What does monohydrogen phosphate combine with when acting as a weak base in the phosphate buffer system?

In the phosphate buffer system, monohydrogen phosphate (HPO4²⁻) acts as a weak base by combining with hydrogen ions (H⁺) from the solution. When monohydrogen phosphate encounters excess hydrogen ions, it accepts these protons, which results in the formation of dihydrogen phosphate (H₂PO4⁻). This reaction is essential for the buffer system as it helps to stabilize pH levels by absorbing excess acidity in the solution.

The role of HPO4²⁻ as a weak base is critical because it provides a mechanism for maintaining the pH in biological systems, which is vital for many physiological processes. The buffer system operates based on the equilibrium between the different forms of phosphate, and the ability of HPO4²⁻ to accept H⁺ is a key aspect of its functionality in this context.