Na3PO4水解有三级：
PO4 3- + H2O = HPO4 2- + OH- Kh1 = Kw / Ka3 = 10^-14 / 2.2*10^-13 = 0.045
HPO4 2- + H2O = H2PO4 - + OH- Kh2 = Kw / Ka2 = 10^-14 / 6.2*10^-8 = 1.61*10^-7
因为Kh1 > 1000 Kh2,所以可以忽略第二级及第三级的水解,以第一级计算：
PO4 3- + H2O = HPO4 2- + OH-
0.1-x x x
x2 / (0.1-x) = 0.045 解出x = [OH-] pOH = -lgx
pH = 14 - pOH即得.