The hydrogen ions in the lumen react with the disodium phosphate (Na₂HPO₄) to produce sodium di-hydrogen phosphate (NaH₂PO₄). The hydrogen ion is derived from the dissociation of carbonic acid. The complementary bicarbonate ion diffuses out of the cell and into the peritubular fluid by passive means. Thus this represents new bicarbonate ions that would replace the bicarbonate ions that were utilised to initially buffer the acid.