- Inorganic phosphate (HPO42-) can reversibly bind free hydrogen ions and in doing so can act as a buffer against changes in pH (Review: Buffer Basics). The pK of the inorganic phosphate buffer is 6.8, providing excellent buffering capacity around the normal ECF pH of 7.4.
- However, due to its low concentration in the extracellular fluid, the importance of inorganic phosphate in ECF buffering is likely low compared to that of bicarbonate. In contrast, inorganic phosphate is critical urinary buffer as its concentration progressively increases as fluid is resorbed within the tubule. In this way, inorganic phosphate buffers against changes in tubular pH as acid is excreted within the tubule as described in Renal Acid Excretion