Phosphate Buffered Saline (PBS) is a commonly used buffer for experiments in biology because its pH and ion concentrations are similar to those in mammalian organisms. It works in a similar fashion to the blood plasma buffer mentioned in the textbook, but using dihydrogen phosphate ions and hydrogen phosphate ions for buffering through the following chemical reaction: H2PO4- (aq) ⇆ H+(aq) + HPO42--(aq)

The equilibrium arrows depict that the phosphate ion (H2PO4- ) is dissociating further into two component ions in solution, but at the same time H+ and HPO42- ions are combining simultaneously to form phosphate in solution. So, at any given point in time, and under the appropriate conditions, there is an equal quantity of dissolved ions and combined ions in solution. There is therefore always a hydrogen ion donor and an acceptor in solution.

Based on the equation above, which ion plays the role of hydrogen-ion donor (acid) and which ion plays the role of hydrogen-ion acceptor (base) in PBS?