Zinc EDDHA

Hydrolysis

Link to relevant study record(s)

Reference

Reference 1

Endpoint:

hydrolysis

Type of information:

experimental study

Adequacy of study:

weight of evidence

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

study well documented, meets generally accepted scientific principles, acceptable for assessment

Qualifier:

no guideline followed

Principles of method if other than guideline:

In order to establish the stability of the products under study against pH, solutions of known concentration (25 mg/L) of each of Zn-EDDHA chelates were prepared, the pH was adjusted to the desired value by means of the MES, HEPES, CAPS and/or AMPSO buffers. The samples were stirred for three days at 25°C and 50 rpm in darkness, filtered and the soluble Zn content was determined by ICP-OES.

GLP compliance:

no

Remarks:

data from a university lab

Radiolabelling:

no

Analytical monitoring:

yes

Buffers:

the pH was adjusted to the desired value by means of the MES, HEPES, CAPS and/or AMPSO buffers

Duration:

3 d

Temp.:

25 °C

Initial conc. measured:

25 mg/L

Remarks:

pH 3-10

Positive controls:

no

Negative controls:

no

Transformation products:

not measured

Remarks on result:

other: The Zn-EDDHA chelate is stable at a pH between 3 and 10

Details on results:

The Zn-EDDHA chelate was shown stable at a pH between 3 and 10 (please refer to table 1).

Table 1: Soluble Zn concentrations of Zn-EDDHA chelate at different pH values. The initial concentration was 25 mg/L.

pH              [Zn] soluble (mg/L)

3                     25.2 ± 0.7

4                     25.1 ± 0.4

5                     26.7 ± 0.3

6                     24.5 ± 0.4

7                     25 ± 1

8                     25.7 ± 0.5

9                     24.8 ± 0.9

10                   25.6 ± 0.2

Conclusions:

The Zn-EDDHA chelate was shown stable at a pH between 3 and 10.

Executive summary:

In order to establish the stability of the products under study against pH, solutions of known concentration (25 mg/L) of each of Zn-EDDHA chelates were prepared, the pH was adjusted to the desired value by means of the MES, HEPES, CAPS and/or AMPSO buffers. The samples were stirred for three days at 25°C and 50 rpm in darkness, filtered and the soluble Zn content was determined by ICP-OES. The Zn-EDDHA chelate was shown stable at a pH between 3 and 10.

Reference 2

Endpoint:

hydrolysis

Type of information:

experimental study

Adequacy of study:

weight of evidence

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

study well documented, meets generally accepted scientific principles, acceptable for assessment

Qualifier:

no guideline followed

Principles of method if other than guideline:

Theoretical speciation studies comparing the stability in solution have been carried out to simulate the possible interactions that can affect Fe, Mn and Zn in aqueous formulations containing these micronutrients. Unknown stability constants of ligands with Zn and Mn have been determined.

GLP compliance:

no

Remarks:

data from publication

Specific details on test material used for the study:

Chelating agents used in the experiment were H4o,o-EDDHA (Promochem), H4 o,p-EDDHA (Syngenta Agro). The titrimetric purity of H4o,o-EDDHA and H4o,p-EDDHA were determined by a photometric titration with Fe3+ solution. It corresponds to 93.9±0.6% and 93.1±0.5% respectively.

Radiolabelling:

no

Analytical monitoring:

yes

Positive controls:

no

Negative controls:

no

Transformation products:

yes

Remarks:

The soluble molecules Zn(OH)2 amorp and ZnCO3 (smithsonite) are formed.

No.:

#1

No.:

#2

Details on hydrolysis and appearance of transformation product(s):

Treatment with o,p-EDDHA show a high stability, maintaining 100% of Zn in solution in all the pH ranges studied (see attached figure).

Remarks on result:

other: please refer to 'Any other information on results'

A satisfactory fit of the experimental potentiometric data was obtained using the computer program HYPERQUAD for o,p-EDDHA/Zn^{2 +} stability constants (Table 1). The formation of the first protonated species probably implies the protonation of the phenolic group in para position and it is the main species below pH 10.22 for o,p-EDDHA/Zn2+. A second protonation constant has also been obtained, occurring in the second ortho phenolic group. However, the close values for the Mn stability constants obtained for o,o-EDDHA and o,p-EDDHA supports the fact that the axial donor groups may be only weakly interacting with the metallic ion, which is also in agreement with existing corresponding data for Zn.

