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Complexometric Titrations

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Presentation on theme: "Complexometric Titrations"— Presentation transcript:

1 Complexometric Titrations
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2 Complex-Formation Titrations General Principles
Most metal ions form coordination compounds with electron-pair donors (ligands) Mn+ + qLm-  MLqn-mq Kf = [MLqn-mq]/[Mn+][Lm-]q The number of covalent bonds formed is called the “coordination number” . e.g., Cu2+ has coordination number of 4 Cu NH3  Cu(NH3)42+ Cu Cl-  Cu(Cl)42- 920220 slides

3 Complex-Formation Titrations General Principles
Ligands are classified regarding the number of donor groups available: e.g., NH3 = “unidentate” :NH3 Glycine = “bidentate” : NH2CH2COO - (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 920220 slides

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5 Nitrilotriacetic acid (NTA)
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6 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) 920220 slides

7 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 920220 slides

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9 Complex-Formation Titrations Solution Chemistry of EDTA(H4Y)
5 forms of EDTA, (H4Y, H3Y-, H2Y2-, HY3-, Y4-) EDTA combines with all metal ions in 1:1 ratio Ag Y4-  AgY3- Fe Y4-  FeY2- Al Y4-  AlY- KMY = [MYn-4]/[Mn+][Y4-] 920220 slides

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pKa1= 1.99 pKa2= 2.67 pKa3= 6,16 pKa4= 10.26 920220 slides

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Y4- complexes with metal ions, and so the complexation equilibria are very pH dependent. 5 forms of EDTA, (H4Y, H3Y-, H2Y2-, HY3-, Y4-) 920220 slides

13 Complex-Formation Titrations Equilibrium Calculations with EDTA
Mn+ + Y4-  MYn Ks = [MYn-4]/[Mn+][[Y4-] Need to know [Y4-], which is pH-dependent pH dependence of Y4-: CT = [Y4-] + [HY3-] + [H2Y2-] + [H3Y-] + [H4Y] Define: a4 = [Y4-]/CT …………… [Y4-] =a4 ×CT a4 = (K1K2K3K4) / ([H+]4 + K1[H+]3 + K1K2[H+]2 + K1K2K3[H+] + K1K2K3K4) Conditional Formation Constant, K’MY Ks = [MYn-4]/[Mn+][[Y4-] Ks =[MYn-4]/[Mn+][[a4CT] K’s = a4 KMY = [MYn-4]/[Mn+][[CT] 920220 slides

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Kf’ = conditional formation constant = Kfa4. It is used at a fixed pH for equilibrium calculations (but varies with pH since a4 does). ©Gary Christian, Analytical Chemistry, 6th Ed. (Wiley) 920220 slides Fig Effect of pH on Kf’ values for EDTA chelates.

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pH=6 920220 slides

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Complex-Formation Titration curve Theoretical titration curves for the reaction of 50.0mL of M metal ion with M EDTA at pH 10.00 920220 slides

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Titration curve 50.0mL 0.020M Ca2+ with 0.050M EDTA, pH 10.0 K’s for specific pH at pH 10.0, K’s = (a4)(KCaY) = (0.35)(5.0 x 1010) = 1.75 x 1010 (a) pCa values before the equivalence point (10.0ml) Ca2+ + Y4-  CaY2- [Ca2+] =((50.0 x 0.020) –(10.0 x 0.050))/(60.0) = M pCa = at 10.0ml EDTA N1V1=N2V2 V2=20 ml 920220 slides

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Titration curve (b) pCa value at the equivalence point (20.0ml) Ca2+ + Y4-  CaY2- [CaY2-] = ((20.0ml x 0.050M)/(70.0ml)) = M K’MY = [CaY2-] / [Ca2+] [CT] = (0.0142)/ [Ca2+]2 [Ca2+] = ((0.0142)/(1.75 x 1010))1/2 = 9.0 x 10-7M; pCa = 6.05 at 20.0ml EDTA 920220 slides

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Titration curve (c) pCa value after the equivalence point (25.0ml) CT = ((25.0 x 0.050)-(50.0 x 0.020))/(75.0) = M [CaY2-] = ((50.0ml x 0.020M)/(75.0ml))= M K’MY = [CaY2-] / [Ca2+] [CT]; [Ca2+] = (0.0133)/(0.0033) K’MY [Ca2+] = x 10-10 pCa = 9.64 at 25.0ml EDTA 920220 slides

