH near pKaHighest capacity within ~1 pH unit of pKa
- Blood bufferCarbonic acid/bicarbonate (HCO₃⁻/H₂CO₃)
- Buffer capacityDepends on absolute concentration
- ExamplesAcetate, phosphate, ammonia, Tris, HEPES
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Why buffers matter
- Biology. Blood pH maintenance.
- Lab research. Constant pH for reactions.
- Pharmaceuticals. Drug stability, IV solutions.
- Industrial. pH control in processes.
- Aquaculture. Maintaining stable pH for organisms.
- Cosmetics. Skin-friendly products.
- Food. Preservation, taste.
Common misconceptions
- Buffers fully prevent pH change. Resist; not infinite capacity.
- Higher pH means stronger buffer. Capacity = concentration; pKa near target.
- Strong acids work as buffers. No — must be weak.
- Buffer pH equals pKa always. Only when [A⁻] = [HA].
- Adding water won't affect. Dilution reduces capacity.
- Buffers always have pKa = pH. pH varies by ratio.
Frequently asked questions
How does a buffer work?
Contains comparable amounts of weak acid (HA) and conjugate base (A⁻). When base added (OH⁻): HA + OH⁻ → A⁻ + H₂O — consumes added base, doesn't change pH much. When acid added (H⁺): A⁻ + H⁺ → HA — consumes added acid. Effective until one component depleted (then capacity exceeded).
What's the Henderson-Hasselbalch equation?
pH = pKa + log([A⁻]/[HA]). Predicts pH from buffer composition. When [A⁻] = [HA]: pH = pKa. To make buffer at desired pH: choose acid with pKa near target pH; adjust ratio. Useful for: making buffers, predicting pH effects.
How do you make a buffer?
Two methods. (1) Mix weak acid and its salt: e.g., acetic acid + sodium acetate. (2) Partial neutralization: weak acid + strong base (e.g., HA + NaOH at 50% conversion gives buffer). Start with target pH; choose acid with pKa near it; calculate ratio from Henderson-Hasselbalch; mix appropriate amounts.
What's buffer capacity?
Amount of acid or base buffer can absorb without significant pH change. Higher capacity = larger concentrations of buffer components. Buffer at pH = pKa: maximum capacity (1:1 ratio resists addition best). Beyond capacity (ratio shifts to extremes): pH changes rapidly. Used in solutions where pH stability is critical.
How does blood maintain pH?
Multiple buffers. Most important: carbonic acid/bicarbonate (H₂CO₃/HCO₃⁻). pKa ≈ 6.1. Despite being far from blood pH 7.4, large bicarbonate concentration provides capacity. Plus: phosphate buffer (intracellular), proteins (histidine pKa ≈ 6.0). System: respiratory (CO₂ release controls H₂CO₃) and renal (kidney HCO₃⁻ regulation) work together.
What are common laboratory buffers?
Tris (pKa 8.1) — biology research, near physiological pH. HEPES (pKa 7.5) — biology, biological cell culture. Phosphate (pKa 7.2) — aqueous biology. Acetate (pKa 4.76) — moderately acidic. Citrate — wide pH range. Choose based on: pKa near desired pH, compatibility with experiment, low metal binding.
When does a buffer fail?
(1) Capacity exceeded: too much acid/base added → one component depleted → pH changes rapidly. (2) Outside ±1 pH unit from pKa: ratio extreme; small additions cause large changes. (3) Dilution: capacity reduced; in extreme dilution, water dominates pH. Solutions: increase concentrations or change buffer components.