Chem Lab Calcium

Chem Lab Calcium - page 1 of 3 Chemistry 305 Laboratory...

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page 1 of 4 Chemistry 305 Laboratory Fall, 2007 EXPERIMENT 1: HARDNESS OF WATER BY EDTA TITRATION INTRODUCTION Water ‘hardness’ is a measure of the amount of hard water cations in water. These hard water cations include calcium, magnesium, iron, zinc and the other polyvalent metal ions. In most water samples, calcium and magnesium are the chief contributors to water hardness. Calcium and magnesium are easily measured by titration with the complexing agent ethylene-diaminetetraacetate (EDTA). The EDTA complexes the Ca 2+ or Mg 2+ metal ion as shown in the equation below. M 2+ Metal ion EDTA EDTA-Metal Complex M 2+ + Y 4- MY 2- The Y 4- ion that forms a 1:1 complex with the metal ion is the completely deprotonated anion of ethylenediaminetetraacetic acid (“H 4 Y”). At pH=10, the EDTA is present in solution primarily as its monoprotonated form, HY 3- . The endpoint of an EDTA titration is determined with a metallochromic indicator. These indicators are themselves complexing agents that change color as they combine with a metal ion. Two popular indicators for titrating Ca 2+ or Mg 2+ are eriochrome black T and calmagite . These indicators (shown as In 3- in the equations below) change from blue to red color when they combine with a metal ion to form a complex ion: M 2+ + HIn 2- + H 2 O <--> MIn - + H 3 O + blue red In a titration, EDTA, a stronger complexing agent than the indicator, displaces the indicator form the metal ion allowing the indicator to return (through shades of violet) to a pure blue color, indicating the end of the reaction.
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This note was uploaded on 04/07/2008 for the course CHEM 305 taught by Professor Dr.h during the Spring '08 term at Cal. Lutheran.

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Chem Lab Calcium - page 1 of 3 Chemistry 305 Laboratory...

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