mecoprop-P (ISO); (R)-2-(4-chloro-2-methylphenoxy)propionic acid

Dissociation constant

Link to relevant study record(s)

Reference

Reference 1

Endpoint:

dissociation constant

Type of information:

experimental study

Adequacy of study:

key study

Study period:

02 February 1999 to 22 June 1999

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

Qualifier:

according to guideline

Guideline:

EPA OPPTS 830.7370 (Dissociation constants in water)

Deviations:

no

GLP compliance:

yes

Dissociating properties:

yes

No.:

#1

pKa:

3.7

Temp.:

25 °C

Conclusions:

Under the conditions of the study the dissociation constant of the test material was found to be 2.0 x 10^-4 (equivalent pKa = 3.7).

Executive summary:

The dissociation constant of the test material was assessed according to OECD Test Guideline 112 and in compliance with GLP.

The titration method was employed in order to determine the dissociation constant of the test material.

A solution of the test material is prepared in purified water such that the concentration does not exceed the lower of 0.01M or half the saturation concentration. The test solution is titrated with standard acid or base solution as appropriate, and the pH measured after each addition of titrant. The pKa is calculated from the points on the titration curve.

Triplicate samples (approximately 43 to 45 mg) of the test material were dissolved in purified water (100 mL) to form the test solutions. The purified water used to prepare the test solutions was freshly drawn from a water purification unit, boiled and cooled, and then stored under nitrogen prior to use. The quality of the water was such that no carbon dioxide should be present - this was confirmed by the fact the blank titration of pure water was consistent with the absence of carbon dioxide.

Portions (25 mL) of the approximately half-saturated solutions were then titrated with 0.01M aqueous potassium hydroxide. An equivalent blank titration was performed. In order to perform the titrations, a Corning 250 pH meter fitted with a Mettler Toledo Inlab 420 electrode was employed. The test temperature during the measurements was 25 °C.

The dissociation constant of the test material was determined directly from the graphical representations of the data. For each titration, the end point volume was determined as the volume at which the rate of change of the pH of the test solution was greatest. The pKa value for the compound is then calculated as the pH at half of the end point volume.

Under the conditions of the study the dissociation constant of the test material was found to be 2.0 x 10^-4 (equivalent pKa = 3.7).

Reference 2

Endpoint:

dissociation constant

Type of information:

experimental study

Adequacy of study:

key study

Study period:

19 January 1996 to 09 February 1996

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 (incl. QA statement)

Dissociating properties:

yes

No.:

#1

pKa:

2.8

Temp.:

20 °C

Determination of the Titrant Concentration

The titrant concentration was determined by titrating three potassium hydrogen phthalate solutions at about 0.35 g/L in water and was found to be 0.0243 mol/L.

 

Determination of pKa for Benzoic Acid

The pKa of benzoic acid was determined in triplicate at about 20 °C using the method of calculation described above and the formulas a - c. The 3 values obtained were respectively 4.07, 4.15 and 4.01. The average value was 4.08 and was consistent with the pKa given in the OECD Guideline 112: pKa of benzoic acid at 20 °C is 4.12.

 

Determination of pKa for the Test Material

The pKa of the test material was determined in triplicate at about 20 °C.

Test	pKa	Standard Deviation	Mean pKa	Deviation
1	2.98	0.02	2.79	0.14
2	2.74	0.03		-0.05
3	2.69	0.04		-0.10

Conclusions:

Under the conditions of the study, the dissociation constant of pure test material has been determined using the potentiometric method. The pKa was estimated to be 2.8 ± 0.1 at 20 °C.

Executive summary:

The dissociation constant of the test material was assessed according to OECD Test Guideline 112 and in compliance with GLP using the potentiometric method.

Three test solutions were prepared by dissolving about 35 mg of test material in 100 mL of boiled water by using mild magnetic stirring. These solutions were titrated with the 0.025 M NaOH solution at about 20 °C and the pH was recorded after each addition of titrant. The pH and cumulative volumes of titrant were recorded. For each titration, the dissociation constant was calculated for ten points. The pH and cumulative volumes of titrant added were recorded by automatic titrator for each titration experiment. Then, for each titration, the dissociation constant was calculated for ten points and the mean value was obtained by averaging the ten pKa values. The calculation was done using the stoichiometric concentrations of HA and A- (schematizing the acid and basic forms of the compound) after each step of neutralisation and correcting these concentrations for the hydrogen ion concentrations.

