Chapter 12 EDTA Titrations

Overview 12-1 Metal-Chelate Complexes 12-2 EDTA 12-3 EDTA Titration Curves 12-4 Do It with a Spreadsheet 12-5 Auxiliary Complexing Agents 12-6 Metal Ion Indicators 12-7 EDTA Titration Techniques

12-2: EDTA Ethylenediaminetetraacetic acid (EDTA) is a compound that forms strong 1:1 complexes with most metal ions. Used in industrial processes and products such as detergents, cleaning agents, and food additives that prevent metal-catalyzed oxidation of food. Metal-EDTA complexes find their way into the environment because they can pass through wastewater treatment plants unscathed.

12-1: Metal-Chelate Complexes Metal ions are Lewis acids that accept electron pairs from electron-donating ligands (Lewis bases). Monodentate ligands bind to a metal ion through only one atom (e.g., CN- through the C atom). A ligand that attaches to a metal ion through more than one ligand atom is said to be multidentate, or a chelating agent.

12-1: Metal-Chelate Complexes Ethylenediamine is a simple chelating agent, which is bidentate. Chelate effect – the ability of multidentate ligands to form more stable metal complexes than those formed by similar monodentate ligands.

12-1: Metal-Chelate Complexes The reaction of with two molecules of ethylenediamine is more favorable than its reaction with four molecules of methylamine, a monodentate ligand:

12-1: Metal-Chelate Complexes Adenosine triphosphate (ATP) is an important tetradentate ligand, which binds divalent metal ions (Mg2+, Mn2+, Co2+, and Ni2+) through four of their six coordination positions. The fifth and sixth positions are occupied by water molecules.

12-1: Analytically Useful Chelating Agents

12-2: EDTA A titration based on complex formation is called a complexometric titration. EDTA is the most widely used chelator in analytical chemistry. By direct titration or through an indirect series of reactions, virtually every element of the periodic table can be measured with EDTA.

12-2: EDTA: Acid-Base Properties EDTA is a hexaprotic system, designated H6Y2+. The first four pK values apply to carboxyl protons; the last two are for the ammonium protons. The neutral acid is tetraprotic, with the formula H4Y, which can be dried at 140oC for 2 h and used as a primary standard.

12-2: EDTA: Acid-Base Properties The fraction of EDTA in each of its protonated forms is plotted in the figure:

12-2: EDTA: Acid-Base Properties We can define a for each species as the fraction of EDTA in that form. For example, aY4- is defined as: where [EDTA] is the total concentration of all free (not complexed to metals) EDTA species in the solution. Therefore, aY4- is given by:

12-1: Example: What Does aY4- Mean?

12-2: EDTA Complexes The equilibrium constant for the reaction of a metal with a ligand is called the formation constant, Kf, or the stability constant: Kf for EDTA is defined in terms of the species Y4- reacting with the metal ion.

12-2: EDTA Complexes The table shows that formation constants for most EDTA complexes are large and tend to be larger or more positively charged cations.

12-2: Conditional Formation Constant Most EDTA is not Y4- below pH 10.37. The species HY3- and H2Y2-, and so on, predominate at lower pH. If the pH is fixed by a buffer, then aY4- is a constant that can be combined with Kf: This is called the conditional formation constant and it describes the formation of MYn-4 at any particular pH.

12-2: Conditional Formation Constant

12-3: EDTA Titration Curves We can calculate the concentration of free Mn+ during its titration with EDTA: If is large, we can consider the reaction to be complete at each point in the titration. The titration curve is a plot of pM versus the volume of EDTA added and has three natural regions.

12-3: EDTA Titration Curves Region 1: Before the Equivalence Point The concentration of free metal is equal to the concentration of excess, unreacted Mn+. Region 2: At the Equivalence Point [Mn+] = [EDTA] Region 3: After the Equivalence Point The concentration of free EDTA can be equated to the concentration of excess EDTA added after the equivalence point.

12-3: Titration Calculations Consider the reaction of 50.0 mL of 0.040 0 M Ca2+ (buffered to pH 10.00) with 0.080 0 M EDTA. Region 1: Before the Equivalence Point After the addition of 5.0 mL of EDTA

12-3 Titration Calculations Region 2: At the Equivalence Point Virtually all the metal is in the form CaY2-. [CaY2-] is equal to the original [Ca2+] with a correction for dilution. The concentration of free Ca2+ is small and unknown.

