Potentiometric determination of calcuimn with EGTA using silver as an indicator electrode

Abstract

Selective titration of calcium in the presence of magnesium with glycoletherdiaminetetraacetic acid (EGTA) was studied.
A small amount of silver ion is added before the titration to establish a reversible potential with the silver electrode.
The potential of the silver electrode is determined by [Ag⁺], and given as follows :
E=E°+0.0592 log [Ag⁺](25°C)
In this titration, the fraction titrated, a=C_Y/C_M, is related with [Ag⁺] by the following equation :
a=1-[Ag⁺]/C_Ag-[Ag⁺]·K_AgY·α(M)/K_MY+C_Ag-[Ag⁺]/[Ag⁺]·α(Y)/K_AgY·C_M
The equation expresses that the smaller the value of K_AgY· α(M)/K_MY, and α(Y)/K_AgY· C_M, the sharper is the end-point obtained.
Using this equation, the optimum conditions for titration such as pH, and concentration of the indicator ion, can be derived theoretically.
Calcium in the presence of ten fold excess of magnesium can be determined satisfactorily at pH 10.