Download Complexing agent

COMPLEXOMETRY
Ashraf M. Mahmoud, Ph.D.
Definition: Titration involving formation of complex.
Complex ions are formed due to interaction
between metal ions, e.g. Ag+, Cu++, Co++
…etc. with negative ions or neutral
molecules (NH3, H2O, CN), e.g.
Ag+ + 2CN
Ag+ + 2NH3
Ex.
[Ag(CN)2]
[Ag(NH3)2]+ Silver amine
[Fe(CN)6]-4
Metal ion
(central ion)
Argento cyanid
Complexing agent
(ligand)
Coordinate bond: Metal ion (cation) is an (e)
acceptor while complexing agent (Ligand) is (e)
donor and direction of donation is represented by an
arrow
Co-ordination complexes: They are neutral or ionic
compounds that involve the formation of at least
one coordinate bond () between the metal ion
(cation: electron acceptor) and a complexing agent
(electron donor).
Metal ion is a Lewis acid (e-acceptor) and ligand is
a lewis base (e-donor).
Complexation Reaction
Formation of a complex between a central metal
atom (M) and a ligand (L) molecule (one or
more: n = coordination number).
Metal ions in solution are surrounded by a sheath
of solvent (H2O).
When the complexing agent = ligands are added
they replace one or two or three mols of H2O
giving complex ion:
M(H2O)n + L = M(H2O)(n-1) L + H2O
L = Ligand
M = Metal
n = no of ligand (coordination number)
Coordination number
Coordination number is the maximum number of
monodentate ligands that can be bound to the
central atom.
 Coordination numbers range from 2 to 8.
6 are the most common.
4 and
 Ions having even coordination numbers are
much more common than those having odd
numbers.
 Some ions may have more than one
coordination number.
Ligand
Definition:
Are neutral molecules or –ve ions contain at
least one pair of unshared (e) N:,O:,S:
The ligands can share these lone pair(e) with
metal ions through the formation of
coordinate bonds to form complex.
Classification of Ligand
1. Monodentate: Bound to the metal ion at only one point
(one atom that can donate its lone pair of electrons
(I.e. form only one coordinate bond): ions
Ex. F, Cl, Br , I, CN, SCN or molecules (H2O & NH3).
Ag+ + 2NH3
[Ag(NH3)2]+ Silver amine
NH2-CH3
H3C-H2N
Cd2+
..
+ 4NH3NH2
Cd
H3C-H2N
NH2-CH3
2+
Classification of Ligand
2- Bidentate: Contain 2 coordinating atoms in the molecule
ex. Ethylenediamine
NH2-CH2-CH2-NH2.
H2
Cd2+
..
..
+ 2NH2-CH2-CH2-H2N
H2
N
N
Cd
N
H2
N
H2
- Formation of strong metal complex.
2+
Classification of Ligand
Bidentate
Co 3+ + NH2-CH2-CH2-NH2
H2
H2
N
N
Co
N
H2
H2 N
N
H2
NH2
Tris (ethylene-diamine)
cobalt III complex
Classification of Ligand
3-Multidentate: Contain more than 2 coordinating
atoms in the molecule
Ex. EthyleneDiaminTetra-Acetic acid (EDTA)
2 donor N atom, 4 donor O-hexadentate
O H
2
HO C C
H2 O
C C OH
N C C N
H2 H2
HO C C
C C OH
H2
H2
O
O
EDTA
- EDTA have 6-center of donation.
- They posses at least 4 acidic group and 2
coordinates
- Powerful complexing.
2-
CO
O
CO
H2
C
O
CH2
N
CH2
M
O
N
CO
CH2
O
C
H2
CH2
CO
Structure of a divalent metal-EDTA chelate.
Structure of a divalent metal-EDTA chelate
Chelating agent
Chelating agent:
Organic product have more than one
position through which coordinate bond
with metal ion formed.
ligands (bi – multidentate) hold the metal
atom like a claw:
Ex; EDTA
Chelate: complex formed between metal and
chelating agent.
Chelate Effect on Stability of the Complex

the ability of multidentate ligands to form
more stable metal complexes than those
formed by similar monodentate ligands

Chelation highly affects Kf of complex
Ex 1
H2NCH3
CH3NH2
Cd2+ + 4CH3NH2
methylamine
K = 3 X 106
Cd
CH3NH2
H2NCH3
NH2 H2N
2+
Cd
+ 2H2NCH2CH2NH2
ethylenediamine
2+
Cd
NH2
H2N
2+
K = 2 X 1010
Ex 2
NH3 (mono)
Ni2+
ethyl-diamine (bidentate)
Penten (hexadentate)
3x108
3.1x1018
3.1x1019
Properties of Chelate Complex:
1- Usually non electrolyte.
