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Water solubility

Link to relevant study record(s)

Reference

Endpoint:

water solubility

Type of information:

experimental study

Adequacy of study:

key study

Study period:

08 March 2021 - 06 October 2021

Reliability:

1 (reliable without restriction)

Rationale for reliability incl. deficiencies:

guideline study

Remarks:

The determination of water solubility was performed using the slow stirring method adapted from OECD Guideline No 123, OECD Guideline No 105 and Method A.6. This study was considered as reliable without restrictions because it was conducted under GLP.

Reason / purpose for cross-reference:

reference to same study

Reason / purpose for cross-reference:

reference to other study

Qualifier:

according to guideline

Guideline:

OECD Guideline 105 (Water Solubility)

Qualifier:

according to guideline

Guideline:

EU Method A.6 (Water Solubility)

Qualifier:

according to guideline

Guideline:

other: adapted from OECD Guideline 123

Deviations:

no

Principles of method if other than guideline:

In a stirring vessel maintained at 20 ± 0.5 °C, ultrapure water containing the test item was slow stirred. The stirring rate was adjusted in order to form a minimal vortex. The aim of this method was to prevent the formation of emulsions when saturating water. Emulsions can be formed with the shake flask method and lead to a solubility overestimation. The water solubility was given by the mass concentration of the substance in water when a plateau was reached as a function of time.
Three experiments were performed in the same conditions.

GLP compliance:

yes (incl. QA statement)

Remarks:

2021-03-17

Type of method:

other: slow-stirring method

Water solubility:

15.17 mg/L

Temp.:

20 °C

Remarks on result:

other: First determination (vessel 1)

Water solubility:

14.33 mg/L

Temp.:

20 °C

Remarks on result:

other: Second determination (vessel 2)

Water solubility:

13.97 mg/L

Temp.:

20 °C

Remarks on result:

other: Third determination (vessel 3)

Key result

Water solubility:

ca. 14.5 mg/L

Temp.:

20 °C

Remarks on result:

other: Mean of three solubility values obtained in three different experiments.

Preliminary test

 

A preliminary test was carried out as a range finding test prior to the main test. The concentration range obtained gave an indication on the necessary analytical method sensitivity.
Increasing volumes of water were added at room temperature to 21.1 mg of the test sample in a 100 mL flask. After each addition of an amount of water, the mixture was shaken for 10 minutes and was visually checked for any undissolved parts of the sample.
The approximate solubility deduced from the necessary volume of pure water in which complete dissolution of the sample occurred is given in the table below.

 

Volume of water added	Solubilisation ?	Approximate solubility
1 mL	NO	-
10 mL	NO	-
100 mL	NO	< 207 mg/L

 

The test item (21.1 mg) was not solubilized by a water volume of 100 mL after 24 hours, corresponding to a solubility inferior to 207 mg/L (since the purity of the test item is to 98.2%).
Two ecotoxicity tests were recently performed on this test item (“Daphnia sp., Acute Immobilisation Test”: see LPL report D20-036, and "Freshwater Alga and Cyanobacteria, Growth Inhibition Test": see LPL report A20-036). Stock solutions of the test item were prepared in test water by adding carefully approximately 500 mg of test substance to 5 L of test water. The mixing vessel was closed and the mixing was initiated. After about 24 hours of gentle stirring in the dark at room temperature, the contents of the vessel were allowed to stand for at least 1 hour before sampling. Three stock solutions were prepared for the tests. The obtained concentrations in test water were 13.51 mg/L, 7.41 mg/L and 7.66 mg/L.
Even though these values were obtained with a fixed equilibration time (24h), at a non-controlled temperature (room temperature) and not in pure water (daphnia and algae test water), the value of about 10 mg/L can be considered as an estimation of the test item water solubility.
It was also observed, during this preliminary test and the ecotoxicity test, that the test item is less dense than water.

 

Main test:

Concentration profiles:

The following table presents the concentrations (in mg/L) obtained during the study for the vessel 1 in ultrapure water.

 

Sampling date (m/d/y)	Vessel 1	Vessel 2	Mean
2021/03/08 9:30 (Preparation T0)	-	-	-
2021/03/09 12:30	9.31	9.44	9.38
2021/03/10 11:25	11.94	12.07	12.01
2021/03/11 11:15	13.41	14.40	13.91
2021/03/12 11:05	12.99	13.03	13.01
2021/03/15 11:00	15.14	15.14	15.14
2021/03/16 10:30	14.83	15.20	15.02
2021/03/17 8:30	14.62	14.56	14.59
2021/03/18 8:30	15.94	15.92	15.93

 

These concentrations are presented as a function of time in the figure.

