2-methylpentyl 2-hydroxybenzoate

Dissociation constant

Link to relevant study record(s)

Reference

Endpoint:

dissociation constant

Type of information:

experimental study

Adequacy of study:

key study

Study period:

25-03-2020 to 08-05-2020

Reliability:

1 (reliable without restriction)

Rationale for reliability incl. deficiencies:

guideline study

Remarks:

Guideline study performed under GLP. All relevant validity criteria were met.

Reason / purpose for cross-reference:

reference to other study

Qualifier:

according to guideline

Guideline:

OECD Guideline 112 (Dissociation Constants in Water)

Deviations:

no

Qualifier:

according to guideline

Guideline:

other: EU Method A.25. Dissociation Constants in Water

Version / remarks:

April 28, 2017

Deviations:

no

GLP compliance:

yes

Dissociating properties:

yes

pKa:

9.82

Temp.:

19.9 °C

(1) Assessment of the spectra of the pure molecular species and the pure dissociated species (pure anion)(Spectroscopic Method)

The phenolic group in test item can dissociate. The pKa value to be determined is thus for an acidic group. From the spectra of the ionized substance and by comparison with the spectra of the non-ionized substance, it was concluded that the difference in optical absorbance is maximal at 330 nm. Hence 330 nm was chosen as the analytical wavelength.From the absorbance and the actual pH values, the approximate pKa value was calculated to be 9.97. The test was performed at a temperature of 19.9 ± 0.1°C.

 

Table 1. Pre-test Absorbance Data

Solution	Absorbance for the test solution [units]	Actual pH
pH 6	0.0126	6.03
pH 7	0.0113	7.01
pH 8	0.0166	8.06
pH 9	0.0279	9.01
pH 10	0.0905	9.99
pH 11	0.1606	10.93
pH 12	0.1648	11.99

 

(2) Exact determination of the pKa value(Spectroscopic Method)

The optical absorbances at 330 nm of all nine test solutions and the seven calculated pKa values from three stock solutions were determined.

 

Table 2. Main test Absorbance Data

Concentration stock [mg/L]	Absorbance for the test solution [units]	Actual pH	pKa value
1010	0.0120	8.03	-
	0.0463	9.42	9.82
	0.0548	9.60	9.86
	0.0719	9.79	9.80
	0.0785	10.00	9.91
	0.0991	10.19	9.78
	0.1079	10.40	9.81
	0.1140	10.60	9.86
	0.1326	12.00	-
1010	0.0137	8.03	-
	0.0465	9.40	9.81
	0.0533	9.61	9.90
	0.0721	9.79	9.79
	0.0747	10.00	9.96
	0.0963	10.19	9.80
	0.1088	10.38	9.73
	0.1158	10.57	9.72
	0.1301	12.00	-
1010	0.0152	8.03	-
	0.0459	9.41	9.85
	0.0560	9.62	9.88
	0.0718	9.80	9.82
	0.0811	10.02	9.90
	0.0982	10.19	9.79
	0.1088	10.38	9.76
	0.1195	10.59	9.63
	0.1310	12.00	-

 

For each stock solution the respective seven calculated pKa were averaged and the standard deviation were calculated (information provided below).

 

Table 3. Definitive Test - Absorbance Data

Concentration stock [mg/l]	pKa	Standard deviation
1010	9.83	0.05
1010	9.81	0.09
1010	9.80	0.09
Mean	9.82

 

The mean calculated pKa value from the three stock solutions was 9.82.

The test was performed at a temperature of 19.9 ± 0.1°C.

Conclusions:

The dissociation constant of the test item was determined using the spectroscopic method to be as follows:
pKa = 9.82 at 20 °C
The optical absorbances at 330 nm of all nine test solutions at various pH (from ca. pH 8 to pH 12) prepared from three 1010 mg/L stock solutions were determined and the seven calculated pKa values were calculated. For each stock solution the respective seven calculated pKa were averaged : (i) pKa 9.83 ± 0.05 ; (ii) pKa 9.81 ± 0.09 and (iii) pKa 9.80 ± 0.09. The mean pKa was 9.82. All relevant validity criteria were considered to be met. Under the conditions of this study, the test item was determined to possess an acidic group with a pKa 9.82.

