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Does the stability of EDTA-metal complex increases or decreases with the pH of the solution?
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Kashaf Umar
Does the stability of EDTA-metal complex increases or decreases with the pH of the solution?
EDTA- ethylene. Diaminetetraacetic acid sodium salt, binds metal ions through the negatively charged caboxylate groups and the neutral amino groups. At lower Ph values the caboxylate groups become neutral and the amino groups may be protonated. If pH is too high, then precipitation may compete with EDTA complexation. Most references seem to suggest solutions buffered at pH around 6.
EDTA- ethylene. Diaminetetraacetic acid sodium salt, binds metal ions through the negatively charged caboxylate groups and the neutral amino groups. At lower Ph values the caboxylate groups become neutral and the amino groups may be protonated. If pH is too high, then precipitation may compete with EDTA complexation. Most references seem to suggest solutions buffered at pH around 6.
Barring any complicating factors, complexation of EDTA (ethylene diamine tetra acetic acid) takes place primarily from the two nitrogens, followed by chelation interactions through the four carboxylic acid (or carboxylate) moieties in this multi-dentate ligand.
As the pH of the solution increases (becomes more basic), the lone-pairs on the nitrogen become more “available”, due to deprotonation of the NH+ species.
~NH+ + OH- <==> ~N: + H2O
This fact alone will improve the ligating ability of the nitrogen, and the consequent stability of the [EDTA-M+] complex.
In addition, at even higher solution pH, the carboxylic acids can get successively de-protonated, and the resulting ~COO- carboxylates can be powerful chelating ligands.
All told, the chelating ability of EDTA ligand, and the consequent stability of the Metal-EDTA complex should increase with pH.
There are always limits and caveats to this general rule. Ability for chelation, pi-backbonding etc. can cause disruption to the general rule.
Barring any complicating factors, complexation of EDTA (ethylene diamine tetra acetic acid) takes place primarily from the two nitrogens, followed by chelation interactions through the four carboxylic acid (or carboxylate) moieties in this multi-dentate ligand.
As the pH of the solution increases (becomes more basic), the lone-pairs on the nitrogen become more “available”, due to deprotonation of the NH+ species.
~NH+ + OH- <==> ~N: + H2O
This fact alone will improve the ligating ability of the nitrogen, and the consequent stability of the [EDTA-M+] complex.
In addition, at even higher solution pH, the carboxylic acids can get successively de-protonated, and the resulting ~COO- carboxylates can be powerful chelating ligands.
All told, the chelating ability of EDTA ligand, and the consequent stability of the Metal-EDTA complex should increase with pH.
There are always limits and caveats to this general rule. Ability for chelation, pi-backbonding etc. can cause disruption to the general rule.
EDTA- ethylene. Diaminetetraacetic acid sodium salt, binds metal ions through the negatively charged caboxylate groups and the neutral amino groups. At lower Ph values the caboxylate groups become neutral and the amino groups may be protonated. If pH is too high, then precipitation may compete with EDTA complexation. Most references seem to suggest solutions buffered at pH around 6.
EDTA- ethylene. Diaminetetraacetic acid sodium salt, binds metal ions through the negatively charged caboxylate groups and the neutral amino groups. At lower Ph values the caboxylate groups become neutral and the amino groups may be protonated. If pH is too high, then precipitation may compete with EDTA complexation. Most references seem to suggest solutions buffered at pH around 6.
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Barring any complicating factors, complexation of EDTA (ethylene diamine tetra acetic acid) takes place primarily from the two nitrogens, followed by chelation interactions through the four carboxylic acid (or carboxylate) moieties in this multi-dentate ligand.
As the pH of the solution increases (becomes more basic), the lone-pairs on the nitrogen become more “available”, due to deprotonation of the NH+ species.
~NH+ + OH- <==> ~N: + H2O
This fact alone will improve the ligating ability of the nitrogen, and the consequent stability of the [EDTA-M+] complex.
In addition, at even higher solution pH, the carboxylic acids can get successively de-protonated, and the resulting ~COO- carboxylates can be powerful chelating ligands.
All told, the chelating ability of EDTA ligand, and the consequent stability of the Metal-EDTA complex should increase with pH.
There are always limits and caveats to this general rule. Ability for chelation, pi-backbonding etc. can cause disruption to the general rule.
reference: Complexation Titration
Barring any complicating factors, complexation of EDTA (ethylene diamine tetra acetic acid) takes place primarily from the two nitrogens, followed by chelation interactions through the four carboxylic acid (or carboxylate) moieties in this multi-dentate ligand.
As the pH of the solution increases (becomes more basic), the lone-pairs on the nitrogen become more “available”, due to deprotonation of the NH+ species.
~NH+ + OH- <==> ~N: + H2O
This fact alone will improve the ligating ability of the nitrogen, and the consequent stability of the [EDTA-M+] complex.
In addition, at even higher solution pH, the carboxylic acids can get successively de-protonated, and the resulting ~COO- carboxylates can be powerful chelating ligands.
All told, the chelating ability of EDTA ligand, and the consequent stability of the Metal-EDTA complex should increase with pH.
There are always limits and caveats to this general rule. Ability for chelation, pi-backbonding etc. can cause disruption to the general rule.
reference: Complexation Titration
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