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Complexometric Titrations 1920220http:\\asadipour.kmu.ac.ir 41 slides.

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2 Complexometric Titrations http:\\asadipour.kmu.ac.ir 41 slides

3 Complex-Formation Titrations General Principles Most metal ions form coordination compounds with electron-pair donors (ligands) M n+ + qL m-  ML q n-mq K f = [ML q n-mq ]/[M n+ ][L m- ] q The number of covalent bonds formed is called the “coordination number”. e.g., Cu 2+ has coordination number of 4 Cu NH 3  Cu(NH 3 ) 4 2+ Cu Cl -  Cu(Cl) http:\\asadipour.kmu.ac.ir 41 slides

4 Complex-Formation Titrations General Principles Ligands are classified regarding the number of donor groups available: e.g., NH 3 = “unidentate” :NH 3 Glycine= “bidentate” : NH 2 CH 2 COO - (also, there are tridentate, tetradentate, pentadentate, and hexadentate chelating agents) Multidentate ligands (especially with 4 and 6 donors) are preferred for titrimetry. –react more completely with metal ion –usually react in a single step –provide sharper end-points http:\\asadipour.kmu.ac.ir 41 slides

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6 Nitrilotriacetic acid (NTA) http:\\asadipour.kmu.ac.ir 41 slides

7 Complex-Formation Titrations General Principles The most useful complex-formation reactions for titrimetry involve chelate formation A chelate is formed when a metal ion coordinates with two of more donor groups of a single ligand (forming a 5- or 6- membered heterocyclic ring) http:\\asadipour.kmu.ac.ir 41 slides

8 Complex-Formation Titrations General Principles Aminopolycarboxylic acid ligands e.g., ethylenediaminetetraacetic acid (EDTA) EDTA is a hexadentate ligand EDTA forms stable chelates with most metal ions http:\\asadipour.kmu.ac.ir 41 slides

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10 Complex-Formation Titrations Solution Chemistry of EDTA(H 4 Y) 5 forms of EDTA, (H 4 Y, H 3 Y -, H 2 Y 2-, HY 3-, Y 4- ) EDTA combines with all metal ions in 1:1 ratio Ag + + Y 4-  AgY 3- Fe 2+ + Y 4-  FeY 2- Al 3+ + Y 4-  AlY - K MY = [MY n-4 ]/[M n+ ][Y 4- ] http:\\asadipour.kmu.ac.ir 41 slides

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12 pK a1 = 1.99 pK a2 = 2.67 pK a3 = 6,16 pK a4 = slides

13 5 forms of EDTA, (H 4 Y, H 3 Y -, H 2 Y 2-, HY 3-, Y 4- ) 12 Y 4- complexes with metal ions, and so the complexation equilibria are very pH dependent. Y 4- complexes with metal ions, and so the complexation equilibria are very pH dependent http:\\asadipour.kmu.ac.ir 41 slides

14 Complex-Formation Titrations Equilibrium Calculations with EDTA M n+ + Y 4-  MY n-4 K s = [MY n-4 ]/[M n+ ][[Y 4- ] Need to know [Y 4- ], which is pH-dependent pH dependence of Y 4- : C T = [Y 4- ] + [HY 3- ] + [H 2 Y 2- ] + [H 3 Y - ] + [H 4 Y] Define:   = [Y 4- ]/C T …………… [Y 4- ]   ×C T                                                Conditional Formation Constant, K ’ MY K s = [MY n-4 ]/[M n+ ][[Y 4- ] K s = [MY n-4 ]/[M n+ ][[   C T ] K ’ s =   K MY =  [MY n-4 ]/[M n+ ][[C T ] http:\\asadipour.kmu.ac.ir 41 slides

15 Fig Effect of pH on K f ’ values for EDTA chelates. K f ’ = conditional formation constant = K f  4. It is used at a fixed pH for equilibrium calculations (but varies with pH since  4 does). K f ’ = conditional formation constant = K f  4. It is used at a fixed pH for equilibrium calculations (but varies with pH since  4 does). ©Gary Christian, Analytical Chemistry, 6th Ed. (Wiley) http:\\asadipour.kmu.ac.ir 41 slides

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17 16 pH= http:\\asadipour.kmu.ac.ir 41 slides

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19 Theoretical titration curves for the reaction of 50.0mL of M metal ion with M EDTA at pH Complex-Formation Titration curve http:\\asadipour.kmu.ac.ir 41 slides

