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1
Chapter 17
Complexation
Reactions and Titrations
Lewis acid-base concept
Lewis acid electron pair acceptor
[metal cations, Mn+]
Lewis base electron pair donor
[ligand, molecules or ions]
Coordinate covalent bond a bond formedwhen both electrons of the bondare donated by one atom.
Complex ion A metal ion with Lewis base attached to it through coordinate covalent bond.
Complex (Coordinate compound) a compound consisting either of complex ions and other ions of opposite charge or of neutral complex species.
2
Complex-Formation Reactions
• some elements (mostly metal ions) can form complexes with molecules/ions which have a “spare pair” of electrons (ligand).
• The central atom, (usually) a metal cation, accepts the pair of electrons from the ligand to form a coordinate covalent bond.
• The empty orbitals of the cation and the orbital occupied by the bonding pair of electrons on the donor ion or molecule (ligand) form a new molecular orbital.
• The number of coordinate covalent bonds that a cation tends to form with individual ligands (or functional groups on ligands) is known as its coordination number.
• Typical values for coordination numbers are 2, 4 and 6.
• The compounds formed can be neutral, positively or negatively charged depending on the charge of the reacting species.
3+ 3+3 3 6Co + 6NH Co(NH )
Central atom Ligand Coordination number (CN)
2+ 2+3 3 4Cu (blue) + 4:NH Cu(NH ) (dark blue)
2+ 2+
3
• When a ligand has a single complexing or donor group in its structure, it is said to be unidentate (single-toothed),
:NH3, I- Co(NH3)62+, CuI2-
• when there are two groups, it is bidentate,
• When there are three (four) groups, it is called tridentate (tetradentate) ligand, etc.
• When a bidentate (or higher number of donor groups present in the ligand) forms a complex with a metal cation, we call the resulting compound a metal chelate (“kee’late”-claw).
• As titrants, multidentate ligands, particularly those with 4 to 6 donor groups have the advantage that they usually react in a single step process, and their reactions with the metal cation are more complete than their unidentate counterparts.
Chelate Effect
• The ability of multidentate ligands to formmore stable metal complexes than thoseformed by similar monodentate ligands
• Often results from the formation of 5-membered "ring" with metal and two atoms on the ligand
4
Complexation Equilibria
1
2
3
4
f
K 23
K 23 2
K 23 3
K 23 4
K 23 4
f 1
2
23
23 2
23
2 3 4
3
3
3
3
3
4
3
2
Cu(NH )Cu(NH )Cu
CuCu(NH )Cu(NH (NH )
Cu(NH )
4 Cu(N
NHNHNH)
Cu(NH )
Cu
NH
NH H )K K K K K
Kf (4) –formation constant
• Complexation reactions occur in a stepwise fasion
1
22 2
32 3 3
2
nn-1 n n
n-1
[ML]M + L ML
[M][L]
[ML ]ML + L ML
[ML][L]
[ML ]ML + L ML
[ML ][L]
...
[ML ]ML + L ML
[ML ][L]
K
K
K
K
Formation Constants (i)
1 1
22 2 1 22
33 3 1 2 33
n1 2 3
[ML]M + L ML
[M][L]
[ML ]M + 2L ML
[M][L]
[ML ]M + 3L ML
[M][L]
...
[ML ]M + L ML ...
[M][L]n n nn
K
K K
K K K
n K K K K
5
2 n
M MLM M
2 nML ML
M M
M 2 n
21 2
21 2
[M] [ML]= =
[ML ] [ML ]= =
[M] [ML] [ML ] ... [ML ]
[M] [M][L] [M][L] ... [M][L]
[M]{1 [L] [L] ... [L] }
nn
nn
c c
c c
c
Alpha () Values Fraction of the Total Metal Concentration
2
n
M 21 2
1ML 2
1 2
22
ML 21 2
ML 21 2
1=
1 [L] [L] ... [L]
[L]=
1 [L] [L] ... [L]
[L]=
1 [L] [L] ... [L]
[L]=
1 [L] [L] ... [L]
nn
nn
nn
nn
nn
Titration Curves of MLn
(A)Tetradentate ligand, 1:1(B)Bidentate ligand 2:1(C)Unidentate ligand, 4:1
Tetradentate or hexadentate ligands are more satisfactory as titrants than ligands with a lesser number of donor groups because their reactions with cations are more complete and they tend to form 1:1 complexes.
6
EDTA Titrations
Most widely used complexometric titrant,Hexadentate ligand (4 –COOH+2 amino groups)
EDTA
• It forms 1:1 complexes with most metals. (Not with Group 1A metals – Na, K, Li)
• Forms stable water soluble complexes.
• High formation constants.
• A primary standard material – a highly purified compound that serves as a reference material.
