CHEM 2400/2480. Lecture 17

Transcription

1 Lecture 17 Compex Formation Titrations (Harris: chapter 13) Compex Equiibria and Cheates Meta ions are Lewis acids which can accept eectron pairs from Lewis bases. The compound formed is caed a coordination compex and the Lewis bases are referred to as Ligands. Types of Ligands monodentate igands e.g. CN -, OH 2, C -, NH 3 - donate 1 pair of eectrons and bind through one atom mutidentate igands e.g. en, phen, EDTA, NTA - donate more than 1 pair of eectrons and bind through more than one atom cheating igands - name given to mutidentate igands Week 9-1

2 exampes of igands and compexes. Week 9-2

3 The cheate effect is the observation that mutidentate igands form more stabe meta compexes than those formed by simiar monodentate igands. e.g. Cd en º Cden 2 2+ Cd CH 3 NH 2 º Cd(CH 3 NH 2 ) 4 2+ K 2 x K 3 x mainy a )S effect )G " )H " - T)S " Reaction is favoured by a arge negative )G " vaue Reaction with )H " (kjmo -1 ) )S " (J.mo -1 K -1 ) )G " (kjmo -1 ) 4CH 3 NH en Week 9-3

4 Anayticay Usefu Cheating Ligands Week 9-4

5 Common reagent is Na 2 H 2 Y.2H 2 O - can be dried at 110 C and used as a primary standard - the reagent grade contains ~ 0.3% moisture in excess of the stoichiometric amount of water - we wi work with Y 4- where α Y [ HY ] + [ HY ] + [ HY] + [ HY ] + [ HY ] + [ HY ] + [ Y ] 4 [ Y ] [ EDTA] Y where [EDTA] is the tota concentration of a species and free - not compexed aso, KKKKKK α Y [ H ] + K [ H ] + K K [ H ] + K K K [ H ] KKKK[ H] + KKKKK[ H] + KKKKKK Week 9-5

6 M n+ + Y 4- º MY n-4 The formation constant,, for a meta.edta compex is n 4 [ MY ] n + 4 [ M ][ Y ] There are many forms of EDTA in soution at any ph and we wi define a conditiona formation constant, in terms of [Y 4- ], [Y 4- ] α [ EDTA] Y 4 K f n + n 4 [ MY ] [ M ] α [ EDTA] Y 4 Since the ph is usuay fixed with a buffer, is constant and α Y 4-1 α Y 4 n 4 [ MY ] n + [ M ][ EDTA] Is the conditiona (or effective) formation constant for a specific ph. Using we can treat EDTA compex formation as if a free EDTA were in one form. Week 9-6

7 Minimum for Compete Titration Let the concentration of MY n-4 be C at the equivaence point for a compete titration 99.9% of M n+ must be compexed eaving 0.1% i.e. [M n+ ] 10-3 C and M n+ + EDTA º MY n C 10-3 C C C 1 f K [MY n + n-4 ] [ M ][ EDTA] 3 ( C 10 C) 3 3 ( 10 C)( 10 C) C C C For C 10-2 M, for 99.9 % compexation Week 9-7

8 Find for Mg(EDTA) 2- at ph9.00. Aso cacuate the concentration of free Mg 2+ in M Na 2 [Mg(EDTA)] at ph α Y at ph9.00 K x x 10 7 f α Y 4- Mg 2+ + EDTA º MgY 2- initia M equi x x x at ph [ MgY ] [ Mg ][ EDTA] 33. x x 2 x [Mg 2+ ] 3.9 x 10-5 M pmg 4.41 Cacuate at ph 8.00,? α Y 4- ph 7.00,? 5.0 x 10-4 α Y 4- ph 6.00,? 2.3 x 10-5 α Y 4- ph 4.00,? 3.8 x 10-9 α Y 4- Week 9-8

9 EDTA TITRATION 1. Dissociated EDTA is treated as the singe species Y 4-, when the conditiona formation constant is used. ' " Y 4-2. ph is fixed ˆ " Y 4- is constant. 3. Meta ion indicators are used to detect the end points. Week 9-9

10 EDTA TITRATION CURVES M n+ + EDTA º MY n-4 ' " Y 4-1. Before the equivaence point - have excess meta - ion eft after EDTA has been consumed - negect the dissociation of the EDTA compex. 2. At the equivaence point - treat as a dissociation reaction: MY n-4 º M + EDTA K diss 1 / " Y 4-3. After the equivaence point - have excess EDTA and [EDTA] free excess EDTA added after the equivaence point - use K diss. Week 9-10

11 e.g. Cacuate the titration curve for the reaction of 50.0 ml of M Mg 2+ with M EDTA. The soution is buffered at ph x 10 8 ; " Y at ph Mg 2+ + EDTA 6 MgY 2- ' " Y x 6.2 x x Before the equivaence point - use 5.0 ml EDTA added Mg 2+ + EDTA 6 MgY 2- init. mo: 2.5x x10-4 fina mo: 2.25x x10-4 M (2.25x10-3 ) / ˆ pmg 1.39 Week 9-11

12 2. At the equivaence point 50.0 ml EDTA added and V tota ml ˆ [MgY 2- ] (2.5x10-3 ) / M MgY 2- º Mg 2+ + EDTA x x x K 1 1 K 9 x 45. x 10 f x 2+ 5 [ Mg ] 106. x 10 pmg 497. Week 9-12

13 3. After the equivaence point -use 51.0 ml added ; V tota ml Mg 2+ + EDTA 6 MgY 2- init. mo: 2.5x x10-3 fina mo: - 5.0x x10-3 and M 4.95x MgY 2- º Mg 2+ + EDTA x x x 4.95x x K 4.5 x [ Mg ]( 495. x 10 ) [ Mg ] 22. x 10 pmg 665. Week 9-13

14 Lecture 18 EDTA titration using Auxiiary Compexing Agents e.g. The titration of Zn 2+ using NH 3 as the auxiiary compexing agent. Zn 2+ + NH 3 º Zn(NH 3 ) 2+ K Zn(NH 3 ) 2+ + NH 3 º Zn(NH 3 ) 2 2+ K Zn(NH 3 ) NH 3 º Zn(NH 3 ) 3 2+ K Zn(NH 3 ) 3 + NH 3 º Zn(NH 3 ) 4 K Zn NH 3 º Zn(NH 3 ) 4 K and from soubiity ectures on meta-igand compexation, for [L] 0.10 M NH 3 α Zn β [ L] + β [ L] + β [ L] + β [ L] x Week 9-14

15 Titration Cacuation in NH 3 (aq) Buffer (say ph 10.0) 50.0 ml of 1.00 x 10-3 M Zn 2+ with 1.00 x 10-3 M EDTA. The foowing titration cacuations wi be done using [NH 3 ] 0.10 M. Use [NH 3 ] 1.0 M to cacuate the other titration curve. 'New' formation constant Week 9-15

16 1. Before the equivaence point (20 ml) Zn 2+ not bound to EDTA is bound to NH 3 C Zn 2+ ( [ Zn(NH 3 ) 4 2+ ] ) Zn(NH 3 ) EDTA 6 ZnY NH 3 mmo 50x x x x10-3 conc 30x x x10-4 M 2.9x10-4 M C Zn 2+ [ Zn(NH 3 ) 4 2+ ] 4.3x10-4 M [Zn 2+ ] " Zn 2+ x C Zn x 10-5 x 4.3x x 10-9 pzn Week 9-16

17 2. At the equivaence point (50 ml) Zn(NH 3 ) EDTA 6 ZnY NH 3 mmo 50x x x x x10-4 M 100 Zn(NH 3 ) EDTA º ZnY 2- " 5.0x10-4 M x x 5.0x x " 2.0 x (5.0x x) / x 2 x C Zn x 10-8 M [Zn 2+ ] " Zn 2+ x C Zn x 10-5 x 5.0 x x pzn Week 9-17

18 3. After the equivaence point (60 ml) Zn(NH 3 ) EDTA 6 ZnY NH 3 mmo 50x x x x10-3 concentration (10/110)x10-3 (50/110)x x10-5 M 4.5x10-4 M 1 2 [ ZnY ] 2 + [ Zn ][ EDTA] x x [ Zn ] x x 10 x 11. x x 10 M pzn Week 9-18