2- Insoluble in H2O, soluble in organic solvent,
3- They exhibit striking colors with metal differ
from colors of normal salts of metal.
4- Chelates are water soluble when ligand contains
one or more hydrophilic groups, ex. sod.chelate.
Binuclear Complexes :
 One containing two metal ions
Ex:
[Z n 2Cl 6]
Zn 2+
+ Cl
high
conc. metal
2-
[Z nC l 3]
[Z nC l 4]
2-
Polynuclear Complexes:
One containing more than wo metal ions
 Stability of the formed complex
Depends on:
• A- the complexing ability of metal ion
• B- characteristics of ligand.
Complexones
Complexones are excellent complexing
agent prepared by Schwarzenbach.
1-He found that acidic ion is able to form
acetate complexes of low stability with all
polyvalent
cations,
that
could
be
reinforced by chelate effect.
2-Aminopolycarboxylic
are
excellent
complexing
agent
which
simplify
complexemetric titrations and facilitate
detection of E.P.
COMPLEXONES
H2
C
COOH
H2
C COOH
H2
HOOC C
+
H N C COO
H2
COOH
C
H2
N C C N
H2 H2
C COOH
HOOC C
H2
H
2
Complexone II (EDTA)
H2
C COOH
Complexone I (NTA)
Nitrilotriacetic acid
H2
H2
C COO
HOOC C
H N C C N H
H2 H2
C COOH
OOC C
H2
H2
H2
C H N
C COOH
H2C
C
H2
H2
C
H2C
C COOH
C H N
H2
C COOH
H2
Complexone III (Na2-EDTA) Complexone IV (CDTA or DCTA)
Trans-1,3-diaminocyclohexanetetracetic acid
CH2COO
HN
HN
(CH2)2
CH2COOH
(CH2)2
CH2COOH
CH2COO
O
N
H
(CH2)2
(CH2)2
CH2COO
O
(CH2)2
HN
CH2COO
CH2COOH
N
H
(CH2)2
HN
CH2COO
CH2COOH
CH2COO
Complexone V (EGTA)
Complexone VI (TTHA)
Ethylene gylcol bis (2-aminoethylether)
tetracetic acid
Triethylenetetramine
hexaacetic acid
Complexones
EDTA and its salts have various names including
Trilon B, Sequestrene, Versene and Chelaton 3. It
is widely used in titrimetric analysis.
CDTA forms stronger complexes than EDTA but
metal –complexes are formed more slowly.
EGTA is superior to EDTA
hardness titration.
in Ca/Mg water
EDTA has
because:
the widest application in analysis
1- Powerful complexing agent
2- Commercially available.
3- The spatial structure of its anion has 6 donor
atoms, enables it to satisfy coordination .no.
around M.
4- Formation of strainless stable 5-membered
rings on chelation.
5- It reacts with all metals in 1:1 ratio.
Na2EDTA (Na2H2Y): in aqueous medium, it is
dissociated to a complex-forming ion (H2Y2 )
M2+ + H2Y2
MY2
+ 2H+
M3+ + H2Y2
MY
+ 2H+
M4+ + H2Y2
MY
+ 2H+
Mn+ + H2Y2
MY(n-4) + 2H+
Dissociation of complex is governed by pH.
 pH  stability of complex except the metal
which has K is high, ex: Bi3+, Zr4+.
EDTA complexes (M2+) are stable in alkaline or
slightly acidic, stability constant.
Complexes with ions M3+ or M4+ need  acidity.
Stability with respect to pH of metalEDTA chelates.
Minimum pH at which
complexes exist
Selected metals
1-3
Zr4+, Hf4+, Th4+, Bi3+, Fe3+, Hg2+,
4-6
Pb2+, Cu2+, Zn2+, Co2+, Ni2+, Al3+,
Mn2+. Fe2+, Al3+, Cd2+, Sn2+
8 - 10
Ca2+, Sr2+, Ba2+, Mg2+
In practice, the stability of metal-EDTA complexes
may be altered by:
1- Variation in pH.
2- Presence of other complexing agents.
