Ferrate(1-), [[N,N'-1,2-ethanediylbis[N-[[2-(hydroxy-.kappa.O)phenyl]methyl]glycinato-.kappa.N,.kappa.O]](4-)]-, sodium (1:1), (OC-6-13)-

Dissociation constant

Link to relevant study record(s)

Reference

Endpoint:

dissociation constant

Type of information:

experimental study

Adequacy of study:

key study

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

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

Qualifier:

no guideline followed

Deviations:

not applicable

Principles of method if other than guideline:

Persistence of chemical compounds is characterised by physical quantities such as equilibrium, stability constant (in the case of complexes - chelates) and the dissociation contant (for soluble salts, acids and bases). In the case of complexes the term dissociation constant does not apply, but a stability constant of complex can be determined instead. The following is a stability constant for FeHBED, which was based on the protonation constant of the chelating agent HBED by potentiometric, spectrophotometric methods and calculation programs.

GLP compliance:

no

Dissociating properties:

not determined

Protonation and thermodynamic stability constants of divalent metals ions with HJB, o,o-EDDHA and HBED chelating agents with HJB, o,o-EDDHA and HBED chelating agents:

Protonation constants	HJB	o,o-EDDHA	HBED
[LH^3-]/[H^+]*[L^4-] => logK1	12.36	11.94	12.64
[LH2^2-]/[H^+]*[LH^3-] => logK2	9.99±0.21	10.73	11.03
[LH3^-]/[H^+]*[LH2^2-] => logK3	8.21±0.29	8.66±0.04	8.34
[LH^4]/[H^+]*[LH3^-] => logK4	4.79±0.30	6.18±0.06	4.40
Stability constants	HJB	o,o-EDDHA	HBED
[CaL^2-]/[Ca^2+]*[L^4-]	8.28±0.07	7.29±0.30	9.29
[CaLH^-]/[Ca^2+]*[H^+]*[L^4-]	17.39±0.23	16.77±0.33	17.98
[CaLH2]/[Ca^2+]*[H^+]^2*[L^4-]	25.31±0.48	25.95±0.50	25.48
[MgL^2-]/[Mg^2+]*[L^4-]	8.95±0.10	9.76±0.05	10.51
[MgLH^-]/[Mg^2+]*[H^+]*[L^4-]	17.83±0.01	18.18±0.15	18.66
[MgLH2]/[Mg^2+]*[H^+]^2*[L^4-]	23.79±0.24	25.79±0.24	27.67

HBED presents a logK₁ higher than o,oEDDHA, but similar to its analogue HJB. LogK₂ is similar to that of o,oEDDHA and higher than in HJB, surely due to a higher interaction between the two phenolate groups in HJB. The amine groups protonation constants (logK₃ and logK₄) are similar to those of the HJB, but lower than those of the o,oEDDHA. It seems that the protonation of the tertiary amines (HJB and HBED) is more difficult than in the secondary amines (o,oEDDHA).

HBED/Ca²⁺stability constants are similar to those of o,oEDDHA and HJB but HBED/Mg²⁺ is more stable than HJB and o,oEDDHA chelates. The lower rigidity of HBED complexes than those the o,oEDDHA complexes is the cause of its higher constants. This effect seems to be more important that the effect of the methyl group in the HJB/Ca²⁺chelate, but no in the HJB/Mg²⁺one. At physiological pH the Ca²⁺predominant species with HBED are CaH₂L⁰ and over pH 7.50 CaHL^-. However the Mg²⁺ predominant species at physiological pH is MgHL^- and at pH over 8.15 is MgL^{2 -.}

The results of HBED/Fe³⁺stability constants compared with those of o,oEDDHA/Fe³⁺ and HJB/Fe³⁺ are presented in the table below. HBED/Fe³⁺ is the chelate with the higher stability. In all the cases, only the FeL^- chelate is the predominant species at agronomic pHs. The o,oEDDHA/Fe³+ has two different diastereoisomers, forming the meso less stable chelates than HJB and the racemic more stables. Both diastereoisomers are less stable than HBED chelates.

 

Stability constants	HJB	o,o-EDDHA	rac-EDDHA	meso-EDDHA	HBED
logKML	34.45±0.04	35.09±0.28	35.86	34.15	39.01
logKMLH	35.27±0.6	36.89±0.21	36.08	36.56	40.52
logKMLOH	-	23.66±0.27	13.12	22.81	-

Conclusions:

Stability constant = 39.01

Executive summary:

In the case of complexes the term dissociation constant does not apply, but a stability constant of the complex was determined instead in a non-GLP study (2009). A stability constant of 39.01 for FeHBED, which was based on the protonation constant of the chelating agent HBED by potentiometric, spectrophotometric methods in KCl supporting electrolyte (0.100 M) at 25.0 °C and calculation programs, was confirmed.

Description of key information

In the case of complexes the term dissociation constant does not apply, but a

stability constant of the complex was determined instead in a non-GLP study (2009). A stability constant of 39.01 for FeHBED, which was based on the protonation constant of the chelating agent HBED by potentiometric, spectrophotometric methods in KCl supporting electrolyte (0.100 M) at 25.0 °C and calculation programs, was confirmed.

Key value for chemical safety assessment

pKa at 20°C:

39.01