The hardness of water is determined by titration with Na₂EDTA. Calcium (Ca²⁺) and magnesium (Mg²⁺) ions form stable complexes with EDTA, allowing for their selective estimation based on the difference in the stability constants of their complexes.
How is the hardness of water determined, and which ions are selectively estimated through complex formation with Na₂EDTA?
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The hardness of water is determined by titration with ethylenediaminetetraacetic acid (EDTA) in the presence of an indicator. Calcium and magnesium ions, responsible for water hardness, form stable complexes with EDTA. The indicator changes color at the endpoint when all the metal ions are complexed. For selective estimation, Mg²⁺ and Ca²⁺ are titrated together, and a masking agent like Mg-EDTA complex is added initially to prevent the precipitation of Mg(OH)₂. This ensures that only Ca²⁺ reacts with EDTA. The difference in the volume of EDTA titrant required before and after adding the masking agent gives the Ca²⁺ concentration, allowing the determination of water hardness.