Why add MgCl2.6H2O crystals and NaOH pellets to an EDTA solution to undergo water hardness determination?

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When you determine the hardness of water you are practically determining the concentration of calcium carbonate. Calcium ions determination via EDTA is called displacement titration because EDTA binds with calcium ions very slowly. So, we add another Metal usually Magnesium or Zinc in form of salt so that it could bind to EDTA quicker forming a Metal-EDTA complex which is less stable than Ca-EDTA complex. Then the free Ca2+ ions in the solution replace the Mg forming Ca-EDTA complex and resulting in free Mg2+ ions which can be titrated easily. The amount of liberated Mg2+ is equivalent to the original concentration of Ca2+. more
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