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EDTA Titrations Introduction 1.)Metal Chelate Complexes  Any reagent which reacts with an analyte in a known ratio and with a large equilibrium constant.

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Presentation on theme: "EDTA Titrations Introduction 1.)Metal Chelate Complexes  Any reagent which reacts with an analyte in a known ratio and with a large equilibrium constant."— Presentation transcript:

1 EDTA Titrations Introduction 1.)Metal Chelate Complexes  Any reagent which reacts with an analyte in a known ratio and with a large equilibrium constant can potentially be used in a titration.  Complexation Titrations are based on the reaction of a metal ion with a chemical agent to form a metal-ligand complex. Metal Ligand Metal-Ligand Complex Metal – Lewis Acid or Electron-pair acceptor Ligand – Lewis Base or Electron-pair donor Note: multiple atoms from EDTA are binding Mn 2+

2 EDTA Titrations Introduction 1.)Metal Chelate Complexes  Complexation Titrations are essentially a Lewis acid-base reaction, in which an electron pair is donated from one chemical to another  The ligands used in complexometric titrations are also known as chelating agents. - Ligand that attaches to a metal ion through more than one ligand atom  Most chelating agents contain N or O - Elements that contain free electron pairs that may be donated to a metal Fe-DTPA Complex

3 EDTA Titrations Metal Chelation in Nature 1.)Potassium Ion Channels in Cell Membranes  Electrical signals are essential for life  Electrical signals are highly controlled by the selective passage of ions across cellular membranes - Ion channels control this function - Potassium ion channels are the largest and most diverse group - Used in brain, heart and nervous system Current Opinion in Structural Biology 2001, 11:408–414 Opening of potassium channel allows K + to exit cell and change the electrical potential across membrane K + channel spans membrane channel contains pore that only allows K + to pass K + is chelated by O in channel

4 EDTA Titrations Metal –Chelate Complexes 1.)Formation Constant (K f )  The equilibrium constant for the reaction between a metal ion (M +n ) and a chelating agent (L -P ) is known as a formation constant or stability constant.  Applying different and specific names to the general equilibrium constant is a common occurrence - Solubility (K sp ), acid-base (K a, K b ), water dissociation (K w ), etc  Chelate effect: ability of multidentate ligands to form stronger metal complexes compared to monodentate ligands. K f = 8x10 9 K f = 4x ethylenediamine molecules binds tighter than 4 methylamine molecules

5 EDTA Titrations Metal –Chelate Complexes 2.)Chelate Effect  Usually chelating agents with more than one electron pair to donate will form stronger complexes with metal ions than chelating agents with only one electron pair. - Typically more than one O or N - Larger K f values  Multidentate ligand: a chelating agent with more than one free electron pair - Stoichiometry is 1:1 regardless of the ion charge  Monodentate ligand: a chelating agent with only one pair of free electrons Multidentate ligand that binds radioactive metal attached to monoclonal antibody (mAb). mAb is a protein that binds to a specific feature on a tumor cell delivering toxic dose of radiation.

6 EDTA Titrations EDTA 1.)EDTA (Ethylenediaminetetraacetic acid)  One of the most common chelating agents used for complexometric titrations in analytical chemistry.  EDTA has 6 nitrogens & oxygens in its structure giving it 6 free electron pairs that it can donate to metal ions. - High K f values - 6 acid-base sites in its structure

7 EDTA Titrations EDTA 2.)Acid-Base Forms  EDTA exists in up to 7 different acid-base forms depending on the solution pH.  The most basic form (Y 4- ) is the one which primarily reacts with metal ions. EDTA-Mn Complex

8 EDTA Titrations EDTA 2.)Acid-Base Forms  Fraction (  ) of the most basic form of EDTA (Y 4- ) is defined by the H + concentration and acid-base equilibrium constants Fraction (  ) of EDTA in the form Y 4- : where [EDTA] is the total concentration of all free EDTA species in solution  Y4- is depended on the pH of the solution

9 EDTA Titrations EDTA 3.)EDTA Complexes  The basic form of EDTA (Y 4- ) reacts with most metal ions to form a 1:1 complex. - Other forms of EDTA will also chelate metal ions  Recall: the concentration of Y 4- and the total concentration of EDTA is solution [EDTA] are related as follows: Note: This reaction only involves Y 4-, but not the other forms of EDTA where  Y4- is dependent on pH

10 EDTA Titrations EDTA 3.)EDTA Complexes  The basic form of EDTA (Y 4- ) reacts with most metal ions to form a 1:1 complex.