K = 6.3 x
1021
Hg-EDTA
K = 1.1 x
1018
Pb-EDTA
Log KH
K=5x
1010
Ca-EDTA
3
Effect of pH on KHZ M-EDTA chelates.
7
9
pH
11
13
EDTA Titration Curve
Titration Curve
pM 16
1016
14
1- KH at the right of the
curve.
2-The  KH, the sharper
E.p at pH constant
.
12
1010
10
8
6
106
4
104
2
0
2
4
6
8
10
12
0.01 M EDTA, mL
EDTA titration curves and the effect of KH values.
pCa 16
pH = 10
14
12
pH = 7
10
8
6
4
2
0
2
4
6
8
10
12
0.01 M EDTA, mL
Titration curves for Ca2+ with EDTA at pH 7 and 10.
2.1
4.1
pM
1.1
Volume of complexing agent
Effect of molar ratio on the metal-ligand
titration curves
Metal Ion (Metallochromic) Indicators
Definition. A compound whose color changes when
binds to a metal ion.
Mg—In
(red)
+
EDTA
(colourless)

Mg —EDTA
(colourless)
+
In
(blue)
 Description:
1- Organic compounds form color with metal.
2- Chelating agents.
3- Change their color on change of pH.
4- M-Indicator. Complex different in color from free form
indicator.
.
Requirements:
1- M-Indicator Complex less stable than M-EDTA complex.
2- The change in equilibrium from M-In to M-EDTA should
be sharp & rapid.
3- The metal-indicator complex must possess sufficient
stability
4- Free indicator color and its M-indicator color easily
observed and used at range of pH of EDTA.
5- The indicator must be very sensitive to metal ions so
that the colour change occurs as near to the equivalence
point as possible.
Ex: Erio-T [H3In]
Mg2+ + HIn2-  Mg-Ind + EDTA  Mg-EDTA + Ind.
red
At adjusted pH
colorless
colorless
blue
Some common metal ion indicators
Indicator
pKa
Eriochrome pK2 = 6.3
Black T
pK3 = 11.6
(Erio T)
Murexide
Xylenol
orange
Colors of free
indicator
Color of metalindicator complex
H2In (red)
HIn2 (blue)
Wine red
In3 (orange)
pK2 = 8.1
pK3 = 12.4
H4In (red)
H3In2 (violet)
H2In3 (blue)
pK2 = 2.32
pK3 = 2.85
pK4 = 6.7
pK5 = 10.0
pK6 = 12.23
H5In (yellow)
H4In2 (yellow)
H3In3 (yellow)
Red
4
H2In (violet)
HIn5 (violet)
In6 (violet)
Red with Ca2+
Yellow with
Co2+, Ni2+ & Cu2+
The structure of these metal indicators are
shown below
OH
O3S
OH
N N
O
O
(H2In)
O
O
Muroxide
Eriochrome Black T
CH3
CH3
OH
O
CO2
CO2
HN
CO2
H
N
N
N
H
O2N
O
O
H
N
NH
SO3
(H3In)
Xylenol orange
CO2
N
H
(H4In)
1-Eriochrome Black T
H2In
HIn2-
Wine red
6
Blue
6-12
Orange
pH  12
OH
Na O3S
In3-
OH
N N
O2N
Most M-indicator are weak acids
a) 2 acidic H
b) Act as acid-base indicator
Free form blue and complex form wine red
Mg-Ind. (red)  Ind. (blue)
2) Xylenol Orange
Free form yellow and complex form red at pH
(acidic)
At pH 7.5 Free form violet and complex form red
 It is useful for titration of metal ion that form
very strong EDTA complex and titrated at pH 1.5Ex. Direct titration of Thorium and Bi3+.
 3) Muroxide:
H4InReddish
violet
9
H3In2-
H2In3-
Violet
Bluish
violet
9-11
 pH 11
H
N
O
O
O
H
N
N
N
H
O
O
O
N
H
(H4In)
Free form violet and complex form red at pH (10)
Blocking of Indicator
An indicator is said to be blocked when :
1- Metal-Ind. Is more stable than Metal-EDTA.
2-Metal-Ind. Complex dissociate slowly during
titration.
3- Metal does not freely dissociate from indicator.
At this case no color change observed at E.P. to
avoid blocking of indicator.
Back titration is used to avoid blocking of indicator
Ex: Erio-T is blocked by Cu2+, Ni2+, Co2+, Cr3+, Fe3+,
Al3+.
