Iron chelates such as ethylenediamine-N,N'-bis(2-hydroxyphenyl)acetic
acid (
EDDHA) and their analogues are the most efficient soil
fertilizers to treat
iron chlorosis in plants growing in calcareous
soils.
EDDHA, EDDH4MA (ethylenediamine-N,N'-bis(2-hydroxy-4-methylphenyl)acetic acid), and EDDCHA (ethylenediamine-N,N'-bis(2-hydroxy-5-carboxyphenyl)acetic acid) are allowed by the European directive, but also EDDHSA (ethylenediamine-N,N'-bis(2-hydroxy-5-sulfonylphenyl)acetic acid) and EDDH5MA (ethylenediamine-N,N'-bis(2-hydroxy-5-methylphenyl)acetic acid) are present in several commercial
iron chelates. In this study, these
chelating agents as well as p,p-
EDDHA (ethylenediamine-N,N'-bis(4-hydroxyphenyl)acetic acid) and EDDMtxA (ethylenediamine-N,N'-bis(2-metoxyphenyl)acetic acid) have been obtained following a new synthetic pathway. Their chemical behavior has been studied to predict the effect of the substituents in the
benzene ring on their efficacy as
iron fertilizers for
soils above pH 7. The purity of the
chelating agents has been determined using a novel methodology through spectrophotometric titration at 480 nm with Fe(3+) as titrant to evaluate the inorganic impurities. The protonation constants were determined by both spectrophotometric and potentiometric methods, and Ca(2+) and Mg(2+) stability constants were determined from potentiometric titrations. To establish the Fe(3+) and Cu(2+) stability constants, a new spectrophotometric method has been developed, and the results were compared with those reported in the literature for
EDDHA and
EDDHMA and their meso- and rac-isomers. pM values have been also determined to provide a comparable basis to establish the relative chelating ability of these
ligands. The purity obtained for the
ligands is higher than 87% in all cases and is comparable with that obtained by (1)H NMR. No significant differences have been found among
ligands when their protonation and stability constants were compared. As expected, no Fe(3+) complexation was observed for p,p-
EDDHA and EDDMtxA. The presence of sulfonium groups in EDDHSA produces an increase in acidity that affects their protonation and stability constants, although the pFe values suggest that EDDHSA could be also effective to correct
iron chlorosis in plants.