Sodium [[α,α'-(ethylenediimino)bis[2-hydroxybenzene-1-acetato]](4-)]ferrate(1-)

Hydrolysis

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Reference

Endpoint:

hydrolysis

Data waiving:

study scientifically not necessary / other information available

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other:

Description of key information

Hydrolysis of EDDHA-moiety containing chelates was studied by means of determination of their stability constants and species distribution in solution, hydroponic and soil condition. The tendency to hydrolysis of EDDHA chelates is a function of the stability, i.e. the more stable the complex, the less prone it is to hydrolyze (Ahrland et al., 1990). The recovery of 100 % was observed for EDDHA/Fe3+ at pH 12.76. The value of pKa(FeL-) (not hydrolysed species) was about 2 units higher than the actual value of pH tested. The author concluded that EDDHA/Fe3+ did not hydrolyse even in the most alkaline soils (pH >9).

A high Fe3+ ion affinity was observed for o,o- and o,p-EDDHA (Yunta et al., 2003a,b). In the studies on determination of stability constants by spectrophotometric and potentiometric measurements, the predominant species of o,o-EDDHA/Fe3+ involved the coordination with the nitrogen atoms, the carboxylate oxygens, and the phenolate groups (FeL- species). The protonated (FeHL) and hydroxylated (FeOHL2-) species were predominant at pH below 3 and above 10, respectively (Yunta et al., 2003b). Both isomers of EDDHA chelate formed stable complexes over the environmental pH range. The stability constants of the hydroxylated species FeOHL2- were significantly lower than the stability constants of the protonated species in both isomers (Yunta et al., 2003a,b). The hydroxylated species FeLOH2- of o,p-EDDHA/Fe3+ was formed also only at pH 9.27 (Yunta et al., 2003a).

According to species distribution studies in nutrient solution, the not hydrolysed FeL species of o,o-EDDHA/Fe3+ was predominant in the whole physiological pH range (Yunta et al., 2003b). 100% of the iron chelate remained as FeL- species at pH below 11. The hydroxylated FeOHL2- species appeared at pH around 11.5. Only at pH above 11.5 became the calcium and magnesium chelates predominant species (iron was displaced from the chelate) (Yunta et al., 2003b). Similarly to o,o-EDDHA/Fe3+, the protonated species of o,p-EDDHAHx/Fe3+ were the predominant species at agronomic pH in the studies on species distribution at agronomic conditions in three theoretical models. Hydroxylated species were formed at pH 10 and 11.5 in solution and nutrient solution models, respectively. According to authors, in soils with limited Cu availability, FeLOH2- of o,p-EDDHA/Fe3+ could also be formed at pH > 11 (Yunta et al., 2003a).

Based on these results, it can be concluded that the target substance o,o-Fe(Na)EDDHA is hydrolytically stable at environmental pH range.