Introduction to Analytical Chemistry CHAPTER 10 COMPLEXATION AND

Introduction to Analytical Chemistry CHAPTER 10 COMPLEXATION AND PRECIPITATION TITRATIONS: TAKING ADVANTAGE OF COMPLEXING AND PRECIPITATING AGENTS

10 A Forming Complexes Most metal ions react with electron-pair donors to form coordination compounds or complexes. The donor species, or ligand, must have at least one pair of unshared electrons. A chelate is produced when a metal ion coordinates with two or more donor groups of a single ligand to form a five- or six-membered heterocyclic ring. 10 -2 Copyright © 2011 Cengage Learning

10 A Forming Complexes 10 -3 Copyright © 2011 Cengage Learning

10 A Forming Complexes A ligand that has a single donor group, is called unidentate, one such as glycine, which has two groups available for covalent bonding, is called bidentate. Tridentate, tetradentate, pentadentate, and hexadentate chelating agents are also known. 10 -4 Copyright © 2011 Cengage Learning

10 A-1 Producing Soluble Complexes Complexation reactions involve a metal ion M reacting with a ligand L to form a complex ML, as shown in Equation 10 -1: (101) 10 -5 Copyright © 2011 Cengage Learning

10 A-1 Producing Soluble Complexes (105) (106) (107) 10 -6 Copyright © 2011 Cengage Learning

10 A-2 Forming Insoluble Species (108) 10 -7 Copyright © 2011 Cengage Learning

Example 10 -1 It is desired to separate Ni and Zn by precipitation as the sulfides Ni. S and Zn. S. The solubility products are (Ksp)Ni. S = 4. 0 × 10¯²⁰ and (Ksp)Zn. S = 3. 0 × 10¯²⁵. If excess KCN is added to a solution initially containing 0. 01 M each of Ni²⁺ and Zn²⁺ and the volume remains approximately constant, both ions form the complex almost exclusively. 10 -8 Copyright © 2011 Cengage Learning

Example 10 -1 If KCN is added until the [CN¯] is 1. 0 M and the [S 2¯] is maintained at 0. 5 M, can we precipitate Zn. S while leaving most of the Ni in solution? Adding CN¯ causes the formation of the Zn²⁺ and Ni²⁺ complexes according to 10 -9 Copyright © 2011 Cengage Learning

Example 10 -1 From mass balance, Since the formation constants of the complexes are so large, we can estimate that most of the metals will be in the form of the complex ion. 10 -10 Copyright © 2011 Cengage Learning

Example 10 -1 • Thus, We can then estimate the free [Ni²⁺] and [Zn²⁺] con-centrations from the β 4 values: 10 -11 Copyright © 2011 Cengage Learning

Example 10 -1 Now the product of the ion concentrations for Ni. S ([Ni²⁺][S²¯] = 6. 25× 10¯³³ × 0. 5 = 3. 1 × 10¯³³) is seen to be much smaller than Ksp (4. 0 × 10¯²⁰) so that the Ni²⁺ remains in solution. The product of the ion concentrations for Zn. S ([Zn²⁺][S²¯] = 2. 38 × 10¯²² × 0. 5 = 1. 2 × 10¯²²), however, exceeds Ksp (3. 0 × 10¯²⁵) so that Zn. S readily precipitates, which then allows a good separation of Zn from Ni. 10 -12 Copyright © 2011 Cengage Learning

10 A-3 Ligands That Can Protonate Complexation with Protonating Ligands For a diprotic acid, like oxalic acid, the fraction of the total oxalate containing species in any given form, Ox²¯, HOx¯, and H₂Ox, is given by an alpha value. (109) 10 -13 Copyright © 2011 Cengage Learning

10 A-3 Ligands That Can Protonate (1010) (10 -11) (1012) Since we are interested in the free oxalate concentration, 10 -14 (1013) Copyright © 2011 Cengage Learning

