chapter-12 edta titrations

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 acceptorLigand – Lewis Base or Electron-pair donor

Note: multiple atoms from EDTA are binding Mn2+

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

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

http://www.bimcore.emory.edu/home/molmod/Wthiel/Kchannel.html

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

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.

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 Kf values- 6 acid-base sites in its structure

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

EDTA Titrations

EDTA

2.) Acid-Base Forms Fraction () of the most basic form of EDTA (Y4-) is defined by the H+

concentration and acid-base equilibrium constants

EDTA

Y

YHYYHYHYHYHYH

Y

4

Y

4322345

26

4

Y

4

4

] [

] [] [] [] [] [] [] [

] [

Fraction () of EDTA in the form Y4-:

where [EDTA] is the total concentration of all free EDTA species in solution

}][][][][][]{[ 23456654321543214321321211

654321Y KKKKKKKKKKKHKKKKHKKKHKKHKHH

KKKKKK4

Y4- is depended on the pH of the solution

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:

]][[

][

4n

4n-

fYM

MYK

Note: This reaction only involves Y4-, but not the other forms of EDTA

EDTAY 4Y4

] [

where Y4-is dependent on pH

EDTA Titrations

EDTA

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

complex.

EDTA Titrations

EDTA

3.) EDTA Complexes Substitute [Y4-] into Kf equation

If pH is fixed by a buffer, then Y4- is a constant that can be combined with Kf

]][[

][

4n

4n-

fYM

MYK EDTAY 4Y

4 ] [

][][

][

-4YEDTAM

MYK

n

4n-

f

where [EDTA] is the total concentration of EDTA added to the solution not bound to metal ions

]][[

][-4Y EDTAM

MYKKK

n

4n-

f'f Conditional or effective formation constant:

(at a given pH)

EDTA Titrations

EDTA

3.) EDTA Complexes Assumes the uncomplexed EDTA were all in one form

-4Yf

'f KK

at any pH, we can find Y4- and evaluate Kf’

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?

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

EDTA Titrations

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+)

Minimum pH for Effective Titration of Metal Ions

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)

-4Yf

'f KK

][ nMlogpM

EDTA Titrations


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

Y4- at pH 10.0 = 0.30

mL00.50V)M0500.0(mL00.5M0500.0)mL(V ee

mL EDTA at equivalence point:

mmol of EDTA mmol of Mg2+

EDTA Titrations


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.

][

)])(( - ))([(][

L0050.0L0500.0

L0050.0M EDTA0500.0L0500.0M Mg0500.0Mg

22

moles of Mg2+ originally present moles of EDTA added

Original volumesolution

Volume titrantadded

39.1MglogpMgM0409.0Mg 222 ][][

Dilution effect

EDTA Titrations


2.) Example (b) At Equivalence Point ( 50.0 mL of EDTA)

Virtually all of the metal ion is now in the form MgY2-

)(

)()(][

L0500.0L0500.0

L0500.0M0500.0MgY 2

Original [Mn+]Original volume of

Mn+ solution


Volume titrantadded

Dilution effect

Moles Mg+ ≡ moles MgY2-

M0250.0MgY 2 ][

EDTA Titrations


2.) Example (b) At Equivalence Point ( 50.0 mL of EDTA)

The concentration of free Mg2+ is then calculated as follows:

Initial Concentration (M) 0 0 0.0250

Final Concentration (M) x x 0.0250 - x

]][[

])[

EDTAMg

EDTA(MgKK

2

2-

Yf'

4f

)x)(x(

)x0250.0()30.0)(102.6( 8

Solve for x using the quadratic equation:

94.4pMg1016.1EDTAMgx 252 ][][

EDTA Titrations


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.

)(

))((][

L0510.0L0500.0

L0010.0M0500.0EDTA

Original [EDTA]Volume excess

titrant


Volume titrantadded Dilution effect

Excess moles EDTA

M1095.4EDTA 4][

Calculate excess [EDTA]:

EDTA Titrations



Calculate [MgY2-]:

)(

)()(][

L0510.0L0500.0

L0500.0M0500.0MgY 2

Original [Mn+]Original volume of

Mn+ solution


Volume titrantadded

Dilution effect

Moles Mg+ ≡ moles MgY2-

M0248.0MgY 2 ][

Only Difference

EDTA Titrations



[Mg2+-] is given by the equilibrium expression using [EDTA] and [MgY2-]:

]][[

])[

EDTAMg

EDTA(MgKK

2

2-

Yf'

4f

)M1095.4)(x(

)M0248.0()30.0)(102.6(

48

57.6pMg107.2Mgx 272 ][

EDTA Titrations


2.) Example Final titration curve for 50.0 ml of 0.0500 M Mg2+ with 0.0500 m EDTA at 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 Y4- and correspondingly pH.

pH is an important factor in setting the completeness and selectivity of an EDTA titration

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

fZnY'' KK 24f

nn

221

M]L[]L[]L[1

1

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:

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

CuSO4 Cu-EDTACu-ammonia

EDTA Titrations

Metal Ion Indicators

1.) Determination of EDTA Titration End Point Four Methods:

1. Metal ion indicator2. Mercury electrode3. pH electrode4. 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) (colorless) (blue)

End Point indicated by a color change from red to blue

EDTA Titrations

Metal Ion Indicators

2.) Illustration Titration of Mg2+ by EDTA

- Eriochrome Black T Indicator

Addition of EDTA

Before Near After Equivalence point

EDTA Titrations Metal Ion Indicators

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

EDTA Titrations Metal Ion Indicators

3.) Common Metal Ion Indicators Useful pH ranges

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, 1969.2) H.A. Flaschka, “EDTA Titrations”, Pergamon Press:New York, 19593) 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:a) Direct Titrationsb) Back Titrationsc) Displacement Titrationsd) Indirect Titrationse) Masking Agents

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

44 Y)ionmetalond(secfY)analyte(f KK


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+]

424n Y)Mg(fY)M(fKK Requires:


5.) Indirect Titration Used to determine anions that precipitate with metal ions

Anion is precipitated from solution by addition of excess metal ion- ex. SO4

2- + 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

Known Titratedetermine


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-

))EDTA(Al(f)AlF(fKK 3

6Requires:

chapter-12 edta titrations

Documents

edta complexes

forms of edta edta y4y4

edta titrationsintroduction

uncomplexed edta

kf edta titrationsedta

solution edta titrationsedta

structure edta titrationsedta

total concentration