虽然 Th-EDTA 和稀土-EDTA 络合物稳定常数之间有相当的差别(1g K_(ThY)=23.2,lg K_(LaY)=15.50,lg K_(DyY)=17.9),当用二甲酚橙(XO)、焦儿茶酚紫、甲基百里酚兰等作指示剂,pH=5-5.5时,不能在滴定钍以后,继续滴定稀土,这是因为在 pH=4时,形成了诸如 Th-EDTA-XO 的有色三元络合物。 Although there is a considerable difference between the stability constants of Th-EDTA and rare earth-EDTA complexes (1g K_ (ThY) = 23.2, 1g K_ (LaY) = 15.50, 1g K_ (DyY) = 17.9) Orange (XO), pyrocatechin, methylthymol blue and the like were used as indicators. When pH = 5-5.5, the titration of thorium could not be continued after the titration of rare earth. This is because at pH = 4, Colored ternary complexes such as Th-EDTA-XO.