2-Pyridinecarboxylic acid, 4-amino-3,6-dichloro-

Dissociation constant

Link to relevant study record(s)

Reference

Endpoint:

dissociation constant

Type of information:

experimental study

Adequacy of study:

key study

Study period:

19 June 2001 to 25 June 2001

Reliability:

1 (reliable without restriction)

Rationale for reliability incl. deficiencies:

guideline study

Qualifier:

according to guideline

Guideline:

OECD Guideline 112 (Dissociation Constants in Water)

Deviations:

no

GLP compliance:

yes

Specific details on test material used for the study:

Purity: 99.5%

Dissociating properties:

yes

No.:

#1

pKa:

2.56

Temp.:

20 °C

Isolation of Ionic Species and Determination of Analytical Wavelength

The absorbance scan for the test material solution in 0.01 N and 0.1 N KOH showed absorbances differences at the peaks of <1 % indicating that in 0.01 N KOH the test material was fully (>99 %) dissociated or ionised.

A large absorbance difference was noted between the UV-Visible scan of the test material in 0.01 N HCl and the scan in 0.1 N HCl, indicating that isolation of the two species was incomplete. A test material solution was then prepared in 1 N HCl and the resulting scan showed absorbances differences at the peaks of <1 % when compared to the scan of the test material solution prepared in 0.1 N HCl. This indicated that the test material was predominantly (>99 %) present as the molecular species in 0.1 N HCl.

UV-Visible Scans from the test material prepared in 0.01 N KOH and 0.1 N HCl were then overlaid and an analytical wavelength of 272.1 nm was selected for further analyses.

Preliminary and Definitive Dissociation Constant Determination

All measurements were made at 20 °C. The absorbance of the test material prepared in 0.01 N KOH was 0.1851 (Ai) and absorbance of the test material prepared in 0.1 N HCl (Am) was 0.9859.

The preliminary pKa determination showed an approximate value of 2.6.

Absorbance, pH and calculated pKa values for the definitive determination are shown in Table 1.

The acid dissociation constant (pKa) for the test material was found to be 2.56 ± 0.03.

Table 1: Values of Measured pH and Absorbance at 20 °C for Nine Buffer Solutions containing Test Material, and Calculated pKa

pH of test material solution	Measured absorbance at 272.1 nm	pKa (calculated from equation 2)
2.06	0.8039	2.59
2.22	0.7441	2.58
2.23	0.7414	2.59
2.56	0.5798	2.55
2.57	0.5759	2.55
2.87	0.4377	2.53
3.16	0.3471	2.56
3.30	0.3105	2.57
3.40	0.2869	2.56

Conclusions:

Under the conditions of the study, the acid dissociation constant (pKa) for the test material was found to be 2.56 ± 0.03.

Executive summary:

The dissociation constant of the test material was determined using UV-Visible spectrophotometry in a study which was conducted under GLP conditions and in accordance with the standardised guideline OECD 112.

During the study a scan of the test material in 0.1 N HCl (test material present as the molecular species) was overlaid with a scan of the test material in 0.01 N KOH (test material present as the ionised species). From these initial scans, an analytical wavelength (wavelength where the greatest difference between the absorbances of the molecular and ionised species was observed) of 272.1 nm was chosen.

The test material was prepared at constant concentration in nine buffer solutions of pH where the compound was only partially ionised. The pH measurements for the nine test material solutions were 2.06, 2.22, 2.23, 2.56, 2.57, 2.87, 3.16, 3.30 and 3.40. The absorbance of these solutions was measured at 272.1 nm, and an average pKa value for the test material of 2.56 ± 0.03 was calculated.

Description of key information

pKa = 2.56 ± 0.03, OECD 112 using UV-Visible spectrophotometry, Cathie (2001)

Key value for chemical safety assessment

pKa at 20°C:

2.56

Additional information

The dissociation constant of the test material was determined using UV-Visible spectrophotometry in a study which was conducted under GLP conditions and in accordance with the standardised guideline OECD 112. The study was assigned a reliability score of 1 in line with the criteria of Klimisch et al. (1997).

During the study a scan of the test material in 0.1 N HCl (test material present as the molecular species) was overlaid with a scan of the test material in 0.01 N KOH (test material present as the ionised species). From these initial scans, an analytical wavelength (wavelength where the greatest difference between the absorbances of the molecular and ionised species was observed) of 272.1 nm was chosen.

The test material was prepared at constant concentration in nine buffer solutions of pH where the compound was only partially ionised. The pH measurements for the nine test material solutions were 2.06, 2.22, 2.23, 2.56, 2.57, 2.87, 3.16, 3.30 and 3.40. The absorbance of these solutions was measured at 272.1 nm, and an average pKa value for the test material of 2.56 ± 0.03 was calculated.