Table 1: Calculated log K^0.1 for the stability constants of o,p-EDDHA/Zn²⁺ chelates

Quotient                                                  o,p-EDDHA/Zn²⁺                          

[ML^n-4] / [Mⁿ⁺] [L^4-]                                  12.44 ± 0.28                                      

[MLH^n-3] / [Mⁿ⁺] [H⁺ ] [L^4-]                      22.66 ± 0.13                                   

[MLH₂^n-2] / [Mⁿ⁺] [H⁺ ]² [L^4-]                  30.11 ± 0.04                                        

Conclusions:

Treatment with o,p-EDDHA show a high stability, maintaining 100% of Zn in solution in all the pH ranges studied (see attached figure).

Executive summary:

The stability constants of o,p-EDDHA/Zn²⁺were determined using the potentiometric method. This method consists of the titration with HCl 0.05 M solution of the chelate formed with metal:ligand 1:1 ratio from pH 10 to pH 2.5 at 25ºC under N₂atmosphere. The pH is continuously monitored by means of a glass electrode. Also, at the end of the titration, total Zn concentration is measured by atomic absorption spectrometry (AAS) (PerkinElmer AAnalystTM 800) to check that the incremental addition of volumes does not produce a large bias and to ensure the adequacy of the procedure. The Zn(II) stability constants were calculated using the program Hyperquad® 2006 utilising mass balance and known equilibrium constant constraints while minimising the least squares differences between the experimental and theoretical potentiometric data.

A satisfactory fit of the experimental potentiometric data was obtained using the computer program HYPERQUAD for o,p-EDDHA/Zn²⁺ stability constants (Table 1). The formation of the first protonated species probably implies the protonation of the phenolic group in para position and it is the main species below pH 10.22 for o,p-EDDHA/Zn2+. A second protonation constant has also been obtained, occurring in the second ortho phenolic group. However, the close values for the Mn stability constants obtained for o,o-EDDHA and o,p-EDDHA supports the fact that the axial donor groups may be only weakly interacting with the metallic ion, which is also in agreement with existing corresponding data for Zn.

Table 1: Calculated log K^0.1for the stability constants of o,p-EDDHA/Zn²⁺chelates

Quotient                                                  o,p-EDDHA/Zn²⁺                         

[ML^n-4] / [Mⁿ⁺] [L^4-]                                  12.44 ± 0.28                                      

[MLH^n-3] / [Mⁿ⁺] [H⁺] [L^4-]                      22.66 ± 0.13                                   

[MLH₂^n-2] / [Mⁿ⁺] [H⁺]²[L^4-]                  30.11 ± 0.04                                        

Treatment with o,p-EDDHA show ahigh high stability, maintaining 100% of Zn in solution in all the pH ranges studied.

Reference 3

Endpoint:

hydrolysis

Type of information:

read-across from supporting substance (structural analogue or surrogate)

Adequacy of study:

weight of evidence

Justification for type of information:

Please refer to Read Across Statement attached in Section 13

Reason / purpose for cross-reference:

read-across source

Preliminary study:

Even in the most alkaline of soils occurring in nature, EDDHA/Fe3+ did not hydrolyze at all.

Transformation products:

no

Details on hydrolysis and appearance of transformation product(s):

EDDHA/Fe3+ does not hydrolyse even in the most alkaline soils (pH >9).

% Recovery:

>= 100

pH:

12.67

Temp.:

25 °C

Remarks on result:

other: the value of pKa(FeL-) is about 2 units higher than the actual value of pH tested.

Key result

Remarks on result:

hydrolytically stable based on preliminary test

For the iron(lll) complexes of EDDHA, both the acidity constants Ka(FeL) and Ka(FeHL) have been determined. A survey of stability and acidity constants of iron(III) chelate complexes show that the relationship is approximately linear, i.e. the more stable the complex, the less prone it is to hydrolyze.

