Pentaerythritol tetrakis(3-mercaptopropionate)

Hydrolysis

Link to relevant study record(s)

Reference

Endpoint:

hydrolysis

Type of information:

experimental study

Adequacy of study:

key study

Study period:

02 October - 17 December 2007

Reliability:

1 (reliable without restriction)

Rationale for reliability incl. deficiencies:

other: GLP guideline study

Qualifier:

equivalent or similar to guideline

Guideline:

EU Method C.7 (Degradation: Abiotic Degradation: Hydrolysis as a Function of pH)

Qualifier:

equivalent or similar to guideline

Guideline:

OECD Guideline 111 (Hydrolysis as a Function of pH)

Principles of method if other than guideline:

The assessment of the hydrolytic stability of the test item was not possible using Method C7 of Commission Directive 92/69/EEC and Method 111 of the OECD Guidelines for Testing of Chemicals, 13 April 2004, due to its susceptibility to oxidation. Based on these guidelines several alternative approaches were performed in order to determine hydrolysis as a function of pH.

GLP compliance:

yes (incl. QA statement)

Remarks:

The Department of Health of the Government of the United Kingdom

Radiolabelling:

no

Analytical monitoring:

yes

Details on sampling:

Sample solutions were taken from the waterbath at various times and the pH of each solution recorded.

Buffers:

For composition of buffers see "any other information on materials and methods".

Buffer solutions were vacuum filtered through a 0.2 µm membrane filter to ensure they were sterile and degassed before commencement of the test. The buffer solutions were then subjected to ultrasonication for 15 minutes and sparging with nitrogen for 5 minutes to further minimise dissolved oxygen content.

Transformation products:

not measured

pH:

9

Temp.:

25 °C

DT50:

1.09 h

Type:

(pseudo-)first order (= half-life)

pH:

9

Temp.:

20 °C

DT50:

5.99 h

pH:

9

Temp.:

30 °C

DT50:

3.77 h

It was concluded from testing performed as a whole that no definitive quantification of the hydrolysis characteristics of the test material could be determined using EU Method C.7 and OECD 111, due to experimental variance attributed to the susceptibility of the test material to oxidation.

The test material contains sulphide functional groups which are known to be susceptible to oxidation, typically resulting in disulfide formation. However, critically the test material also contained a number of ester functional groups known to be susceptible to base catalysed hydrolysis, and thus every effort was made to quantify the rate of hydrolysis of these groups. Testing was significantly hampered by the low aqueous solubility of the test material as typically oxidation of a substance becomes more significant at lower concentrations due to any residual dissolved oxygen then being present in excess.

Although hydrolysis was predicted at pH 9 and experimental evidence supported an accelerated rate in the reduction of the test material concentration in solution at this pH, reduction due to hydrolysis alone could not be confidently isolated from the remaining influence of oxidation. Thus, it is critical to note that oxidation and not hydrolysis may be the primary route of degradation/transformation in the environment.

This conclusion has to be drawn from the practical laboratory work irrespective of the significant steps undertaken to eliminate dissolved oxygen from the sample solutions which included:

- degassing of buffer solution by vacuum filtration, ultrasonication and purging with nitrogen gas before use

- use of nitrogen headspace in each sample vessel during incubation

- use of individual test vessels for each time point, which eliminates exposure of the sample solution to air, and therefore to oxygen, when compared to sampling from a single vessel.

In order to ensure stability of the test material during analysis, a solid phase extraction technique was employed (pre-concentration of sample solutions and a matrix transfer to acetonitrile) prior to analysis. Additionally the anti-oxidant BHT was added to the organic solvent used to elute the SPE cartridges and prepare the standard solutions.

No additional information could be gained on the potential oxidation and / or hydrolysis products of the test material.

At pH 4, where hydrolysis would be expected to be neglible, a second order correlation, typical of oxidation was dominant. At pH 7, where the rate of hydrolysis would increase, neglible correlation was obtained. This was concluded to be due to neither the oxidation or hydrolysis pathway being significantly more dominant than the other. At pH 9, the dominat degradation pathway would be anticipated to be hydrolysis, and this was evident from the increased rate in the reduction of the test material concentration in solution and the first order correlation data generated for the preliminary test at least.

