COMPLEX FORMATION TITRATION

Transcription

1 1 COMPLEX FORMATION TITRATION

2 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 2 Ligand Buffer Sample Metal Indicator

3 Sample Metal 3 The tendency to form complex is inherent property in all metals Aqua complex Therefore Metals form with water (solvated metal ion) as oxygen of water donate electrons to metal ions Complexation reaction is the replacement of solvent molecules by ligand

4 4 Ligand may be: Ligand Mono dentate Bi dentate Tri dentate Multi dentate Complexing agent Chelating agent Sequestering agent Form complex Ring (sol. or Insol complex) Form complex Chelating agent that form soluble chelates

5 5 The most common ligand used is.. EDTA Ethylene Diamine Tetra Acetic acid di sodium salt Na 2 H 2 y. 2H 2 O 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 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 6 M 2+ + H 2 Y 2- MY H + M 3+ + H 2 Y 2- MY - + 2H + M 4+ + H 2 Y 2- MY 0 + 2H + M n+ + H 2 Y 2- MY (n-4) + 2H +

7 Metallochromic Indicators Form colored complex with metals 7 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

8 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 Titration against EDTA EDTA = Complexon III = Sequesterene Compleximetry Titration against any other complexating agent

9 Metal aqua complex (M.H 2 O x ) 2+ Indicator Sample 9 Metal aqua complex EDTA Metal- EDTA complex EDTA M-Ind complex M-Ind complex M/EDTA complex is more stable than M/Ind complex Metal- EDTA complex + Free Indicator

10 Precautions during complexometric titration 10 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 11 DETERMINATION OF NICKEL SAMPLE

12 12 1- Principle Direct Complexometry Ni 2+ e.g. NiSO 4 Directly titrated against EDTA in presence of NH 3 buffer (ph=10) using Murexide as indicator End point: Yellow (Metallized form) Purple (free form) Yellow NH 3 Buffer + Purple

13 13 EDTA Ni/EDTA complex is more stable than Ni/Murexide complex Ni 2+ Murexide Before titration During titration Ni 2+ Murexide EDTA Ni 2+ During titration Ni 2+ Murexide + EDTA Ni 2+ + At End point Murexide Free form + EDTA Ni 2+

14 14 2- Procedure In Conical Flask 10 ml Sample + 2 ml NH 3 buffer + few specks Murexide (yellow color) Titrate against 0.01M EDTA End point: Purple

15 3-Calculation F 15 1ml 0.01M EDTA 2

16 16 DETERMINATION OF COPPER SAMPLE

17 17 1- Principle Direct Complexometry Cu 2+ e.g. CuSO 4.5H 2 O Directly titrated against EDTA in presence of dil. NH 3 using Murexide as indicator End point: Purple (free form) Cu 2+ +NH +NH 3 Cu(OH) 3 2 [Cu(NH 3 ) 4 ] 2+ Copper hydroxide ppt Copper ammine complex Soluble (blue color) dil NH 3 + Purple

18 EDTA 18 Cu/EDTA complex is more stable than both Cu/amine complex and Cu/Ind complex [Cu(NH 3 ) 4 ] 2+ 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 19 2- Procedure In Conical Flask 10 ml Sample + 2 ml dil NH 3 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 NH 3 Auxillary complexing agent Give the suitable ph for formation of Cu/EDTA complex

20 20 3-Calculation Na 2 H 2 Y. 2H 2 O+ CuSO 4.5H 2 O Na 2 CuY+ H 2 SO 4 + 7H 2 O 2

21 21 DETERMINATION OF LEAD SAMPLE

22 22 1- Principle Direct Complexometry Pb 2+ e.g. (CH 3 COO) 2 Pb 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 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 24 3 Calculation 2

25 25 DETERMINATION OF ZINC SAMPLE

26 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) Zn-EBT Violet EDTA Zn-EDTA+ free EBT Full Blue

27 27 2- Procedure 10 ml Sample + 2 ml NH 3 buffer + few speaks of EBT (Violet) Titrate against 0.01M EDTA End point: full blue

28 28 3-Calculation 1 ml 0.01M EDTA = Mwt.of ZnSO 4.7H 2 O = g 100Χ Conc.of Zn = mlsχ fχfχ1000 = 10 g/l

29 29 Thank You T.A. Aya Ahmed Analytical chemistry department