All phenolic iron(III) complexes acting as efficient fertilizers in alkaline soils are complexes of much the same high stability. With the high stability goes a strong resistance to hydrolysis. Even in the most alkaline of soils occurring in nature, they do not hydrolyze at all. The Fe(III)EDTA complex, on the other hand, starts to hydrolyze perceptibly already at values of pH between 5 and 6. In fact, it can be used as fertilizer, but only on extremely acidic soils. One may conclude that an iron(III) complex, in order to be useful as a fertilizer, must not be perceptibly hydrolyzed at the pH prevailing in the soil. This means that the value of pKa(FeL^-) has to be about 2 units higher than the actual value of pH. The hydrolysis generally increases markedly with the temperature. Considering that the values of ptfa(FeLT) listed refer to 25°C, a temperature which is often considerably exceeded in soilds in subtropical countries, it seems safe to choose an even larger difference between pKa(FeLT) and pH.

Validity criteria fulfilled:

not specified

Conclusions:

Even in the most alkaline of soils occurring in nature, EDDHA/Fe3+ does not hydrolyze at all. The same result is expected for the target substance because the core chemical structure of o,o-EDDHA ligand is the same in the source and in the target substance. Thus the same hydrolysis rates are expected.

Executive summary:

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. 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).

Reference 4

Endpoint:

hydrolysis

Type of information:

read-across from supporting substance (structural analogue or surrogate)

Adequacy of study:

weight of evidence

Justification for type of information:

Please refer to Read Across Statement attached in Section 13

Reason / purpose for cross-reference:

read-across source

Preliminary study:

Fe(III) Stability Constants (please see below) indicate that the test substance formed very stable complexes resistant to hydrolysis at environmentally relevant pH range.

Transformation products:

yes

No.:

#1

No.:

#2

No.:

#3

Details on hydrolysis and appearance of transformation product(s):

Fe3+ stability constants were determined using the absorption bands at 480 nm (determined separately, not shown in this RSS). The curves were also determined at 530 and 450 nm, as absorption maxima of o,p-EDDHA/Fe3+ at acid and alkaline pH values, respectively, to detect more clearly the change of the predominant species (see A, B, and C in Figure 6S, Supporting Information). The Fe3+ stability constants are shown in Table 2. These stability constants were compared with those of o,o-EDDHA and p,p-EDDHA. Because Fe3+ is octahedrally coordinated, o,o-EDDHA has six donor groups able to occupy the six positions. The [o,o-EDDHA/Fe]- species is the predominant one in a wide pH range (1.80-11.43). However, in o,p-EDDHA the position corresponding to the p-hydroxy phenolate does not bind the Fe3+, and a water molecule occupies this vacant position. The p-hydroxy phenolate takes a proton to form FeHLxH2O (5 in Figure 3) over an important pH range (up to 6.30). Above this pH a proton from the water can be released, forming the FeL- species (6 in Figure 3) that is in fact Fe(LH)(OH)-. At pH 9.27, the phenol dissociates to give the species FeLOH2- (7 in Figure 3). This pH value could be compared, although the Fe(III) binding can produce a larger acidity over the phenolate protonation, with the second protonation constant of o,p-EDDHA or with the two first protonation constants of p,p-EDDHA. The stronger binding of Fe3+ in o,o-EDDHA produces higher Fe3+ stability constants than o,p-EDDHA (Table 2). However, a higher Fe3+ ion affinity is observed for o,p-EDDHA than for EDTA (log K = 25.10) and their analogues (31), despite the fact that they have the six octahedral positions occupied with the ligand. This is due to the high affinity between Fe3+ and the phenolate donor. Morever, the [o,p-EDDHA/Fe]- stability constant agrees with that of the L3 ligand (log FeL = 27.00) (32). This chelating agent is similar to o,p-EDDHA lacking the hydroxy group in the para-position.

Key result

Remarks on result:

hydrolytically stable based on preliminary test

Details on results:

Species distribution in pH range 2-14 in three models provide information on hydroxylation behaviour of o,p-EDDHA ligand (please see picture attached). Hydroxylated species can be formed at pH 10 and higher.

Species Distribution in Agronomic Conditions.