Types of EDTA titration
1- Direct titration
2- Back titration
3- Displacement titration
4- Alkalimetric titration
5- Indirect titration
6- Masking titration
1. Direct Titrations
The analyte solution is buffered to an appropriate pH
and titrated with standard EDTA:
Stability of metal-EDTA complex is large enough to
produce a shape end point.
To the analyte solution, an auxiliary complexing agent
(ammonia, tartarate, citrate, or triethanolamine) is
added: Prevent the metal ion from precipitating in the
absence of EDTA (Meta—ACA is less stable than Metal
—EDTA).
 Note: At high pH M-OH is ppt. Ex: Pb2+.
Direct titration of Pb2+: is carried at pH 10 (ammonia) –
add auxiliary complexing agent (tartaric acid)  Pbtartarte complex less stable than Pb-EDTA.
2. Back Titrations
A known excess of EDTA is added to the analyte. The
excess EDTA is then titrated with a standard solution of a
second metal ion.
 It is used for when:
1- The analyte precipitates in the absence of EDTA.
2- The analyte reacts too slowly with EDTA under titration
conditions.
3- The analyte blocks the indicator.
N.B. The titrant metal (Mg) should not displace the analyte f
or its EDTA complex.
2. Back Titrations
Ex1: Determination of Al3+:
- Excess EDTA is added, pH is adjusted to 7-8, boil, cool, add
back titrate with standard Zn2+:
Erio—T, &
- Avoid precipitation of Al3+ as Al(OH)3 at pH 7 in the absence
of EDTA.
- Avoid indicator blocking as Al3+—EDTA complex is stable in
solution at this pH.
Ex2:
Ni2+
form
slow
dissociate
complex
with
Ind.
(pyridylazonaphthol) PNA (blocked) and metal cannot
dissociate and react with EDTA.
  Ni + excess EDTA # st Cu2+.
3. Displacement Titrations
 For metal that don’t have suitable indicator.
 For metal not complexed easily with EDTA
M1 (Sample)
+ M2—EDTA (Excess)  M1—EDTA + M2 (Titrated with EDTA)
Determination of Hg2+:
Ex1: Hg2+ has no suitable indicator
Hg2+ + (Mg—EDTA)2+  (Hg—EDTA)2+ + Mg2+
K ((Hg—EDTA)2+ is greater than K (Mg—EDTA)2+
Determination of Ca2+:
Ex2: Ca2+ ion by direct titration by EDTA using Erio-T
Poor E.P. by direct Titration because Ca-Ind complex is
very week.
3. Displacement Titrations
1- Small amount of Mg2+ ion is added.
2- EDTA reacts first with Ca2+ then Mg2+.
3- Ca-EDTA is more stable than Mg-EDTA.
MgY2 + Ca2+  CaY2 + Mg2+
4- Mg2+ react with Erio-T.
5- Mg-Ind. is more stable than Ca-Ind.
6- At E.P. EDTA displace Ind. from Mg2+.
Mg-Ind + EDTA  Mg-EDTA + Ind. (blue)
 Blank Ex. titration of Mg2+ # EDTA (same amount).
Ex3: No available indicator (Ag+)
Ag+ + [Ni(CN)4]2  2 [Ag(CN)2] + Ni2+
 Ni2+ # EDTA using muroxide as indicator.
4- Alkalimetric Titration
Mn+ + H2Y2-
(MY)(n-2)+ + 2H+
1- H+ is titrated with standard NaOH using AcidBase Indicaor or EP detected by poteniometry.
2- Alternative way: IO3 / I mix is added + EDTA
iodine is liberated then titrate I2 by S2O3
There is difficulties:
a) Neutralize solution of metal  may be hydrolyze
salts
b) No buffer can be used in this titration.
5- Indirect Titration: (Anions titration)
For anions that can form ppt with metals
M (excess) + anion (sample)  ppt.
then excess metal titrated by EDTA
pH
Ex1: SO4--:
BaSO4 
1) SO42 + BaCl2
1
2) Filter-wash:
boil
Ba(EDTA)23) BaSO4  + Excess st EDTA
pH 10
2+
4) xx EDTA # st Mg using suitable indicator.
Ex2: PO43
Cool
MgNH4PO4 
1) PO43 + Mg2+ + NH4+
2) MgNH4PO4  + Excess st EDTA  Mg-EDTA
3) Excess EDTA # Zn2+ ion using suitable Ind.