The titrant concentration was determined by titrating three potassium hydrogen phthalate solutions at about 0.35 g/L in water and was found to be 0.0243 mol/L.

The pKa of the test material was determined in triplicate at about 20 °C.

Under the conditions of the study, the dissociation constant of pure test material has been determined using the potentiometric method. The pKa was estimated to be 2.8 ± 0.1 at 20 °C.

Reference 3

Endpoint:

dissociation constant

Type of information:

experimental study

Adequacy of study:

key study

Study period:

11 April 1990 to 20 August 1990

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

Dissociating properties:

yes

No.:

#1

pKa:

2.5

Temp.:

22 °C

Determination	Ve	pKa
1	10.10	2.50
2	10.10	2.49
	Mean pKa value = 2.50

Conclusions:

Under the conditions of the study the dissociation constant (pKa) for the test material was determined to be 2.50 at ambient temperature (approx 22 °C).

Executive summary:

The dissociation constant for the test material was assessed according to OECD method 112 and in compliance with GLP by titration.

The dissociation constant for the test material was determined by titration of a 0.001 M aqueous solution (100 mL) of the test material with sodium hydroxide at ambient temperature.

Under the conditions of the study the dissociation constant (pKa) for the test material was determined to be 2.50 at ambient temperature (approx 22 °C).

Reference 4

Endpoint:

dissociation constant

Type of information:

experimental study

Adequacy of study:

key study

Study period:

August 1991

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

Dissociating properties:

yes

pKa:

>= 3.66 - <= 3.7

Temp.:

20 °C

The pKa-values of the three titrations were in the range of 3.66 - 3.70. The standard deviation of each titration s ≤ 0.05. The mean value was 3.68 (Ka = 2.09 x 10^4).

Titration	1			2			3
Weight A.I. (mg/L)	121.92			120.44			121.28
Weight A.I. (mmol/L)	0.568			0.561			0.565
Vt (mL)	5.83			5.65			5.76
	V (mL)	pH	pKa	V (mL)	pH	pKa	V (mL)	pH	pKa
	2.4	3.60	3.76	2.3	3.60	3.76	2.4	3.60	3.75
	2.5	3.60	3.72	2.4	3.63	3.76	2.5	3.60	3.72
	2.6	3.64	3.73	2.5	3.63	3.73	2.6	3.61	3.69
	2.7	3.64	3.70	2.6	3.65	3.72	2.7	3.63	3.68
	2.8	3.65	3.68	2.7	3.65	3.70	2.8	3.65	3.67
	2.9	3.65	3.65	2.8	3.68	3.69	2.9	3.65	3.64
	3.0	3.68	3.65	2.9	3.70	3.68	3.0	3.68	3.64
	3.1	3.70	3.64	3.0	3.71	3.66	3.1	3.70	3.63
	3.2	3.78	3.68	3.1	3.74	3.66	3.2	3.70	3.60
	3.3	3.78	3.66	3.2	3.75	3.63	3.3	3.73	3.60
	Mean value		3.69	Mean value		3.70	Mean value		3.66
	s		0.04	s		0.04	s		0.05

Conclusions:

Under the conditions of the study the test material has a pKa of 3.68 (s ≤ 0.05) in the concentration range of 0.5 - 0.6 mmol/L at a temperature of 20 °C in an aqueous system. The corresponding Ka = 2.09 x 10^4.

Executive summary:

The dissociation constant was assessed according to OECD Guideline 112 and in compliance with GLP.

A titration method was used with the test material dissolved in DMSO and NaOH.

Under the conditions of the study the test material has a pKa of 3.68 (s ≤ 0.05) in the concentration range of 0.5 - 0.6 mmol/L at a temperature of 20 °C in an aqueous system. The corresponding Ka = 2.09 x 10^4.

Description of key information

Comb (2000b)

Under the conditions of the study the dissociation constant of the test material was found to be 2.0 x 10^-4 (equivalent pKa = 3.7).

Cousin (1996)

Under the conditions of the study, the dissociation constant of pure test material has been determined using the potentiometric method. The pKa was estimated to be 2.8 ± 0.1 at 20 °C.

O'Connor (1990)

Under the conditions of the study the dissociation constant (pKa) for the test material was determined to be 2.50 at ambient temperature (approx 22 °C).

