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Hydrolysis

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Reference
Endpoint:
hydrolysis
Type of information:
experimental study
Adequacy of study:
key study
Study period:
Testing was conducted between 23 October 2009 and 12 June 2010.
Reliability:
1 (reliable without restriction)
Rationale for reliability incl. deficiencies:
guideline study
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)
Remarks:
UK GLP standards (Schedule 1, Good Laboratory Practice Regulations 1999 (SI 1999/3106 as amended by SI 2004/0994)). These Regulations are in accordance with GLP standards published as OECD Principles on Good Laboratory Practice (revised 1997, ENV/MC/CHEM
Radiolabelling:
no
Analytical monitoring:
yes
Details on sampling:
The buffer solutions were filtered through a 0.2 µm membrane filter to ensure they were sterile before commencement of the test. Also these solutionswere subjected to ultrasonication and degassing with nitrogen to minimise dissolved oxygen content.

Preparation of samples:
Sample solutions were prepared in stoppered glass flasks at a nominal concentration of 0.100 g/l in the three buffer solutions, obtained from a stock solution prepared in reverse osmosis grade water at a nominal concentration of 5.00 g/l.
The test solutions were split into individual vessels for each data point.
The solutions were shielded from light whilst maintained at the test temperature.

Preliminary test:
Sample solutions at pH 4, 7 and 9 were maintained at 50.0 ± 0.5°C for a period of 120 hours.

Tier 2:
Results from the preliminary test showed it was necessary to undertake further testing at pH 4 and pH 7. At pH 4, solutions were maintained at 50.0 ± 0.5°C, 60.0 ± 0.5°C and 70.0 ± 0.5°C for periods of 72, 48 and 6 hours respectively. At pH 7, solutions were maintained at 50.0 ± 0.5°C, 60.0 ± 0.5°C and 70.0 ± 0.5°C for periods of 432, 432 and 240 hours respectively.

Testing at pH 1.2:
Sample solutions at pH 1.2 were maintained at 37.0 ± 0.5°C for a period of 20 hours.

Analysis of sample solutions:
Duplicate vessels containing sample solution were taken from the waterbath at various times and the pH of each solution recorded.
The concentration of the sample solution was determined by ion chromatography (IC).

Samples:
Duplicate aliquots (A and B) of the sample solutions were diluted by a factor of 5 using reverse osmosis grade water.

Standards:
Duplicate standard solutions of sodium tripolyphosphate (Sigma Aldrich, lot 12321JI, purity 85% ) were prepared in reverse osmosis grade water at a nominal concentration of 20 mg/l.

Matrix blanks:
Reverse osmosis grade water: relevant buffer solution (80:20 v/v) for each pH.
Buffers:
Buffer Solution (pH 1.2)
Components : Hydrochloric acid and Potassium chloride
Concentrations (mmol dm-3): Hydrochloric acid: 64.4, Potassium chloride: 50

Buffer Solution (pH 4)
Components: Citric acid, Sodium chloride and Sodium hydroxide
Concentrations (mmol dm-3): Citric acid : 12, Sodium chloride, Sodium hydroxide: 14

Buffer Solution (pH7)
Components: Citric acid, Sodium citrate
Concentrations (mmol dm-3): Citric acid: 0.02, Sodium citrate: 9.98

Buffer Solution (pH9)
Components: Disodium tetraborate, Sodium chloride
Concentration (mmol dm-3): Disodium tetraborate: 10, Sodium chloride: 20