12-3 Titration Calculations

12-3 Titration Calculations Region 3: After the Equivalence Point Virtually all of the metal is in the form CaY2-, and there is excess, unreacted EDTA. After the addition of 26.0 mL of EDTA, there is 1.0 mL of excess EDTA.

12-3 Titration Calculations The concentration of Ca2+ is governed by:

12-4: Do It with a Spreadsheet Consider the titration of metal ion M (initial concentration = CM, volume VM) with a solution of ligand L (concentration = CL, volume added = VL) to form a 1:1 complex: The mass balances for metal and ligand are:

12-4: Do It with a Spreadsheet Substituting Kf[M][L] from Equation 12-8 for [ML] in the mass balance: Now substitute the expression for [L] from Equation 12-10 back into Equation 12-9, then solve for the fraction of titration, .

12-4: Do It with a Spreadsheet The equation for titrating L with M:

12-5 Auxiliary Complexing Agents For many metals to be titrated in alkaline solutions with EDTA, we use an auxiliary complexing agent to prevent metal hydroxide precipitation. Reagents are ligands such as ammonia, tartarate, citrate, or triethanolamine They must bind strongly enough to prevent metal hydroxide from precipitating, but weakly enough to give up the metal when EDTA is added.

12-5: Metal-Ligand Equilibria Consider a metal ion that forms two complexes with the auxiliary complexing ligand L: b are the cumulative formation constants. The fraction of metal ion in the uncomplexed state, M, is: Where Mtot is the total concentration of all forms of M (M, ML, and ML2).

12-5: Metal-Ligand Equilibria The mass balance is: Mtot = [M] + [ML] + [ML2] Equations 12-13 and 12-14 allow us to say [ML] = b1[M][L] and [ML2] = b2[M][L]2 Therefore, Mtot = [M] + b1[M][L] + b2[M][L]2 = [M]{1 + b1[L] + b2[L]2} Substituting into Equation 12-15:

12-5: Metal-Ligand Equilibria

12-5: EDTA Titration in the Presence of Ammonia Consider the titration of Zn2+ by EDTA in the presence of NH3. We now need a new conditional formation constant to account for the fact that only some of the EDTA is in the form Y4- and only some of the zinc not bound to EDTA is in the form Zn2+:

12-5: EDTA Titration in the Presence of Ammonia

12-5: EDTA Titration in the Presence of Ammonia

12-5: EDTA Titration in the Presence of Ammonia

12-6: Metal Ion Indicators End-point detection methods: Metal ion indicators (most common) Mercury electrode Ion-selective electrode Glass (pH) electrode Metal ion indicators are compounds that change color when they bind to a metal ion – must bind metal less strongly than does EDTA. Example: the reaction of Mg2+ with EDTA at pH 10 with Calmagite indicator.

12-6: Metal Ion Indicators

12-6: Metal Ion Indicators

12-7: EDTA Titration Techniques Direct titration: Analyte is titrated with standard EDTA. Analyte is buffered to a pH where for the metal-EDTA complex is large and the color of the free indicator is different than that of the metal-indicator complex. Back titration: A known excess of EDTA is added to the analyte. Excess EDTA is then titrated with a standard solution of a second metal ion.

12-7: EDTA Titration Techniques

12-7: EDTA Titration Techniques 3. Displacement Titration: Use when the analyte, such as Hg2+, does not have a satisfactory indicator. Hg2+ is treated with excess Mg(EDTA)2- to displace Mg2+, which is titrated with standard EDTA. There is also no suitable indicator for Ag+, however Ag+ will displace Ni2+ from tetracyanonickelate(II) ion: 2Ag+ + Ni(CN)42- → 2Ag(CN)2- + Ni2+ The Ni2+ liberated can be titrated with EDTA to determine the amount of Ag+ added.

12-7: EDTA Titration Techniques 4. Indirect Titration: Anions that precipitate with certain metal ions can be analyzed with EDTA using an indirect titration. Example: Sulfate can be analyzed by precipitation with excess Ba2+ at pH 1. BaSO4 (s) is washed then boiled with excess standard EDTA at pH 10 to bring Ba2+ back into solution as Ba(EDTA)2-. Excess EDTA is back-titrated with Mg2+. 5. Masking: A masking agent is a reagent that protects a component of the analyte from reaction with EDTA. Example: Al3+ in a mixture of Mg2+ and Al3+ can be measured by masking the Ag+ with F-, leaving only the Mg2+ to react with EDTA.