H3P04=H2P04+H20 K1 [H2P04-][H+] [H3P04] 当反应进行50%时，此时H3PO4]=[H2PO4], Ka1 [H+] pKa1=pHH3PO4 ＝ H2PO4 - + H20 当反应进行50%时，此时[H3PO4 ]=[H2PO4 - ]， Ka1 = [H+] pKa1=pH