As the difficulty of the present hydrolysis study could be clearly identified to be the high susceptibility of the test material towards oxidation, a structural modification of the test substance was considered as a possible solution for testing. However, for several reasons all of the theoretical modification possibilities had to be considered as inappropriate.

Validity criteria fulfilled:

not applicable

Conclusions:

The assessment of the hydrolytic stability of the test item was not possible using EU Method C.7 and OECD 111, due to its susceptibility to oxidation, irrespective of significant steps taken to eliminate dissolved oxygen from the sample solutions. Besides measures taken to eliminate oxygen from solutions also modification of the test material to exclude functional groups known to be susceptible to oxidation was considered. However, all alternatives were concluded not to achieve desired results.
In conclusion, hydrolysis alone could not be confidently isolated from the remaining influence of oxidation and no half-life data exclusively resulting from hydrolysis could be quantified. Therefore, although ranges for the overall stability of the test item in the aquatic environment at different pH values could be delivered by this hydrolysis study, it is critical to note that oxidation and not hydrlysis may be the primary route of degradation/transformation in the environment.

Description of key information

It was concluded from the testing performed as a whole that no definitive quantification of the hydrolysis characteristics of the test material could be determined using EU Method C.7 and OECD Method 111, due to experimental variance attributed to the susceptibility of the test material to oxidation (elimination of test substance).

Additional information

The test material contains sulphide functional groups which are known to be susceptible to oxidation, typically resulting in disulfide formation. However, critically the test material also contained a number of ester functional groups known to be susceptible to base catalysed hydrolysis, and thus every effort was made to quantify the rate of hydrolysis of these groups. Testing was significantly hampered by the low aqueous solubility of the test material as typically oxidation of a substance becomes more significant at lower concentrations due to any residual dissolved oxygen then being present in excess.

Although hydrolysis was predicted at pH 9 and experimental evidence supported an accelerated rate in the reduction of the test material concentration in solution at this pH, reduction due to hydrolysis alone could not be confidently isolated from the remaining influence of oxidation. Thus, it is critical to note that oxidation and not hydrolysis may be the primary route of degradation/transformation in the environment.

This conclusion has to be drawn from the practical laboratory work irrespective of the significant steps undertaken to eliminate dissolved oxygen from the sample solutions which included: (a) degassing of buffer solution by vacuum filtration, ultrasonication and purging with nitrogen gas before use, (b) use of nitrogen headspace in each sample vessel during incubation, and (c) use of individual test vessels for each time point, which eliminates exposure of the sample solution to air, and therefore to oxygen, when compared to sampling from a single vessel.

In order to ensure stability of the test material during analysis, a solid phase extraction technique was employed (pre-concentration of sample solutions and a matrix transfer to acetonitrile) prior to analysis. Additionally the anti-oxidant BHT was added to the organic solvent used to elute the SPE cartridges and prepare the standard solutions.

No additional information could be gained on the potential oxidation and / or hydrolysis products of the test material.

At pH 4, where hydrolysis would be expected to be neglible, a second order correlation, typical of oxidation was dominant. At pH 7, where the rate of hydrolysis would increase, neglible correlation was obtained. This was concluded to be due to neither the oxidation or hydrolysis pathway being significantly more dominant than the other. At pH 9, the dominat degradation pathway would be anticipated to be hydrolysis, and this was evident from the increased rate in the reduction of the test material concentration in solution and the first order correlation data generated for the preliminary test at least.

As the difficulty of the present hydrolysis study could be clearly identified to be the high susceptibility of the test material towards oxidation, a structural modification of the test substance was considered as a further possible solution for testing. However, for several reasons all of the theoretical modification possibilities had to be considered as inappropriate.

The test substance undergoes fast oxidation reducing test substance concentration during hydrolysis test. The resulting experimental difficulties prevent the determination of hydrolysis half-life.