COMPLEX FORMATION TITRATION [PDF]

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COMPLEX FORMATION TITRATION

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Complexometry Volumetric method involves reaction of metal with ligand to form complex M+ Electron acceptor Coordinate bond

Ligand Electron donor

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

Sample

Ligand

Buffer

Metal

Indicator

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Sample

Metal

 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

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Ligand Ligand may be:  Mono dentate  Bi dentate  Tri dentate  Multi dentate

Sequestering Chelating agent that form soluble chelates agent Chelating agent Complexing agent

Form complex Ring (sol. or Insol complex)

Form complex

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The most common ligand used is ..

EDTA Ethylene Diamine Tetra Acetic acid di sodium salt Na2H2y. 2H2O

Na Na

EDTA is a typical sequestering agent EDTA is a Secondary Standard .. Why? Due to the presence of impurities from industrial synthesis

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Advantages of EDTA  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

Disadvantage of EDTA  Its reaction is reversible, requires alkaline buffer  It’s non selective reagent

M2+ + H2Y2-

MY2- + 2H+

M3+ + H2Y2-

MY- + 2H+

M4+ + H2Y2-

MY0 + 2H+

Mn+ + H2Y2-

MY(n-4) + 2H+

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Metallochromic Indicators Form colored complex with metals Examples of metallochromic indicators: 1) EBT (Eriochrome black T or Solochrome Black) 2) Murexide (ammonium salt of purpuric acid) 3) Xylenol Orange

Solid powder Solid powder Solution

 Each indicator has a color in the free state and another color in combined ( metallized) state

Requirments for successful use of metal indicators  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

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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

Titration against any other complexating agent

EDTA = Complexon III = Sequesterene

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Metal aqua complex (M.H2Ox)2+

Sample

Indicator

Metal aqua complex

M-Ind complex M/EDTA complex is more stable than M/Ind complex

EDTA

Metal- EDTA complex

M-Ind complex

EDTA

Metal- EDTA complex

+

Free Indicator

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Precautions during complexometric titration Gentle shaking during first 5 mls of titrant After the first 5 mls, VIGEROUS SHAKING with Rapid titration

 Indicator can be increased any time during titration Once end point is reached (free form of indicator), color do NOT change with addition of excess titrant

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DETERMINATION OF NICKEL SAMPLE

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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)

NH3 Buffer

Yellow

Purple (free form)

+

Purple

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EDTA Ni2+

Murexide

Ni/EDTA complex is more stable than Ni/Murexide complex

Before titration

During titration

Ni2+

Murexide

+

EDTA Ni2+

+

EDTA

During titration

Ni2+

At End point

Murexide

Murexide Free form

+

EDTA

Ni2+

Ni2+

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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

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3-Calculation

F

1ml 0.01M EDTA

2

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DETERMINATION OF COPPER SAMPLE

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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) Cu2++NH3

Cu(OH)2

+NH3

[Cu(NH3)4]2+ Copper ammine complex Soluble (blue color)

Copper hydroxide ppt

dil NH3

+ Purple

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EDTA

Cu/EDTA complex is more stable than both Cu/amine complex and Cu/Ind complex

[Cu(NH3)4]2+ Cu-Murexide Blue + Yellow EDTA reacts first with copper ammine complex because it is the less stable than CuInd complex During titration, color gets lighter

End point: purple (free form of indicator)

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2- Procedure In 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

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3-Calculation Na2H2Y. 2H2O+ CuSO4.5H2O

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Na2CuY+ H2SO4+ 7H2O

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DETERMINATION OF LEAD SAMPLE

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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)

pH =5-6 .. Why ? For maximum stability of Pb/EDTA complex, to increase selectivity

Hexamine

violet red

+

yellow

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2- Procedure In Conical Flask 10 ml Sample + 2 ml Hexamine + 2 dps Xylenol Orange (violet red color)

Titrate against 0.01M EDTA End point: yellow

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3 Calculation

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DETERMINATION OF ZINC SAMPLE

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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 Violet

EDTA

Zn-EDTA+ free EBT Full Blue

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2- Procedure 10 ml Sample + 2 ml NH3 buffer + few speaks of EBT (Violet) Titrate against 0.01M EDTA End point: full blue

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3-Calculation

1 ml 0.01M EDTA

= Mwt.of ZnSO4.7H2O = 0.002874g 100Χ1000

+2

Conc.of Zn = mlsΧ fΧFΧ1000 = 10

g/l

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Thank You T.A. Aya Ahmed Analytical chemistry department