EDTA Titrations

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 Complex Metal Ligand Note: multiple atoms from EDTA are binding Mn2+ Metal – Lewis Acid or Electron-pair acceptor Ligand – Lewis Base or Electron-pair donor

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 K+ is chelated by O in channel channel contains pore that only allows K+ to pass K+ channel spans membrane Opening of potassium channel allows K+ to exit cell and change the electrical potential across membrane Current Opinion in Structural Biology 2001, 11:408–414 http://www.bimcore.emory.edu/home/molmod/Wthiel/Kchannel.html

4. EDTA Titrations • Metal –Chelate Complexes • 1.)Formation Constant (Kf) • 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 (Ksp), acid-base (Ka, Kb), water dissociation (Kw), etc • Chelate effect: ability of multidentate ligands to form stronger metal complexes compared to monodentate ligands. Kf = 8x109 Kf = 4x109 2 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 Kf 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 6nitrogens & oxygens in its structure giving it 6 free electron pairs that it can donate to metal ions. • High Kf 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 (Y4-) is the one which primarily reacts with metal ions. EDTA-Mn Complex

8. EDTA Titrations • EDTA • 2.)Acid-Base Forms • Fraction (a) of the most basic form of EDTA (Y4-) is defined by the H+ concentration and acid-base equilibrium constants Fraction (a) of EDTA in the form Y4-: where [EDTA] is the total concentration of all free EDTA species in solution aY4- is depended on the pH of the solution

9. EDTA Titrations • EDTA • 3.)EDTA Complexes • The basic form of EDTA (Y4-) reacts with most metal ions to form a 1:1 complex. • Other forms of EDTA will also chelate metal ions • Recall: the concentration of Y4- and the total concentration of EDTA is solution [EDTA] are related as follows: Note: This reaction only involves Y4-, but not the other forms of EDTA where aY4-is dependent on pH

10. EDTA Titrations • EDTA • 3.)EDTA Complexes • The basic form of EDTA (Y4-) reacts with most metal ions to form a 1:1 complex.

11. EDTA Titrations • EDTA • 3.)EDTA Complexes • Substitute [Y4-] into Kf equation • If pH is fixed by a buffer, then aY4- is a constant that can be combined with Kf 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 aY4- and evaluate Kf’

13. EDTA Titrations • EDTA • 4.)Example: • What is the concentration of free Fe3+ 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 • Kf decreases • [Fe3+] = 5.4x10-7 at pH 2.0 -> [Fe3+] = 1.4x10-12 at pH 8.0 • In order to get a “complete” titration (Kf ≥106), 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 Minimum pH for Effective Titration of Metal Ions • EDTA • 5.)pH Limitation • By adjusting the pH of an EDTA titration: • one type of metal ion (e.g. Fe3+) can be titrated without interference from others (e.g. Ca2+)

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 Mn+) • At the equivalence point ([EDTA]=[Mn+] • After the equivalence point (excess EDTA)

17. EDTA Titrations • EDTA Titration Curves • 2.)Example • What is the value of [Mn+] and pM for 50.0 ml of a 0.0500 M Mg2+ solution buffered at pH 10.00 and titrated with 0.0500 m EDTA when (a) 5.0 mL, (b) 50.0 mL and (c) 51.0 mL EDTA is added? Kf = 108.79 = 6.2x108 aY4- at pH 10.0 = 0.30 mL EDTA at equivalence point: mmol of Mg2+ mmol of EDTA

18. EDTA Titrations • EDTA Titration Curves • 2.)Example • (a) Before Equivalence Point ( 5.0 mL of EDTA) Before the equivalence point, the [Mn+] is equal to the concentration of excess unreacted Mn+. Dissociation of MYn-4 is negligible. moles of Mg2+ 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 MgY2- Original volume of Mn+ solution Moles Mg+ ≡ moles MgY2- Original [Mn+] Original volume solution Volume titrant added Dilution effect

20. EDTA Titrations • EDTA Titration Curves • 2.)Example • (b) At Equivalence Point ( 50.0 mL of EDTA) The concentration of free Mg2+ is then calculated as follows: 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 MgY2- and there is excess, unreacted EDTA. A small amount of free Mn+ exists in equilibrium with MgY4- and EDTA. Calculate excess [EDTA]: Volume excess titrant Excess moles EDTA Original [EDTA] Original volume solution Volume titrant added Dilution effect

22. EDTA Titrations • EDTA Titration Curves • 2.)Example • (c) After the Equivalence Point ( 51.0 mL of EDTA) Calculate [MgY2-]: Original volume of Mn+ solution Moles Mg+ ≡ moles MgY2- Original [Mn+] Only Difference Original volume solution Volume titrant added Dilution effect

23. EDTA Titrations • EDTA Titration Curves • 2.)Example • (c) After the Equivalence Point ( 51.0 mL of EDTA) [Mg2+-] is given by the equilibrium expression using [EDTA] and [MgY2-]:

24. EDTA Titrations • EDTA Titration Curves • 2.)Example • Final titration curve for 50.0 ml of 0.0500 M Mg2+with 0.0500 m EDTAat pH 10.00. • Also shown is the titration of 50.0 mL of 0.0500 M Zn2+ Note: the equivalence point is sharper for Zn2+ vs. Mg2+. This is due to Zn2+ having a larger formation constant. The completeness of these reactions is dependent on aY4- 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 Kf. • Larger change at the equivalence point. • Exception:If Mn+ reacts with OH- to form an insoluble metal hydroxide • Auxiliary Complexing Agents: a ligand can be added that complexes with Mn+ strong enough to prevent hydroxide formation. • Ammonia, tartrate, citrate or triethanolamine • Binds metal weaker than EDTA Fraction of free metal ion (aM) depends on the equilibrium constants (b) 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 (CuSO4) 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 Cu-ammonia CuSO4 Cu-EDTA

27. EDTA Titrations • Metal Ion Indicators • 1.)Determination of EDTA Titration End Point • Four Methods: • Metal ion indicator • Mercury electrode • pH electrode • 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 End Point indicated by a color change from red to blue (red) (colorless) (colorless) (blue)

28. EDTA Titrations • Metal Ion Indicators • 2.)Illustration • Titration of Mg2+ 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: • G. Schwarzenbach and H. Flaschka, “Complexometric Titrations”, Methuen:London, 1969. • H.A. Flaschka, “EDTA Titrations”, Pergamon Press:New York, 1959 • C.N. Reilley, A.J. Bernard, Jr., and R. Puschel, In: L. Meites (ed.) “Handbook of Analytical Chemistry”, McGraw-Hill:New York, 1963; pp. 3-76 to 3-234. • Some Common Techniques used in these titrations include: • Direct Titrations • Back Titrations • Displacement Titrations • Indirect Titrations • 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 (Mn+) is treated with excess Mg(EDTA)2-, causes release of Mg2+. • Amount of Mg2+ released is then determined by titration with a standard EDTA solution • Concentration of released Mg2+ equals [Mn+] 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. SO42- + excess Ba2+ • 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 Mg2+ solution [Total EDTA] = [MYn-4] + [Y4-] complex free determine Known Titrate

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 Al3+ is not available to bind EDTA because of the complex with F- Requires: