Complexation Reactions and Titrations

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Chapter 17 Complexatio Reactios ad Titratios Lewis acid-base cocept Lewis acid electro pair acceptor [metal catios, M + ] Lewis base electro pair door [ligad, molecules or ios] Coordiate covalet bod

1 Chapter 17 Complexatio Reactios ad Titratios Lewis acid-base cocept Lewis acid electro pair acceptor [metal catios, M + ] Lewis base electro pair door [ligad, molecules or ios] Coordiate covalet bod a bod formed whe both electros of the bod are doated by oe atom. Complex io A metal io with Lewis base attached to it through coordiate covalet bod. Complex (Coordiate compoud) a compoud cosistig either of complex ios ad other ios of opposite charge or of eutral complex species. 1

The empty orbitals of the catio ad the orbital occupied by the bodig pair of electros o the door io or molecule (ligad) form a ew molecular orbital.

2 Complex-Formatio Reactios some elemets (mostly metal ios) ca form complexes with molecules/ios which have a spare pair of electros (ligad). The cetral atom, (usually) a metal catio, accepts the pair of electros from the ligad to form a coordiate covalet bod. The empty orbitals of the catio ad the orbital occupied by the bodig pair of electros o the door io or molecule (ligad) form a ew molecular orbital. The umber of coordiate covalet bods that a catio teds to form with idividual ligads (or fuctioal groups o ligads) is kow as its coordiatio umber. Typical values for coordiatio umbers are, 4 ad 6. The compouds formed ca be eutral, positively or egatively charged depedig o the charge of the reactig species.

3 Whe a ligad has a sigle complexig or door group i its structure, it is said to be uidetate (sigle-toothed), :NH, I - Co(NH ) 6 +, CuI - whe there are two groups, it is bidetate, Whe there are three (four) groups, it is called tridetate (tetradetate) ligad, etc. Whe a bidetate (or higher umber of door groups preset i the ligad) forms a complex with a metal catio, we call the resultig compoud a metal chelate ( kee late -claw). As titrats, multidetate ligads, particularly those with 4 to 6 door groups have the advatage that they usually react i a sigle step process, ad their reactios with the metal catio are more complete tha their uidetate couterparts. Chelate Effect The ability of multidetate ligads to form more stable metal complexes tha those formed by similar moodetate ligads Ofte results from the formatio of 5- membered "rig" with metal ad two atoms o the ligad

4 Complexatio Equilibria Cu Cu(NH ) NH NH K1 Cu(NH ) K Cu(NH ) Cu(NH ) NH u(nh ) Cu( NH ) NH K C K4 Cu(NH ) 4 Cu 4NH Cu(N K K K K K f 1 Kf H) 4 K f ( 4 ) formatio costat 4 4 Complexatio reactios occur i a stepwise fasio M + L ML ML + L ML ML + L ML ML + L ML [ML] K1 [M][L] [ML ] K [ML][L] [ML ] K [ML ][L] K [ML ] [ML ][L] -1 Formatio Costats ( i ) M + L ML M + L ML M + L ML [ML] K [M][L] 1 1 [ML ] [M][L] 1 [ML ] [M][L] 1 M + L ML K1KK... K... [ML ] [M][L] KK KKK 4

5 Alpha () Values Fractio of the Total Metal Cocetratio [M] [ML] M= ML= c c c M [ML ] [ML ] = = ML ML cm cm [M] [ML] [ML ]... [ML ] M M [M] 1[M][L] [M][L]... [M][L] [M]{1 1[L] [L]... [L] } 1 = 1 [L] [L]... [L] M 1 [L] = 1 [L] [L]... [L] 1 ML 1 [L] = 1 [L] [L]... [L] ML 1 [L] = 1 [L] [L]... [L] ML 1 Titratio Curves of ML (A)Tetradetate ligad, 1:1 (B)Bidetate ligad :1 (C)Uidetate ligad, 4:1 Tetradetate or hexadetate ligads are more satisfactory as titrats tha ligads with a lesser umber of door groups because their reactios with catios are more complete ad they ted to form 1:1 complexes. 5

6 EDTA Titratios Most widely used complexometric titrat, Hexadetate ligad (4 COOH+ amio groups) EDTA It forms 1:1 complexes with most metals. (Not with Group 1A metals Na, K, Li) Forms stable water soluble complexes. High formatio costats. A primary stadard material a highly purified compoud that serves as a referece material. Octahedro Structure of EDTA-M 6

7 Acidic Properties of EDTA EDTA is a weak tetrafuctioal acid: K 1 = 1.0 x 10 - ; K =.14 x 10 - K = 6.9 x 10-7 ; K 4 = 5.50 x There are 4 possible acidic bidig sites. Additioally, each itroge ca act as a e- pair door. There are six potetial bidig sites: Hexadetate Ligad 7

