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在络合滴定中,使用如Cu-EDTA-PAN、Mg-EDTA-EBT、Zn-EOTA-Zincon(锌试剂)这样的指示剂,可以补金属指示剂之不足。在使用间接指示剂对,除了金属离子M与络合剂Y之间的平衡外,引人了另一种平衡,即作为指示剂用的金属离子Q与M离子浓度之间的平衡。指示剂是以QY形式加入的,QY用量不仅与K_(MY)、C_M及K_(QY)、K_(QIn)有关,也与酸度、共存金属离子N的K_(NY)、C_N以及可能存在的辅助络合剂等有关。选择一个合适的滴定条件要进行不少计算。如使用对数图解法求取QY合适用量,在一定酸度与辅助络合剂条件下,K′_(MY)、C_M、K′_(QIn)、K′_(QY)、K′_(NY)及C_N一定,仅需一次简单的图解。在欲选择酸度等条件时,也仅需在坐标纸上作几次钩划,即可确定。下面举EGTA滴定Ca~(2+)离子,用Zn-EGTA-Zincon作指示剂为例,说明这种图解方法。
In complexometric titration, the use of indicators such as Cu-EDTA-PAN, Mg-EDTA-EBT and Zn-EOTA-Zincon can compensate for the lack of metal indicator. With the use of an indirect indicator couple, in addition to the balance between the metal ion M and the complexing agent Y, another balance is introduced, namely the balance between the metal ion Q and the M ion concentration for the indicator. The indicator is added in the form of QY. The amount of QY is not only related to K_ (MY), C_M and K_ (QY) and K_ (QIn), but also to acidity, K_ (NY) and C_N of coexisting metal ions N, Auxiliary complex agents and so on. Choose a suitable titration conditions to make a lot of calculations. Such as the use of logarithmic graphic method to find the appropriate amount of QY, with a certain acidity and auxiliary complexing agent conditions, K’_ (MY), C_M, K’_ (QIn), K’_ (QY), K’_ NY) and C_N, just a simple diagram. In the choice of conditions such as acidity, but also only a few times in the coordinate paper hooked, you can determine. The following EGTA titration of Ca ~ (2+) ions, with Zn-EGTA-Zincon indicator as an example to illustrate this graphical method.