Executive summary:

The dissociation constant of the substance was determined using the spectroscopic method of OECD TG 112 and EU Method A.25 under GLP. Prior to testing the substance was structurally assessed. The pKa value to be determined was for an acidic group. Initially, the titration method was attempted. A test solution of the test item was titrated with standard acid or base, as appropriate with the pKa values to be determined based on the titration curve obtained. During the performance of the titration method, it was noticed that due to low water solubility of the test item the titration method was not the appropriate method to determine dissociation constant. Therefore, it was decided to perform the spectrophotometric method for the determination of the test item pKa values. A 1010 mg/L stock solution of test item was prepared in methanol. Two test solutions (all 1.01 mg/L) were prepared by diluting the stock solution by factor of 1000 with two buffers (pH 2 and pH 12). Blank solutions were prepared by diluting methanol by factor of 1000 with each of the applicable buffer solutions. From each of the test solutions, an absorption spectrum between 200 and 900 nm was recorded with the appropriate reference solution as reference. The pH of each solution (test and reference solution) was measured using a pH meter. The analytical wavelength was chosen from the absorption spectra recorded from the test item for pH 2 and pH 12. From the absorption spectra of the ionized substance and by comparison with the spectra of the non-ionized substance, it was concluded that the difference in optical absorbance is maximal at 330 nm. Hence 330 nm was chosen as the analytical wavelength. The definitive test was performed at a temperature of 19.9 ± 0.1°C. The optical absorbances at 330 nm of all nine test solutions at various pH (from ca. pH 8 to pH 12) prepared from three 1010 mg/L stock solutions were determined and the seven calculated pKa values were calculated. For each stock solution the respective seven calculated pKa were averaged : (i) pKa 9.83 ± 0.05 ; (ii) pKa 9.81 ± 0.09 and (iii) pKa 9.80 ± 0.09. The mean pKa was 9.82. All relevant validity criteria were considered to be met. Under the conditions of this study, the test item was determined to possess an acidic group with a pKa 9.82.

Applicant assessment indicates: the test item would not significantly ionise at environmentally relevant pH 2 to pH 8 (i.e. < 10% ionisation). The test item would be approximately 10% ionised at pH 8.8. Using the Henderson-Hasselbalch equation the substance would be ca. 15% ionized at pH 9.

 

Within relevant ECHA guidance environmentally relevant pH is considered to be: fresh surface waters: pH 4 to 9, marine environments: stable approximately pH 8, agricultural soils and sewage treatment plant tanks : pH variable between 5.5 and 7.5. However, based on fresh surface waters pH information presented in WaterBase (European Environment Agency (EEA)) and/or information collated and analysed by Blue Frog Scientific Limited (UK), for EU counties and the UK : datapoints ‘n’ = 22242 ; mean pH = 7.76 ; 2xSD range = 6.73 – 8.79 ; with 5th percentile: pH 6.83, lowest 5th percentile for any country: pH 6.57 and/or 95th percentile: pH 8.31. The realistic environmentally relevant pH range for fresh surface waters would be: pH = ca. 6.6 to ca. 8.3. Consequentially, it is considered the test item will not be significantly ionized (>10%) in relevant fresh waters.

 

Furthermore, when the substance was tested at pH 7 and pH 9 within an available log Pow : HPLC study to OECD TG 117 (2020), the log Pow observed was found to be insignificantly affected by the effects of ionisation between pH 7 and pH 9 (i.e. ± 0.1 log units). Thus, supporting inconsequential levels of ionisation of the substance being observed at up to pH 9.

 

It is considered that the substance will not significantly ionize (>10%) in all environmentally relevant pHs.

 

References:

1. WaterBase (https://eea.europa.eu ; European Environment Agency (EEA)) and/or information collated by Blue Frog Scientific Ltd. (UK)

2. ECHA Guidance on Information Requirements and Chemical Safety Assessment (Chapter R.7a: Endpoint Specific Guidance, July 2017)

3. ECHA Guidance on Information Requirements and Chemical Safety Assessment (Chapter R.7b: Endpoint Specific Guidance, June 2017)

4. ECHA Guidance on Information Requirements and Chemical Safety Assessment (Chapter R.7c: Endpoint Specific Guidance, June 2017)

Description of key information

pKa = 9.82 at 20 °C, OECD TG 112, 2020

Further information: the test item would not significantly ionise at environmentally relevant pH 2 to pH 8 (i.e. < 10% ionisation). The test item would be approximately 10% ionised at pH 8.8. Using the Henderson-Hasselbalch equation the substance would be ca. 15% ionized at pH 9. From data within WaterBase (European Environment Agency) database, the realistic environmentally relevant pH range for European fresh surface waters would be: pH = ca. 6.6 to ca. 8.3. Therefore, it is considered that the substance will not significantly ionize (>10%) in all environmentally relevant pHs.

