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Harris Chapter 13

Harris Chapter 13. E thylene D iamine T etra A cetic acid Chapter 16 starts with slide 6. EDTA-Mg 2 –. Sticking Points. 3 ED + Mg 2+  Mg(ED) 3 2+ p ~ –2 EDTA 4– + Mg 2+  Mg(EDTA) 2– pK f = – 8.8 EDTA has higher K due to both O – ligand and reduction in entropy change.

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Harris Chapter 13

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  1. Harris Chapter 13 EthyleneDiamineTetraAcetic acid Chapter 16 starts with slide 6

  2. EDTA-Mg 2–

  3. Sticking Points • 3 ED + Mg2+ Mg(ED)32+ p ~ –2 • EDTA4– + Mg2+  Mg(EDTA)2– pKf = –8.8 • EDTA has higher K due to both O– ligand and reduction in entropy change. • Ca2+ (–10.69) Sr2+ (–8.73) Ba2+ (–7.86) • Implications for seawater analysis. • Mg 1,272 ppm; Ca 400 ppm; Sr 13 ppm.

  4. Availability of Y4– • Best binding is when all 4 EDTA’s carboxyl groups are ionized, but (Y4–) is only 0.36 at pH 10 and Ca(OH)2 a problem for pH>10. • Fortunately, Kf is so high that quantitative binding occurs since K’f = (Y4–) Kf > 108. • K’f = “conditional” formation constant • (Y4–) Kf = [MY n – 4 ] / { [M n+ ] [EDTA] free }

  5. Complexation Indicators • Just as acid-base indicators are weak acids, compleximetric titration indicators are weak ligands. • KM-ind < KM-EDTA for the analyte ion, M. • pK Mg-EDTA = – 8.8 • pK Mg-Eriochrome black T = – 6.3

  6. Harris Chapter 16 Redox Titrations

  7. Iodimetry • Not an example of electrochemical endpoint. • Instead of reference electrode showing changes in concentration of analyte, presence of I2 titrant is shown by Starch-I6 intense blue-black! • Iodimetry is a back titration. Excess from measured I3– is titrated with S2O32– • S2O32– standardized with weighed IO3– to what? • I3– + 2 S2O32– 3 I – + S4O62–

  8. Thiosulfate Standardization • IO3– + 6H+ + 6e – I – + 3H2O • 2S2O32–  S4O62– + 2e – • Scale latter by 3 and add • IO3– + 6H+ + 6S2O32–  3S4O62– + I– + 3H2O • Note the enormous molar advantage of iodate. • So you must weigh it with great care! • We ignore the correct (weak) acid forms here.

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