11 EDTA Titrations EDTA 3.)EDTA Complexes  Substitute [Y 4- ] into K f equation  If pH is fixed by a buffer, then  Y4- is a constant that can be combined with K f where [EDTA] is the total concentration of EDTA added to the solution not bound to metal ions Conditional or effective formation constant: (at a given pH)

12 EDTA Titrations EDTA 3.)EDTA Complexes  Assumes the uncomplexed EDTA were all in one form at any pH, we can find  Y4- and evaluate K f ’

13 EDTA Titrations EDTA 4.)Example:  What is the concentration of free Fe 3+ in a solution of 0.10 M Fe(EDTA) - at pH 8.00?

14 EDTA Titrations EDTA 5.)pH Limitation  Note that the metal –EDTA complex becomes less stable as pH decreases - K f decreases - [Fe 3+ ] = 5.4x10 -7 at pH 2.0 -> [Fe 3+ ] = 1.4x at pH 8.0  In order to get a “complete” titration (K f ≥10 6 ), EDTA requires a certain minimum pH for the titration of each metal ion End Point becomes less distinct as pH is lowered, limiting the utility of EDTA as a titrant

15 EDTA Titrations EDTA 5.)pH Limitation  By adjusting the pH of an EDTA titration:  one type of metal ion (e.g. Fe 3+ ) can be titrated without interference from others (e.g. Ca 2+ ) Minimum pH for Effective Titration of Metal Ions

16 EDTA Titrations EDTA Titration Curves 1.)Titration Curve  The titration of a metal ion with EDTA is similar to the titration of a strong acid (M + ) with a weak base (EDTA)  The Titration Curve has three distinct regions: - Before the equivalence point (excess M n+ ) - At the equivalence point ([EDTA]=[M n+ ] - After the equivalence point (excess EDTA)

17 EDTA Titrations EDTA Titration Curves 2.)Example  What is the value of [M n+ ] and pM for 50.0 ml of a M Mg 2+ solution buffered at pH and titrated with m EDTA when (a) 5.0 mL, (b) 50.0 mL and (c) 51.0 mL EDTA is added? K f = = 6.2x10 8  Y4- at pH 10.0 = 0.30 mL EDTA at equivalence point: mmol of EDTA mmol of Mg 2+

18 EDTA Titrations EDTA Titration Curves 2.)Example  (a) Before Equivalence Point ( 5.0 mL of EDTA) Before the equivalence point, the [M n+ ] is equal to the concentration of excess unreacted M n+. Dissociation of MY n-4 is negligible. moles of Mg 2+ originally present moles of EDTA added Original volume solution Volume titrant added Dilution effect

19 EDTA Titrations EDTA Titration Curves 2.)Example  (b) At Equivalence Point ( 50.0 mL of EDTA) Virtually all of the metal ion is now in the form MgY 2- Original [M n+ ] Original volume of M n+ solution Original volume solution Volume titrant added Dilution effect Moles Mg + ≡ moles MgY 2-

20 EDTA Titrations EDTA Titration Curves 2.)Example  (b) At Equivalence Point ( 50.0 mL of EDTA) The concentration of free Mg 2+ is then calculated as follows: Initial Concentration (M) Final Concentration (M)xx x Solve for x using the quadratic equation:

21 EDTA Titrations EDTA Titration Curves 2.)Example  (c) After the Equivalence Point ( 51.0 mL of EDTA) Virtually all of the metal ion is now in the form MgY 2- and there is excess, unreacted EDTA. A small amount of free M n+ exists in equilibrium with MgY 4- and EDTA. Original [EDTA] Volume excess titrant Original volume solution Volume titrant added Dilution effect Excess moles EDTA Calculate excess [EDTA]:

22 EDTA Titrations EDTA Titration Curves 2.)Example  (c) After the Equivalence Point ( 51.0 mL of EDTA) Calculate [MgY 2- ]: Original [M n+ ] Original volume of M n+ solution Original volume solution Volume titrant added Dilution effect Moles Mg + ≡ moles MgY 2- Only Difference

23 EDTA Titrations EDTA Titration Curves 2.)Example  (c) After the Equivalence Point ( 51.0 mL of EDTA) [Mg 2+- ] is given by the equilibrium expression using [EDTA] and [MgY 2- ]:

24 EDTA Titrations EDTA Titration Curves 2.)Example  Final titration curve for 50.0 ml of M Mg 2+ with m EDTA at pH Also shown is the titration of 50.0 mL of M Zn 2+ Note: the equivalence point is sharper for Zn 2+ vs. Mg 2+. This is due to Zn 2+ having a larger formation constant. The completeness of these reactions is dependent on  Y4- and correspondingly pH. pH is an important factor in setting the completeness and selectivity of an EDTA titration

