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EC number: 225-768-6 | CAS number: 5064-31-3
Dissociation constant
Administrative data
Link to relevant study record(s)
Description of key information
Schwarzenbach estimated the stability constant for Na3NTA to log(Kf) = 2.15 at 20 °C and pH = 10.
Anderegg estimated the dissociation constant from the measured acidity constant (at 20 degree C). A range of 1.10 to 9.87 was estimated for NTA with different electrolytes, ion strength and electrodes: pKa1: 8.96 to 9.87, pKa2: 2.14 to 2.5, pKa3: 1.7 to 1 .99, pKa4: 1.10.
Handbook data deliver a value of 1.2 for log(Kf) at 20 °C and 25 °C independent of the concentration of background electrolyte.
Key value for chemical safety assessment
- pKa at 20°C:
- 1.22
Additional information
The dissociation constant was estimated from the measured acidity constant (at 20 degree C; Anderegg). A range of 1.10 to 9.87 was estimated for NTA with different electrolytes, ion strength and electrodes: pKa1: 8.96 to 9.87, pKa2: 2.14 to 2.5, pKa3: 1.7 to 1 .99, pKa4: 1.10.
Schwarzenbach estimated the stability constant for Na3NTA to -log(Kf) = -2.15 at 20 °C and pH = 10. The experiment was performed at pH 10. The dissociation constant is a function of the pH-value.
Handbook data show a value for the stability constant of Na3NTA of: -log(Kf) = -1.22 at 20 °C (and 25 °C) in a solution of background electrolyte (0.1 mol/l and 1.0 mol/l at 25 °C). The variation of background electrolyte concentration from 0.1 to 1.0 mol/l does not affect the value of the stability constant.
As the pH-value in the Schwarzenbach experiment is very high the data from the handbooks are considered more reliable. But the value of Schwarzenbach could be used in case of strongly alkaline solutions.
The pKd-value for Na3NTA at 20 °C is approximately: pKd = -log Kd = log Kf = 1.22
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