Carbonic acid functions as the pivotal weak acid in biological pH homeostasis, forming the cornerstone of the bicarbonate buffer system that maintains physiological pH (7.35–7.45) in blood plasma and regulates CO₂ transport. It exists primarily in equilibrium as the transient hydrated form of CO₂, playing a central role in respiratory acid–base balance.
Chemical Properties
Carbonic acid (H₂CO₃, MW 62.03 g/mol) forms instantaneously from CO₂ hydration according to the reversible reaction:
CO₂ + H₂O ⇌ H₂CO₃
This reaction is relatively slow in the absence of catalysis (k = 0.037 s⁻¹) but is dramatically accelerated by carbonic anhydrase (up to 10⁶ s⁻¹). Carbonic acid exhibits diprotic acidity through the following equilibria:
H₂CO₃ ⇌ H⁺ + HCO₃⁻ (pKa₁ = 6.35)
HCO₃⁻ ⇌ H⁺ + CO₃²⁻ (pKa₂ = 10.33)
These equilibria define its buffering capacity. While an equimolar mixture of H₂CO₃/HCO₃⁻ yields a pH around 4.5, physiological conditions maintain pH ~7.4 through a ratio of approximately 20:1 (24 mM HCO₃⁻ to 1.2 mM dissolved CO₂). Pure carbonic acid is highly unstable and decomposes within milliseconds into CO₂ and H₂O, remaining stable only under low-temperature conditions or in theoretical models.
Biochemical Applications
In blood physiology, the H₂CO₃/HCO₃⁻/CO₂ system is essential for CO₂ transport, with approximately 70% of CO₂ carried as bicarbonate ions following carbonic anhydrase-catalyzed conversion in red blood cells. This equilibrium also plays a critical role in buffering pH and compensating for respiratory acidosis.
In enzymology, carbonate buffers (10–50 mM, pH 9.0–10.6) are widely used for studying enzymes such as histidine kinases and CO₂-dependent carboxylases. In molecular biology, CO₂ derived from dry ice sublimation enables rapid pH reduction for protein precipitation without the use of trichloroacetic acid (TCA).
In cell culture systems, physiological pH is maintained through CO₂-dependent buffering, typically using 5–10% CO₂ in combination with approximately 25 mM bicarbonate, providing an effective alternative to HEPES-based buffering systems.
