Lecture 9 Complexometric Titrations Chapter 13: Problems 13:1,8,9,13 Corresponding Lab: Ca+ and Mg2= in Seawater Anal Chem 251 Lect 9 Complexometric Titrations 1 Complexometric Titrations The formation of well-defined (that means of known stoichiometry) complex ion by the reaction of <a href="/keyword/metal-ions/" >metal ions</a> (Lewis acids) with electron pair donor ligands (Lewis bases) with large equilibrium constants, constitutes the basis for complexometric titrations. Anal Chem 251 Lect 9 Complexometric Titrations 2 General Considerations H+ + OHAcid Base H2O K = 1x 1014=1/kw Cu2+ 4:NH3 Cu(NH3)4 2+ K =8.1x1012 Lewis Lewis acid base Ag+ + 2 :CN:Lewis Lewis acid base Anal Chem 251 Ag(CN)2- K =1.0 x1021 Lect 9 Complexometric Titrations 3 Review - First Definition of Acid/Base Classical Definition (Arrhenius) of Acids and Bases In the classical definition, acids and bases are defined according to their formulas and their behavior in water: An acid is a substance that has H in its formula and dissociates in water to yield H3O+ A base is a substance that has OH in its formula and dissociates in water to yield OH When an acid and a base react, they undergo neutralization Anal Chem 251 Lect 9 Complexometric Titrations 4 Review Second Definition of Acid/Base - Br nsted-Lowry Acid-Base Definition An acid is a proton donor, any species which donates a H+. A base is a proton acceptor, any species which accepts a H+. An acid must contain H in its formula. HNO3, H2PO4- A base must contain a lone pair of electrons to bind the H+ ion. NH3 CO32- An acid-base reaction can now be viewed from the standpoint of the reactants AND the products. An acid reactant will produce a base product and the two will constitute an acid-base conjugate pair. Anal Chem 251 Lect 9 Complexometric Titrations 5 Review Third Definition of Acids and Bases, Lewis Acids and Bases An acid is an electron-pair acceptor. The third definition of acid/base is used in complexometric titrations F B F F acid + N HH base F A base is an electron-pair donor. H F H B F N HH adduct M(H2O)42+(aq) M2+ H2O(l) Anal Chem 251 Lect 9 Complexometric Titrations adduct 6 Review - Coordination Compounds <a href="/keyword/metal-ions/" >metal ions</a> (Lewis acids) form complexes or coordination compounds with neutral or charged electron pair donors (Lewis bases are also called ligands). The ligands of complex ions are molecules or anions with one or more donor atoms that each donate a lone pair of electrons to the metal ion to form a covalent bond. Anal Chem 251 Lect 9 Complexometric Titrations 7 Review Structures of Complex Ions: Coordination Numbers, Geometries, and Ligands Coordination Number - the number of ligand atoms that are bonded directly to the central metal ion. The coordination number is specific for a given metal ion in a particular oxidation state and compound. Geometry - the geometry (shape) of a complex ion depends on the coordination number and nature of the metal ion. Ligand - molecules and/or anions with one or more donor atoms (teeth) that each donate a lone pair of electrons to the metal ion to form a covalent bond. Anal Chem 251 Lect 9 Complexometric Titrations 8 Review Figure 23.9 Components of a coordination compound. models Anal Chem 251 wedge diagrams Lect 9 Complexometric Titrations chemical formulas 9 Review Anal Chem 251 Lect 9 Complexometric Titrations 10 Ligands Donor Atoms per Ligand (Teeth) Ligands are classified in terms of the number of donor atoms, or &quot;teeth&quot;, that each uses to bond to the central metal ion. Dentate (Latin) means having toothlike projections. Monodentate ligand (Latin &quot;one-toothed&quot; ligand) such as NH3 use a single donor atom (ammonia) Bidentate ligands have two donor atoms, each of which bonds to the metal ion. (ethylene diamine) Polydentate ligands have more two than donor atoms. (EDTA) Anal Chem 251 Lect 9 Complexometric Titrations 11 Ligands - Donor Atoms per Ligand (Teeth) Anal Chem 251 Lect 9 Complexometric Titrations 12 Used in our Experiment Ethylenediaminetetraacetic Acid (EDTA) EDTA, a hexadentate (polydentate) ligand, is among the most important and widely used reagents in titrimetry. Anal Chem 251 Lect 9 Complexometric Titrations 13 p.458 EDTA - Chelating Agent Because EDTA4- has 6 donor atoms, it forms very stable complexes with many metal ion. The property makes EDTA useful in treating heavy-metal poisoning . Once ingested by the patient, the ion acts as a scavenger to remove lead and other heavy-<a href="/keyword/metal-ions/" >metal ions</a> from the blood and other body fluids. Such ligands seem to grab the metal ion like claws. A complex ion that contains them is also called a chelate (Greek chela &quot;crab's claw&quot;). The ligand is referred to as a chelating agent. Anal Chem 251 Lect 9 Complexometric Titrations 14 Species Present in a Solution of EDTA Figure Structure of H4Y and its dissociation products. Note the fully protonated species H4Y exists as the double zwitterion with the amine nitrogens and two of the carboxylic acid groups protonated. Correct the C-N bonds The first two protons dissociate from the carboxyl groups , while the last two come from the amine groups. Anal Chem 251 Lect 9 Complexometric Titrations 15 Fig 17-f1, p.460 EDTA Figure -Structure of a metal/EDTA complex. Note EDTA behaves here as a hexadentate ligand in that six donor groups are involved in bonding the divalent metal cation Anal Chem 251 Lect 9 Complexometric Titrations 16 Fig 17-3, p.461 EDTA-Ni Complex Figure -Molecular model of NiY2-. This complex is typical of the strong complexes that EDTA forms with <a href="/keyword/metal-ions/" >metal ions</a> . The formation constant of the Ni 2+ complex is 4.2x1018. Anal Chem 251 Lect 9 Complexometric Titrations 17 p.465 Determination of Water Hardness with EDTA Water hardness is a measure of the total Ca 2+ and Mg 2+ in water. It is an important parameter of water that is used for municipal or industrial purposes. Water hardness is conveniently determined by titration of total Ca 2+ and Mg 2+ with EDTA. EDTA is used to titrate many ions, it is used to titrate Mg 2+ &amp; Ca 2+ in water to determine the &quot;hardness&quot; of water. MgY2- K = 1.8x108 Mg 2+ + Y4Ca 2+ + Y4CaY2- K = 1.8x1010 The pH of the solution is adjusted to 10 to prevent excessive competition of H+ for Y4-. At pH 10, the predominant species of EDTA is: Ca 2+ + HY3CaY2- + H+ At lower pH's, the above reaction shifts to the left Higher pH values would cause precipitation of CaCO3 (s) or Mg(OH)2 (s). Sample test kits are available for this determination Lect 9 Complexometric Titrations 18 Anal Chem 251 Test kit for the Determination of Water Hardness Test kits for determining the hardness of household water are available at stores selling water softeners and plumbing supplies. Figure Typical kit for testing for water hardness in houehold water Anal Chem 251 Lect 9 Complexometric Titrations 19 Fig 17-f2, p.482 Complexation Indicators Complexation indicators function in much the same way as acid-base indicators. These indicators are weak complexing agents that exhibit different colors in their complexed and uncomplexed form. Choice of an Indicator 1. Choose one that changes color at the equivalence point 2. The indicator should react last after the titration When such an indicator is added to the solution to be titrated, it forms a colored complex with the analyte: M + In MIn (color A) Where M and In refer to metal ion and indicator, respectively. The charges have been omitted for the sake of clarity. Anal Chem 251 Lect 9 Complexometric Titrations 20 Complexation Indicators The titration flask now contains M and MIn. When titrant is added, it reacts with the metal until essentially none is left, at which point it takes the metal away from Min (charges omitted): MIn + Y MY + In (color A) (colorless) (color B) This constitutes the end-point reaction and is responsible for the color change that signals the analyst to stop the titration. Clearly if the above reaction is to occur, the metai-titrant complex must be more stable than metal-indicator complex Anal Chem 251 Lect 9 Complexometric Titrations 21 Complexation Indicators Most indicators are affected by pH in the same manner as EDTA. They form different protonated species that may exhibit different reactivities toward <a href="/keyword/metal-ions/" >metal ions</a> . In addition, these species often have different colors. Anal Chem 251 Lect 9 Complexometric Titrations 22 Eriochrome Black T Indicator Eriochrome Black T is one of the oldest and most widely used complexometric indicators. It is used exclusively in the pH range of 7 to 11, where the blue form of the indicator predominates in the absence of <a href="/keyword/metal-ions/" >metal ions</a> . EBT forms red complexes with about 30 metals such as Mg 2+ , and Ca 2+ . The indicator Eriochrome Black T is a triprotic acid that may be represented as H3In. Anal Chem 251 Lect 9 Complexometric Titrations 23 Structure and Molecular Model of Eriochrome Black T . The compound contains a sulfonic acid group that completely dissociates in water and two phenolic groups that only partially dissociate Anal Chem 251 Lect 9 Complexometric Titrations 24 Fig 17-11, p.476 Eriochrome Black T Indicator In aqueous solutions of EBT, one proton is always completely dissociated. The acid-base properties of the indicator are summarized by