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CHEM 287 (1)

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CHEM 287
Janine Mauzeroll

Chapter 13: EDTA Titrations EDTA: can be used to titrate most metal ions by forming strong 1:1 complexes 13-1: Metal-Chelate Complexes Ligand: atom or groups of atoms bound to whatever atom youre interested in Lewis acids: electron pair acceptors Lewis bases: electron pair donors Monodentate ligand: binds to a metal ion through only one atom Multidentate ligand: (aka chelating ligand chelate) binds to a metal ion through more than one ligand atom EDTA is hexadentate (bind to metal through 2N atoms + 4O atoms) ATP - tetradentate Chelates excreted by microbes to gather iron called siderophores Complexometric titration: titration based on complex formation 13-2: EDTA EDTA most widely used chelator in analytical chemistry 2+ H 6 Below pH 10.24, most EDTA is protonated and isnt in the form Y that binds to metal ions Neutral EDTA tetraprotic, H Y4 Formation constant: K orfstability constant equilibrium constant for reaction of metal with a ligand. Kf M + Y MY n-4 - Metal-EDTA complex becomes unstable at low pH b/c H+ competes w/ metal ion for EDTA - Too high pH, EDTA complex is unstable b/c OH- competes w/ EDTA for the metal ion and may precipitate the metal hydroxide - Auxiliary complexing agent: forms weak complex w/ metal ion and keeps it in solution displaced by EDTA during the titration - Ex: ammonia, tartrate, citrate, triethanolamine prevent metal ions from precipitating in absence of EDTA - Metal-aux. complexing agent must be less stable than metal-EDTA complex 13-3: Metal Ion Indicators - Metal ion indicator: compound whose colour changes when it binds to a metal ion - For indicator to be useful, must bind metal less strongly than EDTA does - Most metal ion indicators also acid-base indicators - Used only in certain pH ranges (colour of free indicator is pH dependent) - Block: metal doesnt freely dissociate from indicator metal blocks the indicator 13-4: EDTA titration techniques - Direct titration: analyte titrated w/ standard EDTA - Analyte buffered to appropriate pH, at which rxn with EDTA is essentially complete and free indicator has color distinctly different from that of the metal-indicator complex - Aux. complexing agent might be needed to keep metal ion in soln in the absence of EDTA - Back titration: known excess of EDTA added to analyte excess EDTA titrated w/ standard soln of metal ion (back titration necessary if analyte precipitates in absence of EDTA, ir analyte reacts too slowly w/ EDTA, or if analyte blocks the indicator) - Metal used in back titration must not displace analyte from EDTA - Displacement titration: analyte treated with excess Mg(EDTA) to displace Mg 2+ - b/c no suitable indicator 2+ - Mg later titrated with standard EDTA M + MhY MY 2- n-4+ Mg 2+ - Formation constant of (metal)(EDTA) must be greater than K of Mg(EDTA) (orfelse displacement of Mg wont happen) - Indirect Titration: anions that precipitate metal ions can be analyzed with EDTA by indirect titration 2+ - E.g. sulfate can be analyzed by precipitation w/ excess Ba at pH 1. - BaSO (s) filtered, washed, boiled w/ excess EDTA at pH 10 to bring Ba back into solution as Ba(EDTA) 2- 4 - Excess EDTA back-titrated w/ Mg 2+ - Alternatively, anion can be precipitated w/ excess metal ion precipitate filtered, washed, excess metal ion in filtrate is titrated w/ EDTA - Masking agent: reagent that protects some component of the analyte from rxn w/ EDTA 2+ 2+ 3+ 3+ 3- - E.g. Mg in soln containing Mg and Al can be titrated by masking Al w/ F- to produce AlF , which doesnt react w6 EDTA (only 2+ Mg reacts w/ EDTA) - Masking prevents one element from interfering in the analysis of another element 13-5: The pH-Dependent Metal-EDTA Equilibrium 4- - Fraction of free EDTA in form of Y [Y ] = [EDTA] Y4- - [EDTA] refers to total concentration of all EDTA species not bound to metal ion (pg. 300 equations) - Kf= Y4-K falled the conditional formation constant effective formation constant n-4 - Describes formtion of MY at any particular pH - W/ conditional formation constant, can treat EDTA complex formation as if all free EDTA were in one form 13-6: EDTA Titration Curves - If Kf is large, can consider rxn to be complete at each point in the titration - Titration curve is graph of pM (= log[M]) versus volume of added EDTA Region 1: Before the Equivalence Point n+ - In this region, excess M after EDTA has been consumed n+ - Concentration of free metal ions equal to concentration of excess, unreacted M - Dissociation of MY n-4is negligible Calculations - Calculate pM using fraction of metal ion that will be left (after all present EDTA has reacted) Region 2: At the Equivalence Point - There is as much EDTA as metal in the solution - Can treat soln as if it were made by dissolving pure MY n-4 n+ n-4 - Some free M is generated by the slight dissociation of MY n-4 n+ MY M + EDTA - In this rxn, EDTA refers to total concentration of free EDTA in all of its forms. At equiv. pt., [M ] = [EDTA] n+ Region 3: After the Equivalence Point n-4 - Excess EDTA and virtually all metal ion is in the form MY - Concentration of free EDTA can be equated to concentration of excess EDTA added after the equivalence point The Titration Curve - Completeness of rxn (and sharpness of equivalence point) determined by conditional formation constant, K f - Y4-decrease as pH is lowered, therefore pH is important variable determining whether a titration is feasible - End point is more distinct at high pH - However, pH cant be too high that metal hydroxide precipitatesChapter 14: Electrode Potentials 14-1: Redox Chemistry and Electricity - Redox rxn: electrons transferred from one species to another - Oxidized: molecule loses electrons - Reduced: molecule gains electrons - Oxidizing agent: also called oxidant; takes electrons from another substance and becomes reduced - Reducing agent: also called reductant; gives electrons to another substance and is oxidized Chemistry and Electricity - Electric charge (q) measured in coulombs (C) - Magnitude of charge of a single electron (or proton) = 1.602x10 -19C 4 - Mole of electrons has charge = 9.649x10 C/mol Faradays constant, F - q = n x F - Quantity of electrons flowing from a rxn is proportional to the quantity of analyte that reacts Electric Current Is Proportional to the Rate of a Redox Reaction - Electric current (I) is quantity of charge flowing each second past a point in an electric circuit - Unit of current is ampere (A) flow of 1 C/s - Electrode: device to conduct electrons into or out of the chemicals involved in the redox reaction - Platinum is an inert electrode (does not participate in the reaction except as a conduct of electrons - Electroactive species: molecule that can donate or accept electrons at an electrode Voltage and Electrical Work - Difference in electrical potential between two points measures the work that is needed (or can be done) when electrons move from one point to another - Greater potential difference between two points, more work can be done (or must be done)
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