1) why color of Zn EDTA complex is colourless?
2) what is meant by masking?
1) The Zn EDTAcomplex is colorless due to the fact that the energy of the quantum of electromagnetic radiation, which is necessary for making the transition between the split d-orbitals, lies outside the visible range.
2)Masking of the anion is the binding of the ion into a complex that is more stable than the titrating complex and indicator. If the ion is masked, it will almost not affect the titration.