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We know that in amino acids, carboxylic acid groups deprotonate first than the amine groups. For example, in alanine, $\mathrm{p}K_\mathrm{a,\ce{COOH}} = 2.34$, $\mathrm{p}K_\mathrm{a,\ce{NH3+}} = 9.69$ according to this page.

But according to the structures of EDTA as searched from Google, and from this extract from p.325 of Analytical Chemistry, 8e by Christian, et al.:

At sufficiently low pH, the nitrogens can also be protonated and this diprotonated EDTA can be considered a hexaprotic acid. However, this occurs at a very low pH and EDTA is almost never used under such conditions.

the hydrogens of these amino groups seem to be deprotonated first before those of the four carboxylic acid groups. Why is this so?

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    $\begingroup$ Before you ask "why", ask "if". $\endgroup$ Nov 21, 2017 at 5:14
  • $\begingroup$ How many covalent bonds can N form with other elements? $\endgroup$
    – wolphram
    Nov 21, 2017 at 8:33
  • $\begingroup$ There used to be a place on the internet where you could enter compounds and things such as acidity constants were calculated, including which proton deprotonated in which order. Alas, I can’t find the site any more. $\endgroup$
    – Jan
    Nov 21, 2017 at 14:31
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    $\begingroup$ I mean you should not blindly trust some random images brought to you by Google image search. Trust your logic; it is sound. $\endgroup$ Nov 24, 2017 at 12:02
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    $\begingroup$ @IvanNeretin, okay, I added another reference that supports this ionization behavior of EDTA. $\endgroup$
    – Acnologia
    Nov 25, 2017 at 3:24

2 Answers 2

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Dissolved in water, EDTA can have net charges varying from -4 to +2, from fully deprotonated to all four carboxylic acid groups and two amine groups protonated.

It is the fully deprotonated form that binds to divalent cations (see e.g. https://chemistry.stackexchange.com/a/81988/72973). If necessary, deprotonation occurs upon binding (see e.g. https://doi.org/10.1016/S0301-4622(99)00047-2).

The intermediate protonation states occur as zwitterions both in aqueous solution and in some of the crystal forms. Disodium EDTA is an example:

enter image description here

Source: https://pubchem.ncbi.nlm.nih.gov/compound/57339238#section=2D-Structure

[OP] the hydrogens of these amino groups seem to be deprotonated first before those of the four carboxylic acid groups. Why is this so?

In aqueous solution, this carboxylic acid deprotonates before the ammonium. If you see other statements, they ignored to mention the prevalent tautomeric state. The situation in organic solvents might be different. The neutral species might have the carboxylic acid groups protonated and the amines deprotonated when EDTA is dissolved in a no-ionic organic solvent. The cationic or anionic species would probably not be soluble, so for those species, the question is moot.

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The carboxylates deprotonate first followed by the amines. The pKa values for the carboxylates ar 0.0, 1.5, 2.0, and 2.66. The amines have pKa values of 6.16 and 10.24. Ref. Harris. Quantitative Chemical Analysis.

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  • $\begingroup$ Yeah, that was wrong, I edited it now, but my question still stands. $\endgroup$
    – Acnologia
    Apr 20, 2018 at 1:44

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