Aminopolycarboxylic acids

The aminopolycarboxylic acids are used principally as chelating agents, and a large proportion is used in water treatment and cleaning formulations. The data of Table 5 include nitnlotriacetic acid (NTA), most of which was exported. 

The formation of a single complex species rather than the stepwise production of such species will clearly simplify complexometric titrations and facilitate the detection of end points. Schwarzenbach2 realised that the acetate ion is able to form acetato complexes of low stability with nearly all polyvalent cations, and that if this property could be reinforced by the chelate effect, then much stronger complexes would be formed by most metal cations. He found that the aminopolycarboxylic acids are excellent complexing agents the most important of these is 1,2-diaminoethanetetra-aceticacid (ethylenediaminetetra-acetic acid). The formula (I) is preferred to (II), since it has been shown from measurements of the dissociation constants that two hydrogen atoms are probably held in the form of zwitterions. The values of pK are respectively pK, = 2.0, pK2 = 2.7,... 

Edta and other aminopolycarboxylic acids as chromogenic agents. F. Bermejo-Martinez, Chelates Anal. Chem., 1976, 5, 1-161 (266). 

Figure 10.5 Effect of pH on the conditional stability constants at 25 °C of Fe(m) and Fe(n) chelates of aminopolycarboxylic acids [20]...

Another aminopolycarboxylic acid, closely related to IMDA and forming strong complexes with the lanthanide ions, is nitrilotriacetic acid (NTA). The hy-... 

Bucheli-Witschel, M., and T. Egli, Environmental fate and microbial degradation of aminopolycarboxylic acids , FEMS Microbiol. Reviews, 25, 69-106 (2001). 

Egli, T., Biodegradation of metal-complexing aminopolycarboxylic acids , J. Biosci. andBioeng., 92, 89-97 (2001). 

Nitrilotriacetic acid N(CH2C02H)3 (nta) was first obtained by Heintz in 1862 from an ammon-iacal solution of monochloroacetic acid. Other products, i.e. glycolic, aminoacetic and iminodiacetic acids, were also formed. The next paper concerned with the preparation of aminopolycarboxylic acids appeared after 1930. A procedure similar to that given above, based on the treatment of ethylenediamine with monochloroacetic acid in presence of sodium hydroxide, was reported in 1935 by I.G.-Farbenindustrie.5 In a more recent version of the chloroacetic acid process, reaction (1) is carried out at 40-90 °C. For the preparation of edta, good yields of the tetrasodium salt can be obtained using appropriate conditions. The conversion to the insoluble free acid is achieved by acidification with a mineral acid. 

Smith, R.M. and Martell, A.E. (1987) Critical stability constants, enthalpies and entropies for the formation of metal complexes of aminopolycarboxylic acids and carboxylic acids. Sci. Total Environ., 64, 125. 

EDTA has been determined in a wide variety of sample matrices by HPLC. These matrices include waste waters, natural waters, sediments, fertilizers, chemical cleaning solutions, radioactive waste solutions, and pharmaceutical preparations. Chinnick reported the separation and identification of EDTA and other aminopolycarboxylic acid sequestrants by a high performance liquid chromatographic (HPLC) method [57]. 

A reversed-phase HPLC method was used by Yamaguchi et al for the determination of EDTA and other aminopolycarboxylic acids as their Fe(III) complexes [58]. The conditions used in other HPLC methods [58-83] are listed in Table 4. 

Up to pH 2, edta (LH4) forms (TiOHL)-, and (TiOL)2 above this pH.190 Ternary complex formation in the titanium-polyphenol-aminopolycarboxylic acid system has been studied.191 Tiron forms ternary chelates with nitrilotriacetic acid, edta, and [(H02CCH2)2NCH2CH2]2NCH2C02H. Pyrocatechol, dibromogallic acid, and gallic acid form mixed complexes only with nitrilotriacetic acid. Chromotropic acid forms a complex192 with Ti,v but not with the Ti -polyphenol system. 

All of the processes used to make aminopolycarboxylic acid chelating agents are similar. They can be illustrated by the route to the tetrasodium salt of... 

Other aminopolycarboxylic acids are produced by substituting organic polyamines (e.g., diethylenetriamine) or ammonia for the ethylenediamine. NTA is made via the two-step process using ammonia (instead of ethylenediamine), formaldehyde and HCN as starting materials. The resulting cyanomethyl intermediate is subsequently hydrolyzed in a separate step to the free acid or sodium salt250. 

In the case of Fem complexes with chelating ligands, generation of ligand radicals is observed. Thus, in the case of the Fem complexes with aminopolycarboxylic acids, eg [Femedta] , photoreduction of Fem - Fe11 is accompanied by decarboxylation and formation of the respective organic radical [11,20-22. ... 

The complex formation constants of rare earth ions with a variety aminopolycarboxylic acids are sufficiently different for the purpose of separation of the ions by ion-exchange methods. 

End-points can be determined more easily when a single complex is formed rather than when the complex is formed in a step-wise fashion. This can be achieved by using the aminopolycarboxylic acid, EDTA (Fig. 23.2). 

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