Abstract
A spiral platinum wire electrode coated with manganese dioxide can be used as an indicator electrode for the potentiometric titration of various metal cations with EDTA or EGTA.
When a small amount of manganese ion is added before the titration, the potential of the indicator electrode at 25°C is given by the following expression:
E=E°-0.0296 log [Mn2+] -0.1183 pH
where the concentration is assumed to be equal to the activity.
It was shown that the electrode potential is dependent on the manganese ion concentration in the solution and its pH value.
The titration must be carried out in neutral or alkaline solutions, because stability constant of manganese-EDTA complex is not so great, and the indicator electrode attains equilibrium more rapidly in alkaline solutions than in neutral.
Chloride ion does not interfere with the MnO2 electrode, permitting the use of an Ag-AgCl reference electrode, and the equivalence point is determined from the maximum slope of the titration curve.
In the case of titrations with EDTA, Cu, Cd, Pb, Zn and Co can be determined at pH about 7 in a succinate medium, and Cu, Cd, Zn, Co, Ca and Sr at pH about 9 in ammoniacal medium. In the case with EGTA, Ca can be selectively determined in ammoniacal medium (pH about 9) in the presence of five fold excess of Mg, which forms only a less stable complex with EGTA.
