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EDTA Chelation and Titrations

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    Reading: Chapter 13 (sections 1-6), Exploring Chemical Analysis 5th ed., D. Harris




    • Practically what does this fractional composition diagram mean for EDTA titrations?






    • Calculate the concentration of free Ca2+ in a solution of 0.10 M CaY2- at pH 10.00 and at pH 6.00 with the conditional formation constant.
      • What is the rxn we are thinking about?


      • Use an ice table to determine the amount of free (uncomplexed) Ca2+









    • Calculate the pCa2+ vs. mL EDTA for the reaction of 50.0 mL of 0.0400 M Ca2+, buffered to pH 10.00, with 0.0800 M EDTA at the following volumes.
      • Before the equivalence point: 5.0 mL EDTA







      • At the equivalence point








      • After the equivalence point: 26.0 mL




    Titration Techniques. Direct titration: titration of the analyte with standard EDTA solution. Like previous example. Back titration: known excess of EDTA is added then the solution is titrated with a second metal ion solution and the difference shows the concentration of the first metal ion. Used when the analyte can not be stabilized in solution without EDTA, reacts too slowly under normal titrations conditions, or blocks the indicator.

    • Ni2+ can be analyzed by back titration, using standard Zn2+ at pH 5.5 w/ xylenol orange indicator. A solution containing 25.00 mL of Ni2+ in dilute HCl is treated w/ 25.00 mL of 0.05283 M Na2EDTA. Titration w/ 0.02299 M Zn2+ requires 17.61 mL to reach the red end point. What is the molarity of the Ni2+?






















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