[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Biochemistry](https://durgical.com/en/biochemistry/) › pH, buffers and the Henderson–Hasselbalch equation [Biochemistry](https://durgical.com/en/biochemistry/) · Topic # 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. ## In detail - 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. **Also called** Buffer systems of the body, pKa and buffering capacity ## Clinical notes - 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. ## Related conditions - [Diabetic ketoacidosis](https://durgical.com/en/conditions/diabetic-ketoacidosis/) - [Lactic acidosis](https://durgical.com/en/conditions/lactic-acidosis/) - [Chronic obstructive pulmonary disease (COPD)](https://durgical.com/en/conditions/chronic-obstructive-pulmonary-disease/) - [Chronic kidney disease](https://durgical.com/en/conditions/chronic-kidney-disease/) ## Related - [Amino acids: structure, charge and pKa](https://durgical.com/en/biochemistry/topics/amino-acids-structure-charge-and-pka/) - [Hemoglobin and 2,3-BPG](https://durgical.com/en/biochemistry/enzymes/hemoglobin-and-2-3-bpg/) - [Oxygen binding: haemoglobin versus myoglobin](https://durgical.com/en/biochemistry/topics/oxygen-binding-haemoglobin-versus-myoglobin/) - [Ammonia (ammonium)](https://durgical.com/en/biochemistry/molecules/ammonia/) - [Glutamine](https://durgical.com/en/biochemistry/molecules/glutamine/) - [Histidine](https://durgical.com/en/biochemistry/molecules/histidine/) - [Calcium and phosphate](https://durgical.com/en/biochemistry/topics/calcium-and-phosphate/) - [Bicarbonate–CO₂ buffer system](https://durgical.com/en/biochemistry/topics/bicarbonate-co-buffer-system/) - [Anion gap and delta ratio](https://durgical.com/en/biochemistry/topics/anion-gap-and-delta-ratio/) - [Acid–base interpretation (step by step)](https://durgical.com/en/biochemistry/topics/acid-base-interpretation/) ## Test yourself 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. [Add to my cards (5)](https://durgical.app/estudiar/?add=/en/biochemistry/topics/ph-buffers-and-the-henderson-hasselbalch-equation/) Educational information, not medical advice. For any symptom, see a health professional. Sources: [Blood gases: MedlinePlus Medical Encyclopedia](https://medlineplus.gov/ency/article/003855.htm) (MedlinePlus, US National Library of Medicine, 2026); [Acidosis: MedlinePlus Medical Encyclopedia](https://medlineplus.gov/ency/article/001181.htm) (MedlinePlus, US National Library of Medicine, 2026); [Metabolic acidosis: MedlinePlus Medical Encyclopedia](https://medlineplus.gov/ency/article/000335.htm) (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). [← All of biochemistry](https://durgical.com/en/biochemistry/) ## **Spotted a mistake?** We are always improving for the community. Tell us what is wrong and our team will review it. 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