Chapter 12, & 6.4 Complexation chemistry & EDTA titrations

Transcription

1 Chapter 12, & 6.4 Complexation chemistry & EDTA titrations First Section 6.4, an introduction to complex chemistry. The strange behavior of PbI 2 solubility: PbI 2 Pb I - K sp = [Pb 2+ ][I - ] 2 1 Don t concern yourself with all this other stuff, just look at the blue line. At low I - concentration the solubility of PbI 2 decreases with increasing I -, as you would expect from the common ion effect. But at high I - concentration the opposite occurs. This is because of complex formation.

2 2 Strangely enough this is still considered acid-base chemistry, but under another broader theory Lewis acid/base chemistry. Here the electron acceptor is a Lewis Acid, and the electron donor is a Lewis base. This is different from a regular covalent bond where 2 atoms share electrons, 1 from each atom. Both electrons come from the Lewis base, or ligand, to form a coordinate covalent bond. A ligand will always have a non-bonding pair of electrons. Regardless, you can still write an equilibrium constant expression for the above reaction: Chapter 12 Molecules can also be ligands. Ex. Ethylamine This molecule has 1 atom with 1 unshared pair of electrons - a monodentate ligand. Complex reaction with Cd 2+ Cd CH 3 CH 2 NH 2 {Cd(CH 3 CH 2 NH 2 ) 4 } 2+

3 3 Some ligands have >1 atom with 1 unshared pair of electrons. Ex. Ethylenediamine This molecule has 2 atoms that can behave as a ligand a bidentate ligand. Complex reaction with Cd 2+ Cd NH 2 (CH 2 ) 2 NH 2 {Cd[NH 2 (CH 2 ) 2 NH 2 ] 2 } 2+ K for the reaction of a bidentate ligand with a Lewis acid is > K for the reaction of a monodentate ligand with the same Lewis acid. The chelate effect. ATP, EDTA are multidentate ligands, so K is very large for complexation with Lewis acids (i.e. metal ions). So great for titrations to determine metal ion concentrations. EDTA = ethylenediaminetetraacetic acid.

4 4 Think of EDTA as reacting in the completely deprotonated form: Y 4-. M n+ + Y 4- MY n-4 So K is defined in terms of the [Y 4- ]. Dissolve Na 2 H 2 Y in water: 2Na + + H 2 Y 2- Which can then exist in 7 different forms depending on ph (it s a hexaprotic acid) To determine K need to know the fraction of EDTA existing in the Y 4- form: α Y4-. We skipped how to do this quantitatively in Section 10-5, but did discuss this qualitatively in Section 10 4 when covering principal species.

5 5 Saw something like this for monoprotic and triprotic acids then. Using the quantitative values that made the graph above, can get α Y4- values as a function of ph, shown below. If you dissolve 0.1 M Na 2 H 2 Y: Since

6 6 At a given ph α Y4- is constant (above table) Define K f which is the conditional formation constant. It is conditional on ph. We know that K must be large for a titration reaction to be useful, >99.9% complete. Above table implies that the ph for EDTA titrations for some elements must be greater than for others. A couple of examples illustrating a few points. 1. Calculate the [Mg 2+ ] in a 0.10 M MgY 2- solution. a) at ph 10

7 7 b) at ph 6 Titration cannot be done at ph = 6. Buffer at high ph. c) For Mg 2+ must buffer to at least ph = 10 (NH 3 /NH 4 + ). What if you also wanted to titrate Zn 2+ at the same ph? It looks easy, but there are complications.

8 8 Section 12 6 How complex ion indicators work Just like acid/base indicators are weak acids or bases themselves whose acid/base forms are different colors, complex (metal) ion indicators are chelating agents whose uncomplexed and complexed forms have different colors. So a useful metal ion indicator (i.e. one that will change color at the equivalence point) must bind to the metal, or analyte, less strongly than EDTA. If EDTA does not displace the indicator at the equivalence point the color will not change. One way to get around this is to do a back titration, the same technique used in the Vitamin C lab (not an EDTA titration). Ex. Eriochrome Black T is blocked by Ni 2+. But Ni 2+ can still be analyzed with this indicator by: 1) Add a known mmol EXCESS of EDTA

9 9 2) Back-titrate the excess (unreacted) EDTA with a standard Zn 2+ solution which does not block Eriochrome Black T. Color change from Blue Red The number of mmole of Zn 2+ to this color change = mmole EXCESS EDTA added. mmole Ni 2+ = mmol EDTA added mmol EDTA excess Following in an example of a back titration calculation, similar but easier than the one for the Vitamin C lab. This example taken from your text on p ml of Ni 2+ is treated with ml of M EDTA. Titration with M Zn 2+ requires ml to reach the end point. Find the Ni 2+ solution molarity. Chapter 12 Problems 1, 3, 13, 22, 30, 31