The first model is shown in Figure 5A (attached to this file); the distribution of the different species of o,p-EDDHA is in accordance with their stability constants. The species of o,p-EDDHAHx/Fe3+ are the predominant species at agronomic pH. Only at pH >11.5 are the o,p-EDDHA/Mg2+ and o,p-EDDHA/Ca2+ species important. When the second model is considered (nutrient solution; see Figure 5B), the same tendency is found. Because precipitation of Fe-(OH)3 (amorp) is allowed in this medium, the presence of the FeOHL species is reduced at high pH values, although the stability is enough to maintain all of the iron in solution at pH <8.5, which is a normal limit for agronomic purposes. The o,p-EDDHA/Cu2+ is very low when compared with o,p-EDDHA/ Fe3+ (see Figure 5A,B) because the Cu2+ molar concentration (3.15 x 10^-7) is lower than that of Fe(III) (1.00 x 10^-4) in Hoagland nutrient solution. However, if the levels of chelated Cu2+ among the three chelating agents (o,p-EDDHA, o,o-EDDHA, and EDTA) are compared, o,p-EDDHA/Cu2+ is the most stable because almost 100% of the soluble Cu2+ remains chelated at all pH ranges (data not shown).

For the results of soil theorietical model please refer to the attached publication.

Comparisons between o,p-EDDHA/Fe3+, o,o-EDDHA/Fe3+, and EDTA/Fe3+ in solution, nutrient solution, and soil conditions are shown in panels A, B, and C, respectively, of in Figure 6. The behavior of o,p-EDDHA/Fe3+ is close to that of o,o- EDDHA/Fe3+ in solution, hydroponic conditions, and soil with low Cu2+ level (see Figure 6A-C) because ferric chelate is the main component in agronomic conditions. EDTA maintains soluble Fe3+ in solution conditions and in hydroponic conditions at pH <7.

Table 2. log Stability constants for Fe chelates

Quotient	o,p-EDDHA	o,o-EDDHA
[FeL^-] / [Fe3^+][L4^-]	28.72 ± 0.05	35.09 ± 0.28
[FeHL] / [Fe3^+][L4^-][H^+]	35.02 ± 0.05	36.89 ± 0.21
[FeH2L^+] / [Fe3^+][L4^-][H^+]^2	37.35 ± 0.10
[FeOHL^2-] / [Fe3^+][L4^-][OH^-]	19.45 ± 0.19	23.66 ± 0.27

The stability constants of the hydroxylated species FeOHL^2- is significantly lower than the stability constants of the protonated species.

Validity criteria fulfilled:

not specified

Conclusions:

A high Fe3+ ion affinity is observed for o,p-EDDHA. The chelate forms stable complexes over the environmental pH range. Hydroxylated species FeLOH2- was formed only at pH 9.27.The stability constants of the hydroxylated species FeOHL2- is significantly lower than the stability constants of the protonated species. According to species distribution in three theoretical models, the protonated species of o,p-EDDHAHx/Fe3+ are the predominant species at agronomic pH. Hydroxylated species are formed at pH 10 and 11.5 in solution and nutrient solution models, respectively. In soils with limited Cu availability, FeLOH2- can also be formed at pH > 11.
In conlcusion, hydrolysis of o,p-EDDHA/Fe3+ is not relevant for the environmetal pH range.
The same result is expected for the target substance because the core chemical structure of o,p-EDDHA ligand is the same in the source and in the target substance. Thus the same hydrolysis rates are expected.

Executive summary:

In this study, chemical behavior of o,p-EDDHA has been studied. The speciation of o,p-EDDHA was determined by the determination of the complexing capacity, protonation, and Ca2+, Mg2+, Cu2+, and Fe3+ stability constants. The pM values and species distribution in solution, hydroponic, and soil conditions were obtained.

With regard to hydrolysis of o,p-EDDHA, during the spectrophotometric and potentiometric measurements of the stability constants, a formation of hydroxylated species was observed only at pH 9.27. Generally, a high Fe3+ ion affinity was reported for o,p-EDDHA. The chelate formed stable complexes over the environmental pH range. The stability constants of the hydroxylated species FeOHL^2-were significantly lower than the stability constants of the protonated species.

