complexation reaction 2.0

• Complexometric titration(sometimes chelatometry)

  • a form of volumetric analysis in which the formation of a colored complex is used to indicate the endpoint of a titration.
  • useful in the determination of a mixture of different metal ions in solution.
• • Inorganic products containing metal ions (Al, Bi, Ca, Mg, Zn) are analyzed using gravimetric methods(time consuming-precipitation, filtration, washing, drying)
  • The analytical reagent disodium ethylenediamine tetraacetate (EDTA), a new volumetric procedure evolved for metal determination.
• COMPLEX

  • a molecule or ion formed by the reaction of 2 or more ions or molecules capable of independent existence
  • metal ion combines w/ a molecule which can donate electrons, the resulting compound is termed complex
• CHELATE

  • the complex formed
  • derived from the greek term meaning “clawlike”
  • literally wrap themselves around a metal ion
  • If the combining molecule contains 2 or more groups that donate electrons, this complex is called chelate
• COMPLEXING AGENTS

  • are also called CHELATING AGENTS
  • an agent that has 2 or more groups capable of complexing with a metal ion
• EDTA – the most important chelating agent
• • The most important complexation reaction is between a metal ion and a complexing agent
  • Ethylenediamine tetraacetic acid
  • EDTA reacts w/ metal ions to form a water soluble, stable complex or chelate compound
• • Reaction is rapid and quantitative w/such polyvalent metal ions as Al, Bi, Ca, Cu, Hg, Mg and Zn
  • The 4 oxygen and 2 nitrogen atoms of EDTA capable of entering a complexation reaction w/ a metal ion would make it a hexadentate molecule
• The EDTA molecule w/c provides groups for attachment to metal ions is called Ligand
• EDTA + Ca → Ca-EDTA complex
• USES OF EDTA

  • used in foods
  • promote color retention in dried bananas, beans, frozen potatoes, canned shrimp
  • improve flavor retention in canned beverage, beer, mayonnaise, sauces
  • inhibits rancidity in salad dressings, mayonnaise, sauces
• REAGENT

  • The disodium salt is preferred over the free acid in preparing the standard solution of EDTA. It is much more soluble, non hygroscopic, and very stable
• Indicators

  • Forms colored complex ions w/ the metal ion in high dilution
  • sharpness of color change at endpoint
  • Must give up the metal ion to the titrant EDTA for complexing and not compete w/ it
• TYPES OF COMPLEXOMETRIC TITRATIONS

  • direct titration methods
  • residual titration methods
  • replacement/displacement titration methods
• DIRECT TITRATION METHODS

  • The metal ions determined are Ca, Mg and Zn
  • titrant used: EDTA solution
  • indicators: hydroxynaphthol blue, eriochrome black, xylenol orange
  • used for the analysis of calcium lactate tablet, calcium gluconate
  • it is the simplest and the most convenient method
• RESIDUAL TITRATION METHOD

  • for the analysis of Al, Bi compounds, Mn since direct titration may introduce errors because of the precipitation of the metal as hydroxides in alkaline media
  • Bi forms a highly stable complex and can be titrated at a pH as low as 1 or 2; at a pH of 4, Bi reacts w/ EDTA forming stable complex
  • excess standard EDTA is added
  • titrate with zinc sulfate
  • indicators: dithizone, eriochrome black
• REPLACEMENT/DISPLACEMENT TITRATION

  • when direct or back titration do not give sharp end point, the metal may be determined by displacement
  • the analyte is treated with an excess of a second metal bound to EDTA.
  • the analyte ion displaces the second metal from the EDTA complex, and then the second metal is titrated with EDTA. A typical displacement titration involves Hg2+ as the analyte and MgEDTA at the displacement titrant
• MASKING

  • determination of a metal in the presence of another metal
  • you can also add masking agent that will bind so tightly to a metal ion that it will not titrate with EDTA. These can be used to prevent other ions from interfering in a given titration
• • The masking agent itself is compund capable of entering a complexation reaction
  • Masking can be accomplished by adjusting pH of titration medium so that it will be favorable for complexation of the metal being determined and not of the other metal
• • Bi will complex at pH 2 in the presence of most other metals
  • Zn will not interfere w/ Ca titration at a pH of 13
• Examples of Masking Agent:

  • Potassium cyanide can be used to inactivate or mask or screen such metals as Co, Ni, Cu and Zn
  • Ammonium fluoride will mask Ca, Mg and Al to allow titration of Zn
  • Thioglycols will inactivate metals such as Hg and Cu which react w/sulfhydryl groups thus allowing the titration of Zn at pH 6
  • Other masking agents: ascorbic acid, citrates, tartrates
• Note: the most important aspect is that the masking agent must not form complexes with the metal ion under analysis
• Only Ca is titrated in the sample:

  • Cu + Cyanide ion complex
  • Cd + Cyanide ion complex
  • Ca + Cyanide ion – no reaction
  • titrate w/ EDTA forming Ca +EDTA complex
• Indicators for complexometric titrations:

  • Diphenylamine TS
  • Dithizone TS
  • Eriochrome Black TS – o.o5% aq. soln.
  • Hydroxynaphthol blue
  • Murexide (Acid Ammonium Purpurate)
  • Naphthol green TS
• FERRIC CHLORIDE TITRATION

  • Sodium fluoride determination is based on the reaction of sodium fluoride w/ ferric chloride to form stable complex
  • The sample sodium fluoride is dissolve in water, neutralize, treated w/ NaCl; potassium thiocyanate is used as indicator
• DETERMINATION OF WATER HARDNESS

  • BY ATOMIC ABSORPTION SPECTROPHOTOMETER
  • BY COMPLEXOMETRIC TITRATION
• HARD WATER

  • is due to metal ions (minerals) that are dissolved in the ground water
  • these minerals include Ca, Mg, Fe, SO4and HCO3
  • the concentration of the Ca ion is greater than the concentration of any other metal ions in water
• TYPES OF WATER HARDNESS:

  • Temporary Hardness
  • Permanent Hardness
• Temporary Hardness

  • Due to bicarbonate ion, HCO3 ¯
  • Removed by boiling water to expel carbon dioxide
  • HCO3 ¯ → H2O + CO2 (g)
• Permanent Hardness

  • Due to the presence of ions Ca, Mg, Fe and SO4
  • This type of hardness cannot be eliminated by boiling