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Hydrolysis

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Reference
Endpoint:
hydrolysis
Type of information:
experimental study
Adequacy of study:
key study
Study period:
01 May - 29 Jul 2009
Reliability:
1 (reliable without restriction)
Rationale for reliability incl. deficiencies:
guideline study
Reference:
Composition 0
Qualifier:
according to
Guideline:
OECD Guideline 111 (Hydrolysis as a Function of pH)
Deviations:
no
Qualifier:
according to
Guideline:
EU Method C.7 (Degradation: Abiotic Degradation: Hydrolysis as a Function of pH)
Deviations:
no
GLP compliance:
yes (incl. certificate)
Remarks:
The Department of Health of the Government of the United Kingdom, date of inspection 19 Aug 2008
Test material information:
Composition 1
Radiolabelling:
no
Analytical monitoring:
yes
Buffers:
- pH: 4, 7, 9
- Composition of buffer: pH 4, 61 mM citric acid and 44 mM NaCl; pH 7, 30 mM Na2HPO4, 20 mM KH2PO4 and 20 mM NaCl; pH9, 10 mM disodium tetraborate and 20 mM NaCl
Details on test conditions:
TEST SYSTEM
- Type, material and volume of test flasks: capped glass bottles
- Sterilisation method: The buffer solutions were filtered through 0.2 µm membrane filters.
- Measures to exclude oxygen: The buffer solutions were subjected to ultrasonication and degassing with nitrogen.
- If no traps were used, is the test system closed/open: closed

TEST MEDIUM
- Identity and concentration of co-solvent: 1% acetonitrile
Duration:
240 h
pH:
4
Initial conc. measured:
> 0.284 - < 0.301 mg/L
Duration:
209 h
pH:
4
Initial conc. measured:
> 0.219 - < 0.223 mg/L
Duration:
168 h
pH:
4
Initial conc. measured:
> 0.36 - < 0.426 mg/L
Duration:
144 h
pH:
7
Initial conc. measured:
> 0.182 - < 0.27 mg/L
Duration:
120 h
pH:
7
Initial conc. measured:
> 0.425 - < 0.446 mg/L
Duration:
336 h
pH:
7
Initial conc. measured:
> 0.32 - < 0.383 mg/L
Duration:
168 h
pH:
7
Initial conc. measured:
> 0.245 - < 0.308 mg/L
Duration:
96 h
pH:
7
Initial conc. measured:
> 0.389 - < 0.414 mg/L
Number of replicates:
2 replicates per temperature and pH level
Positive controls:
no
Negative controls:
no
Transformation products:
no
% Recovery:
78.4
pH:
4
Duration:
> 168 - < 240 h
% Recovery:
66.2
pH:
7
Duration:
> 96 - < 144 h
% Recovery:
82
pH:
9
Duration:
> 5 - < 50 h
pH:
4
Temp.:
25 °C
Half-life:
> 1 yr
Type:
other: Due to effects other than hydrolysis
pH:
7
Temp.:
25 °C
Half-life:
> 1 yr
Type:
other: Due to effects other than hydrolysis
pH:
9
Temp.:
25 °C
Half-life:
6.61 h
Type:
(pseudo-)first order (= DT50)
Remarks on result:
other: For component PEMB
pH:
9
Temp.:
25 °C
Half-life:
9.33 h
Type:
(pseudo-)first order (= DT50)
Remarks on result:
other: For component PE3MB
Details on results:
Testing at pH 7 demonstrated a significant degree of variation in the analytical results for duplicate samples and across the time points over the test period. Even though this test was performed at five different temperatures plus some limited testing at 25 °C, consistent data could not be obtained. The variation in analytical data was not considered to be due to solubility issues. It was concluded that there was no evidence of hydrolysis at pH 7, since losses observed were not of a pseudo-first order kinetic nature and were considered to be due to other mechanisms or effects.
For pH 4, no definitive signs of hydrolysis are reported.
The kinetics of the study for pH 9 has been determined to be consistent with that of a pseudo-first order reaction as the graphs of log concentration versus time are straight lines.
As the test material was determined to be hydrolytically stable under acidic conditions (T ½ > 1 year at 25 °C), no additional testing was performed at pH 1.2 and 37 °C.