10 A-3 Ligands That Can Protonate Accounting for p. H Effects with Conditional Formation Constants (1014) At a particular p. H value, α 2 is constant, and we can combine K 1 and α 2 to yield a new conditional constant K’ 1 : (1015) 10 -15 Copyright © 2011 Cengage Learning

10 B-1 Complexation Titrations As titrants, multidentate ligands, particularly those having four or six donor groups, have two advantages over their unidentate counterparts. First, they generally react more completely with cations and thus provide sharper end. Second, they ordinarily react with metal ions in a single-step process. 10 -16 Copyright © 2011 Cengage Learning

Figure 10 -1 Titration curves for complexometric titrations. Titration of 60. 0 m. L of a solution that is 0. 020 M in M with (curve A) a 0. 020 -M solution of the tetradentate ligand D to give MD as the product; (curve B) a 0. 040 -M solution of the bidentate ligand B to give MB 2 ; and (curve C) a 0. 080 - M solution of the unidentate ligand A to give MA 4. The overall formation constant for each product is 1020. 10 -17 Copyright © 2011 Cengage Learning

10 B-2 Precipitation Titrations with silver nitrate are sometimes called argentometric titrations. 10 -18 Copyright © 2011 Cengage Learning

Example 10 -2 Calculate the p. Ag of the solution during the titration of 50. 00 m. L of 0. 0500 M Na. Cl with 0. 1000 M Ag. NO₃ after the addition of the following volumes of reagent: (a) 0. 00 m. L, (b) 24. 50 m. L, (c) 25. 00 m. L, (d) 25. 50 m. L. (a) Because no Ag. NO₃ has been added, [Ag⁺] 0 and p. Ag is indeterminate. (b) At 24. 5 m. L, [Ag⁺] is very small and cannot be computed from stoichiometric considerations, but [Cl¯] can be obtained readily. 10 -19 Copyright © 2011 Cengage Learning

Example 10 -20 Copyright © 2011 Cengage Learning

Example 10 -2 (c) This volume corresponds to the equivalence point where [Ag⁺]=[Cl¯] and (d) 10 -21 Copyright © 2011 Cengage Learning

Figure 10 -2 Effect of titrant concentration on precipitation titration curves. Curve A shows 50. 00 m. L of 0. 0500 M Na. Cl with 0. 1000 M Ag. NO 3 , and curve B shows 50. 00 m. L of 0. 00500 M Na. Cl with 0. 01000 M Ag. NO 3. Note the increased sharpness of the break for the more concentrated solution, A. 10 -22 Copyright © 2011 Cengage Learning

Figure 10 -3 Effect of reaction completeness on precipitation titration curves. For each curve, 50. 00 m. L of a 0. 0500 M solution of the anion was titrated with 0. 1000 M Ag. NO 3. Note that smaller values of Ksp give much sharper breaks at the end point. 10 -23 Copyright © 2011 Cengage Learning

10 B-2 Precipitation Titrations Signaling the End Point for Argentometric Titrations Three types of end points are encountered in titrations with silver nitrate: (1) chemical, (2) potentiometric, and (3) amperometric. 10 -24 Copyright © 2011 Cengage Learning

10 B-2 Precipitation Titrations Formation of Colored Precipitate: The Mohr Method. 10 -25 Copyright © 2011 Cengage Learning

10 B-2 Precipitation Titrations The high chromate ion concentration imparts such an intense yellow color to the solution. Lower concentrations of chromate ion are generally used, and, as a consequence, excess silver nitrate is required before precipitation begins. An additional excess of the reagent must also be added to produce enough silver chromate to be seen. 10 -26 Copyright © 2011 Cengage Learning

10 B-2 Precipitation Titrations The Mohr titration must be carried out at a p. H of 7 to 10 because chromate ion is the conjugate base of the weak chromic acid. Consequently, in acidic solutions, where the p. H is less than 7, the chromate ion concentration is too low to produce the precipitate. 10 -27 Copyright © 2011 Cengage Learning