Octahedron Structure of EDTA-M
5 –five membered rings
7
Acid-Base Properties (H6Y2+)
The first four values apply to carboxyl protons, and the last twoare for the ammonium protons. The neutral acid is tetraprotic,with the formula H4Y. A commonly used reagent isthe disodium salt, Na2H2Y2H2O.
EDTA (H4Y) Disassociation
1 2
3 5 64
4Y
2+ +6 5
- 2- 3- 4-4 3 2
0 1 2 3 4
n+
H Y H Y
H Y H Y H Y HY Y
(or )
M + Y
K K
K K KK
(n-4)+4- (n-4)+
MY n+ 4-
[MY ] MY =
[M ][Y ]K
4- 4-
4T
4-
2+ + - 2- 3- 4-6 5 4 3 2
[Y ] [Y ]
[EDTA]
[Y ]
[H Y ] [H Y ] [H Y] [H Y ] [H Y ] [HY ] [Y ]
c
8
(n-4)+ (n-4)+
MY n+ 4- n+
(n-4)+'MY MYn+
'MY MY
4 T
4T
4
[MY ] [MY ] =
[M ][Y ] [M ]
Conditional formation constant:
[MY ] =
[M
1.0
]
c
c
K
K K
K K as
K’MY is pH dependent!
4- 4-
4 2+ + - 2- 3- 4-T 6 5 4 3 2
+ 6 + 5 + 4 + 31 1 2 1 2 3
1 2 3 4 5 6 + 2 +1 2 3 4 1 2 3 4 5 1 2 3 4 5 6
[Y ] [Y ]
[H Y ] [H Y ] [H Y] [H Y ] [H Y ] [HY ] [Y ]
[H ] + [H ] [H ] + [H ] = /
[H ] [H ]+
c
K K K K K KK K K K K K
K K K K K K K K K K K K K K K
4
Composition of EDTA solution as a function of pH.
Distribution of unprotonated form of Y4- as a function of pH
-2.00E-01
0.00E+00
2.00E-01
4.00E-01
6.00E-01
8.00E-01
1.00E+00
1.20E+00
0.00 5.00 10.00 15.00
Alpha4
Alpha4
Excel Computing of Y4- as a function of pH
9
Example 17-4 Use speadsheet to construct the titration curve of pCa versus volume of EDTA for 50.0 mL of 0.00500 M Ca2+ being titrated with 0.0100 M EDTA in a solution buffered to a constant pH of 10.0
(1) pH = 10.0, 4 = 0.35,KCaY = 5.0 e10, K’CaY = 4 KCaY = 1.75e10
(2) Equivalence point: vEDTA = 50x0.00500/0.0100 = 25.0 mL
(3) Initial pCa: [Ca2+] = 0.00500 M, pCa = 2.30
(4) Pre-equivalence point: Ca (excess) + Y CaY (disassociation negligible)
[Ca] = [50.00x0.00500-vEDTAx0.0100]/(50.00+vEDTA)
vEDTA (mL) [Ca] pCa5.00 3.64e-3 2.4410.00 2.46e-3 2.6120.00 7.14e-4 3.1524.00 1.35e-4 3.87
(5) At equivalence point:[CaY]/[Ca]^2 = K’MY, [Ca] = {[CaY]/K’MY}^(1/2)[Ca] = {[50.00x0.00500/(50.00+25.00)]/1.75e10}^0.5
=4.36e-7M , pCa = 6.36
(6) Post-equivalence point:Ca + Y CaYx excess ~equivalence concentration
(50.00x0.00500)/(50.00+vEDTA)/{[Ca][(0.0100xvEDTA-50.00x0.00500)/(50.00+vEDTA)]}=K’MY
[Ca]=0.25/{[0.0100xvEDTA-0.25]xK’MY}=1.43e-11/(0.0100xvEDTA-0.25)
VEDTA(mL) [Ca] pCa26.00 1.42e-9 8.8530.00 2.86-10 9.5440.00 9.53e-11 10.0250.00 5.71e-11 10.24
10
0
2
4
6
8
10
12
0 10 20 30 40 50
V (EDTA, mL)
pC
a
Ca-EDTA Titration Curve at pH 10
EDTA Titration Curves forCa (K’2.75e10) and Mg (K’1.72e8) at pH 10
Larger the K’MY
Larger the pM change
Eriochrome Black T
Typical Metal Ion Indicator--Eriochrome Black T(EBT)
A Weak Acid and azo dye
11
Indicators for EDTA Titrations
At the end point:MgIn + EDTA MgEDTA + In(red) (colorless) (colorless) (Blue)
Before Titration:Mg2+ + In MgIn
(colorless) (blue) (red)
During Titration: Before the end pointMg2+ + EDTA MgEDTA
(free Mg2+ ions) (Solution red due to MgIn complex)
Compounds changing colour when binding to metal ion.Kf for Metal-In < Kf for Metal-EDTA.