Ex3: CN
1) CN + xx Ni2+  [Ni(CN)4]2
2) Excess Ni # (st) EDTA.
Ex4: Halides
(X)
1) X + Ag+  AgX
2) AgX + [Ni(CN4)] dissol.  Ni2+
3) Ni2+ # EDTA
(Ag+ replace Ni2+ ion cyanide in complex)
Ex5: F
1) F + Ca2+  CaF2 
2) CaF2  + Excess EDTA . . . . . . .
Ex6: CO32, Cr2O42, SO32, S2 - indirect.
6- Masking
 Def.: Process in which some component of
analyte is protected from reaction with EDTA
without being physically separated from
medium.
 There are 3 methods:
(1) Masking agent
(2) Kinetic masking
(3) Masking by adjustment of oxidation state of
element F mask Al3+ form stable complex.
(1) Masking agent:
Ex1: F
Mix of Al3+ + Mg2+: mask Al3+ with F leave Mg2+
Mg2+ # EDTA.
Ex2: CN can mask the following metals: Cd2+,
Zn2+, Co2+, Ni2+….Fe2+, Fe3+ but not Mg2+, Ca2+,
Mn2+ or Pb2+.
Mix Cd2+ & Pb2+, Cd2+ masked with CN, Pb2+ #
EDTA.
Ex3: Triethanolamine N(Et)3 masks Al3+, Fe3+, Mn2+
Ex4: 2,3-dimercaptopropanol CH3-CH(SH)CH(SH)OH
masks Bi3+, Cd2+, Cu2+, Hg2+ and Pb2+
(2) Kinetic Masking:`A Cr3+ # EDTA  slow
Ex: Cr3+ & Fe3+ # EDTA
So no interference from 3+
Fe # EDTA  rapid
3+
Cr
(3) Oxidation reduction (adjust oxidation state)
Ex1: Fe3+ & Fe2+  EDTA prefere higher oxidation
state of metal to complex with it.
Ex2: Cu2+
Ex3: Hg2+
red
H+
red
Cu+ Ascorbic acid or NH2OH
Hg°
Ex4: Fe3+ & Cr3+
 Add ascorbic acid
 Titrate Cr3+ with EDTA
Fe3+
red
Fe2+
Ex5: Removing Fe3+  Fe2+
Mix of Fe3+ & M4+
Ex6: Cr3+ (oxid.)  CrO42+ not react with EDTA
Demasking:
 Release of M from masking agent.
1- CH2O / HAc
2- Chloral hydrate
[Zn(CN)4]2 + H+ 4CH2O  Zn2+ + 4 HOCH2CN
Selectivity of EDTA
 EDTA is unselective reagent react with all metals.
 To increase selectivity:
I) Masking and Demasking: Ex: Mg2+, Zn2+, Cu2+
a) Mix 10 ml + known excess of EDTA  M-EDTA.
b) xx EDTA # st Mg2+ using Erio-T pH 10.
c) V1 = Mg, Zn, Cu
d) Same volume of mix + KCN # EDTA
V2 = Mg+
(Cu & Zn, CN  masking complex)
e) Demask the previous solution with chlorhydrate # EDTA
(Zn2+ librate for CN)
V3 = Zn+
 Cu2+ = V1 (V2 + V3)
II) Control pH: Ex: Bi3+ & Pb2+
Mix
pH 2
Xylenol.O
On same sol.
# EDTA
hexamine
pH 5
# EDTA
V1 = Bi3+
V2 = Pb2+
III) Classical Separation:
 Ppt. ion of cations and redissolved the ppt #
EDTA

Ox
pH
redissolved
2+

2+
Ex: 1) Ca
CaOx 
Ca
# EDTA
12
2)
Ni2+
3) Mg2+
D.M. gluoxime
red ppt
Alk.
Alk.
MgNH4PO4
redissolved
(Meuroxide
ind.)
# EDTA
IV) Solvent Excretion:
1) Mix
Zn2+,
Cu2+
+ Excess
Extract Zn(SCN)2
dil
H 2O
SCN
IsobutylZn(SCN)2
Methyl keton
# EDTA
2) Zn2+ & Pb2+ . . . . . . . . . .
V) Choice of Indicator:
Choice of M-Ind.  rapid complex with Metal.
VI) Removal of Anions:
O-Phosphate removed by ion exchanging resin.