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Hydrolysis

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Reference
Endpoint:
hydrolysis
Type of information:
experimental study
Adequacy of study:
key study
Study period:
23rd October 2001 to 10th September 2002
Reliability:
1 (reliable without restriction)
Rationale for reliability incl. deficiencies:
guideline study
Qualifier:
according to guideline
Guideline:
OECD Guideline 111 (Hydrolysis as a Function of pH)
Deviations:
no
Qualifier:
according to guideline
Guideline:
EU Method C.7 (Degradation: Abiotic Degradation: Hydrolysis as a Function of pH)
Deviations:
no
GLP compliance:
yes (incl. QA statement)
Radiolabelling:
yes
Analytical monitoring:
yes
Details on sampling:
Samples were taken after the following days of incubation:

25 ºC
pH 7 - 0, 3, 11, 20 and 29
pH 9 - 0, 1, 2, 5, 9, 20 and 30

40 ºC
pH 7 - 0, 1, 2, 7, 11, 20 and 29
pH 9 - 0, 0,08, 0,17, 0.25, 1, 2, 5, 12, 20 and 30
Buffers:
- pH: 4
- Type and final molarity of buffer: 0.01 M Citrate
- Composition of buffer: 0.357 g citric acid (monohydrate), 0.078 g sodium chloride and 0.083 g sodium hydroxide

- pH: 7
- Type and final molarity of buffer: 0.01 M Phosphate
- Composition of buffer: 0.105 or 0.106 g potassium dihydrogenphosphate and 0.159 g di-sodium hydrogenphosphate

- pH: 9
- Type and final molarity of buffer: 0.01 M Borate
- Composition of buffer: 0.072 g potassium dihydrogenphosphate and 0.512 g sodium tetraborate decahydrate
Details on test conditions:
TEST SYSTEM
- Type, material and volume of test flasks, other equipment used: glass incubation vessels
- Measures taken to avoid photolytic effects: incubations were performed in the dark
- Is there any indication of the test material adsorbing to the walls of the test apparatus? yes, therefore solutions of test material were prepared in 10 % acetonitrile

TEST MEDIUM
- Volume used/treatment: 10 mL sterile buffer solution containing test material and 10 % acetonitrile
Duration:
29 d
pH:
7
Temp.:
25 °C
Duration:
30 d
pH:
9
Temp.:
25 °C
Duration:
29 d
pH:
7
Temp.:
40 °C
Duration:
30 d
pH:
9
Temp.:
40 °C
Number of replicates:
25 ºC - duplicate; 40 ºC - single
Positive controls:
no
Negative controls:
no
Statistical methods:
The amount of radioactivity recovered was expressed in percent of the initial radioactivity applied:
Radioactivity recovered Rv(%) = (dpm in buffer solution / initial dpm applied) x 100

The amount of test material and hydrolysis products in the buffer solutions was calculated in percent of the initial radioactivity applied by considering Rv and the amount of each radioactive fraction obtained by HPLC (peak %):
Radioactive fraction (% applied) = (Rv(%) x Peak(%) / 100%

CALCULATION OF DT50 AND DT90 VALUES
The rates of hydrolysis were determined by applying first-order kinetics using the following equation:
-dC/dt = k1 · C

Integration of this equation yields:
C = C0 exp (-k1 · t)

where:
C = amount of test material at any incubation time (percent of applied radioactivity)
C0 = amount of test material obtained on day 0 (percent of applied radioactivity)
K1 = first-order rate constant
t = incubation time (days)

DT50 and DT90 values were obtained using the following equations:
DT50 = ln(2) / k1-
DT90 = ln(10)/ k1
Preliminary study:
Mean recoveries of radioactivity were 96.5 ± 9.8 % (pH 4), 97.1 ± 5.2 % (pH 7) and 100.0 ± 2.3 % (pH 9).

At pH 4, the test material was stable to hydrolysis with 91 % of the initial amount of the substance remaining after 5 days at 50 ºC. At pH 7, the test material was steadily hydrolysed with an average of about 80 % of the initial amount hydrolysed after 5 days at 50 ºC. At pH 9, the test material was completely hydrolysed after 1 day. The major hydrolysis product was characterised as TBPC. The results from pH 7 showed that increasing the acetonitrile content from 1 to 10 % had no effect on the rate of hydrolysis of the test material.
Transformation products:
yes
Remarks:
main degradate TBPC
No.:
#1
Details on hydrolysis and appearance of transformation product(s):
The test material was more rapidly hydrolysed at pH 9 than pH 7. At pH 9, the test material was no longer detected after 1 day (40 ºC) and 9 days (25 ºC) of incubation with corresponding half-lives of less than 1.1 days for both temperatures. For pH 9, incubation was prolonged to investigate the hydrolysis of the main degradate TBPC. For pH 7, at 40 ºC, the test material was completely hydrolysed within less than 29 days. At 25 ºC (pH 7), the test material represented 72.6 % of the applied radioactivity at the end of incubation (day 29).