 

Sampling in vessels 2 and 3 occurred on March 19, 2021 at 10:30, 10 days after their preparation. The concentrations measured (in mg/L) for these two vessels were:

 

	Rep.1	Rep.2	Mean
Vessel 2	13.60	15.05	14.33
Vessel 3	14.39	13.55	13.97

 

 

The pH controlled with the indicator strips for each specimen was 5 for all the samples.
The temperature variations were included in the range 19.8 °C - 20.4 °C.

 

Equilibrium demonstration:

 

Two conditions had to be fulfilled in order to demonstrate the equilibrium.
First, the graph representing the evolution of the concentrations of the test item against time has to reach a plateau. The test needed to be continued until four successive time points yielded a slope that was significantly different from zero at a p-level of 0.05, indicating that the concentration was independent of time.
A hypothesis test for regression slope was carried out to demonstrate the equilibrium state of the system. The first step was to state the null hypothesis and an alternative hypothesis:
H0: The slope of the regression line was equal to zero.
HA: The slope of the regression line was not equal to zero.
If the relationship between the concentration and time was significant, the slope would not be equal to zero. For this analysis, the significance level was 0.05. Using sample data, a linear regression t-test was conducted to determine whether the slope of the regression line differed significantly from zero. To apply the linear regression t-test to sample data, the slope of the regression line, the standard error of the slope, the degrees of freedom, the t-score test statistic, and the P-value of the statistical test were required.
Two degrees of freedom were chosen due to the four observations used. According to the student distribution tables with two degrees of freedom and a significance level of 0.05, H0 could be accepted if:
- 4.303 < t < 4.303
The sampling time period used for this equilibrium demonstration was the four last sampling times, i.e. from March 15 at 11:00 to March 18 at 8:30.
The results of the statistical test are presented in the following tables:

 

Slope	0.2069
Standard error	0.283
t value	0.731
Equilibrium ?	YES

The t value was obtained by dividing the slope by the standard error. Since this value was included in the [-4.303;4.303] interval, the equilibrium state can be declared for the vessel 1 between March 15 and March 18.

The second condition to verify in order to declare the equilibrium is that the maximum difference (MAX–MIN / MEAN(MAX;MIN)) between the four last samples is below 15 %. The following table demonstrates that this equilibrium criteria is valid.

Maximum difference observed in the 4 last samples	Min: 14.59 mg/L Max: 15.93 mg/L Difference: 9% < 15%
Conclusion	Equilibrium reached

Since the two conditions were fulfilled, the equilibrium state was declared on March 18 at 8:30 for the vessel 1.

Water solubility calculation:

The equilibrium concentrations for each vessels were:
- 15.17 mg/L for vessel 1;
- 14.33 mg/L for vessel 2;
- 13.97 mg/L for vessel 3.
The maximum difference observed between the three determinations is 8%, which proves that an acceptable repeatability is obtained (the OECD 105 acceptability criterion for the maximum difference is 15% for the shake flask method).
The test item solubility in water is given by the mean of the equilibrium concentrations determined for each vessel: 14.5 ± 0.5 mg/L.

Conclusions:

The solubility of test item in water at 20 ± 0.5 °C, based on the mean of three determinations with the slow-stirring method was 14.5 ± 0.5 mg/L.

Executive summary:

A study was performed to assess the water solubility of test item by the slow-stirring method.

Three determinations in ultrapure water were carried out. 13 μL of the test item were carefully poured at the top of 250 mL of water thermostated at 20 ± 0.5 °C (corresponding to a loading rate of 50.7 mg/L). The solutions were allowed to equilibrate 24 hours before starting the sampling and up to 10 days. The test item concentrations were monitored by GC-FID.

The solubility of test item in water at 20 ± 0.5 °C, based on three determinations with the slow-stirring method was 14.5 ± 0.5 mg/L.

Description of key information

Water solubility: 14.5 mg/L at 20 ºC, OECD TG 123; slow stirring method 

Key value for chemical safety assessment

Water solubility:

14.5 mg/L

at the temperature of:

20 °C

Additional information

A study was performed to assess the water solubility of test item by the slow-stirring method.

The solubility of test item in water at 20 ± 0.5 °C, based on three determinations with the slow-stirring method was 14.5 ± 0.5 mg/L.