Harris Chapter 13

EthyleneDiamineTetraAcetic acid

Chapter 16 starts with slide 6

EDTA-Mg 2–

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.

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 }

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

Harris Chapter 16

Redox Titrations

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 S2O3

2– S2O3

2– standardized with weighed IO3– to what?

I3– + 2 S2O32– 3 I – + S4O6

2–

Thiosulfate Standardization

IO3– + 6H+ + 6e – I – + 3H2O

2S2O32– S4O6

2– + 2e –

Scale latter by 3 and add

IO3– + 6H+ + 6S2O3

2– 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.