At pH 7, TBPC steadily increased with incubation time to reach maximum levels of 28.9 % (25 ºC) and 108.8 % (40 ºC) of the applied radioactivity at the end of the 29 day incubation period. At pH 9, TBPC accounted for all of the radioactivity applied from days 1 (40 ºC) and days 9 (25 ºC) onwards.
% Recovery:
99.6
St. dev.:
2
pH:
7
Temp.:
25 °C
% Recovery:
100.8
St. dev.:
5
pH:
7
Temp.:
40 °C
% Recovery:
102.7
St. dev.:
2
pH:
9
Temp.:
25 °C
% Recovery:
100.3
St. dev.:
1.7
pH:
9
Temp.:
40 °C
Key result
pH:
7
Temp.:
25 °C
DT50:
66.3 d
Type:
(pseudo-)first order (= half-life)
Remarks on result:
other: DT90 220.6 days
Key result
pH:
7
Temp.:
40 °C
DT50:
9 d
Type:
(pseudo-)first order (= half-life)
Remarks on result:
other: DT90 29.9 days
Key result
pH:
9
Temp.:
25 °C
DT50:
1.1 d
Type:
(pseudo-)first order (= half-life)
Remarks on result:
other: DT90 3.7 days
Key result
pH:
9
Temp.:
40 °C
DT50:
0.2 d
Type:
(pseudo-)first order (= half-life)
Remarks on result:
other: DT90 0.6 days
Details on results:
TEST CONDITIONS
- pH, sterility, temperature, and other experimental conditions maintained throughout the study: Yes

TBPC
No degradation of TBPC was observed at pH 4, 7 and 9 after 5 days of incubation at 50 ºC; TBPC is therefore considered to be hydrolytically stable under environmentally relevant basic, neutral and acidic conditions.
Validity criteria fulfilled:
not specified
Conclusions:
Under the conditions of the test, the test material was determined to be stable to hydrolysis at pH 4. At pH 7 and 9, the test material was hydrolytically unstable with the rate of hydrolysis being fastest at pH 9. The DT50 values were determined to be 66.3 and 9.0 days (pH 7, 25 and 40 ºC, respectively) and 1.1 and 0.2 (pH 9, 25 and 40 ºC, respectively). The only hydrolysis product was TBPC which was considered to be hydrolytically stable.
Executive summary:

The hydrolytic stability of the 14C-test material was investigated in aqueous solutions at three different pH values.

Initially, a preliminary test (pre-test) was performed by incubating samples containing sterile buffer solution treated with 14C-test material at pH 4, 7 and 9. The samples were treated with the test material at a concentration between 0.2 and 0.3 mg/L buffer solution, less than half of the saturation concentration, and incubated at 50 ºC in the dark for up to 7 days. Due to the high adsorption of the test material to the glass, the organic solvent concentration was increased from 1 to 10 %; it was subsequently established that increasing the co-solvent percentage did not affect the rate or nature of the hydrolysis of the test material.

Mean recoveries of total radioactivity during the 5 day pre-test incubation period were 96.5 ± 9.8 % (pH 4), 97.1 ± 5.2 % (pH 7) and 100.0 ± 2.3 % (pH 9). Recoveries of certain samples were below 90 % of the initially applied amount due to loss of the test material by adsorption to glass. However for pH 7, where two incubations were conducted, one consisting of samples with only 1 % acetonitrile and one consisting of samples with 10 % acetonitrile, it was shown that the use of about 10 % acetonitrile reduced this adsorption to glass and showed almost no effect on the rate of hydrolysis of the test material. The major hydrolysis product, TBPC, represented 82.6 % (1 % acetonitrile) and 83.8 % (10 % acetonitrile) of the applied radioactivity at the end of incubation (day 7, pH 7).

The test material was hydrolytically stable at pH 4 with less than 10 % hydrolysed after 5 days incubation at 50 ºC. At pH 7, the test material was steadily hydrolysed, with only 5 % of the initial amount remaining after 7 days at 50 ºC. For the buffer solution at pH 7 containing 10 % acetonitrile, 12.1 % of the initial amount remained after this period. At pH 9, the test material was completely hydrolysed after 1 day.

No further testing was carried out for pH 4. The main test was performed at pH 7 and 9, using a concentration of 0.1 mg/L, an organic solvent content of 10 %, and two temperature (25 and 40 ºC) for up to 30 days.

The main test was performed by incubating duplicate (25 ºC) or individual (40 ºC) samples for each time point. Each 10 mL sample, consisting of sterile buffer solution containing the test material and 10 % acetonitrile was incubated in a tightly closed glass vessel placed in a thermo-regulated incubation room or oven under constant stirring. At each sampling interval, the samples were diluted 1:1 with methanol, to reduce any adsorption to glassware, and submitted for radiochemical quantification by LSC and chromatographic analysis by HPLC. Selected samples were additionally submitted to 1D- and 2D-TLC analysis in order to confirm the HPLC results.

For the main test at 25 and 40 ºC, mean recoveries were 99.6 ± 2.0 %, 100.8 ± 5.0 % (pH 7), 102.7 ± 2.0 % and 100.3 ± 1.7 % (pH 9) respectively.