EDTA Titration Solution pH must be Controlled for EBT Indicator
2 1
32- -
2--
Factors Influencing Chemical Specification of Eriochrome Black T (EBT):
1. pH, as EBT is a weak acid,
HIn (blue)
rcolo varies on pH:
= 5e-
HIn (blue)
rH In ( ed)
In (orange)
7K 2
f [M-In]
2+f [M-In]
f [M-In] f [M-Y]
2-
= 2.8e-12
so pH must be controlled ~7-12 [dominated by ]
2. of M-In complex,
Mg
+ =1.0e
HIn (blue)
In (
n
MgI 7
must
blue
be smaller tha
n) (red)
K
K
K
K K
Influence of pH on Titration Curves of Ca-EDTA
The higher pHThe larger pCa changepH should > 8 forCa-EDTA titrations
12
Lower pHs OKfor Large KMY Complexes
pH = 6.0
Minimum pH needed for satisfactory titration of
various cations with EDTA.
Effect of Other Complexing Agentson EDTA Titration Curves
• A second complexing agent added to maintain theanalyte metal ion in solution (many metal ions forminsoluble hydroxides or oxides at slightly high pH).
• To "mask" or remove interfering ions present in thesample matrix.
• The second complexing agent (“masking agent“) usuallyhas a higher affinity for the interfering ion than the EDTAto prevent it from reacting with the EDTA.
• Most buffers will complex metal ions because they alsocontain functional groups (-OH, -COOH, -NH2) which canform coordinate covalent bonds and their effect on thefree metal ion concentration must be considered.
13
Influence of [NH3] on the Zn-EDTA Titration at pH 9.00
2+ 2+3 3 4
3-
2-3 4
Before Titration Zn + 4NH Zn(NH )
HY EDTA (during Titration)
ZnY + 3NH NH
Ammonia decreases the change in pZn in the
equivalence-point region.
n
M
4 T 4 T
4
M
Y
MY
Y
M
Y M
M
M
M
M
M + Y MY (EDTA complexing)
M + nL ML (second complexing agent)
[MY] = ([M],[Y]--free concentration)
[M][Y]
[MY] [MY] =
( )(
Conditional formation constant:
[MY]"
() )
=
c
K
c
K K
c
c
Kc
21
M T
M2M
1Where
1 [L] [L] ... [L
[M]
]nn
c
c
EDTA Titration Techniques
1. Direct Titration
*Buffer analyte to pH where Kf’ for MY is large,*and M-In colour distinct from free In color.
*Auxiliary complexing agent may be used.
2. Back Titration
*Known excess std EDTA added.
*Excess EDTA then titrated with a std sol’n of a secondmetal ion.
*Note: Std metal ion for back titration must not displace analyte from MY complex.
14
2. Back Titration: When to apply it
*Analyte precipitates in the absence of EDTA.
*Analyte reacts too slowly with EDTA.
*Analyte blocks indicator
3. Displacement Titration
*Analyte treated with excess Mg(EDTA)
M + MgY MY + Mg
* Kf’ for MY > Kf’ for MgY
*Metal ions with no satisfactory indicator.
4. Indirect Titration
*Anions analyzed: CO32-, CrO4
2-, S2-, and SO42-.
Precipitate SO42- with excess Ba2+ at pH 1.
*BaSO4(s) washed & boiled with excess EDTA at pH 10.
BaSO4(s) + EDTA(aq) BaY2-(aq) + SO42-(aq)
Excess EDTA back titrated:EDTA(aq) + Mg2+MgY2-(aq)
Alternatively: *Precipitate SO42- with excess
Ba2+ at pH 1.
*Filter & wash precipitate.
*Treat excess metal ion in filtrate with EDTA.
5. Masking
*Masking Agent: Protects some component of analytefrom reacting with EDTA.
*F- masks Hg2+, Fe3+, Ti4+, and Be2+.
*CN- masks Cd2+, Zn2+, Hg2+, Co2+, Cu+, Ag+, Ni2+, Pd2+, Pt2+, Hg2+, Fe2+, and Fe3+,
but not Mg2+, Ca2+, Mn2+, Pb2+.
*Triethanolamine: Al3+, Fe3+, and Mn2+.
*2,3-dimercapto-1-propanol: Bi3+, Cd2+, Cu2+, Hg2+, and Pb2+.
15
*Demasking: Releasing masking agent from analyte.
mHCOmHCNM mnm 2
mH2C
CN
OH
Mn+
Metal-CyanideComplex
Formaldehyde
*Oxidation with H2O2 releases Cu2+ from Cu+-Thiourea complex.
*Thus, analyte selectivity:1. pH control2. Masking3. Demasking
Recommended