1.1.1               Preliminary test/Tier 1

The mean peak areas relating to the pH 4 and pH 9 standard and sample solutions are shown in the following table:

Table 4.2

Solution

Component A

MeanArea

Component B

MeanArea

Standard 46.2 mg/L

376.809

2.570 x 103

Standard 46.7 mg/L

387.843

2.587 x 103

Initial Sample A, pH 4

349.774

2.672 x 103

Initial Sample B, pH 4

351.318

2.882 x 103

2.4 hour Sample A, pH 4

364.178

2.852 x 103

2.4 hour Sample B, pH 4

240.039

1.607 x 103

4 hour Sample A, pH 4

353.094

3.062 x 103

4 hour Sample B, pH 4

330.860

2.725 x 103

Standard 46.2 mg/L

384.043

2.511 x 103

Standard 46.7 mg/L

397.867

2.616 x 103

24 hour Sample A, pH 4

146.740

1.412 x 103

24 hour Sample B, pH 4

278.857

2.930 x 103

Initial Sample A, pH 9

256.161

1.330 x 103

Initial Sample B, pH 9

272.743

1.463 x 103

2.4 hour Sample A, pH 9

ND

ND

2.4 hour Sample B, pH 9

44.970

76.761

24 hour Sample A, pH 9

ND

ND

24 hour Sample B, pH 9

ND

ND

Standard 46.2 mg/L

360.395

2.490 x 103

Standard 46.7 mg/L

354.935

2.496 x 103

120 hour Sample A, pH 4

65.561

479.563

120 hour Sample B, pH 4

92.852

860.987

ND – none detected


The mean peak heights relating to the pH 7 standard and sample solutions are shown in the following table:

Table 4.3

Solution

Component B

MeanHeight

Standard 46.2 mg/L

234.921

Standard 46.7 mg/L

248.334

Initial Sample A, pH 7

156.610

Initial Sample B, pH 7

145.797

2.4 hour Sample A, pH 7

18.970

2.4 hour Sample B, pH 7

41.456

50 hour Sample A, pH 7

ND

50 hour Sample B, pH 7

ND

96 hour Sample A, pH 7

ND

96 hour Sample B, pH 7

ND

ND – none detected

The test material concentrations at the given time points are shown in the following tables:

Table 4.4      pH 4 at 50.0 ± 0.5ºC - Component A

Time (Hours)

Concentration (g/L)

Log10[concentration (g/L)]

% of mean initial concentration

A

B

A

B

A

B

0

4.25 x 10-4

4.27 x 10-4

-3.37

-3.37

-

-

2.4

4.43 x 10-4

2.92 x 10-4

-3.35

-3.54

104

68.5

4

4.29 x 10-4

4.02 x 10-4

-3.37

-3.40

101

94.4

24

1.74 x 10-4

3.31 x 10-4

-3.76

-3.48

40.9

77.8

120

8.51 x 10-5

1.12 x 10-4

-4.07

-3.92

20.0

28.3

Result:           Slope =         -4.99 x 10-3     (see Figure 4.1)
r2        =         0.850
kobs     =         1.15 x 10-2hour-1, 3.19 x 10-6second-1
t½       =         60.4 hours, 2.52 days


Table 4.5      pH 4 at 50.0 ± 0.5ºC - Component B

Time (Hours)

Concentration (g/L)

Log10[concentration (g/L)]

