Complex Formation Titration

1 COMPLEX FORMATION TITRATION 2 Complexometry Volumetric method involves reaction of metal with ligand to form complex M+ Ligand Electron acceptor Coordinate bond 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 Ligand Buffer Sample Metal Indicator 3 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 4 Ligand Ligand may be: Mono dentate Bi dentate Tri dentate Multi dentate Sequestering Chelating agent that agent form soluble chelates Chelating Form complex Ring (sol. or agent Insol complex) Complexing agent Form complex 5 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 6 Advantages of EDTA Disadvantage of EDTA Cheap, commercially available Its reaction is reversible, Versatile, reacts with most requires alkaline buffer metals Form stable complexes with It’s non selective most metals (stable than metal reagent indicator complex) Reacts in ratio 1:1 with metals 2+ 2- 2- + M + H2Y MY + 2H 3+ 2- - + M + H2Y MY + 2H 4+ 2- 0 + M + H2Y MY + 2H n+ 2- (n-4) + M + H2Y MY + 2H 7 Metallochromic Indicators Form colored complex with metals Examples of metallochromic indicators: 1) EBT (Eriochrome black T or Solochrome Black) Solid powder 2) Murexide (ammonium salt of purpuric acid) Solid powder 3) Xylenol Orange 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 8 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 EDTA = Complexon III = agent Sequesterene 9 Metal aqua complex Sample 2+ (M.H2Ox) Indicator Metal aqua M-Ind complex complex EDTA M/EDTA complex is more stable than M/Ind complex Metal- EDTA M-Ind complex complex EDTA Free Metal- EDTA complex + Indicator 10 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 11 DETERMINATION OF NICKEL SAMPLE 12 1- Principle Direct Complexometry 2+ Ni 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) NH 3 + Buffer Yellow Purple 13 Ni/EDTA complex is more stable than EDTA Ni/Murexide complex Ni2+ Murexide Before titration During titration Ni2+ Murexide + EDTA Ni2+ During titration Ni2+ Murexide + EDTA Ni2+ At End 2+ point Murexide + EDTA Ni Free form 14 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 15 3-Calculation F 1ml 0.01M EDTA 2 16 DETERMINATION OF COPPER SAMPLE 17 1- Principle Direct Complexometry 2+ Cu e.g. CuSO4.5H2O Directly titrated against EDTA in presence of dil. NH3 using Murexide as indicator End point: Purple (free form) 2+ +NH3 2+ Cu +NH3 Cu(OH)2 [Cu(NH3)4] Copper Copper ammine complex hydroxide ppt Soluble (blue color) dil + NH3 Purple 18 Cu/EDTA complex is more stable EDTA than both Cu/amine complex and Cu/Ind complex 2+ [Cu(NH3)4] Cu-Murexide Blue + Yellow EDTA reacts first with copper ammine complex because it is the less stable than Cu- Ind complex During titration, color gets lighter End point: purple (free form of indicator) 19 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 20 3-Calculation Na2H2Y. 2H2O+ CuSO4.5H2O Na2CuY+ H2SO4+ 7H2O 2 21 DETERMINATION OF LEAD SAMPLE 22 1- Principle Direct Complexometry 2+ Pb 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 23 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 24 3 Calculation 2 25 DETERMINATION OF ZINC SAMPLE 26 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) EDTA Zn-EBT Zn-EDTA+ free EBT Violet Full Blue 27 2- Procedure 10 ml Sample + 2 ml NH3 buffer + few speaks of EBT (Violet) Titrate against 0.01M EDTA End point: full blue 28 3-Calculation 1 ml 0.01M EDTA = Mwt.of ZnSO4.7H2O = 0.002874g 100Χ1000 +2 Conc.of Zn = mlsΧ fΧFΧ1000 = g/l 10 29 Thank You T.A. Aya Ahmed Analytical chemistry department.

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