2. Harris Chapter 13 E thylene D iamine T etra A cetic acid Chapter 16 starts with slide 6

3. EDTA-Mg 2–

4. 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.  Ca 2+ (–10.69) Sr 2+ (–8.73) Ba 2+ (–7.86)  Implications for seawater analysis. Mg 1,272 ppm; Ca 400 ppm; Sr 13 ppm.

5. Availability of Y 4–  Best binding is when all 4 EDTA’s carboxyl groups are ionized, but  (Y 4– ) is only 0.36 at pH 10 and Ca(OH) 2 a problem for pH>10.  Fortunately, K f is so high that quantitative binding occurs since K’ f =  (Y 4– ) K f > 10 8. K’ f = “conditional” formation constant  (Y 4– ) K f = [MY n – 4 ] / { [M n+ ] [EDTA] free }

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

7. Harris Chapter 16 Redox Titrations

8. Iodimetry  Not an example of electrochemical endpoint. Instead of reference electrode showing changes in concentration of analyte, presence of I 2 titrant is shown by Starch-I 6 intense blue-black!  Iodimetry is a back titration. Excess from measured I 3– is titrated with S 2 O 3 2– S 2 O 3 2– standardized with weighed IO 3 – to what? I 3– + 2 S 2 O 3 2–  3 I – + S 4 O 6 2–

9. Thiosulfate Standardization  IO 3 – + 6H + + 6e –  I – + 3H 2 O  2S 2 O 3 2–  S 4 O 6 2– + 2e – Scale latter by 3  and add  IO 3 – + 6H + + 6S 2 O 3 2–  3S 4 O 6 2– + I – + 3H 2 O Note the enormous molar advantage of iodate. So you must weigh it with great care! We ignore the correct (weak) acid forms here.