pH, buffers and the Henderson–Hasselbalch equation !

A buffer is a weak acid with its salt that soaks up added acid or base, keeping pH steady. Blood stays near pH 7.4 thanks to several buffers.

Updated 2026-11-01

Key facts

pH = −log[H⁺]; normal arterial pH 7.35–7.45 (H⁺ about 40 nmol/L). Each unit of pH is a tenfold change in H⁺.
Henderson–Hasselbalch: pH = pKa + log ([A⁻]/[HA]). When pH = pKa the acid is half dissociated and the buffer works best (useful range about pKa ± 1).
Main body buffers: bicarbonate–CO₂ (extracellular, open system, pKa 6.1), haemoglobin (red cells; histidine imidazole groups), proteins (cells and plasma), phosphate (pKa 6.8; cells and urine) and ammonia/ammonium (urine).
Defence in three lines: buffers (seconds), lungs (minutes to hours, CO₂) and kidneys (hours to days, HCO₃⁻ reabsorption and H⁺ excretion as titratable acid and NH₄⁺).
Bicarbonate buffers well despite a pKa far from 7.4 because its acid (CO₂) is removed by the lungs and its base (HCO₃⁻) is regulated by the kidneys.

1. A weak acid only partly dissociates: HA ⇌ H⁺ + A⁻. Added H⁺ combines with A⁻; added OH⁻ takes H⁺ from HA, so pH changes little.
2. Buffer capacity depends on the concentration of the buffer pair and is greatest at pH = pKa.
3. Phosphate (H₂PO₄⁻/HPO₄²⁻, pKa 6.8) is the main intracellular and urinary buffer; in urine it forms most of the titratable acid.
4. Haemoglobin: deoxygenated Hb binds H⁺ better than oxygenated Hb (Haldane effect), so H⁺ produced from CO₂ in tissues is buffered with little pH change (isohydric transport); the chloride shift exchanges HCO₃⁻ for Cl⁻.
5. Proteins buffer mostly through histidine (imidazole pKa about 6.0) and terminal amino groups.
6. The kidney excretes the daily acid load (about 1 mmol/kg/day of non-volatile acid from protein metabolism) as titratable acid and as NH₄⁺ made from glutamine; each H⁺ excreted regenerates one new HCO₃⁻.
7. Worked example: an acid with pKa 6.8 at pH 7.8 has [A⁻]/[HA] = 10, so it is about 91 % in the base form.

The bicarbonate system (pH = 6.1 + log [HCO₃⁻]/(0.03 × pCO₂)) is used to read blood gases and to classify acid–base disorders.
In chronic kidney disease, ammonium excretion falls and metabolic acidosis appears; the bone also buffers acid, contributing to bone loss.
In diabetic ketoacidosis, ketoacids consume bicarbonate; deep, fast breathing (Kussmaul) lowers CO₂ to compensate.

Think of the answer, then tap the question to see it.

At what pH does a buffer work best?

At a pH equal to its pKa, where the acid and base forms are equal; it buffers well within about one unit of the pKa.

What is the main intracellular and urinary buffer?

Phosphate (H₂PO₄⁻/HPO₄²⁻, pKa 6.8), together with proteins inside cells and ammonia/ammonium in urine.

Why is the bicarbonate system an effective blood buffer even though its pKa is 6.1?

It is an open system: the lungs remove CO₂ and the kidneys adjust HCO₃⁻, so both members of the pair are regulated.

Which amino acid gives proteins and haemoglobin most of their buffering power at physiological pH?

Histidine, whose imidazole group has a pKa near 6–7.

What fraction of a weak acid with pKa 6.8 is in the base form at pH 7.8?

About 91 %: the base-to-acid ratio is 10 to 1, because pH − pKa = 1.

Educational information, not medical advice. For any symptom, see a health professional.

Sources: Blood gases: MedlinePlus Medical Encyclopedia (MedlinePlus, US National Library of Medicine, 2026); Acidosis: MedlinePlus Medical Encyclopedia (MedlinePlus, US National Library of Medicine, 2026); Metabolic acidosis: MedlinePlus Medical Encyclopedia (MedlinePlus, US National Library of Medicine, 2026); Textbook of Biochemistry for Medical Students, 6th ed. (Jaypee Brothers Medical Publishers (Vasudevan DM, Sreekumari S, Vaidyanathan K), 2011).

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Representative model, for educational purposes only.