2. COMPLEXOMETRY
Ashraf M. Mahmoud, Ph.D.

3. 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.
Argento cyanid
Silver amine
Ex. [Fe(CN)6]-4
Metal ion
(central ion)
Complexing agent
(ligand)

4. 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).

5. Complexation Reaction M(H2O)n + L = M(H2O)(n-1) L + H2O
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)

6. 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 and 6 are the most common.
Ions having even coordination numbers are much more common than those having odd numbers.
Some ions may have more than one coordination number.

7. 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.

8. Classification of Ligand
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).
Silver amine

9. Classification of Ligand
2- Bidentate: Contain 2 coordinating atoms in the molecule.
ex. Ethylenediamine NH2-CH2-CH2-NH2.
Formation of strong metal complex. 9

10. Classification of Ligand
Bidentate
Co NH2-CH2-CH2-NH2
Tris (ethylene-diamine) cobalt III complex

11. 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
EDTA
- EDTA have 6-center of donation.
- They posses at least 4 acidic group and 2 coordinates
- Powerful complexing.

12. Structure of a divalent metal-EDTA chelate.12

13. Structure of a divalent metal-EDTA chelate

14. 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.

15. 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

16. Ex 2 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.

17. Binuclear Complexes : One containing two metal ions Ex:
Polynuclear Complexes:
One containing more than wo metal ions [ Z n 2C l 6 ] C 3 4 Zn 2+
+ Cl
high
conc. metal 2-

18. Stability of the formed complex
Depends on:
A- the complexing ability of metal ion
B- characteristics of ligand.

19. 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.

20. COMPLEXONES Complexone II (EDTA) Complexone I (NTA)
Nitrilotriacetic acid
Complexone IV (CDTA or DCTA)
Trans-1,3-diaminocyclohexanetetracetic acid
Complexone III (Na2-EDTA)

21. Complexone V (EGTA) Complexone VI (TTHA)
Ethylene gylcol bis (2-aminoethylether) tetracetic acid
Complexone VI (TTHA)
Triethylenetetramine hexaacetic acid

22. 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 in Ca/Mg water hardness titration.

23. EDTA has the widest application in analysis because:
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 )

24. 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.

25. Stability with respect to pH of metal-EDTA chelates.
Minimum pH at which complexes exist
Selected metals
1 - 3
4 - 6
8 - 10
Zr4+, Hf4+, Th4+, Bi3+, Fe3+, Hg2+,
Pb2+, Cu2+, Zn2+, Co2+, Ni2+, Al3+,
Mn2+. Fe2+, Al3+, Cd2+, Sn2+
Ca2+, Sr2+, Ba2+, Mg2+

26. Effect of pH on KHZ M-EDTA chelates.
In practice, the stability of metal-EDTA complexes may be altered by:
1- Variation in pH.
2- Presence of other complexing agents. pH
Log KH
Ca-EDTA
K = 5 x 1010
Pb-EDTA
K = 1.1 x 1018
Hg-EDTA
K = 6.3 x 1021
Effect of pH on KHZ M-EDTA chelates.

27. EDTA Titration Curve

28. EDTA titration curves and the effect of KH values. 16 14 12 10 8 6 4 2
0.01 M EDTA, mL pM
104
106
1010
1016
1- KH at the right of the curve.
2-The  KH, the sharper E.p at pH constant .
EDTA titration curves and the effect of KH values.

29. Titration curves for Ca2+ with EDTA at pH 7 and 10. 16 14 12 10 8 6 4 2
0.01 M EDTA, mL
pCa
pH = 7
pH = 10
Titration curves for Ca2+ with EDTA at pH 7 and 10.

30. Volume of complexing agent
1.1
2.1
4.1 pM
Effect of molar ratio on the metal-ligand titration curves

31. Metal Ion (Metallochromic) Indicators
Definition. A compound whose color changes when binds to a metal ion.
Mg—In EDTA  Mg —EDTA In
(red) (colourless) (colourless) (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. .

32. Mg2+ + HIn2-  Mg-Ind + EDTA  Mg-EDTA + Ind.
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 colorless colorless blue
At adjusted pH

33. Some common metal ion indicators
pKa
Colors of free indicator
Color of metal-indicator complex
Eriochrome
Black T
(Erio T)
pK2 = 6.3
pK3 = 11.6
H2In (red)
HIn2 (blue)
In3 (orange)
Wine red
Murexide
pK2 = 8.1
pK3 = 12.4
H4In (red)
H3In2 (violet)
H2In3 (blue)
Red with Ca2+
Yellow with Co2+, Ni2+ & Cu2+
Xylenol orange
pK2 = 2.32
pK3 = 2.85
pK4 = 6.7
pK5 = 10.0
pK6 = 12.23
H5In (yellow)
H4In2 (yellow)
H3In3 (yellow)
H2In4 (violet)
HIn5 (violet)
In6 (violet)
Red

34. The structure of these metal indicators are shown below
(H2In)
Eriochrome Black T
Muroxide
(H4In)
Xylenol orange
(H3In)

35. Mg-Ind. (red)  Ind. (blue)
1-Eriochrome Black T
Wine red Blue Orange
pH  12
Most M-indicator are weak acids
2 acidic H b) Act as acid-base indicator
Free form blue and complex form wine red
Mg-Ind. (red)  Ind. (blue)

36. Free form yellow and complex form red at pH (acidic)
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 1.5-Ex. Direct titration of Thorium and Bi3+.
3) Muroxide:
(H4In)
Reddish
violet 9
Violet
9-11
Bluish
violet
 pH 11
Free form violet and complex form red at pH (10)

37. 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+.

