Quality Of Commercial Iron Chelates

Commercial products are obtained by carrying out first the industrial synthesis of the chelating agents and then incorporating Fe from inorganic salts. While the first synthesis pathways proposed produced quite pure compounds (Kroll et al., 1957), the industrial synthesis pathways used nowadays (Dexter, 1958; Petree et al., 1978) yield commercial products with quite different purities, and also lead to the presence of by-products. For instance, the condensation product 2,6-di [CH(COOH)NHCH2 CH2NHCH(COOH)Ar] phenol (Ar) hydroxyphenyl (Cremonini et al., 2001) and o,p-EDDHA (Gómez-Gallego et al., 2002) have been detected in EDDHA commercial products, by using one- and two dimensional nuclear magnetic resonance and ion pair HPLC. Also, main impurities in the EDDHSA/Fe3+ and EDDCHA/Fe3+ containing products have been studied by HPLC (Álvarez-Fernández et al., 2002a), revealing that unreacted starting materials (p-hydroxybenzenesulfonic acid and p-hydroxybenzoic acid, respectively) were always present. 1D and 2D NMR experiments (Álvarez-Fernández et al., 2002a) showed that commercial fertilizers based on EDDHMA/Fe3+ contained different methyl substitutes of EDDHMA. These findings suggest that current production processes of Fe-chelates used in agriculture are far from providing the very pure compounds that can be obtained with other methods (Kroll et al., 1957; Sierra et al., 2002).

Eddha Production

Figure 5-10. A typical chromatogram for the quantification of the o,o-EDDHA/Fe3+ in commercial products using the method described by Lucena et al. (1996). I and II are the geometric isomers (meso and racemic) of o,o-EDHA/Fe3+. III correspond with degradation products of the p,p-EDDHA and peak IV is due to the presence of op -EDDHA/Fe3+. The visible-UV spectra are also included.

Figure 5-10. A typical chromatogram for the quantification of the o,o-EDDHA/Fe3+ in commercial products using the method described by Lucena et al. (1996). I and II are the geometric isomers (meso and racemic) of o,o-EDHA/Fe3+. III correspond with degradation products of the p,p-EDDHA and peak IV is due to the presence of op -EDDHA/Fe3+. The visible-UV spectra are also included.

Analytical methods have been an important source of information on the quality of the commercial products. Several analysis methods for the quality control of the Fe-chelates have been developed (Boxema, 1979; Barak and Chen, 1987; Deacon et al., 1994: Lucena et al., 1996). A comparative study about the efficacy of the different analysis methods of Fe chelates concluded that high performance liquid chromatography (HPLC) is the most reliable means to quantify the amount of chelated Fe. Among them, the method proposed by Lucena et al. (1996) was the simplest and most user-friendly (Hernandez-Apaolaza et al., 2000). An example including a chromatogram obtained for a commercial product containing EDDHA/Fe3+ is shown in

Figure 5-10. Using this methodology, Hernandez-Apaolaza et al. (1997, 2000) and Alvarez-Fernandez (2000) pointed out that all tested commercial Fe-chelate formulations containing o,o-EDDHA and EDDHMA and sold in Spain when these studies were carried out, did not meet the declared Fe-chelated content. In fact, using 42 products available before 2000 in Spain, with a declared content of 6% Fe in the form of o,o- EDDHA/Fe3+, the average chelated Fe content was only 2.59%. Now that a standard analytical method has been approved and is widely used, the actual and declared contents are quite similar in commercial products, purities of the products declaring o,o- EDDHA/Fe3+ being in the range from 3 to 6%.

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