10 B-2 Precipitation Titrations Adsorption Indicators: The Fajans Method. Fluorescein is a typical adsorption indicator In the early stages of the titration of chloride ion with silver nitrate, the colloidal silver chloride particles are negatively charged because of adsorption of excess chloride ions. The dye anions are repelled from this surface by electrostatic repulsion. 10 -28 Copyright © 2011 Cengage Learning

10 B-2 Precipitation Titrations Beyond the equivalence point, the silver chloride particles strongly adsorb silver ions and thereby acquire a positive charge. Fluoresceinate anions are now attracted into the counter-ion layer. The net result is the appearance of the red color of silver fluoresceinate in the surface layer of the solution surrounding the solid. 10 -29 Copyright © 2011 Cengage Learning

10 B-2 Precipitation Titrations Forming a Colored Complex: The Volhard Method. Iron(III) serves as the indicator. The titration must be carried out in acidic solution to prevent precipitation ofiron(III) as the hydrated oxide. 10 -30 Copyright © 2011 Cengage Learning

10 B-2 Precipitation Titrations The most important application of the Volhard method is for the indirect determination of halide ions. A measured excess of standard silver nitrate solution is added to the sample, and the excess silver ion is determined by back-titration with a standard thiocyanate solution. 10 -31 Copyright © 2011 Cengage Learning

10 B-2 Precipitation Titrations A distinct advantage over other titrimetric methods of halide analysis because such ions as carbonate, oxalate, and arsenate (which form slightly soluble silver salts in neutral media but not in acidic media) do not interfere. Silver chloride is more soluble than silver thiocyanate. 10 -32 Copyright © 2011 Cengage Learning

10 B-2 Precipitation Titrations This reaction causes the end point to fade. This error can be circumvented by filtering the silver chloride before undertaking the back-titration. 10 -33 Copyright © 2011 Cengage Learning

10 C-1 Reagents for Precipitating Metals One important type of reaction involving an organic complexing agent is that in which an insoluble, uncharged complex is formed. (1016) Precipitation occurs when the solubility of the species MXn has been exceeded. (1017) 10 -34 Copyright © 2011 Cengage Learning

10 C-1 Reagents for Precipitating Metals and the solubility product expression as (1018) 10 -35 Copyright © 2011 Cengage Learning

10 C-2 Forming Soluble Complexes for Extractions and Other Uses Many organic reagents are useful in converting metal ions into forms that can be readily extracted from water into an immiscible organic phase. Organic complexing agents for extractions are listed in Table 10 -4. 10 -36 Copyright © 2011 Cengage Learning

10 D-1 Versatile Ethylenediaminetetraacetic Acid(EDTA) EDTA, is the most widely used complexometric titrant. EDTA is a hexadentate ligand. The various EDTA species are often abbreviated H₄Y, H₃Y¯, H₂Y²¯, HY³¯, and Y⁴¯. 10 -37 Copyright © 2011 Cengage Learning

Figure 10 -5 Structure of H 4 Y and its dissociation products. Note that the fully protonated species H 4 Y exist as the double zwitterion with the amine nitrogens and two of the carboxylic acid groups protonated. The first two protons dissociate from the carboxyl groups, whereas the last two come from the amine groups 10 -38 Copyright © 2011 Cengage Learning

10 D-2 The Nature of EDTA Complexes with Metal Ions Solutions of EDTA are particularly valuable as titrants because the reagent combines with metal ions in a 1 : 1 ratio regardless of the charge on the cation. One of the common structures for metal/EDTA complexes is shown in Figure 10 -6. 10 -39 Copyright © 2011 Cengage Learning

Figure 10 -6 Structure of a metal /EDTA complex. Note that EDTA behaves here as a hexadentate ligand in that six donor atoms are involved in bonding the divalent metal cation. 10 -40 Copyright © 2011 Cengage Learning

10 D-2 The Nature of EDTA Complexes with Metal Ions Formation constants KMY (1020) 10 -41 Copyright © 2011 Cengage Learning