38. Types of EDTA titration
1- Direct titration
2- Back titration
3- Displacement titration
4- Alkalimetric titration
5- Indirect titration
6- Masking titration

39. 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)  Pb-tartarte complex less stable than Pb-EDTA.

40. 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.

41. 2. Back Titrations Ex1: Determination of Al3+:
Excess EDTA is added, pH is adjusted to 7-8, boil, cool, add Erio—T, & back titrate with standard Zn2+:
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+.

42. 3. Displacement Titrations
For metal that don’t have suitable indicator.
For metal not complexed easily with EDTA
Determination of Hg2+:
Ex1: Hg2+ has no suitable indicator
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.
M1 (Sample) + M2—EDTA (Excess)  M1—EDTA + M2 (Titrated with EDTA)
Hg2+ + (Mg—EDTA)2+  (Hg—EDTA) Mg2+

43. 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.
Determination of Ag+:
2 Ag+ + Ni(CN)42-  2Ag(CN) Ni (Titrated with EDTA)

44. 4- Alkalimetric Titration
1- H+ is titrated with standard NaOH using Acid-Base 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.

45. 5- Indirect Titration: (Anions titration)
For anions that can form ppt with metals
M (excess) + anion (sample)  ppt.
then excess metal titrated by EDTA
Ex1: SO4--:
1) SO42 + BaCl2
BaSO4  pH 1
2) Filter-wash:
3) BaSO4  + Excess st EDTA
Ba(EDTA)2-
boil
pH 10
4) xx EDTA # st Mg2+ using suitable indicator.
Ex2: PO43
1) PO43 + Mg2+ + NH4+
MgNH4PO4 
Cool
2) MgNH4PO4  + Excess st EDTA  Mg-EDTA
3) Excess EDTA # Zn2+ ion using suitable Ind.

46. (Ag+ replace Ni2+ ion cyanide in complex)
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.

47. 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.

48. Ex3: Triethanolamine N(Et)3 masks Al3+, Fe3+, Mn2+
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
Ex: Cr3+ & Fe3+ # EDTA
So no interference from Cr3+
Cr3+ # EDTA  slow
Fe3+ # EDTA  rapid

49. (3) Oxidation reduction (adjust oxidation state)
Ex1: Fe3+ & Fe2+  EDTA prefere higher oxidation state of metal to complex with it.
Cu Cu+
red H+
Ascorbic acid or NH2OH
Ex2:
Ex3:
Hg Hg°
red
Ex4: Fe3+ & Cr3+
Add ascorbic acid
Titrate Cr3+ with EDTA
Fe Fe2+
red
Ex5: Removing Fe3+  Fe2+
Mix of Fe3+ & M4+
Ex6: Cr3+ (oxid.)  CrO42+ not react with EDTA

50. [Zn(CN)4]2 + H+ 4CH2O  Zn2+ + 4 HOCH2CN
Demasking:
Release of M from masking agent.
1- CH2O / HAc 2- Chloral hydrate
[Zn(CN)4]2 + H+ 4CH2O  Zn HOCH2CN
Selectivity of EDTA
EDTA is unselective reagent react with all metals.
To increase selectivity:
I) Masking and Demasking: Ex: Mg2+, Zn2+, Cu2+
Mix 10 ml + known excess of EDTA  M-EDTA.
xx EDTA # st Mg2+ using Erio-T pH 10.
V1 = Mg, Zn, Cu
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)

51. II) Control pH: Ex: Bi3+ & Pb2+
Mix # EDTA
pH 2
Xylenol.O
V1 = Bi3+
On same sol # EDTA
hexamine
pH 5
V2 = Pb2+
III) Classical Separation:
Ppt. ion of cations and redissolved the ppt # EDTA
Ca CaOx Ca # EDTA
Ox
redissolved pH 12
(Meuroxide ind.)
Ex: 1)
2) Ni red ppt # EDTA
D.M. gluoxime
Alk.
redissolved
3) Mg MgNH4PO4
Alk.

52. IV) Solvent Excretion:
1) Mix Zn2+, Cu2+ + Excess SCN
Isobutyl-
Methyl keton
Zn(SCN)2
Extract Zn(SCN)2
dil
H2O
# EDTA
2) Zn2+ & Pb
V) Choice of Indicator:
Choice of M-Ind.  rapid complex with Metal.
VI) Removal of Anions:
O-Phosphate removed by ion exchanging resin.

53. Good Luck
Ashraf M. Mahmoud