J. Technology and Education, Vol.28, No.1, pp.27-36 (2021)

総　説

EDTAの化学構造表記にみられる混乱―分子か双性イオンか

 

野口　大介

長崎大学大学院工学研究科教育研究支援部（〒852-8521　長崎県長崎市文教町1-14）

* a.chemist.noguchi.d@gmail.com

 

Confusion in the Notation of the Chemical Structure of EDTA:

Molecule or Zwitterion?

 

Daisuke NOGUCHI

Division of Education and Research Support, Graduate School of Engineering, Nagasaki University

(Bunkyo-machi 1-14, Nagasaki 852-8521, Japan)

 

(Received April 28, 2021; Accepted May 24, 2021)

 

Abstract

Ethylenediaminetetraacetic acid (EDTA) is one of the well-known chelating ligands learned in analytical chemistry and widely used in such as industry and molecular biology to support our daily lives. It is also demonstrated as an acidic reagent in neutralization titration. The chemical structure indicates that EDTA is a tetrabasic acid; however, confusion of its notation exists in some textbooks. Some authors represent it as a neutral molecule, while others say it is a zwitterion. As the appropriate structure of EDTA seemed unclear, I surveyed some published research papers; herein I show in detail the historical controversy concerning the various proposed structures of EDTA, including non-ionized molecule and double zwitterion with ammonium carboxylate groups, both in solid state and aqueous solution. In the 1960s, some structures based on IR spectroscopic analyses were proposed; earlier studies exhibiting non-ionized EDTA molecular structure were almost overturned. Regrettably, without sufficient reference to each other, the idea of EDTA as a neutral molecule in a solid state still remained. After that, X-ray crystallographic studies confirmed the existence of EDTA as a double zwitterion also in a solid state, whereas an XPS investigation deduced that there are two kinds of nitrogen atoms, =N⁺H– and =N– types, in an EDTA molecule. Recent studies have revisited a double zwitterion form of EDTA in solution; however, some textbooks of analytical chemistry have shown the chemical structure of non-ionized EDTA, subsequently, as if “it returns to the beginning”. The reasons for this difficulty in interpreting the structure of EDTA in aqueous solution are considered to be as follow: (1) the poor solubility of EDTA itself in water made it quite difficult to be measured by IR spectroscopy in solution; (2) unexpected hydrogen bonds could change the frequencies of certain IR-active bonding; (3) there was some unawareness about its states, whether in solid or in solution; (4) the fact that EDTA has two different crystalline modifications with different IR spectra was almost unknown. I hope that the present study will be helpful in making the precise structure of EDTA in solution easily available based on literature for students, with a result that also would be of significance for those who teach and educate chemical analysis and technology.

Key words: Acid-base titration, Chemical structure, Infrared spectroscopy, X-ray analysis, Zwitterion.