End point detection in complexometric titration

end point detection , masking &demasking agent

1. METHOD OF END POINT DETECTION IN COMPLExOMETRIC TITRATION RENJITHA J R ASSISTANT PROFESSOR MOUNT ZION COLLEGE OF PHARMACEUTICAL SCIENCES AND RESEARCH,ADOOR

2. End point detection in complexometric titration is mainly performed by two methods. Visual method Physical method

3. 1) Visual methods Includes; Metal / metallochromic indicators p H indicators Redox indicators

4. a) Metal or Metallochromic indicators The simplest and most convenient method of metal analysis is the direct titration of a metal ion with a standardized EDTA solution using a visual colour change of a metal indicator to direct the end point. Metal ion indicators are substances ,usually dyes which produce one colour in the presence of free metal ions and a different colour ( or no colour ) when they are absent. Requirement of indicators The compound must be chemically stable The metal indicators should from a 1: 1 complex which must be weaker than the metal chelon complex. Should have sufficient stability to withstand dilution Colour reaction should be selective to the metal being titrated.

5. b) pH Indicators

6. C) Redox Indicators This method is very limited and can be applied only to special systems. The metal ion must be a component of a redox couple The metal such as Iron must be able to exist in two different oxidation states which are in equilibrium in the solution. Eg ; variamine blue B, is used in titration of ferric ion by EDTA & at the end point the violet blue shade becomes colorless.

7. II) PHYSICAL METHODS Spectrophotometric detection Amperometric titration Potentiometric titration High frequency titrator Coulometric analysis Conductometric titration

8. 1) Spectrophotometric detection The change in absorption spectrum when a metal ion of a complexing agent is converted to the metal complex is observed. 2) Amperometric titration The change in diffusion current is recorded with respect to the addition of titrant. 3) Potentiometric titration The end point of the titration is indicated by a large and rapid change of potential . The change of potential is measured at indicator electrode. 4) High frequency titrator The method is particularly suitable for dilute solution. The ions can titrated directly in buffered solution.

9. pM indicators (metal ion indicators) Classification & eg ; 1) Hydroxy azo compound : mordant black – II Solochrome dark blue 2) Phenolic compounds: xylenol orange catechol violet methyl & thymol blue 3) Anthraquinone dye: alizarin 4) others: murexide diphenyl carbazone variamine blue

10. MASKING AND DEMASKING AGENTS

11. MASKING AGENTS Masking may be defined as the process in which a substance , with out physical separation of it or its reaction products, is so transformed that it does not enter into a particular reaction. Masking agents acts either by precipitation or by formation of complexes more stable than the interfering ion- edetate complex. Masking agent can be defined as a complexing agent that reacts selectively with a component in a solution and in so doing prevents that component from interfering in an analysis. The term masking is used to indicate the determination of a metal in the presence of another metal. The masking can be accomplished by adjusting the pH of the titration medium so that it will be favorable for complexation of the metal being determined and not of the other metal. Eg 1; bismuth will complex at ph 2 in the presence of most other metal and without interference with them. Eg 2; Zinc will not interfere with calcium titration at ph of 13.

12. Eg of masking agents; Ammonium fluoride (mask aluminium , iron, and titanium by complex formation.) Ascorbic acid( mask iron,copper etc..) Dimercaprol ( cobalt and copper but not nickel are displaced form their edetate complexes by dimercaprol ) CH 2 SH – CHSH – CH 2 OH Potassium cyanide (in alkaline solution they form complexes with silver, copper, mercury, iron, zinc, cadmium etc..) Potassium iodide (mask mercuric ion) Tiron (mask aluminium and titanium as colourless complexes) Triethanolamine (forms colourless complex with aluminium ) [N(CH 2 - CH 2 - OH) 3

13. MASKING BY PRECIPITATION Many heavy metals including cobalt, copper, lead can be separated either in the form of insoluble sulphides using sodium sulphide or as insoluble sulphides using sodium sulphide or insoluble complexes using thioacetamide . These are filtered decomposed and titrated with disodium edetate . Common precipitating agents are; Sulphate for lead and barium Oxate for calcium and lead Fluoride for calcium, magnesium and lead Ferrocycinide for zinc and copper 8- hydroxyl quinoline for many heavy metals etc.. p H CONTROL A simple method of masking is based on the fact that the alkaline earth metals don't form edetate complexes below pH 7 . While most transition elements form edetate complexes stable down pH 3.

14. DEMASKING AGENT Demasking is a process in which a masked element or compound is released from its masked form, and regains its activity to enter into reactions with certain substances. Eg ; formaldehyde chloralhydrate Methods Used for Demasking Decomposition of the masking reagent Replacement of the masked ion in a complex by anotherion with which the masking agent forms a stronger complex By change of pH to alter the stability of a complex. By changing the oxidation state of the completed ion. By volatilization of one of the components of the masked system.

15. THANK YOU