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Effect of complexing buffer : auxiliary complexing agent  At pH 10.00 Zn OH–  Zn(OH)2  Ksp = 3.0×10–16 Auxiliary complexing agent : ammonia (0.10~0.02M), NH3 tartrate, citrate, triethanolamine Zn2+ + NH3  [Zn(NH3)] 2+  [Zn(NH3)2] 2+  [Zn(NH3)3] 2+  [Zn(NH3)4] 2+ 920220 slides

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Complexation with auxiliary complexing agent = K1 = K1K2 M nL MLn βn = [MLn] / [M][L]n = K1K2 ··· Kn The equivalent constants, βi, are called overall or cumulative formation constants. 920220 slides

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Effect of complexing buffer : auxiliary complexing agent  Zn2+ + Y4 –  ZnY2– complexing agent : ammonia (0.10~0.02M), tartrate, citrate,triethanolamine At pH 10.00 Zn OH–  Zn(OH)2  Ksp = 3.0×10–16 Zn2+ + NH3  Zn(NH3) 2+  Zn(NH3)2 2+  Zn(NH3)3 2+  Zn(NH3)4 2+ [Zn2+] =αM .C Zn 920220 slides

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EDTA Titration with an Auxiliary Complexing Agent Now consider a titration of Zn2+ by EDTA in the presence of NH3. The EDTA is in the form Y4- The Zinc bound to EDTA is in the form Zn2+ K’’f is the effective formation constant at a fixed pH and fixed concentration of auxiliary complexing agent. 920220 slides

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auxiliary complexing agent  Zn NH3  Zn(NH3)42+ Zn2+ + Y4-  ZnY2- Zn(NH3) Y4 –  ZnY2– + 4 NH3 920220 slides

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Influence of ammonia concentration on the end point for the titration of 50.0ml of M Zn2+. 920220 slides

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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 Mg2+ with EDTA, using Eriochrome black T as the indicator. H2I-  HI2-  I3- pka= pKa= MgIn EDTA  MgEDTA HI2- (red) (colorless) (colorless) (blue) Structure and molecular model of Eriochrome Black T(left) and Calmagite (right). 920220 slides

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EDTA titration curves for 50.0 ml 0f M Ca2+ (K’CaY = 1.75 ×1010) and Mg2+ (K’MgY= 1.72 × 108) at pH 920220 slides

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EDTA titration techniques Direct titration 2) Back titration 3) Displacement titration 920220 slides

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EDTA titration techniques Direct titration A) There is suitable indicator Analyte Suitable indicator Buffer EDTA B) There isn’t suitable indicator Mg2+ Or Mg-EDTA Indicator 920220 slides

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Added Mg2+ or Mg-EDTA Ca-E >Mg-E >Mg-I >Ca-I 100 Ca2+ 1 I Kf Ca-I 99 Ca2+ 1 Ca-I 100 E 100 E Rplacement 99 Ca2+ 1 Ca-I 100 Ca2+ 4 Mg2+ 1 Mg-I 105 E - = 100 E 5 Mg2+ 10 Ca-E Mg-I Ca2+ Mg2+ 99 Ca2+ 1 Ca-I 1 90 10 Mg-E 9 920220 slides

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EDTA titration techniques Back titration : Slow reaction Analyte Excess EDTA Time Standard Zn2+ or Mg2+ 920220 slides

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EDTA titration techniques Displacement titration :: Suitable indicator????? Analyte Excess EDTA- Mg Indicator EDTA • determine Fe3+: Fe3+ + MgY2- ⇌ FeY- + Mg2+ • liberated Mg2+ can be titrated with standard EDTA where: Fe3+ ≡ Mg2+ 920220 slides

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Masking Agents auxillary ligand that forms stable complex with potential interferenceat pH=10, CN- masks Co2+ , Ni2+ , Cu2+ , Zn2+ , Cd2+ , Hg2+ Kf Co-CN>Kf Co-EDTA 920220 slides

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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 920220 slides

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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 CO32-, CrO42-, S2-, and SO42- 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. 920220 slides

39 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. 920220 slides

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…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. 920220 slides

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…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: MgY2- + M2+ MY2- + Mg2+ where M2+ represents the analyte cation. The liberated Mg2+ or, in some cases Zn2+ is then titrated with a standard EDTA solution. Displacement titrations are used when no indicator for an analyte is available. 920220 slides

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