Question

explain why tetradentate or hexadentate ligands are more satisfactory as titrants in EDTA titrations than unidentate ligands

Answer 1

• Multidentate ligands like EDTA have more electron pairs available and thus they form more bonds. They form Chelate complex with Transition Metals.
• Multidentate ligands tend to form 1:1 complexes producing a clear end point.
  
  1:1 Complexes formed with EDTA, for example, have a very simple formation constant.
  For the following reaction :
  
  M + EDTA ->(equilibrium) M.EDTA
  
  The formation constant for above reaction
  K_f= [M.EDTA]/[M][EDTA]
  which is simple, as compared to reactions where a unidentate ligand reacts with a metal ,
  
  M + nL --> ML_n
  K_f for this reaction is more complex than the one we obtained above.
  
  Therefore, We can conclude that EDTA or simply any multi-dentate ligand simplifies the equilibrium equation.
• For multi-dentate ligands, the overall interaction between the central metal atom and the ligands is very strong as compared to single interaction.