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Dissociation constant

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Description of key information

The proton donator unit shows dissociation of the proton starting at approx. pH 9,  the corresponding pKAis 10.00 (pKB=4.00). 
The proton acceptor unit shows protonation starting at approx. pH 3.5 the corresponding pKBis 12.13 (pKA=1.87).

Key value for chemical safety assessment

pKa at 20°C:
10

Additional information

The photometric method using HCl/NaCl buffers was chosen.

As result of the evaluation of the photometric method, the test item1,2,4-Triazole should be considered as amphiprotic with one proton donator unit and two proton acceptor units.

The proton donator unit shows dissociation of the proton starting at approx. pH 9, the corresponding pKAis 10.00 (pKB=4.00). The proton acceptor unit shows protonation starting at approx. pH 3.5 the corresponding pKBis 12.13 (pKA=1.87).

  

Both values correspond with the literature values for pKAof approx 2.2 and 10.3 :

Information regarding dissociation constants was sourced from a handbook which is known to contain peer-reviewed data of Klimisch-2 reliability, and also from a TGD-approved model. The CRC Handbook of Chemistry and Physics (81st Ed.) states that the substance has a pKa of 2.27 at 20°C. However, this rather acidic value is interpreted as representing the pH at which 50% of the neutral molecules have accepted a proton to become cationic, and not to represent their having donating a proton to become anionic, as is known to occur during the formation of the sodium salt of the substance. The Chemical Abstract Service (CAS) Registry file contains two estimated pKa values for the substance, which were calculated using the Advanced Chemistry Development (ACD) model. The first pKa at 3.00 ± 0.45 at 25°C is again interpreted to represent the protonation of the neutral molecules to form cations, and is consistent with the CRC data. The second pKa at 10.18 ± 0.20 at 25°C is interpreted to represent the deprotonation of the neutral molecules to form anions, which would be consistent with what would need to occur in order to form a sodium salt, which is known to exist for this substance.