25 EDTA Titrations Auxiliary Complexing Agents 1.)Metal Hydroxide  In general, as pH increases a titration of a metal ion with EDTA will have a higher K f. - Larger change at the equivalence point.  Exception: If M n+ reacts with OH - to form an insoluble metal hydroxide  Auxiliary Complexing Agents: a ligand can be added that complexes with M n+ strong enough to prevent hydroxide formation. - Ammonia, tartrate, citrate or triethanolamine - Binds metal weaker than EDTA Fraction of free metal ion (  M ) depends on the equilibrium constants (  ) or cumulative formation constants: Use a new conditional formation constant that incorporates the fraction of free metal:

26 EDTA Titrations Auxiliary Complexing Agents 2.)Illustration:  Titration of Cu +2 (CuSO 4 ) with EDTA  Addition of Ammonia Buffer results in a dark blue solution - Cu(II)-ammonia complex is formed  Addition of EDTA displaces ammonia with corresponding color change CuSO 4 Cu-EDTA Cu-ammonia

27 EDTA Titrations Metal Ion Indicators 1.)Determination of EDTA Titration End Point  Four Methods: 1.Metal ion indicator 2.Mercury electrode 3.pH electrode 4.Ion-selective electrode  Metal Ion Indicator: a compound that changes color when it binds to a metal ion - Similar to pH indicator, which changes color with pH or as the compound binds H +  For an EDTA titration, the indicator must bind the metal ion less strongly than EDTA - Similar in concept to Auxiliary Complexing Agents - Needs to release metal ion to EDTA Potential Measurements (red) (colorless) (blue) End Point indicated by a color change from red to blue

28 EDTA Titrations Metal Ion Indicators 2.)Illustration  Titration of Mg 2+ by EDTA - Eriochrome Black T Indicator Addition of EDTA Before Near After Equivalence point

29 EDTA Titrations Metal Ion Indicators 3.)Common Metal Ion Indicators  Most are pH indicators and can only be used over a given pH range

30 EDTA Titrations Metal Ion Indicators 3.)Common Metal Ion Indicators  Useful pH ranges

31 EDTA Titrations EDTA Titration Techniques 1.)Almost all elements can be determined by EDTA titration  Needs to be present at sufficient concentrations  Extensive Literature where techniques are listed in: 1)G. Schwarzenbach and H. Flaschka, “Complexometric Titrations”, Methuen:London, )H.A. Flaschka, “EDTA Titrations”, Pergamon Press:New York, )C.N. Reilley, A.J. Bernard, Jr., and R. Puschel, In: L. Meites (ed.) “Handbook of Analytical Chemistry”, McGraw-Hill:New York, 1963; pp to  Some Common Techniques used in these titrations include: a)Direct Titrations b)Back Titrations c)Displacement Titrations d)Indirect Titrations e)Masking Agents

32 EDTA Titrations EDTA Titration Techniques 2.)Direct Titrations  Analyte is buffered to appropriate pH and is titrated directly with EDTA  An auxiliary complexing agent may be required to prevent precipitation of metal hydroxide. 3.) Back Titrations  A known excess of EDTA is added to analyte - Free EDTA left over after all metal ion is bound with EDTA  The remaining excess of EDTA is then titrated with a standard solution of a second metal ion  Approach necessary if analyte: - precipitates in the presence of EDTA - Reacts slowly with EDTA - Blocks the indicator  Second metal ion must not displace analyte from EDTA

33 EDTA Titrations EDTA Titration Techniques 4.)Displacement Titration  Used for some analytes that don’t have satisfactory metal ion indicators  Analyte (M n+ ) is treated with excess Mg(EDTA) 2-, causes release of Mg 2+.  Amount of Mg 2+ released is then determined by titration with a standard EDTA solution - Concentration of released Mg 2+ equals [M n+ ] Requires:

34 EDTA Titrations EDTA Titration Techniques 5.)Indirect Titration  Used to determine anions that precipitate with metal ions  Anion is precipitated from solution by addition of excess metal ion - ex. SO excess Ba 2+ - Precipitate is filtered & washed  Precipitate is then reacted with excess EDTA to bring the metal ion back into solution  The excess EDTA is titrated with Mg 2+ solution [Total EDTA] = [MY n-4 ] + [Y 4- ] complexfree Known Titrate determine

35 EDTA Titrations EDTA Titration Techniques 6.)Masking Agents  A reagent added to prevent reaction of some metal ion with EDTA  Demasking: refers to the release of a metal ion from a masking agent Al 3+ is not available to bind EDTA because of the complex with F - Requires:


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