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MGDA is a tertiary amine with 3 acetic acid groups. I would like to know whether the amine Nitrogen of MGDA is protonated or not for a given pH value. I could not find any literature data which specifies the molecular form of the amine see here.

Conjugated acids of tertiary amines have a pKa of around 10.8. Even if the pKa value reduce a bit in MGDA (due to electron withdrawing carboxylates), I feel the pKa should still be high enough for the amine to be protonated. However, none of the reported structures of MGDA reports this amine to be protonated. can this be due to steric effects ?

MGDA structure

The same was observed for GLDA, a very similar amine + carboxylate structure See here

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  • $\begingroup$ related chemistry.stackexchange.com/questions/86137/… $\endgroup$
    – Mithoron
    Commented Dec 11, 2023 at 18:21
  • $\begingroup$ It's not a structure of acid but salt. Neutral acid would be mostly a zwitterion - sterics would only slightly hamper protonation. $\endgroup$
    – Mithoron
    Commented Dec 11, 2023 at 18:25

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