10 D-3 Equilibrium Calculations Involving EDTA where c (1021) is the total molar concentration of uncomplexed EDTA: 10 -42 T Copyright © 2011 Cengage Learning

10 D-3 Equilibrium Calculations Involving EDTA Conditional Formation Constants Conditional formation constant K’MY: (1022) (1023) 10 -43 Copyright © 2011 Cengage Learning

10 D-3 Equilibrium Calculations Involving EDTA Computing α 4 Values for EDTA Solutions (1024) (1025) where K₁ , K₂ , K₃ , and K₄ are the four dissociation constants for H₄Y and D is the denominator of Equation 10 -24. 10 -44 Copyright © 2011 Cengage Learning

Example 10 -4 Calculate the molar Y⁴¯ concentration in a 0. 0200 M EDTA solution buffered to a p. H of 10. 00. At p. H 10. 00, α 4 is 0. 35 (Figure 10 -7). Thus, 10 -45 Copyright © 2011 Cengage Learning

Example 10 -5 Calculate the equilibrium concentration of Ni²⁺ in a solution with an analytical Ni. Y²¯ concentration of 0. 0150 M at p. H (a) 3. 0 and (b) 8. 0. 10 -46 Copyright © 2011 Cengage Learning

Example 10 -5 If we assume that [Ni²⁺] << 0. 0150, an assumption that is almost certainly valid in light of the large formation constant of the complex, Since the complex is the only source of both Ni²⁺ and the EDTA species, 10 -47 Copyright © 2011 Cengage Learning

Example 10 -5 Substitution of this equality into Equation 10 -23 gives α 4 is 2. 5 × 1011 at p. H 3. 0. 10 -48 Copyright © 2011 Cengage Learning

Example 10 -5 (b) At p. H 8. 0, 10 -49 Copyright © 2011 Cengage Learning

Example 10 -6 Calculate the concentration of Ni²⁺ in a solution that was prepared by mixing 50. 0 m. L of 0. 0300 M Ni²⁺ with 50. 0 m. L of 0. 0500 M EDTA. The mixture was buffered to a p. H of 3. 0. The solution has an excess of EDTA, 10 -50 Copyright © 2011 Cengage Learning

Example 10 -6 Assume that [Ni²⁺]<<[Ni. Y²¯] so that 10 -51 Copyright © 2011 Cengage Learning

10 D-4 EDTA Titration Curves Calculating the Conditional Constant 10 -52 Copyright © 2011 Cengage Learning

10 D-4 EDTA Titration Curves Preequivalence-Point Values for p. Ca The equilibrium concentration of Ca²⁺ is equal to the untitrated excess of the cation plus any dissociation of the complex, the latter being equal numerically to c. T. c is small relative to the analytical concentration of the uncomplexed calcium ion. 10 -53 T Copyright © 2011 Cengage Learning

10 D-4 EDTA Titration Curves After 5. 00 m. L of EDTA has been added, 10 -54 Copyright © 2011 Cengage Learning

10 D-4 EDTA Titration Curves The Equivalence-Point p. Ca The only source of Ca²⁺ ions is the dissociation of the complex. 10 -55 Copyright © 2011 Cengage Learning

10 D-4 EDTA Titration Curves To obtain [Ca 2+], we substitute into the expression for K'Ca. Y , 10 -56 Copyright © 2011 Cengage Learning

10 D-4 EDTA Titration Curves Postequivalence-Point p. Ca After the addition of 26. 0 m. L of EDTA, 10 -57 Copyright © 2011 Cengage Learning

10 D-4 EDTA Titration Curves Postequivalence-Point p. Ca 10 -58 Copyright © 2011 Cengage Learning

Figure 10 -9 Figure 10 -10 Influence of p. H on the titration of 0. 0100 M Ca 2 with 0. 0100 M EDTA. Note that the end point becomes less sharp as the p. H decreases because the complex-formation reaction is less complete under these circumstances. 10 -59 Copyright © 2011 Cengage Learning