The results of the main test show that the test material hydrolyses faster at pH 9 than at pH 7. The test material completely disappeared in less than 1 (40 ºC) and 9 (25 ºC) days compared to 29 days at pH 7 (40 ºC). At pH 7 (25 ºC), the parent compound was still detected after 29 days of incubation (72.6 %). The following DT50 and DT90 values were obtained:

   pH 4  pH 7  pH 9
 50 ºC  25 ºC  40 ºC  25 ºC  40 ºC
 Half-life (days)  Stable  66.3  9.0  1.1  0.2
 DT90 (days)    220.6  29.9  3.7  0.6
 Correlation coefficient (r2)    0.9913  0.9858  0.9986  0.9977

At both pH values, only one hydrolysis product was detected (M1). M1 was characterised to be TBPC and was shown to be stable to hydrolysis at both pH values and both temperatures.

14C-test material may be considered to be hydrolytically stable at pH 4, however under neutral or alkaline conditions it is hydrolysed to form TBPC which is considered to be hydrolytically stable under all pH conditions.

Description of key information

DT50 66.3 and 9.0 days (pH 7) at 25 and 40 ºC, respectively and 1.1 and 0.2 days (pH 9) at 25 and 40 ºC, respectively; study conducted in accordance with OECD Guideline 111 and EU Method C.7; van der Gaauw (2002)

Key value for chemical safety assessment

Half-life for hydrolysis:
66.3 d
at the temperature of:
25 °C

Additional information

A preliminary test was performed by incubating samples containing sterile buffer solution treated with 14C-propargite at pH 4, 7 and 9. The samples were treated with the test material at a concentration between 0.2 and 0.3 mg/L buffer solution and incubated at 50 ºC in the dark for up to 7 days. Due to the high adsorption of the test material to the glass, the organic solvent concentration was increased from 1 to 10 %; it was subsequently established that increasing the co-solvent percentage did not affect the rate or nature of the hydrolysis of the test material.

Mean recoveries of total radioactivity during the 5 day preliminary test were 96.5 ± 9.8 % (pH 4), 97.1 ± 5.2 % (pH 7) and 100.0 ± 2.3 % (pH 9). Recoveries of certain samples were below 90 % of the initially applied amount due to loss of the test material by adsorption to glass. However for pH 7, where two incubations were conducted, it was shown that the use of about 10 % acetonitrile reduced this adsorption and showed almost no effect on the rate of hydrolysis of the test material. The major hydrolysis product, TBPC, represented 82.6-83.8 % of the applied radioactivity at the end of the incubation period (day 7, pH 7).

Propargite was hydrolytically stable at pH 4 with less than 10 % of the initial amount of test material hydrolysed after 5 days incubation at 50 ºC. At pH 7, propargite was steadily hydrolysed, with only 5 % of the initial amount of the test material remaining after 7 days at 50 ºC. For the buffer solution at pH 7 containing 10 % acetonitrile, 12.1 % of the initial amount remained as propargite after this period. At pH 9, propargite was completely hydrolysed after 1 day. No further testing was carried out for pH 4. The main test was performed at pH 7 and 9, using a concentration of 0.1 mg/L, an organic solvent content of 10 %, and two temperatures (25 and 40 ºC) for up to 30 days.

The main test was performed by incubating duplicate (25 ºC) or individual (40 ºC) samples for each time point. Each 10 mL sample, consisting of sterile buffer solution containing the test material and 10 % acetonitrile was incubated in a tightly closed glass vessel placed in a thermo-regulated incubation room or oven under constant stirring. At each sampling interval, the samples were diluted 1:1 with methanol, to reduce any adsorption to glassware, and submitted for radiochemical quantification by LSC and chromatographic analysis by HPLC. Selected samples were additionally submitted to 1D- and 2D-TLC analysis in order to confirm the HPLC results.

For the main test at 25 and 40 ºC, mean recoveries were 99.6 ± 2.0 %, 100.8 ± 5.0 % (pH 7), 102.7 ± 2.0 % and 100.3 ± 1.7 % (pH 9) respectively.

The results of the main test show that propargite hydrolyses faster at pH 9 than at pH 7. Propargite completely disappeared in less than 1 (40 ºC) and 9 (25 ºC) days compared to 29 days at pH 7 (40 ºC). At pH 7 (25 ºC), the parent compound was still detected after 29 days of incubation (72.6 %). The following DT50 and DT90 values were obtained:

   pH 4  pH 7  pH 9
 50 ºC  25 ºC  40 ºC  25 ºC  40 ºC
 Half-life (days)  Stable  66.3  9.0  1.1  0.2
 DT90 (days)    220.6  29.9  3.7  0.6
 Correlation coefficient (r2)    0.9913  0.9858  0.9986  0.9977

At both pH values, only one hydrolysis product was detected (M1). M1 was characterised to be TBPC and was shown to be stable to hydrolysis at both pH values and both temperatures.

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