30.14 : Phosphate Buffer

The phosphate buffer system is a critical biological mechanism for maintaining pH stability in the body. This system operates primarily through two components: sodium dihydrogen phosphate (NaH₂PO₄), which acts as a weak acid, and sodium hydrogen phosphate (Na₂HPO₄), which serves as a weak base.

Sodium dihydrogen phosphate does not fully dissociate in neutral or acidic solutions. When a strong base, such as sodium hydroxide (NaOH), is introduced into the solution, sodium dihydrogen phosphate donates a proton (H⁺), forming water and sodium hydrogen phosphate:

NaH₂PO₄ + OH^- → Na₂HPO₄ + H₂O

This reaction also produces sodium hydrogen phosphate, a weak base, which helps neutralize the strong base. As a result, the phosphate buffer system minimizes pH changes, even in the presence of a strong base, effectively stabilizing the solution.

Sodium hydrogen phosphate, the weak base in the system, neutralizes strong acids by reacting with hydrogen ions (H⁺) released by the acid, forming sodium dihydrogen phosphate:

Na₂HPO₄ + H⁺ → NaH₂PO₄

This reaction binds the excess hydrogen ions, preventing a significant drop in pH. The strong acid is effectively converted into sodium dihydrogen phosphate, a weak acid, causing only a slight pH reduction. This mechanism highlights the phosphate buffer system's ability to maintain pH balance in the presence of strong acids.

The phosphate buffer system is essential in renal physiology, where it helps excrete hydrogen ions and stabilize urine pH. Its dual capacity to neutralize both acids and bases makes it vital to maintaining the body's acid-base balance.