29. The hexaprotic acid ethylenediaminetetraacetic acid (EDTA) is widely used in

Question

29. The hexaprotic acid ethylenediaminetetraacetic acid (EDTA) is widely used in complexation reactions with metal ions. It is frequently used for titrations of Ca and Mg ions in water as a measurement of water hardness. In order to observe typical titration behavior at the equivalence point, the pH of the solution must be buffered so that most of the EDTA exists in solution in the fully deprotonated form. Given the information about EDTA below, choose a pH value for optimal titration. Consult a table of acid dissociation constants to choose a buffer that can provide the necessary pH for titration. HOOCH2C 0.0 H2COOH 3 2.00 4 2.69 6.13 6 10.37 HOOCH2C EDTA: H‘Y2. 30. Consider cis-Butenedioic acid (Maleic Acid, pK.-1.92, pK.g-6.27). Sketch the distribution graph for solution containing 0.01 M this weak acid. Label the lines with the respective species to tell them apart. Circle the regions on the graph where Maleic Acid would be a good buffer. Explain why it is in these pH regions that Maleic Acid is a good buffer. Maleic Acid 0.010 0.008 0.006 .004 0.002 10 12 6 pH8 10 The Buffer Zone 2/21/09

Explanation / Answer

29. EDTA is a strong chelating agent which forms a stable complex with divalent metal ions in basic medium. Hence alkaline buffer of NH4OH and NH4Cl of pH 10 can be used.

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