Complexometry

complexometric titration

1. COMPLEXOMETRIC TITRATIONS SHILPA JOY ASSISTANT PROFESSOR DEPARTMENT OF CHEMISTRY KURIAKOSE ELIAS COLLEGE MANNANAM

2. Complexometric titrations or Chelatometry Volumetric analysis in which the formation of colored complex is used to indicate the end point of a titration. Estimation of metal ion in solution Stability of the complex formed is the determining factor Indicator- unambiguous color change.

3. Reaction reaches equilibrium rapidly after each portion of titrant is added 2. Interfering situations do not arise 3. Complexometric indicator capable of locating equivalence point with fair accuracy is available 4. the technique involves titrating metal ions with a complexing agent or chelating agent( ligand ) Metal ion + complexing agent complex ion

4. EDTA – Ethylene diamine tetraacetic acid- titrant Erio -chrome black-T --- indicator

5. 4- hydroxyl groups and two amine groups- act as electron pair donor The ability of EDTA to donate 6 lone pairs of electrons for the formation of coordinate covalent bonds to metal cations makes EDTA a hexadentate ligand . EDTA is usually only partially ionized form , thus forms fewer than 6 coordinate covalent bonds with the metal.

6. Disodium EDTA is commonly used to standardise aqueous solutions of metal cations . Na2H2Y forms 4 coordinate covalent bond at a Ph <12 . EDTA forms an octahedral complex with most M+2 ions in aqueous solution

7. EDTA is extensively used in the standardisation of metal cation solutions since the formation constant for the most metal cation –EDTA complexes are very high. M 2+ + H 4 Y MH 2 Y + 2H + equilibrium lies far to the right. carrying out the reaction in basic buffer solution removes H + as it is formed , which also favours the formation of the complex product . Formation of metal cation –EDTA complex goes on completion- advantage

8. Magnesium Calcium Cadmium Lead Copper Nickel cobalt Iron Bismuth Thorium Zironium Direct determination Analyte is titrated with standard EDTA solution buffered at a pH where kf is large .

9. Back titration 4 dozen of elements- direct titration with EDTA with a suitable metal ion indicator non availability of suitable indicator– direct titration is not possible In back titration , excess standard solution of EDTA is added to the analyte and excess EDTA or unreacted EDTA is back titrated against a standard solution of Mg+2 or Zn+2 using eriochrome black T

10. MASKING OF IONS Mg+2 or Ca+2 ions can be titrated in the presence of interfering Cd+2,Cu+2,or Ni+2 by adding cyanide ion reaction mixture Cyanide act as masking agent It can form complexes with Cd+2,Cu+2,or Ni+2 thereby eliminating interference

11. Advantages Estimates hardness of water –calcium and magnesium ions. EDTA is a convenient reagent , it always react with metals 1:1 basis, making calculation easy. Selection of solution Ph allows to determine one metal in the presence of other Cations with high charges (Fe+3, Bi+3) have large stability constant,so they can be titrated at low pH , in the presence of divalent cations which will not interfere in this condition .

12. Indicators Eriochrome Black T Eriochrome Red B Murexide Fast Sulphon Black .

13. Triethanolamine – comlpexant to mask Aluminium ions- before performing titration Multiple metal ins can be subsequently titrated by careful control of the pH

14. These dyes binds metal cations in the solution to form colored complexes . Since EDTA binds to the metal cations more strongly than does the dye used as an indicator, the EDTA will displace the dye from the metal cations as it is added to the solution of analyte MgEbT (red) + EDTA(colourless) Mg Edta (colourless)+ EbT (blue)

15. Titration curve