20 Titration curve 50.0mL 0.020M Ca 2+ with 0.050M EDTA, pH 10.0 K ’ s for specific pH at pH 10.0, K ’ s = (  4 )(K CaY ) = (0.35)(5.0 x ) = 1.75 x (a) pCa values before the equivalence point (10.0ml) Ca 2+ + Y 4-  CaY 2- [Ca 2+ ] =((50.0 x 0.020) –(10.0 x 0.050))/(60.0) = M pCa = 2.08 at 10.0ml EDTA http:\\asadipour.kmu.ac.ir 41 slides N1V1=N2V2 V2=20 ml

21 Titration curve (b) pCa value at the equivalence point (20.0ml) Ca 2+ + Y 4-  CaY 2- [CaY 2- ] = ((20.0ml x 0.050M)/(70.0ml))  M K ’ MY = [CaY 2- ] / [Ca 2+ ] [C T ] = (0.0142)/ [Ca 2+ ] 2 [Ca 2+ ] = ((0.0142)/(1.75 x )) 1/2 = 9.0 x M; pCa = 6.05 at 20.0ml EDTA http:\\asadipour.kmu.ac.ir 41 slides

22 Titration curve (c) pCa value after the equivalence point (25.0ml) C T = ((25.0 x 0.050)-(50.0 x 0.020))/(75.0) = M [CaY 2- ] = ((50.0ml x 0.020M)/(75.0ml))= M K ’ MY = [CaY 2- ] / [Ca 2+ ] [C T ]; [Ca 2+ ] = (0.0133)/(0.0033) K ’ MY [Ca 2+ ] = 2.30 x pCa = 9.64 at 25.0ml EDTA http:\\asadipour.kmu.ac.ir 41 slides

23 Effect of complexing buffer : auxiliary complexing agent  At pH Zn OH –  Zn(OH) 2  Ksp = 3.0×10 –16 Auxiliary complexing agent : ammonia (0.10~0.02M), NH 3 tartrate, citrate, triethanolamine Zn 2+ + NH 3  [Zn(NH 3 )] 2+  [Zn(NH 3 ) 2 ] 2+  [Zn(NH 3 ) 3 ] 2+  [Zn(NH 3 ) 4 ] http:\\asadipour.kmu.ac.ir 41 slides

24 Complexation with auxiliary complexing agent The equivalent constants, β i, are called overall or cumulative formation constants. = K 1 = K 1 K 2 M + nL ML n β n = [ML n ] / [M][L] n = K 1 K 2 ··· K n http:\\asadipour.kmu.ac.ir 41 slides

25 Effect of complexing buffer : auxiliary complexing agent  Zn 2+ + Y 4 –  ZnY 2– complexing agent : ammonia (0.10~0.02M), tartrate, citrate,triethanolamine http:\\asadipour.kmu.ac.ir 41 slides Zn 2+ + NH 3  Zn(NH 3 ) 2+  Zn(NH 3 ) 2 2+  Zn(NH 3 ) 3 2+  Zn(NH 3 ) 4 2+ [Zn 2+ ] =α M.C Zn At pH Zn OH –  Zn(OH) 2  Ksp = 3.0×10 –16

26 EDTA Titration with an Auxiliary Complexing Agent K ’’ f is the effective formation constant at a fixed pH and fixed concentration of auxiliary complexing agent. Now consider a titration of Zn 2+ by EDTA in the presence of NH 3. The EDTA is in the form Y 4- The Zinc bound to EDTA is in the form Zn 2+  http:\\asadipour.kmu.ac.ir 41 slides

27 auxiliary complexing agent  http:\\asadipour.kmu.ac.ir 41 slides Zn(NH 3 ) Y 4 –  ZnY 2– + 4 NH 3 Zn NH 3  Zn(NH 3 ) 4 2+ Zn 2+ + Y 4-  ZnY 2-

28 Influence of ammonia concentration on the end point for the titration of 50.0ml of M Zn http:\\asadipour.kmu.ac.ir 41 slides

29 Metal Ion Indicators   Metal ion indicators are compounds whose color changes when they bind to a metal ion. Useful indicators must bind metal less strongly than EDTA does. A typical titration is illustrated by the reaction of Mg 2+ with EDTA, using Eriochrome black T as the indicator. H 2 I -  HI 2-  I 3- pka=7 pKa=12 MgIn + EDTA  MgEDTA + HI 2- (red) (colorless) (colorless) (blue) Structure and molecular model of Eriochrome Black T(left) and Calmagite (right) http:\\asadipour.kmu.ac.ir 41 slides

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31 EDTA titration curves for 50.0 ml 0f M Ca 2+ (K’ CaY = 1.75 ×10 10 ) and Mg 2+ (K’ MgY = 1.72 × 10 8 ) at pH http:\\asadipour.kmu.ac.ir 41 slides

32 EDTA titration techniques 1)Direct titration 2) Back titration 3) Displacement titration http:\\asadipour.kmu.ac.ir 41 slides

