Canonical Question
Acid Base – Buffers
Master answer
Buffer
- Weakly ionised acid or base in equilibrium with its full ionised salt
- A buffer can “resist” change in pH by absorbing or releasing H+ ions
- Works best when pKa is closest to the target pH (7.4)
- Isohydric Principle
- All buffer systems which participate in defence of acid-base changes are in equilibrium with each other. There is after all only one value for [H+] at any moment. This is known as the Isohydric Principle.
Effectiveness:
- buffer pKa (most effective if pKa = pH of carrying solution)
- pH of carrying solution
- amount of buffer present
- open (physiological) vs closed (chemical) system
Buffer Systems
| Name | Location |
|---|---|
| Protein | Intra cellular |
| Haemo globin | Blood |
| Bicarb | Plasma, Interstitium, Urine (minimal) |
| Ammonia | Urine |
| Phosphate | Urine, Bone |
| CaCO3 | Bone |
Compare and Contrast:
Bicarbonate, Phosphate, Protein buffer systems
| Bicarbonate | Phosphate | Protein |
|---|---|---|
| H+ + HCO3– ⇌ H2CO3 ⇌ H2O + CO2 | H+ + HPO42- ⇌ H2PO4– | H+ + Prot– ⇌ HProt |
| Extracellular (80% of plasma buffer capacity) | Nil extracellular. Important in tubular fluid | Extracellular (20% of plasma buffer capacity) |
| poor intracellular buffers (poor membrane diffusion of H+, HCO3– (except RBC)) | Some Intracellular buffering | Major Intracellular (60~70% of total body buffering capacity occurs intracellularly and most of this is due to proteins) |
| pKa 6.1 | pKa 6.8 | pKa of histidine residues 6.8 |
| pKa far from physiological pH (7.4) However Good buffer due to abundance of substrates | pKa close to physiological pH However extracellular concentration 1.8 of bicarbonate | pKa close to physiological pH |
| Open ended system – CO2 eliminated by lungs – HCO3 eliminated renally Requires carbonic anhydrase | Becomes important buffer in renal tubules due to H2O reabsorption and consequent concetration of phosphate. | Hb important buffer in blood due to 38 histidine residues per tetramer Deoxyhaemoglobin further increases buffering capacity by increasing pKa to 7.8 |
Gladwin / Sakurai / JC 2020
Exam appearances
| Exam | Exact wording | Relationship | Success |
|---|---|---|---|
| 2008B Q03 | Compare and contrast the body’s bicarbonate, phosphate and protein buffer systems. | historical_member | — |