Titrations and Buffers

Titration determines unknown concentration via titrant (known) added to titrand (unknown) until the equivalence point (moles titrant = moles titrand): NₐVₐ = N_bV_b.
Equivalence point pH depends on strength: strong+strong=pH 7; strong acid+weak base<7; weak acid+strong base>7; weak+weak variable.
Endpoint (indicator color change) ideally coincides with equivalence point (theoretical stoichiometric point) — they can differ slightly.
A buffer (weak acid/conjugate base, or weak base/conjugate acid) resists pH change via common ion effect: H⁺+A⁻→HA, HA+OH⁻→A⁻+H₂O.
Henderson-Hasselbalch: pH = pKa + log([A⁻]/[HA]). Buffer capacity maximized near pH≈pKa (1:1 ratio), scales with total concentration.

Titrant / Titrand: Titrant: solution of known concentration added from a burette. Titrand: solution of unknown concentration being tested.
Equivalence point: Point where moles (or equivalents) of titrant added exactly equal moles/equivalents of titrand present.
Endpoint: Point where the indicator visibly changes color, signaling the titration should stop — ideally close to the equivalence point.
Buffer: A solution built from a weak acid/conjugate base (or weak base/conjugate acid) pair that resists pH change.
Buffer capacity: Amount of acid or base a buffer can absorb before undergoing a significant pH change; maximal when pH is within ~1 unit of pKa.

NₐVₐ = N_bV_b

Worked example: 20.0 mL HCl (unknown N) titrated by 15.0 mL 0.200 N NaOH → Nₐ=(0.200×15.0)/20.0=0.150 N.

pH = pKa + log([A⁻]/[HA])

Worked example: 0.30 M acetic acid (Ka=1.8×10⁻⁵, pKa≈4.74) + 0.30 M acetate → ratio=1, log(1)=0 → pH=4.74.

Combination	Equivalence point pH
Strong acid + strong base	pH 7 — only water and a neutral, non-hydrolyzing salt formed.
Strong acid + weak base	< pH 7 — product (e.g., NH₄⁺) is a weak acid that hydrolyzes.
Weak acid + strong base	> pH 7 — product (e.g., ClO⁻) is a weak base that hydrolyzes.
Weak acid + weak base	Variable, depends on relative strengths — rarely titrated in practice.

1Each ionizable proton has its own pKa and produces its own equivalence point.
2Before each equivalence point lies a buffer region — the weak acid and its conjugate base (or weak base/conjugate acid) coexist and resist pH change.
3Differences between successive pKa values create distinctly separate buffer regions and equivalence points along the curve.

The bicarbonate buffer system (H₂CO₃/HCO₃⁻, pKa≈6.1) holds blood pH ≈7.4 via a ~20:1 HCO₃⁻:H₂CO₃ ratio — despite sitting >1 pH unit from its own pKa, it works because it's an open system actively regulated by respiration (CO₂ exhalation) and the kidneys (H⁺ excretion, HCO₃⁻ reabsorption).
Buffer capacity depends on TWO factors: total concentration of buffer components (higher=more capacity) and ratio of components (closest to 1:1 = most capacity).
Phenolphthalein (colorless below ~pH 8.2, pink above ~pH 10) suits strong acid/strong base titrations — the curve is so steep near pH 7 that the indicator still changes right at the true equivalence point.

Equivalence point ≠ endpoint — equivalence point is theoretical (stoichiometric), endpoint is when the indicator visibly changes. They usually nearly coincide but aren't defined as identical.
Weak acid/weak base titrations lack a sharp inflection point, making indicators unreliable — don't expect a clean equivalence point the way you would for a strong-strong titration.
The bicarbonate buffer's pKa (6.1) is NOT close to blood pH (7.4) — don't assume physiological buffers always sit at pH≈pKa; open-system active regulation can compensate.

What is the pH at the equivalence point of a strong acid titrated with a strong base?

What two organ systems actively regulate the bicarbonate buffer system?

When is buffer capacity at its maximum?

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