33 EDTA titration techniques 1)Direct titration A) There is suitable indicator Analyte Suitable indicator Buffer EDTA B) There isn’t suitable indicator Analyte Mg 2+ Or Mg-EDTA Indicator Buffer EDTA http:\\asadipour.kmu.ac.ir 41 slides

34 http:\\asadipour.kmu.ac.ir 41 slides 100 Ca 2+ 1 I 99 Ca 2+ 1 Ca-I 100 E 105 E 100 Ca 2+ 4 Mg 2+ 1 Mg-I 5 Mg 2+ -= 99 Ca 2+ 1 Ca-I 99 Ca 2+ 1 Ca-I 10 Mg-E Ca-E Mg-I Ca 2+ Mg Ca-E >Mg-E >Mg-I >Ca-I Rplacement K f Ca-I 100 E Added Mg 2+ or Mg-EDTA

35 EDTA titration techniques 2)Back titration : Slow reaction Analyte Excess EDTA Time Standard Zn 2+ or Mg http:\\asadipour.kmu.ac.ir 41 slides

36 EDTA titration techniques 3)Displacement titration :: Suitable indicator????? Analyte Excess EDTA- Mg Indicator EDTA determine Fe3+: Fe 3+ + MgY 2- ⇌ FeY - + Mg 2+ liberated Mg2+ can be titrated with standard EDTA where: Fe 3+ ≡ Mg http:\\asadipour.kmu.ac.ir 41 slides

37 Masking Agents auxillary ligand that forms stable complex with potential interferenceat pH=10, CN - masks Co 2+, Ni 2+, Cu 2+, Zn 2+, Cd 2+, Hg 2+ K f Co-CN>K f Co-EDTA http:\\asadipour.kmu.ac.ir 41 slides

38 EDTA Titration Techniques Direct Titration In a direct titration, analyte is titrated with standard EDTA. Conditional formation constant for the metal-EDTA complex is large The color of the free indicator is distinctly different from that of the metal-indicator complex. Back Titration In a back titration, a known excess of EDTA is added to the analyte. The excess EDTA is then titrated with a standard solution of a second metal ion. Necessary if the analyte precipitates in the absence of EDTA, if it reacts too slowly with EDTA under titration conditions, or if it blocks the indicator. The metal ion used in the back titration must not displace the analyte metal ion from its EDTA complex http:\\asadipour.kmu.ac.ir 41 slides

39 Displacement Titration For metal ions that do not have a satisfactory indicator, a displacement titration maybe feasible. Indirect Titration Anions that precipitate with certain metal ions can be analyzed with EDTA by indirect titration. Alternatively, an anion can be precipitated with excess metal ion. The precipitate is filtered and washed, and the excess metal ion in the filtrate is titrated with EDTA. Anions such as CO 3 2-, CrO 4 2-, S 2-, and SO 4 2- can be determined by indirect titration with EDTA. Masking A masking agent is a reagent that protects some component of the analyte from reaction with EDTA. Demasking release metal ion from a masking agent http:\\asadipour.kmu.ac.ir 41 slides

40 Titration Methods Employing EDTA Direct Titration: Many of the metals in the periodic table can be determined by titration with standard EDTA solution. Some methods are based on indicators that respond to the analyte itself, whereas others are based on an added metal ion. Methods Based on Indicators for an Added metal Ion: In case where a good, direct indicator for the analyte is unavailable, a small amount of a metal ion for which a good indicator is available can be added. The metal ion must form a complex that is less stable than the analyte complex http:\\asadipour.kmu.ac.ir 41 slides

41 …continued… Back-Titration Methods: Back-titrations are useful for the determination of cations that form stable EDTA complexes and for which a satisfactory indicator is not available; the determination of thallium is an extreme example. The method is also useful for cations such as Cr(III) and Co(III) that react only slowly with EDTA. A measured excess of standard EDTA solution is added to the analyte solution. After the reaction is judged complete, the excess EDTA is back-titrated with a standard magnesium or zinc ion solution to an Eriochrome Black T or Calmagite end point http:\\asadipour.kmu.ac.ir 41 slides

42 …continued… Displacement methods: In displacement titrations, an unmeasured excess of a solution containing the magnesium or zinc complex of EDTA is introduced into the analyte solution. If the analyte forms a more stable complex than that of magnesium or zinc, the following displacement reaction occurs: MgY 2- + M 2+ MY 2- + Mg 2+ where M 2+ represents the analyte cation. The liberated Mg 2+ or, in some cases Zn 2+ is then titrated with a standard EDTA solution. Displacement titrations are used when no indicator for an analyte is available http:\\asadipour.kmu.ac.ir 41 slides


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