Potassium hexadecyl hydrogen phosphate

Hydrolysis

Link to relevant study record(s)

Reference

Endpoint:

hydrolysis

Type of information:

experimental study

Adequacy of study:

key study

Study period:

2012-07-03 to 2013-02-19

Reliability:

1 (reliable without restriction)

Rationale for reliability incl. deficiencies:

guideline study

Remarks:

The study was regarded as reliable without restrictions because it was conducted in compliance with GLP regulation and guideline.

Qualifier:

according to guideline

Guideline:

OECD Guideline 111 (Hydrolysis as a Function of pH)

Version / remarks:

adopted 2004-04-13

Deviations:

no

Qualifier:

according to guideline

Guideline:

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

Version / remarks:

dated 31 May 2008

Deviations:

no

GLP compliance:

yes (incl. QA statement)

Radiolabelling:

no

Analytical monitoring:

yes

Buffers:

The buffer solutions were prepared as described below. The pH of each buffer solution at the relevant temperatures was checked with a pH meter. Each buffer solution was purged with nitrogen gas and sterilized by heating in an autoclave before using it to prepare the test solution.

pH 4:
A) KH2PO4 buffer (0.01 M)
340 - 347 mg KH2PO4 were dissolved in 250 mL pure water resulting in a concentration of 0.01 M. The pH of the solution was adjusted by adding dropwise a solution of H3PO4 (85 %) diluted 1:50 v/v with pure water.
B) Citric acid buffer
Commercially available citric acid buffer (Merck, Germany, Certipur, citric acid, sodium hydroxide, hydrogen chloride, pH 4.00 at 20 °C)

pH 7:
KH2PO4 buffer (0.001 M)
50 mL potassium dihydrogen phosphate solution (1.36 g KH2PO4 / 100 mL pure water) were mixed with 29.6 mL sodium hydroxide solution (0.1 mol/L NaOH) and filled up to 100 mL with pure water. For preparing the test solution, the buffer solution (0.1 M) was diluted 1:100 v/v with pure water.

pH 9:
Boric acid buffer (0.01 M)
50 mL boric acid solution (618 mg H3BO3 / 100 mL KCl 0.1 M) was mixed with 21.3 mL sodium hydroxide solution (0.1 M NaOH) and filled up to 100 mL with pure water. For preparing the test solution, the buffer solution was diluted 1:10 v/v with pure water.

Details on test conditions:

TEST MEDIUM
A stock solution of the test item was prepared in methanol at a concentration level of approximately 1 g/L. 0.6 mL of the stock solution were transferred into a 200 mL volumetric flask and filled up to the mark with the respective buffer solution resulting in a final concentration of about 3 mg/L. The concentration was below 0.01 M or half of the saturation concentration of the test item in water (water solubility: 198 mg/L at 20 °C).
- Identity and concentration of co-solvent: The amount of the co-solvent methanol was below 1 % (0.3 %) of the total volume.

Sterility Control:
A sterility confirmation test was performed at the end of the incubation period. To do this, separate test vessels were incubated under test conditions as described above. At the end of the maximal incubation period dip slides for fluids (Hycon GK-T/HS, Heipha Dr. Müller GmbH, Eppelheim, Germany) were wetted completely with samples of each pH value. Excess sample was allowed to run off. The dip slides were placed back into their tube and incubated at 20 °C to 22 °C for 4 - 7 days. No colonies on the agar were observed (expect for the replicate A (with test item) at pH 7 where one colony was counted after 4 days of incubation).

Sampling:
Three samples of solutions of each pH value were taken after 0 h for the determination of the applied concentration whereas two samples were taken at each following sampling point.

Sampling points:
0, 2.4 - 2.8, 24, 120 - 130 hours.

Duration:

29 d

pH:

4

Temp.:

18 °C

Initial conc. measured:

30 mg/L

Duration:

29 d

pH:

4

Temp.:

18 °C

Initial conc. measured:

3 mg/L

Duration:

15 d

pH:

4

Temp.:

50 °C

Initial conc. measured:

30 mg/L

Duration:

15 d

pH:

4

Temp.:

50 °C

Initial conc. measured:

3 mg/L

Duration:

11 d

pH:

4

Temp.:

60 °C

Initial conc. measured:

30 mg/L

Duration:

11 d

pH:

4

Temp.:

60 °C

Initial conc. measured:

3 mg/L

Number of replicates:

3

Positive controls:

no

Negative controls:

no

Preliminary study:

In a preliminary test the test item was dissolved in aqueous solutions buffered at pH 4, 7 and 9 and incubated at 50 °C ± 4.5 °C for a maximum of 5 days.
No significant reduction of the test item concentration was observed in the samples incubated at pH 7 and 9 after 5 days of incubation (mean recovery > 90 % of the applied concentration). In the incubated samples at pH 4 a significant reduction of the test item concentration was observed after 5 days of incubation (mean recovery 63 % of the applied concentration). To assess the impact of the buffer solution on the degradation process the preliminary test at pH 4 was repeated. Two different buffer solutions were used. To verify the results of the first pre-test an equal phosphate buffer was used, whereas the commercially available citric acid buffer was chosen due to the absence of phosphate molecules which were expected to be a degradation product during hydrolytic cleavage of the test item. A significant reduction of the test item concentration was observed after 5 days of incubation for both buffer solutions (mean recovery 75 % of the applied concentration). Thus, it is evident that the buffer solution had no impact on the degradation process and the results of the first pre-test are valid. According to the OECD guideline a main test to determine the reaction rate constant and half-life of potassium hexadecyl hydrogen phosphate at pH 4 had to be performed. The test item was found to be stable at pH 7 and 9, thus, no main test was performed for those pH-values.