In hydroponic conditions, o,p-EDDHA/ Fe3+ can be used as iron chelate because it is completely formed at normal agronomic pH. The protonated species predominated. Hydroxylated species were formed at pH 10 and 11.5 in solution and nutrient solution models, respectively. In soils with limited Cu availability, FeLOH^2-can also be formed at pH > 11. Thus, in soil conditions with limited Cu2+ availability o,p-EDDHA/Fe3+ is stable, but when the soil presents high availability of Cu2+, it can displace the Fe3+ from the chelate.

In conlcusion, hydrolysis of o,p-EDDHA/Fe3+ is not relevant at the environmetal pH range.

Reference 5

Endpoint:

hydrolysis

Type of information:

read-across from supporting substance (structural analogue or surrogate)

Adequacy of study:

weight of evidence

Justification for type of information:

Please refer to Read Across Statement attached in Section 13

Reason / purpose for cross-reference:

read-across source

Preliminary study:

Fe(III) Stability Constants (please see below) indicate that the test substance formed very stable complexes resistant to hydrolysis at environmentally relevant pH range.

Transformation products:

yes

No.:

#1

No.:

#2

No.:

#3

Details on hydrolysis and appearance of transformation product(s):

According to the studies on stability constants, the predominant species involves the coordination with the nitrogen atoms, the carboxylate oxygens, and the phenolate groups (11 in Scheme 4 and as No. 1 in the field "Identity of transformation products" above). The protonated (10) and hydroxylated (12) species are predominant at pH below 3 and above 10, respectively.

% Recovery:

>= 100

pH:

11

Temp.:

25 °C

Remarks on result:

hydrolytically stable based on preliminary test

Remarks:

The FeL species is predominant in the whole physiological pH range. Thus, 100% of the iron chelate remains as FeL- species at pH below 11. The hydroxylated FeOHL2- species appear at pH around 11.5 in those ligands in which it has been possible to determine it. Only at pH above 11.5 iron dissociates from chelate.

Key result

Remarks on result:

hydrolytically stable based on preliminary test

Fe³⁺Stability Constants.

Fe³⁺stability constants are shown in the following table 3.

Table 3.log Stability Constants^afor Fe³⁺EDDHA with Reference Values Given in Brackets

	[ML]/ [L][M]	[MHL]/ [H][L][M]	[MH2L]/ [H]^2[L][M]	[MOHL]/ [H]^-1[L][M]
EDDHA	35.09 ± 0.28	36.89 ± 0.21	--	23.66 ± 0.27
	33.91^d
rac-EDDHA	35.86	35.08	--	13.12
	(35.54)^b	--	--	(23.76)^b
meso-EDDHA	34.15	36.56		22.81
	(33.28)^b	(36.00)^b		(22.83)^b

^bRef 9; µ = 0.1 M (KCl); t = 25 °C.

^dRef 27; µ = 0.1 M (KNO3); t = 20 °C.

The stability of the hydroxylated species is much lower than the stabilities of ML- or protonated species.

The stability constant were determined from plots of absorbance against pH at 480 nm.

The Fe3+ chelate species are represented in Scheme 4 (please refer to the attched publication). The predominant species involves the coordination with the nitrogen atoms, the carboxylate oxygens, and the phenolate groups (11 in Scheme 4) except for p,p-EDDHA and EDDMtxA that are not able to form the chelate. The protonated (10) and hydroxylated (12) species are predominant at pH below 3 and above 10, respectively.

Species distribution

In Figure 3 (please refer to the attached publication), the species distribution curves for some chelating agents in Hoagland nutrient solution are shown. For EDDHA and other phenolic ligands (the results are not shown in this RSS; with exception made for p,p- EDDHA and EDDMtxA), the FeL species is predominant in the whole physiological pH range. Thus, 100% of the iron chelate remains as FeL- species at pH below 11. The hydroxylated FeOHL2- species appear at pH around 11.5 in those ligands in which it has been possible to determine it. Only at pH above 11.5 do the calcium and magnesium chelates become predominant species.