The buffer solutions were filtered through a 0.2 µm membrane filter to ensure they were sterile before commencement of the test. Also these solutions were subjected to ultrasonication and degassing with nitrogen to minimise dissolved oxygen content.
Estimation method (if used):
Not used.
Details on test conditions:
Preliminary Test
Sample solutions at pH 4, 7 and 9 were maintained at 50.0 ± 0.5°C for a period of 120 hours.
Tier 2
Results from the preliminary test showed it was necessary to undertake further testing at pH 4 and pH 7. At pH 4, solutions were maintained at 50.0 ± 0.5°C, 60.0 ± 0.5°C and 70.0 ± 0.5°C for periods of 72, 48 and 6 hours respectively. At pH 7, solutions were maintained at 50.0 ± 0.5°C, 60.0 ± 0.5°C and 70.0 ± 0.5°C for periods of 432, 432 and 240 hours respectively.
Testing at pH 1.2
Sample solutions at pH 1.2 were maintained at 37.0 ± 0.5°C for a period of 24 hours.
Duration:
120 h
pH:
4
Initial conc. measured:
0.046 g/L
Duration:
72 h
pH:
4
Initial conc. measured:
0.046 g/L
Duration:
48 h
pH:
4
Initial conc. measured:
0.046 g/L
Duration:
6 h
pH:
4
Initial conc. measured:
0.046 g/L
Duration:
120 h
pH:
7
Initial conc. measured:
0.05 g/L
Duration:
432 h
pH:
7
Initial conc. measured:
0.045
Duration:
432 h
pH:
7
Initial conc. measured:
0.045 g/L
Duration:
240 h
pH:
7
Initial conc. measured:
0.045 g/L
Duration:
120 h
pH:
9
Initial conc. measured:
0.05 g/L
Duration:
24 h
pH:
1.2
Initial conc. measured:
0.048 g/L
Number of replicates:
Duplicate aliquots (A and B) of the sample solutions were diluted by a factor of 5 using reverse osmosis grade water.
Positive controls:
no
Negative controls:
no
Statistical methods:
Not specified.
Preliminary study:
The mean peak areas relating to the standard and sample solutions are shown in tables 5.2 and 5.6 and the test material concentrations at the given time points are shown in tables 5.3 to 5.5, and 5.7 to 5.12 (see attachment- Abiotic Results).
The graphs from plotting the natural logarithm of the rate constants against the reciprocal of the temperature (K), the rate constant and half-life at 25°C are shown in the attachment - Abiotic Graphs
The Arrhenius data for pH 4 and pH 7 is shown in the attachment - Abiotic Arrhenius Data
The results for ph 1.2 is shown in the attachment - Abiotic pH 1.2 results and graph
Test performance:
Validation:
The linearity of the detector response with respect to concentration was assessed over the nominal standard concentration range of 0 to 75 mg/l, equivalent to approximately 0 to 44 mg/l as the triphosphate anion. This was satisfactory with a correlation coefficient of 1.000 being obtained.
Transformation products:
yes
No.:
#1
No.:
#2
Details on hydrolysis and appearance of transformation product(s):
Peaks were observed at the anticipated retention time of the phosphate and pyrophosphate anions, confirming hydrolysis of the triphosphate anion into these two subunits.
The half-life of the pyrophosphate anion has been addressed using OECD Method 111 under Harlan Laboratories Ltd, Shardlow, UK, project number 2920/0047. Sample solutions of tetrapotassium pyrophosphate were prepared at a nominal concentration of 30 mg/l with respect to the pyrophosphate anion in pH 4 and pH 7 sodium citrate buffer solutions and a pH 9 sodium tetraborate buffer solution and incubated at 50°C. Greater than 90% of the initial concentration of pyrophosphate anion remained after 120 hours incubation at 50°C for pH 7 and pH 9, indicating an estimated half-life of greater than 1 year at 25°C. Less than 90% remained at pH 4, and full testing was performed at test temperatures of 50°C, 60°C and 70°C. However on application of the Arrhenius plot, the final extrapolated half-life at 25°C again remained greater than 1 year (527 days). Therefore further significant hydrolysis of the pyrophosphate hydrolysis product is not anticipated under the environmentally relevant pH range of 4 to 9 at 25°C.

pH:
1.2
Temp.:
37 °C
Hydrolysis rate constant:
0.111 h-1
DT50:
6.25 h
Type:
(pseudo-)first order (= half-life)
Remarks on result:
other: Result: Slope = -4.82 x 10-2 kobs = 0.111 hour-1 = 3.08 x 10-5 second-1
pH:
4
Temp.:
25 °C
Hydrolysis rate constant:
0.002 h-1
DT50:
14.5 d
Type:
(pseudo-)first order (= half-life)
Remarks on result:
other: The rate constant and half-life at 25°C have been estimated to be as follows: kobs = 1.99 x 10-3 hour-1 = 5.54 x 10-7 second-1
pH:
7
Temp.:
25 °C
Hydrolysis rate constant:
0 h-1
DT50:
> 1 yr
Type:
(pseudo-)first order (= half-life)
Remarks on result:
other: The rate constant and half-life at 25°C have been estimated to be as follows: kobs = 2.12 x 10-5 hour-1 = 5.89 x 10-9 second-1 t½ = 3.27 x 103 hours = 1363 days (>1 year)
pH:
9
Temp.:
25 °C
DT50:
> 1 yr
Type:
not specified
Remarks on result:
other: .
Other kinetic parameters:
None.
Details on results:
Due to the high volume of tables and figures please see the attachment - Abiotic Full Set of results

