29
COMPLEX FORMATION TITRATION 1
COMPLEX FORMATION
TITRATION
1
Complexometry
Volumetric method involves reaction of metal with ligand
to form complex
M+
Electron acceptor
Ligand
Electron donorCoordinate bond
Complex formation is a type of acid base reaction according to lewis
concept,
where metal ion is lewis acid (electron acceptor) and ligand is lewis
base electron donor
Ligand Buffer
Metal IndicatorSample
2
The tendency to form complex is inherent property in all metals
Therefore Metals form with water Aqua complex (solvated metal ion) as oxygen of water donate electrons to metal
ions
Complexation reaction is the replacement of solvent molecules by ligand
SampleMetal
3
Ligand
Complexing
agent
Chelating
agent
Sequestering
agent
Form complex
Form complex Ring (sol. or
Insol complex)
Chelating agent that
form soluble chelates
Ligand may be:
Mono dentate
Bi dentate
Tri dentate
Multi dentate
4
The most common ligand used is ..
EDTAEthylene Diamine Tetra Acetic acid di sodium salt
Na2H2y. 2H2O
EDTA is a Secondary Standard .. Why?
Due to the presence of impurities from industrial synthesis
Na
Na
EDTA is a typical sequestering agent
5
Cheap, commercially available
Versatile, reacts with most
metals
Form stable complexes with
most metals (stable than metal
indicator complex)
Reacts in ratio 1:1 with metals
Advantages of EDTA
Mn+ + H2Y2- MY(n-4) + 2H+
Its reaction is reversible,
requires alkaline buffer
It’s non selective
reagent
Disadvantage of EDTA
M4+ + H2Y2- MY0 + 2H+
M3+ + H2Y2- MY- + 2H+
M2+ + H2Y2- MY2- + 2H+
6
Metallochromic Indicators
Examples of metallochromic indicators:
1) EBT (Eriochrome black T or Solochrome Black)
2) Murexide (ammonium salt of purpuric acid)
3) Xylenol Orange
Each indicator has a color in the free state and another color
in combined ( metallized) state
Form colored complex with metals
M/EDTA complex is more stable than M/Ind complex Indicator Free color should be distinguished from M/Ind color
Most metal indicators are acid-base indicators so, their color
change due to pH
Indicator is not necessary to be specific but at least selective
Solid powder
Solid powder
Solution
Requirments for successful use of metal indicators
7
Alkaline buffer is used in complexometric
titrations .. Why
1. Shift reaction between EDTA and metal forward, to prevent
the reversibility of the reaction
2. Make color change at end point due to change in metal
concn. not due to pH as most metal indicators are also acid
base indicators
ComplexOmetry Compleximetry
Titration against EDTA
EDTA = Complexon III =
Sequesterene
Titration against any
other complexating
agent
8
Metal aqua complex
(M.H2Ox)2+
Metal aqua
complex
M-Ind complex
Metal- EDTA
complex
M-Ind complex
Metal- EDTA complexFree
Indicator
Sample
Indicator
EDTA
EDTA
+
M/EDTA complex is more
stable than M/Ind complex
9
Precautions during complexometric titration
Gentle shaking during first 5 mls of titrant
After the first 5 mls, VIGEROUS SHAKING
with Rapid titration
Once end point is reached (free form of
indicator), color do NOT change with
addition of excess titrant
Indicator can be increased any time during titration
10
DETERMINATION
OF
NICKEL SAMPLE
11
1- Principle
Direct Complexometry
Ni2+ e.g. NiSO4
Directly titrated against EDTA in presence of NH3 buffer
(pH=10) using Murexide as indicator
End point: Yellow (Metallized form) Purple (free form)
NH3
Buffer+
Yellow Purple
12
Ni2+ Murexide
Ni2+ Ni2+
Ni2+
Ni2+
Ni2+MurexideAt End
point
EDTA
EDTA
EDTA
EDTA
Before titration
+
+
+
During titration
During titration
Ni/EDTA complex is more stable than
Ni/Murexide complex
Murexide
Murexide
Free form
13
2- Procedure
In Conical Flask
10 ml Sample
+ 2 ml NH3 buffer
+ few specks Murexide (yellow color)
Titrate against 0.01M EDTA
End point: Purple
14
3-CalculationF
2
1ml 0.01M EDTA
15
DETERMINATION
OF
COPPER SAMPLE
16
1- Principle
Direct Complexometry
Cu2+ e.g. CuSO4.5H2O
Directly titrated against EDTA in presence of dil. NH3
using Murexide as indicator
End point: Purple (free form)
dil
NH3
Cu2++NH3 Cu(OH)2 [Cu(NH3)4]2++NH3
Copper
hydroxide ppt
Copper ammine complex
Soluble (blue color)
+
Purple
17
[Cu(NH3)4]2+ Cu-Murexide
EDTA
EDTA reacts first with copper ammine
complex because it is the less stable than Cu-
Ind complex
Blue + Yellow
End point: purple
(free form of
indicator)
Cu/EDTA complex is more stable
than both Cu/amine complex and
Cu/Ind complex
During titration, color gets lighter
18
2- ProcedureIn Conical Flask
10 ml Sample
+ 2 ml dil NH3 drop wise
till the ppt formed dissolve to give Copper
ammine complex (Blue color)
+ few specks Murexide (Dark green color)
Titrate against 0.01M EDTA
End point: Purple
Role of dil NH3
Auxillary complexing agent
Give the suitable pH for formation of Cu/EDTA complex
19
3-Calculation
Na2H2Y. 2H2O+ CuSO4.5H2O Na2CuY+ H2SO4+ 7H2O
2
20
DETERMINATION
OF
LEAD SAMPLE
21
1- Principle
Direct Complexometry
Pb2+ e.g. (CH3COO)2Pb
Directly titrated against EDTA in presence of Hexamine
(pH=5-6) using Xylenol orange as indicator
End point: violet red yellow
(metallized form) (free form)
Hexamine +
pH =5-6 .. Why ?
For maximum stability of Pb/EDTA complex, to increase selectivity
violet red yellow
22
2- ProcedureIn Conical Flask
10 ml Sample
+ 2 ml Hexamine
+ 2 dps Xylenol Orange (violet red color)
Titrate against 0.01M EDTA
End point: yellow
23
3 Calculation
2
24
DETERMINATION
OF
ZINC SAMPLE
25
1- Principle
zinc is determined by direct complexometric titration
against EDTA using EBT as indicator in presence of
ammonia buffer (pH=10)
End point: Violet Full Blue
(metallized form) (free form)
Zn-EBT Zn-EDTA+ free EBTEDTA
Violet Full Blue
26
2- Procedure
10 ml Sample
+ 2 ml NH3 buffer
+ few speaks of EBT (Violet)
Titrate against 0.01M EDTA
End point: full blue
27
3-Calculation
1 ml 0.01M EDTA = Mwt.of ZnSO4.7H2O = 0.002874g
100Χ1000
Conc.of Zn = mlsΧ fΧFΧ1000 = g/l
10
+2
28
Thank YouT.A. Aya Ahmed
Analytical chemistry department
29