% of mean initial concentration

A

B

A

B

A

B

0

4.81 x 10-4

5.19 x 10-4

-3.32

-3.29

-

-

2.4

5.14 x 10-4

2.90 x 10-4

-3.29

-3.54

103

57.9

4

5.52 x 10-4

4.91 x 10-4

-3.26

-3.31

110

98.1

24

2.56 x 10-4

5.31 x 10-4

-3.59

-3.28

51.1

106

120

8.94 x 10-5

1.60 x 10-4

-4.05

-3.80

17.9

32.1

Result:           Slope =         -5.00 x 10-3     (see Figure 4.2)
r2        =         0.794
kobs     =         1.15 x 10-2hour-1, 3.20 x 10-6second-1
t½       =         60.2 hours, 2.51 days

 

Table 4.6      pH 7 at 50.0 ± 0.5ºC - Component B

Time (Hours)

Concentration (g/L)

Log10[concentration (g/L)]

% of mean initial concentration

A

B

A

B

A

B

Initial

3.01

2.80

-3.52

-3.55

-

-

2.4

3.65

7.97

-4.44

-4.10

12.5

27.4

50

-

-

-

-

-

-

96

-

-

-

-

-

-

Result:           Slope =         -0.305(see Figure 4.3)
r2        =         0.903
kobs     =         0.702 hour-1, 1.95 x 10-4second-1
t½       =         0.988 hours, 4.12 x 10-2days


Table 4.7      pH 9 at 50.0 ± 0.5ºC - Component A

Time (Hours)

Concentration (g/L)

Log10[concentration (g/L)]

% of mean initial concentration

A

B

A

B

A

B

Initial

3.04 x 10-4

3.24 x 10-4

-3.52

-3.49

-

-

2.4

-

5.34 x 10-5

-

-4.27

-

17.0

24

-

-

-

-

-

-

Result:           Slope =         -0.321(see Figure 4.4)
r2        =         1.00
kobs     =         0.738 hour-1, 2.05 x 10-4second-1
t½       =         0.939 hours, 3.91 x 10-2days

 

Table 4.8      pH 9 at 50.0 ± 0.5ºC - Component B

Time (Hours)

Concentration (g/L)

Log10[concentration (g/L)]

% of mean initial concentration

A

B

A

B

A

B

Initial

2.41 x 10-4

2.65 x 10-4

-3.62

-3.58

-

-

2.4

-

1.39 x 10-5

-

-4.86

-

5.50

24

-

-

-

-

-

-

Result:           Slope =         -0.525(see Figure 4.5)
r2        =         1.00
kobs     =         1.21 hour-1, 3.36 x 10-4second-1
t½       =         0.574 hours, 2.39 x10-2days

 

For each pH, the extent of hydrolysis indicated that further tests (Tier 2), conducted at three temperatures[1]were required to estimate the rate constants and half-lives.



[1]Testing at pH 7 was initially performed at three temperatures (20, 30 and 40°C). Due to inconclusive data, further testing was performed at elevated temperatures of 50 and 60°C plus some testing at 25°C.

1.1                      Validation

The linearity of the detector response with respect to concentration was assessed over the nominal concentration range of 0 to 50 mg/L. This was satisfactory with a correlation coefficient of 1.00 being obtained for Components A and B.

Recovery of analysis of the sample procedure was assessed and proved adequate for the test, with the exception of Component A in pH 7 buffer.

Recovery data is shown in the following table:

Table 4.39

Buffer

Nominal concentration
(mg/L)

Recovery range (%)

Mean recovery (%)

Component A

Component B

Component A

Component B

pH 4

0.37

87.3 to 92.7

75.8 to 82.3

90.8

78.4

pH 7

0.37

0

62.0 to 74.1

0

66.2

pH 9

0.37

62.9 to 80.3

78.2 to 84.7

72.7

82.0

Concentrations have not been corrected for recovery of analysis.

