By Marie‑Luise Blue, Updated Mar 24, 2022
The pH scale quantifies the acidity or alkalinity of a solution by measuring the logarithm of the hydrogen ion concentration. A higher concentration of H⁺ ions lowers the pH, whereas a lower concentration raises it. On a scale ranging from 0 to 14, a pH of 7 denotes neutrality, values below 7 are acidic, and those above 7 are alkaline.
Buffers stabilize pH by balancing a weak acid and its conjugate weak base. When excess H⁺ ions appear, the weak base sequesters them, converting into its acidic form and maintaining the pH. Conversely, when a base is added, the weak acid donates H⁺ ions, shifting back toward neutrality.
Inside cells, the phosphate buffer—comprising dihydrogen phosphate (H₂PO₄⁻) and hydrogen phosphate (HPO₄²⁻)—keeps the intracellular pH near physiological values. With a dissociation constant (pKa) of 7.21, the equilibrium of this pair aligns closely with the typical cytosolic pH of 7.2–7.4, making it highly effective for intracellular homeostasis (standard biochemical references).
In the circulatory system, the limited concentrations of dihydrogen and hydrogen phosphate cannot buffer blood effectively. Here, the bicarbonate buffer—formed from dissolved CO₂ as bicarbonate (HCO₃⁻) and carbonic acid (H₂CO₃)—maintains blood pH at approximately 7.4. Respiratory exhalation of CO₂ removes excess carbonic acid, allowing the bicarbonate system to operate efficiently.
