Analysis of Tap Water for Metallic Cations

[adenine135]

Assume that the tap water to be analyzed contains a certain amount of an unknown cation which forms a complex with EDTA at pH 10 but not at pH 12. The formation constant for the EDTA complex of this unknown cation at pH 10 is 1×10^13. Neither the EBT nor Calcon indicators are affected by the presence of this cation. How would the values of the concentration of Ca2+ ions and the concentration of Mg2+ ions determined in this experiment be affected by the presence of this unknown cation impurity?



I have absolutely no idea where to even start with this one :(

 

[chemisttree]

What experiment? What pH? Details!