Validity criteria fulfilled:

not specified

Conclusions:

In the studies on determination of stability constants, the predominant species of EDDHA involves the coordination with the nitrogen atoms, the carboxylate oxygens, and the phenolate groups (FeL- species). The protonated (FeHL) and hydroxylated (FeOHL2-) species are predominant at pH below 3 and above 10, respectively.
According to species distribution in nutrient solution, similarly, the FeL species is predominant in the whole physiological pH range. Thus, 100% of the iron chelate remains as FeL- species at pH below 11. The hydroxylated FeOHL2- species appear at pH around 11.5 in those ligands in which it has been possible to determine it. Only at pH above 11.5 do the calcium and magnesium chelates become predominant species (iron is displaced from the chelate). Based on these results, it can be concluded that o,o-EDDHA/Fe3+ is hydrolytically stable at environmental pH range. The same result is expected for the target substance because the core chemical structure of o,o-EDDHA ligand is the same in the source and in the target substance. Thus the same hydrolysis rates are expected.

Executive summary:

Hydrolysis as function of pH of EDDHA/Fe3+ chelate could be deduced from the studies dealing with the determination of stability constants and species distribution determined by means of theoretical models. Stability constants were determined by potentiometric and spectrophotometric measurements. For species distribution, a model was employed to know the behavior of the chelating agents in solution in the 4-13 pH range. Species distribution was established using the same methodology as that used to calculate pFe in agronomic conditions.

In the studies on determination of stability constants, the predominant species of EDDHA involves the coordination with the nitrogen atoms, the carboxylate oxygens, and the phenolate groups (FeL^-species). The protonated (FeHL) and hydroxylated (FeOHL^2-) species are predominant at pH below 3 and above 10, respectively. According to species distribution in nutrient solution, similarly, the FeL species is predominant in the whole physiological pH range. Thus, 100% of the iron chelate remains as FeL^-species at pH below 11. The hydroxylated FeOHL^2-species appear at pH around 11.5 in those ligands in which it has been possible to determine it. Only at pH above 11.5 do the calcium and magnesium chelates become predominant species (iron is displaced from the chelate). Based on these results, it can be concluded that o,o-EDDHA/Fe3+ is hydrolytically stable at environmental pH range.

Description of key information

Based on the available data for the registered substance itself and the structural analogues implemented in this weight-of-evidence approach, the registered substance is considered as stable to hydrolysis at environmentally relevant pH values.

Additional information

The stability of the EDDHA complexes is pH-dependent. While at low pH values protonation of the EDDHA complexes is decisive for their stability, it is determined by hydrolysis at higher pH values. Protonation of the hydroxyl- and carboxyl-groups binding the metal leads to instability of the complex. The formation of the first protonated species was shown to imply the protonation of the phenolic group in para position (López-Rayo (2012), Yunta (2003a, b)). This results in the opening of the structure. A second protonation occurs in the ortho phenolic group. The formation of the first protonated species which probably implies the protonation of the phenolic group in para position and is the main species below pH 10.22 for o,p-EDDHA/Zn²⁺. A second protonation constant has also been obtained, occurring in the second ortho phenolic group. However, the close values for the Mn stability constants obtained for o,o-EDDHA and o,p-EDDHA supports the fact that the axial donor groups may be only weakly interacting with the metallic ion, which is also in agreement with existing corresponding data for Zn (López-Rayo 2012).

Iron, on the other hand, was shown to form stable unprotonated complexes with EDDHA and EDDHMA over wide ranges of pH (Ahrland 1990). With increasing pH, manganese and iron are likely to precipitate as a result of hydrolysis. Generally, the stability constants for o,o-EDDHA and o,p-EDDHA complexed with Fe³⁺, Mn²⁺and Zn²⁺are in the following order: Fe³⁺> Zn²⁺> Mn²⁺.This is due to the larger electropositive character, the higher oxidation state and the smaller size of Fe³⁺with respect to Mn²⁺and Zn²⁺, and also because Zn²⁺is smaller than Mn²⁺(López-Rayo 2012).

In conclusion, based on the available data for the structure itself and the structural analogues, the target substance is considered as stable to hydrolysis at environmentally relevant pH values.