Key value for chemical safety assessment

pKa at 20°C:

9.82

Additional information

Key study : OECD TG 112, 2020 : The dissociation constant of the substance was determined using the spectroscopic method of OECD TG 112 and EU Method A.25 under GLP. Prior to testing the substance was structurally assessed. The pKa value to be determined was for an acidic group. Initially, the titration method was attempted. A test solution of the test item was titrated with standard acid or base, as appropriate with the pKa values to be determined based on the titration curve obtained. During the performance of the titration method, it was noticed that due to low water solubility of the test item the titration method was not the appropriate method to determine dissociation constant. Therefore, it was decided to perform the spectrophotometric method for the determination of the test item pKa values. A 1010 mg/L stock solution of test item was prepared in methanol. Two test solutions (all 1.01 mg/L) were prepared by diluting the stock solution by factor of 1000 with two buffers (pH 2 and pH 12). Blank solutions were prepared by diluting methanol by factor of 1000 with each of the applicable buffer solutions. From each of the test solutions, an absorption spectrum between 200 and 900 nm was recorded with the appropriate reference solution as reference. The pH of each solution (test and reference solution) was measured using a pH meter. The analytical wavelength was chosen from the absorption spectra recorded from the test item for pH 2 and pH 12. From the absorption spectra of the ionized substance and by comparison with the spectra of the non-ionized substance, it was concluded that the difference in optical absorbance is maximal at 330 nm. Hence 330 nm was chosen as the analytical wavelength. The definitive test was performed at a temperature of 19.9 ± 0.1°C. The optical absorbances at 330 nm of all nine test solutions at various pH (from ca. pH 8 to pH 12) prepared from three 1010 mg/L stock solutions were determined and the seven calculated pKa values were calculated. For each stock solution the respective seven calculated pKa were averaged : (i) pKa 9.83 ± 0.05 ; (ii) pKa 9.81 ± 0.09 and (iii) pKa 9.80 ± 0.09. The mean pKa was 9.82. All relevant validity criteria were considered to be met. Under the conditions of this study, the test item was determined to possess an acidic group with a pKa 9.82.

 

Applicant assessment indicates: the test item would not significantly ionise at environmentally relevant pH 2 to pH 8 (i.e. < 10% ionisation). The test item would be approximately 10% ionised at pH 8.8. Using the Henderson-Hasselbalch equation the substance would be ca. 15% ionized at pH 9.

 

Within relevant ECHA guidance environmentally relevant pH is considered to be: fresh surface waters: pH 4 to 9, marine environments: stable approximately pH 8, agricultural soils and sewage treatment plant tanks : pH variable between 5.5 and 7.5. However, based on fresh surface waters pH information presented in WaterBase (European Environment Agency (EEA)) and/or information collated and analysed by Blue Frog Scientific Limited (UK), for EU counties and the UK : datapoints ‘n’ = 22242 ; mean pH = 7.76 ; 2xSD range = 6.73 – 8.79 ; with 5th percentile: pH 6.83, lowest 5th percentile for any country: pH 6.57 and/or 95th percentile: pH 8.31. The realistic environmentally relevant pH range for fresh surface waters would be: pH = ca. 6.6 to ca. 8.3. Consequentially, it is considered the test item will not be significantly ionized (>10%) in relevant fresh waters.

 

Furthermore, when the substance was tested at pH 7 and pH 9 within an available log Pow : HPLC study to OECD TG 117 (2020), the log Pow observed was found to be insignificantly affected by the effects of ionisation between pH 7 and pH 9 (i.e. ± 0.1 log units). Thus, supporting inconsequential levels of ionisation of the substance being observed at up to pH 9.

 

It is considered that the substance will not significantly ionize (>10%) in all environmentally relevant pHs.

 

References:

1. WaterBase (https://eea.europa.eu ; European Environment Agency (EEA)) and/or information collated by Blue Frog Scientific Ltd. (UK)

2. ECHA Guidance on Information Requirements and Chemical Safety Assessment (Chapter R.7a: Endpoint Specific Guidance, July 2017)

3. ECHA Guidance on Information Requirements and Chemical Safety Assessment (Chapter R.7b: Endpoint Specific Guidance, June 2017)

4. ECHA Guidance on Information Requirements and Chemical Safety Assessment (Chapter R.7c: Endpoint Specific Guidance, June 2017)