Redeker (1991)

Under the conditions of the study the test material has a pKa of 3.68 (s ≤ 0.05) in the concentration range of 0.5 - 0.6 mmol/L at a temperature of 20 °C in an aqueous system. The corresponding Ka = 2.09 x 10^4.

Additional information

Comb (2000b)

The dissociation constant of the test material was assessed according to OECD Test Guideline 112 and in compliance with GLP. The study was awarded a reliability score of 1 in accordance with the criteria set forth by Klimisch et al. (1997).

The titration method was employed in order to determine the dissociation constant of the test material.

A solution of the test material is prepared in purified water such that the concentration does not exceed the lower of 0.01M or half the saturation concentration. The test solution is titrated with standard acid or base solution as appropriate, and the pH measured after each addition of titrant. The pKa is calculated from the points on the titration curve.

Triplicate samples (approximately 43 to 45 mg) of the test material were dissolved in purified water (100 mL) to form the test solutions. The purified water used to prepare the test solutions was freshly drawn from a water purification unit, boiled and cooled, and then stored under nitrogen prior to use. The quality of the water was such that no carbon dioxide should be present - this was confirmed by the fact the blank titration of pure water was consistent with the absence of carbon dioxide.

Portions (25 mL) of the approximately half-saturated solutions were then titrated with 0.01M aqueous potassium hydroxide. An equivalent blank titration was performed. In order to perform the titrations, a Corning 250 pH meter fitted with a Mettler Toledo Inlab 420 electrode was employed. The test temperature during the measurements was 25 °C.

The dissociation constant of the test material was determined directly from the graphical representations of the data. For each titration, the end point volume was determined as the volume at which the rate of change of the pH of the test solution was greatest. The pKa value for the compound is then calculated as the pH at half of the end point volume.

Under the conditions of the study the dissociation constant of the test material was found to be 2.0 x 10^-4 (equivalent pKa = 3.7).

Cousin (1996)

The dissociation constant of the test material was assessed according to OECD Test Guideline 112 and in compliance with GLP using the potentiometric method. The study was awarded a reliability score of 1 in accordance with the criteria set forth by Klimisch et al. (1997).

Three test solutions were prepared by dissolving about 35 mg of test material in 100 mL of boiled water by using mild magnetic stirring. These solutions were titrated with the 0.025 M NaOH solution at about 20 °C and the pH was recorded after each addition of titrant. The pH and cumulative volumes of titrant were recorded. For each titration, the dissociation constant was calculated for ten points. The pH and cumulative volumes of titrant added were recorded by automatic titrator for each titration experiment. Then, for each titration, the dissociation constant was calculated for ten points and the mean value was obtained by averaging the ten pKa values. The calculation was done using the stoichiometric concentrations of HA and A- (schematizing the acid and basic forms of the compound) after each step of neutralisation and correcting these concentrations for the hydrogen ion concentrations.

The titrant concentration was determined by titrating three potassium hydrogen phthalate solutions at about 0.35 g/L in water and was found to be 0.0243 mol/L.

The pKa of the test material was determined in triplicate at about 20 °C.

Under the conditions of the study, the dissociation constant of pure test material has been determined using the potentiometric method. The pKa was estimated to be 2.8 ± 0.1 at 20 °C.

O'Connor (1990)

The dissociation constant for the test material was assessed according to OECD method 112 and in compliance with GLP by titration.The study was awarded a reliability score of 1 in accordance with the criteria set forth by Klimisch et al. (1997).

The dissociation constant for the test material was determined by titration of a 0.001 M aqueous solution (100 mL) of the test material with sodium hydroxide at ambient temperature.

Under the conditions of the study the dissociation constant (pKa) for the test material was determined to be 2.50 at ambient temperature (approx 22 °C).

Redeker (1991)

The dissociation constant was assessed according to OECD Guideline 112 and in compliance with GLP.The study was awarded a reliability score of 1 in accordance with the criteria set forth by Klimisch et al. (1997).

A titration method was used with the test material dissolved in DMSO and NaOH.

Under the conditions of the study the test material has a pKa of 3.68 (s ≤ 0.05) in the concentration range of 0.5 - 0.6 mmol/L at a temperature of 20 °C in an aqueous system. The corresponding Ka = 2.09 x 10^4.