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Dissociation constant

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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
  [LH3-]/[H+]*[L4-] => logK1 12.36 11.94 12.64
 [LH22-]/[H+]*[LH3-] => logK2 9.99±0.21 10.73 11.03
 [LH3-]/[H+]*[LH22-] => logK3 8.21±0.29 8.66±0.04 8.34
 [LH4]/[H+]*[LH3-] => logK4 4.79±0.30 6.18±0.06 4.40
 Stability constants  HJB  o,o-EDDHA  HBED
[CaL2-]/[Ca2+]*[L4-] 8.28±0.07 7.29±0.30 9.29
[CaLH-]/[Ca2+]*[H+]*[L4-] 17.39±0.23 16.77±0.33 17.98
[CaLH2]/[Ca2+]*[H+]2*[L4-] 25.31±0.48 25.95±0.50 25.48
[MgL2-]/[Mg2+]*[L4-] 8.95±0.10 9.76±0.05 10.51
[MgLH-]/[Mg2+]*[H+]*[L4- 17.83±0.01 18.18±0.15 18.66
[MgLH2]/[Mg2+]*[H+]2*[L4-] 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 MgL2 -.

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

Additional information