已知H3PO4的pKa1=2,12,pKa2=7.20,pKa3=12.36,今有一磷酸盐溶液,测得其PH=7.0,则主

直接用分布系数计算
H3PO4的分布系数=[H+]^3 /{ [H+]^3 +[H+]^2*Ka1 +[H+]*Ka1 *Ka2 +Ka1*Ka2*Ka3}
=(10^-7)^3 / { (10^-7)^3 +(10^-7)^2 *10^-2.12 +(10^-7)*(10^-2.12)* (10^-7.21 )+ (10^-2.12)* (10^-7.21 )*(10^-12.66)
=8.15*10^-6
H2PO4-的分布系数=[H+]^2*Ka1 /{ [H+]^3 +[H+]^2*Ka1 +[H+]*Ka1 *Ka2 +Ka1*Ka2*Ka3}
=(10^-7)^2 *10^-2.12 / { (10^-7)^3 +(10^-7)^2 *10^-2.12 +(10^-7)*(10^-2.12)* (10^-7.21 )+ (10^-2.12)* (10^-7.21 )*(10^-12.66)
=0.6185
HPO42-的分布系数=[H+]*Ka1*Ka2 /{ [H+]^3 +[H+]^2*Ka1 +[H+]*Ka1 *Ka2 +Ka1*Ka2*Ka3}
=(10^-7) *(10^-2.12)* (10^-7.21 ) / { (10^-7)^3 +(10^-7)^2 *10^-2.12 +(10^-7)*(10^-2.12)* (10^-7.21 )+ (10^-2.12)* (10^-7.21 )*(10^-12.66)
=0.3814
PO43-的分布系数=Ka1*Ka2*Ka3 /{ [H+]^3 +[H+]^2*Ka1 +[H+]*Ka1 *Ka2 +Ka1*Ka2*Ka3}
=(10^-2.12)* (10^-7.21 ) *(10^-12.66) / { (10^-7)^3 +(10^-7)^2 *10^-2.12 +(10^-7)*(10^-2.12)* (10^-7.21 )+ (10^-2.12)* (10^-7.21 )*(10^-12.66)
=8.344*10^-7
因此 H2PO4- > HPO42- > H3PO4 > PO43-
即主要是：H2PO4- 、 HPO42- >
很少：H3PO4 、 PO43-