1.2                      Discussion

Initial experiments demonstrated poor recoveries of analyte in buffer solutions, but acceptable recoveries in pH adjusted water. Therefore, preliminary testing was performed in reverse osmosis water adjusted to pH 4, 7 and 9 using 0.1 M hydrochloric acid or 0.1 M sodium hydroxide. However, due to the absence of buffering, the pH of these solutions was unstable. Further investigation revealed that acceptable recoveries could be achieved in the buffers specified in Section 4.2.1, with the exception of pH 7 phosphate buffer in which no recovery of Component A could be achieved. This anomaly could not be readily explained since the pH 7 matrix blank did not demonstrate interfering peaks and the analytical procedure was shown to be successful for both analytes in pH 4 and pH 9 buffers. Therefore, no data for Component A at pH 7 could be generated.

Tier 2 testing at pH 7 demonstrated a significant degree of variation in the analytical results for duplicate samples and across the time points over the period of the test.  Even though this test was performed at five different temperatures plus some limited testing at 25°C, consistent data could not be obtained. The variation in analytical data was not considered to be due to solubility issues. It was concluded that there was no evidence of hydrolysis at pH 7, since losses observed were not of a pseudo-first order kinetic nature and were considered to be due to other mechanisms or effects.

For additional information, an estimation of the hydrolytic stability of the test material was performed using the specialised predictive software, HYDROWIN v 2.00 (September 2008), © 2000 US Environmental Protection Agency. The estimated half-lives at 25°C are shown in the following table:

Component

pH 7

pH 8

A (PE3MB)

13.1 years

1.31 years

B (PEMB)

9.85 years

360 days

For calculation of the test material concentration, peak area was used throughout the
pH 4 and pH 9 tests, whereas peak height was the pH 7 tests. The use of peak height in the pH 7 calculations gave initial concentrations consistent with the fortified concentrations.

The kinetics of the study for pH 9 has been determined to be consistent with that of a pseudo-first order reaction as the graphs of log10 concentration versus time are straight lines.

As the test material was determined to be hydrolytically stable under acidic conditions (t½ > 1 year at 25°C), no additional testing was performed at pH 1.2, 37.0 ± 0.5°C.

Validity criteria fulfilled:
yes
Conclusions:
No definitive signs of hydrolysis at pH 4 or pH 7. Testing performed at various temperatures indicated that losses observed were not pseudo-first order and therefore most likely to be due other mechanisms or effects rather than hydrolysis.
Executive summary:

Assessment of hydrolytic stability was carried out using Method C7 Abiotic Degradation, Hydrolysis as a Function of pH of Commission Regulation (EC) No 440/2008 of 30 May 2008 and Method 111 of the OECD Guidelines for Testing of Chemicals, 13 April 2004. The results are as follows:

pH

Rate constant (s-1)

Estimated half-life at 25°C

PE3MB

PEMB

PE3MB

PEMB

4

not applicable

not applicable

>1 year

>1 year

7

not applicable

not applicable

>1 year

>1 year

9

2.07 x 10-5

2.91 x 10-5

9.33 hours

6.61 hours

No definitive signs of hydrolysis at pH 4 or pH 7. Testing performed at various temperatures indicated that losses observed were not pseudo-first order and therefore most likely to be due other mechanisms or effects rather than hydrolysis. 

Description of key information

DT50 = 6.61 h (pH 9) and > 1 year (pH 4 and 7) (OECD 111)

Key value for chemical safety assessment

Half-life for hydrolysis:
6.61 h
at the temperature of:
25 °C

Additional information

Hydrolysis of the substance was investigated in one key study according to OECD 111 and GLP (Showa Denko, 2009). The tests were carried out at pH 4, 7 and 9 and at five different temperatures ranging from 20 to 60 °C. No definitive signs of hydrolysis are reported for the pH values 4 and 7. The kinetics of the study for pH 9 was of pseudo-first order and allowed the determination of a hydrolysis rate constant of 2.91E-05/s and the estimation of a half-life of 6.61 h at 25 °C.