Figure 10 -10 Influence of p. H on the titration of 0. 0100 M Ca 2 with 0. 0100 M EDTA. Note that the end point becomes less sharp as the p. H decreases because the complexformation reaction is less complete under these circumstances. 10 -60 Copyright © 2011 Cengage Learning

10 D-5 How Do Other Complexing Agents Affect EDTA Titrations? Many cations form precipitates with hydroxide when the p. H is raised to the level required for their successful titration with EDTA. When this problem is encountered, an auxiliary complexing agent is needed to keep the cation in solution. For example, zinc(II) is ordinarily titrated in a medium that has fairly high concentrations of ammonia and ammonium chloride. Ammonia forms ammine complexes with zinc(II) and prevents formation of the sparingly soluble zinc hydroxide, 10 -61 Copyright © 2011 Cengage Learning

Figure 10 -11 Titration curves for 50. 0 m. L of 0. 0100 M solutions of various cations at p. H 6. 0. 10 -62 Copyright © 2011 Cengage Learning

Figure 10 -12 Minimum p. H needed for satisfactory titration of various cations with EDTA. (From C. N. Reilley and R. W. Schmid, Anal. Chem. , 1958, 30, 947. With permission of the American Chemical Society. ) 10 -63 Copyright © 2011 Cengage Learning

Figure 10 -13 Influence of ammonia concentration on the end point for the titration of 50. 0 m. L of 0. 00500 M Zn 2. Solutions are buffered to p. H 9. 00. The shaded region shows the transition range for Eriochrome Black T. Note that ammonia decreases the change in p. Zn in the equivalence-point region. 10 -64 Copyright © 2011 Cengage Learning

10 D-6 Indicators for EDTA Titrations Eriochrome Black T is a typical metal-ion indicator The metal complexes of Eriochrome Black T are generally red. It is necessary to adjust the p. H to 7 or above so that the blue form of the species, HIn²¯, predominates 10 -65 Copyright © 2011 Cengage Learning

10 D-6 Indicators for EDTA Titrations With the first slight excess of EDTA, the solution turns blue as a consequence of the reaction 10 -66 Copyright © 2011 Cengage Learning

10 D-7 Titration Methods Employing EDTA Direct Titration Many of the metals in the periodic table can be determined by titration with standard EDTA solutions. Some methods are based on indicators that respond to the analyte itself, whereas others are based on an added metal ion. 10 -67 Copyright © 2011 Cengage Learning

10 D-7 Titration Methods Employing EDTA Direct Titration In cases 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. Potentiometric Methods. 10 -68 Copyright © 2011 Cengage Learning

10 D-7 Titration Methods Employing EDTA Direct Titration Spectrophotometric Methods. Back-Titration Methods 10 -69 A measured excess of standard EDTA solution is added to the analyte solution. The excess EDTA is back-titrated with a standardmagnesium or zinc ion solution to an Eriochrome Black T or Calmagite end point. Copyright © 2011 Cengage Learning

10 D-7 Titration Methods Employing EDTA Direct Titration It is necessary that the magnesium or zinc ions form an EDTA complex that is less stable than the corresponding analyte complex. 10 -70 Copyright © 2011 Cengage Learning

10 D-7 Titration Methods Employing EDTA Displacement Methods where M²⁺ represents the analyte cation. The liberated Mg²⁺ or, in some cases Zn²⁺, is then titrated with a standard EDTA solution. 10 -71 Copyright © 2011 Cengage Learning

10 D-9 Determining Water Hardness Water hardness is ordinarily determined by an EDTA titration after the sample has been buffered to p. H 10. Magnesium, which forms the least stable EDTA complex of all the common multivalent cations in typical water samples, does not form a stable EDTA complex until enough EDTA has been added to complex all the other cations in the sample. 10 -72 Copyright © 2011 Cengage Learning

10 D-9 Determining Water Hardness A magnesium ion indicator can serve as indicator in water-hardness titrations. 10 -73 Copyright © 2011 Cengage Learning

THE END 10 -74 Copyright © 2011 Cengage Learning