Due to the high volume of tables and figures please see the attachment - Abiotic Full Set of results

Validity criteria fulfilled:
yes
Conclusions:
The rate constant and estimated half-life at 25°C of the test material are shown in the following table:


pH Rate constant (s-1) Estimated half-life at 25°C
4 5.54 x 10-7 14.5 days
7 5.89 x 10-9 >1 year
9 not applicable >1 year

Under the physiologically relevant conditions of pH 1.2, 37.0 ± 0.5°C, the half-life of the test material was determined to be 6.25 hours.

This study is conducted according to an appropriate guideline and under the conditions of GLP and therefore the study is considered to be acceptable and to adequately satisfy both the guideline requirement and the regulatory requirement as a key study for this endpoint.
Executive summary:

Summary

Abiotic Degradation, Hydrolysis as a Function of pH. Assessnt 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. The results are as follows:

pH

Rate constant (s-1)

Estimated half-life at 25°C

4

5.54 x 10-7 

14.5 days

7

5.89 x 10-9 

>1 year

9

not applicable

>1 year

Under the physiologically relevant conditions of pH 1.2, 37.0 ± 0.5°C, the half-life of the test material was determined to be 6.25 hours.

Description of key information

A number of studies exist to assess the hydrolysis of pentapotassium triphosphate or the analogue substance pentasodium triphosphate (see 'discussion for read-across arguments) under both laboratory and natural conditions.
The key study for the endpoint ‘hydrolysis as a function of pH’ (O’Connor BJ, 2010 ) has been selected on the basis that the study is conducted to the recommended OECD guideline and under the conditions of GLP and therefore meets the regulatory requirements for this endpoint. However the data does not necessarily reflect a ‘real world’ situation as phosphates and essential cations such as K+ are rapidly assimilated by microorganisms in soil and waters.
The key study reports the estimated half-life’s at 25°C of the test material were determined to be; 14.5 days at pH 4 and > 1 year at pH 7 and 9. Under the physiologically relevant conditions of pH 1.2, 37.0 ± 0.5°C, the half-life of the test material was determined to be 6.25 hours. As the substance is ionic it will naturally dissociate into a cation and anion. The triphosphate anion was shown to undergo further hydrolysis to orthophosphate and pyrophosphate subunits.
The additional supporting literature provides data to show that the rate of hydrolysis in natural waters is far greater than in distilled water. Potassium tripolyphosphate will not persist in natural waters it will first dissociate to its cationic and anionic forms, subsequently the triphosphate anion will be undergo hydrolysis (as discussed in the key study) and at a great rate breakdown by biotic degradation and assimilation by algae and/or microorganisms. The breakdown products of such reactions are pyrophosphate anions and the ubiquitous orthophosphate anion. Furthermore, via similar processes the pyrophosphate anion will ultimately hydrolyse to orthophosphate.

Key value for chemical safety assessment

Additional information

The hydrolytic half-life was determined for Pentapotassium triphosphate (EC No: 237-574-9). The ionic substance was assumed to undergo dissociation in aqueous solutions (resulting in potassium cations and triphosphate aninons) therefore the cation was assumed to have a negligible influence on the rate of hydrolysis of the triphosphate anion. This is expected to be replicated at environmentally relevant concentrations and on dissolution into complex environmental matrices. Although cation of the final matrix may retain some minor influence on the hydrolytic rate, such a relationship is considered beyond the scope of a standard test method and would not have been addressed or identified when testing in accordance with OECD method 111. Therefore, the rate of hydrolysis can be read across to the following substances:

- pentapotassium pentasodium bis(triphosphate)

- pentasodium triphosphate

No further testing is considered necessary.