Transcript Chapter 12: EDTA Titrations - UNL

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
Note: multiple atoms from
EDTA are binding Mn2+
Metal – Lewis Acid or Electron-pair acceptor
Ligand – Lewis Base or Electron-pair donor
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
channel contains
pore that only
allows K+ to pass
K+ is chelated by O
in channel
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
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.
-

Multidentate ligand: a chelating agent with more than one free electron pair
-

Typically more than one O or N
Larger Kf values
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

aY 4  
 6
Fraction (a) of the most basic form of EDTA (Y4-) is defined by the H+
concentration and acid-base equilibrium constants
 5
 4
 3
K1K2 K 3 K 4 K5 K6
{[H ]  [H ] K1  [H ] K1K2  [H ] K1K2 K 3  [H  ]2 K1K2 K 3 K 4  [H  ]K1K2 K 3 K 4 K5  K1K2 K 3 K 4 K5 K6 }
Fraction (a) of EDTA in the form Y4-:
aY 4  
aY 4 
[Y 4  ]
[H6Y 2  ]  [H5Y  ]  [H 4Y ]  [H 3Y  ]  [H 2Y 2  ]  [HY 3  ]  [Y 4  ]
[Y 4  ]

EDTA
where [EDTA] is the total concentration of all free EDTA species in solution
aY4- 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
Kf 
[MY n- 4 ]
[M n  ][Y 4  ]
Note: This reaction only involves Y4-, but not the other forms of EDTA

Recall: the concentration of Y4- and the total concentration of EDTA is
solution [EDTA] are related as follows:
[Y 4  ]  aY 4  EDTA
where aY4-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

[Y
4
Substitute [Y4-] into Kf equation
]  aY 4  EDTA
Kf 
Kf 

[MY n- 4 ]
[M n  ]a Y 4- [EDTA]
[MY n- 4 ]
[M n  ][Y 4  ]
where [EDTA] is the total
concentration of EDTA added
to the solution not bound to
metal ions
If pH is fixed by a buffer, then aY4- is a constant that can be combined with Kf
Conditional or effective formation constant:
(at a given pH)
K'f
 K  K f a Y 4- 
[MY n- 4 ]
[M n  ][EDTA]
EDTA Titrations
EDTA
3.) EDTA Complexes

Assumes the uncomplexed EDTA were all in one form
K'f  K f a Y 4at any pH, we can find aY4- 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
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+)
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)
K'f  K f a Y 4
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)
pM   log [M n  ]
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:
Ve ( mL )0.0500 M   5.00 mL ( 0.0500 M )  Ve  50.00 mL
mmol of EDTA
mmol of Mg2+
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
[Mg
2
moles of EDTA added
[(0 .0500 M Mg2  )(0 .0500 L) - (0 .0500 M EDTA)(0 .0050 L)]
]
[0.0500 L  0.0050 L]
Original volume
solution
Volume titrant
added
Dilution effect
[Mg 2  ]  0.0409 M  pMg 2    log [Mg 2  ]  1.39
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
Original [Mn+]
Mn+ solution
[MgY 2  ]  (0 .0500 M )
(0 .0500 L)
(0.0500 L  0.0500 L)
Original volume
solution
[MgY 2  ]  0.0250 M
Moles Mg+ ≡ moles MgY2-
Volume titrant
added
Dilution effect
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:
Initial Concentration (M)
0
0
0.0250
Final Concentration (M)
x
x
0.0250 - x
K'f  K f aY 4  
[Mg( EDTA)- 2 ]
[Mg2  ][EDTA]
( 0.0250  x )
8
 ( 6.2  10 )( 0.30 ) 
( x )( x )
Solve for x using the quadratic equation:
 x  [Mg 2  ]  [EDTA ]  1.16  10 5  pMg 2   4.94
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
Original [EDTA] titrant
[EDTA] 
Excess moles EDTA
(0 .0500 M )(0 .0010 L)
(0.0500 L  0.0510 L)
Original volume
solution
[EDTA ]  4.95  10 4 M
Volume titrant
added
Dilution effect
EDTA Titrations
EDTA Titration Curves
2.) Example

(c) After the Equivalence Point ( 51.0 mL of EDTA)
Calculate [MgY2-]:
Original volume of
Original [Mn+]
Mn+ solution
[MgY 2  ]  (0 .0500 M )
(0 .0500 L)
(0.0500 L  0.0510 L)
Original volume
solution
[MgY 2  ]  0.0248 M
Moles Mg+ ≡ moles MgY2-
Volume titrant
added
Only Difference
Dilution effect
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-]:
K f  K f aY 4  
'
8
 ( 6.2  10 )( 0.30 ) 
[Mg( EDTA)- 2 ]
[Mg2  ][EDTA]
( 0.0248 M )
( x )( 4.95  10  4 M )
 x  [Mg 2  ]  2.7  10 7  pMg 2   6.57
EDTA Titrations
EDTA Titration Curves
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 aY4- 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
Fraction of free metal ion (aM) depends on the
equilibrium constants () or cumulative formation
constants:
Use a new conditional formation constant that
incorporates the fraction of free metal:
aM 
1
1  1 [ L ]   2 [ L ] 2    n [ L ] n
K'f'  aY 4  a Zn 2  K f
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-ammonia Cu-EDTA
EDTA Titrations
Metal Ion Indicators
1.) Determination of EDTA Titration End Point

Four Methods:
1.
2.
3.
4.

Potential
Measurements
Metal Ion Indicator: a compound that changes color when it binds to a metal
ion
-

Metal ion indicator
Mercury electrode
pH electrode
Ion-selective electrode
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
End Point indicated by a color
change from red to blue
(red)
(colorless)
(colorless)
(blue)
EDTA Titrations
Metal Ion Indicators
2.) Illustration

Titration of Mg2+ by EDTA
-
Eriochrome Black T Indicator
Addition of EDTA
Before
Near
Equivalence point
After
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)
2)
3)

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:
a)
b)
c)
d)
e)
Direct Titrations
Back Titrations
Displacement Titrations
Indirect Titrations
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
Kf ( analyte )aY 4   Kf (sec ond metal ion )aY 4 
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+.
Requires:

Kf ( M n  )aY 4   Kf ( Mg2  )aY 4 
Amount of Mg2+ released is then determined by titration with a standard EDTA
solution
Concentration of released Mg2+ equals [Mn+]
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
Known
determine
free
Titrate
EDTA Titrations
EDTA Titration Techniques
6.) Masking Agents

A reagent added to prevent reaction of some metal ion with EDTA
Al3+ is not available to bind EDTA because of the complex with F-
Requires:

Kf ( AlF 3  )  Kf ( Al ( EDTA ))
6
Demasking: refers to the release of a metal ion from a masking agent