8 EDTA Disassociatio HYHY HY HY Y K K K K (-4) (-4)+ [MY ] M +Y MY KMY = [M + ][Y 4- ] 4-4- [Y ] [Y ] ct 4 [H Y] [H Y ] [H Y ] [HY ] [Y ] K (-4)+ (-4)+ [MY ] [MY ] MY = [M + ][Y 4- ] [M + ] 4cT Coditioal formatio costat: K [MY ] (-4)+ ' MY = 4KMY + [M ] ct K MY is ph depedet! 4-4- [Y ] [Y ] ct K1 K1K K1KK K1KKK ; ct D ct D [HY ] K1[H ] ; T [H Y] [H Y ] [H Y ] [HY ] [Y ] KKKK = [H ] + [H ] + [H ] + [H ] + KKKK 1 4 = D [HY ] KKK[H ] [H Y ] KK[H ] c D [H4Y] [H ] 0 c D T + 4 8

Compositio of EDTA solutio as a fuctio of ph. 1 0.8 alpha4, Y4- alpha 4 0.

9 Compositio of EDTA solutio as a fuctio of ph alpha4, Y4- alpha ph alpha4 Distributio of uprotoated form of Y 4- as a fuctio of ph Example 17-4 Use speadsheet to costruct the titratio curve of pca versus volume of EDTA for 50.0 ml of M Ca + beig titrated with M EDTA i a solutio buffered to a costat ph of 10.0 (1) ph = 10.0, 4 = 0.5, K CaY = 5.0 e10, K CaY = 4 K CaY = 1.75e10 () Equivalece poit: v EDTA = 50x / = 5.0 ml () Iitial pca: [Ca + ] = M, pca =.0 9

10 (4) Pre-equivalece poit: Ca (excess) + Y CaY (disassociatio egligible) [Ca] = [50.00x v EDTA x0.0100]/(50.00+v EDTA ) v EDTA (ml) [Ca] pca e e e e-4.87 (5) At equivalece poit: [CaY]/[Ca]^ = K MY, [Ca] = {[CaY]/K MY }^(1/) [Ca] = {[50.00x /( )]/1.75e10}^0.5 =4.6e-7M, pca = 6.6 (6) Post-equivalece poit: Ca + Y CaY x excess ~equivalece cocetratio (50.00x )/(50.00+v EDTA )/ {[Ca][(0.0100xv EDTA x )/(50.00+v EDTA )]}=K MY [Ca]=0.5/{[0.0100xv EDTA -0.5]xK MY }=1.4e-11/(0.0100xv EDTA -0.5) V EDTA (ml) [Ca] pca e e e pca V (EDTA, ml) Ca-EDTA Titratio Curve at ph 10 10

Idicators for EDTA Titratios Compouds chagig colour whe bidig to metal io. K f for Metal-I < K f for Metal-EDTA.

11 EDTA Titratio Curves for Ca (K.75e10) ad Mg (K 1.7e8) at ph 10 Larger the K MY Larger the pm chage Eriochrome Black T Typical Metal Io Idicator--Eriochrome Black T Weak Acid Idicators for EDTA Titratios Compouds chagig colour whe bidig to metal io. K f for Metal-I < K f for Metal-EDTA. Before Titratio: Mg + + I MgI (colourless) (blue) (red) Durig Titratio: Before the ed poit Mg + + EDTA MgEDTA (free Mg + ios) (Solutio red due to MgI complex) At the ed poit: MgI + EDTA MgEDTA + I (red) (colourless) (colourless) (Blue) 11

12 Example

13 Ifluece of ph o Titratio Curves of Ca-EDTA The higher ph The larger pca chage ph should > 8 for Ca-EDTA titratios Lower phs OK for Large K MY Complexes ph = 6.0 Miimum ph eeded for satisfactory titratio of various catios with EDTA. 1

14 Effect of Other Complexig Agets o EDTA Titratio Curves Addig a secod complexig aget to the titratio medium to maitai the aalyte metal io i solutio (may metal ios form isoluble hydroxides or oxides at slightly acidic, eutral or basic ph values). To "mask" or remove iterferig ios also preset i the sample matrix. The secod complexig aget usually has a higher affiity for the iterferig io (greater K MY ) tha the EDTA ad prevets it from reactig with the EDTA. I this case, the secod complexig aget is called a "maskig aget". Most buffers will complex metal ios because they also cotai fuctioal groups which ca form coordiate covalet bods (-OH, -COOH, -NH, etc.) ad their effect o the free metal io cocetratio must be cosidered whe calculatig the pm. Before Titratio Z + 4NH Z(NH ) HY - EDTA (durig Titratio) NH - ZY + NH 4 Ammoia decreases the chage i pz i the equivalece-poit regio. Ifluece of [NH ] o the Z-EDTA Titratio at ph 9.00 M + Y MY (EDTA complexig) M + L ML (secod complexig aget) K K MY MY M M [MY] = ([M],[Y]--free cocetratio) [M][Y] [MY] [MY] = ( c )( c ) ( ) c c 4 T M 4 M T Coditioal formatio costat: K" MY = K M 4 M MY [MY] c c M T [M] 1 Where M c 1 1[L] [L]... [L] 14

