The reaction given:
H2PO4-(aq) ⇆ H+(aq) + HPO42-( aq)
Here, H2PO4- ion serves as a proton donor, while HPO42- ion is a proton acceptor. In a PBS solution, Na2HPO4 dissociates into Na+ and HPO42-. As a result, this molecule serves as a proton acceptor. In contrast, KH2PO4 dissociates into K+ and H2PO4-. As H2PO4- is a proton donor, KH2PO4 serves as a proton donor in a PBS solution.