the following ionization reactions: H2In(RED) H3O+ + HIn2(BLUE) Ka2 = 5 x 10-7 (pKa2 = 6.3) + H2O HIn2- + H2O (BLUE) H3O+ + In3(Orange) Ka3 = 3 x 10-12 (pKa3 = 11.5) Anal Chem 251 Lect 9 Complexometric Titrations 25 Eriochrome Black T Indicator Metal complexes of EBT are generally red. Thus if a color change is to be observed with this indicator, the pH of the solution must be between about 7 and 11, so that the blue form of the indicator dominates when the titrant breaks up the the red metal-EBT complex at the endpoint, At a pH of 10, the endpoint reaction is: MIn- + Y4- + H+ (RED) MY2- + HIn2(BLUE) Anal Chem 251 Lect 9 Complexometric Titrations 26 Chemistry of EDTA Titration EDTA is usually standardized against a solution of calcium ions. When EBT is used as the indicator, a minor difficulty is encountered. EBT is actually not a suitable indicator for the titration of calcium because it forms too weak a complex, that means it wants to dissociate. However, in the early stages of the titration, the calcium-indicator complex will not dissociate appreciably, due to the presence of a large excess of untitrated calcium ions. That is the position of the equilibrium lies to the right for the reaction as per LeChatelier's Principle. Ca 2+ + HIn2CaIn- + H+ (blue) (red) and the solution remains red. As the titration proceeds and more Ca 2+ is complexed with the titrant, the position of the equilibrium shifts to the left, causing a gradual color change to occur. Anal Chem 251 Lect 9 Complexometric Titrations 27 Chemistry of EDTA Titration Chemists have devised a solution to this problem based on three facts: 1. At pH 10, Ca-EDTA complex is more stable than Mg-EDTA complex since the formation constant of CaY2- is 1.8 x 1010 the formation constant of MgY2- is 1.8 x 108 At pH 10, MgIn- complex is more stable than CaIn- complex. the formation constant of MgIn- is 1.3 x 107 the formation constant of CaIn- is 1.3 x 105 MgIn- is sufficiently stable that it will not dissociate appreciably prior to the equivalence point 2. 3. SOLUTION: A small amount of 1:00 to 1:00 EDTA:Mg is added (this does not affect the stoichiometry of the titration reaction because the quantities of EDTA and Mg are equimolar 28 Chemistry of EDTA Titration When 1:00 to 1:00 EDTA:Mg is added to the Ca 2+ analyte containing indicator, the following reactions take place: MgY2- + Ca 2+ Mg 2+ + CaInMg 2+ + CaY2- (more stable) Ca 2+ + MgIn- (more stable) RED Anal Chem 251 Lect 9 Complexometric Titrations 29 Chemistry of EDTA Titration When EDTA titrant is added, it first binds to all of the Ca 2+ (KCaY2- = 1.8 x 1010) CaY2- + H+ Ca 2+ + HY3Until at the endpoint, EDTA replaces the less strongly bound Eriochrome Black T from Mg 2+ (KMgIn- = 1.3 x 107) HIn2- + MgY2MgIn- + HY3RED BLUE Anal Chem 251 Lect 9 Complexometric Titrations 30 Figure -EDTA Titration Curves for 50.0 mL of 0.00500 M Ca 2+ and Mg 2+ at pH 10.0. This is similar to the titration of H+ with OH-. (pH vs.vol of base added) Note because of the larger formation constant, the reaction of calcium ion with EDTA is more complete, and a larger change occurs in the equivalence-point region. The shaded areas show the transition range for the indicator Eriochrome Black T. 31 Fig 17-6, p.470 Figure - Influence of pH on the titration of 0.0100 M Ca 2+ with 0.0100 M EDTA . Note the endpoint becomes less sharp as the pH decreases because the complex formation reaction is less complete under these circumstances. Anal Chem 251 Lect 9 Complexometric Titrations 32 Fig 17-7, p.470 Lab Experiment Ca 2+ and Mg 2+ in Seawater Standardization of EDTA against a primary standard containing a known [Ca 2+ ]. The primary standard is CaCO3 (source of Ca 2+ ). Weigh accurately 0.2 g of CaCO3, transfer it to 500 mL volumetric flask and dissolve in concentrated HCl CaCO3 (s) + 2HCl Ca 2+ (aq) + 2Cl- + CO2 (g) And dilute to the mark Anal Chem 251 Lect 9 Complexometric Titrations 33 Lab Experiment Ca 2+ and Mg 2+ in Seawater [Ca 2+ ] = # mole CaCO3 dissolved/0.500 L Titrate 25 mL of this known [Ca 2+ ] against EDTA From stoichiometry: CaY2Ca 2+ + Y4MEDTA x VEDTA = M Ca 2+ x V Ca 2+ (V = 25 mL) Solve for MEDTA Anal Chem 251 Lect 9 Complexometric Titrations 34 Lab Experiment Ca 2+ and Mg 2+ in Seawater At pH 10, the predominant species of EDTA is HY3 HY3- + Ca 2+ CaY 2- + H+ To maintain the pH at 10, a buffer of NH3/NH4Cl is used At lower pH's, the above reaction shifts to the left At higher pH's (high OH- concentration), Mg(OH)2 precipitates in hard water Anal Chem 251 Lect 9 Complexometric Titrations 35 Lecture 9 Perspective What do these prictures all have in common with your experiment? Forensic Chemistry Food Chemistry Medicinal Chemistry 36