15 Example Cosider the titratio of 50.0 ml of M Z + with M EDTA at ph10 i the presece of 0.10 M NH. (This is the cocetratio of NH. There is also NH 4+ i the solutio.) Fid pz after additio of 0.0, 50.0, ad 60,0 ml of EDTA. Note: We always assume that EDTA is a much stroger complexig aget tha NH. K f for EDTA > K f for NH. Solutio Z + -NH complexes: Z(NH ) +, Z(NH ) +, Z(NH ) +, ad Z(NH ) 4 + Z [ L] [ L] [ L] 4[ L] 1 = 1.51 x 10, =.69 x 10 4, = 5.50 x 10 6, ad 4 = 5.01 x 10 8 [L] = [NH ] = 0.10 M Z [Z]= pz = Very little free Z + i the presece of 0.10 M NH. Most Z + complexed by NH At ph 10, Y K '' f ' K Z f 4 Z Y K f = (1.8 x10-5 ) (0.5) ( ) =.05 x Additio of 0.0 ml EDTA solutio: c Z ( ) M [ Z ] M c (1.810 )(4.10 ) M Z pz = -log[z + ] =

16 . Equvalece poit: Additio of 50.0 ml EDTA: 50.0 [ZY] ( M) x 4 x '' 11 K f = 5.00 x 10-4 M x = c Z =4.9 x 10-8 M [Z ] M c ( )(4.910 ) M Z pz = -log[z + ] = After the equivalece poit: 60.0 ml EDTA c Y 10.0 ( ) = 9.1 x 10-5 M [ZY] ( M) = 4.5 x 10-4 M [ZY] K K (0.5)(10 ) c c c Z Y " f Z 4 Y f Z Z [ZY] Z 16 Z1.110 cy [Z] = c [ZY] Z Z cy [Z + ] = 4. x M pz = 15.6 EDTA Titratios at Differet Cocetratios of Auxiliary Complexig Reaget (NH ). Small pz ear equivalece poit. Sigificat pz Near equiv. Poit. (More distict ed poit) 16

17 1. Direct Titratio EDTA Titratio Techiques *Buffer aalyte to ph where K f for MY is large, *ad M-I colour distict from free I color. *Auxiliary complexig aget may be used.. Back Titratio *Kow excess std EDTA added. *Excess EDTA the titrated with a std sol of a secod metal io. *Note: Std metal io for back titratio must ot displace aalyte from MY complex.. Back Titratio: Whe to apply it *Aalyte precipitates i the absece of EDTA. *Aalyte reacts too slowly with EDTA. *Aalyte blocks idicator. Displacemet Titratio *Metal ios with o satisfactory idicator. *Aalyte treated with excess Mg(EDTA) M + MgY MY + Mg * K f for MY > K f for MgY 4. Idirect Titratio *Aios aalysed: CO -, CrO 4 -, S -, ad SO 4 -. Precipitate SO 4 - with excess Ba + at ph 1. *BaSO 4 (s) washed & boiled with excess EDTA at ph 10. BaSO 4 (s) + EDTA(aq) BaY - (aq) + SO - 4 (aq) Excess EDTA back titrated:edta(aq) + Mg + MgY - (aq) Alteratively: *Precipitate SO - 4 with excess Ba + at ph 1. *Filter & wash precipitate. *Treat excess metal io i filtrate with EDTA. 17

18 5. Maskig *Maskig Aget: Protects some compoet of aalyte from reactig with EDTA. *F - masks Hg +, Fe +, Ti 4+, ad Be +. *CN - masks Cd +, Z +, Hg +, Co +, Cu +, Ag +, Ni +, Pd +, Pt +, Hg +, Fe +, ad Fe +, but ot Mg +, Ca +, M +, Pb +. *Triethaolamie: Al +, Fe +, ad M +. *,-dimercapto-1-propaol: Bi +, Cd +,Cu +, Hg +, ad Pb +. *Demaskig: Releasig maskig aget from aalyte. M m CN m mh CO mh mh C OH M + Metal-Cyaide Formaldehyde Complex CN *Oxidatio with H O releases Cu + from Cu + -Thiourea complex. *Thus, aalyte selectivity: 1. ph cotrol. Maskig. Demaskig 18

CHAPTER 11. Practice Questions (a) OH (b) I. (h) NH 3 CH 3 CO 2 (j) C 6 H 5 O - (k) (CH 3 ) 3 N conjugate pair

CHAPTER 11. Practice Questions (a) OH (b) I. (h) NH 3 CH 3 CO 2 (j) C 6 H 5 O - (k) (CH 3 ) 3 N conjugate pair CAPTER 11 Practice Questios 11.1 (a) O (b) I (c) NO 2 (d) 2 PO 4 (e) 2 PO 4 (f) 3 PO 4 (g) SO 4 (h) N 3 (i) C 3 CO 2 (j) C 6 5 O - (k) (C 3 ) 3 N 11.3 cojugate pair PO 3 4 (aq) C 3 COO(aq) PO 2 4 (aq)