Pre-Lab Questions for Unknown Calcium and Water Hardness A) Draw the structure of the EDTA/Ca2+ complex ion. In terms of denticity, what type of ligand is it? Are there any special experimental conditions necessary for the use of EDTA in complexometric titrations? B) Comment on the effect that other transition metals have on EDTA ions at pH 10. You should look at some of the stability constants for transition metals such as A13+ and Fe3+, which are often found in tap water. What else is found in tap water that is likely to react with EDTA? C) What does the term "masking" mean in analytical chemistry? How do the ions mentioned i.e. CN, S2- and triethanolamine, act as masking agents for transition metals? Why dıo these ions not mask Ca2+ or Mg2+? D) In the determination of Ca2+ by complexometric titration with EDTA, why is it necessary to add MgC12 to the EDTA titrant when using Eriochrome black-T as the indicator? Is it necessary to do this in a similar determination of Mg2+?