Transformation products:

not measured

Key result

pH:

7

Temp.:

50 °C

Remarks on result:

hydrolytically stable based on preliminary test

Key result

pH:

9

Temp.:

50 °C

Remarks on result:

hydrolytically stable based on preliminary test

Key result

pH:

4

Temp.:

50 °C

Remarks on result:

not determinable

Remarks:

See details on results

Details on results:

The test item is hydrolytically stable at pH 7 and 9.
At pH 4 - 6 a test on the hydrolytic properties of potassium hexadecyl hydrogen phosphate was not feasible as the test item partly precipitated in course of incubation.

Due to the tenfold lower concentration of the 3 mg/L samples, a turbidity of the solution could not been recognized but both concentration levels were far below the water solubility, thus, it can be assumed that the behaviour of the test item at pH 4 was equal for both applied concentrations. As no precipitation could be observed after application the main test was started. However, in course of the test and also in course of sample storage the test item precipitated by the formation of white solid components. The results in table 5 and 6 indicate that with potassium hexadecyl hydrogen phosphate buffered at pH 4 the mean recoveries did not fit to a common hydrolytic degradation pattern, as the values e.g. increase in course of incubation or reach a steady state after the first few sampling points. In this context it may be possible that potassium hexadecyl hydrogen phosphate interacted on the one hand with counter ions of the buffer solution (e.g. sodium) or on the other hand with itself by the formation of micelles resulting in both cases in the formation of the mentioned precipitate. As it cannot be differentiate between hydrolytic degradation and precipitation, which both leads to a reduction in the measured test item concentration, an evaluation of the obtained data regarding hydrolysis rate constant, half-life or the calculation of Arrhenius equations was not possible. With all the efforts done including a change of the buffer system (acetate buffer) or without a buffering additive (HCl was used to adjust the pH of the test solution in pure water) to exclude any counter ions it can be concluded that it was not feasible to performed the hydrolysis test with potassium hexadecyl hydrogen phosphate at pH 4.

Validity criteria fulfilled:

yes

Conclusions:

Potassium hexadecyl hydrogen phosphate is hydrolytically stable at pH 7 and 9. At pH 4 - 6 a test on the hydrolytic properties of potassium hexadecyl hydrogen phosphate was not feasible as the test item partly precipitated in course of incubation.

Executive summary:

A study was conducted to determine the rate of hydrolysis of potassium hexadecyl hydrogen phosphate at different environmentally relevant pH-values by quantifying the test item concentration after different incubation periods and at different temperatures according to OECD TG 111 and Regulation (EC) No 440/2008 method C.7. In a preliminary test the test item was dissolved in aqueous solutions buffered at pH 4, 7 and 9 and incubated at 50 °C ± 4.5 °C for a maximum of 5 days. No significant reduction of the test item concentration was observed in the samples incubated at pH 7 and 9 after 5 days of incubation (mean recovery > 90 % of the applied concentration). In the incubated samples at pH 4 a significant reduction of the test item concentration was observed after 5 days of incubation in phosphate and citric acid buffer, respectively (mean recovery 63 – 75 % of the applied concentration).

A main test was performed with a commercially available citric acid buffer solution at pH 4. The test item was dissolved and incubated at 18 °C ± 1.3 °C, 50 °C ± 0.6 °C and 60 °C ± 1.8 °C in the dark. Two test item concentrations (approx. 3 mg/L and 30 mg/L) were used. The concentration of the test item was determined after different incubation times. A LC-MS/MS method was used for quantification of the test item. In course of the test and also in course of sample storage the test item precipitated by the formation of white solid components. In order to execute the test at a pH between 4 to 7, test solutions were prepared with a concentration of approx. 3 mg/L and 30 mg/L. As buffer systems citric acid and acetate solutions were used, both at pH-values of 5 and 6. The solutions were stored at 20 °C ± 1.3 °C in the dark over a period of 13 days. In each case a precipitate was observed. With regard to the observations it was not feasible to investigate the hydrolytic properties of potassium hexadecyl hydrogen phosphate at a pH range of 4 - 6. Neither degradation rate nor corresponding half-life values were calculated as the test item was found to be stable at pH 7 and 9. At pH values of 4, 5 and 6 the test item (partly) precipitated in course of incubation, thus, hydrolysis endpoints could not be determined.

Description of key information

The test item is hydrolytically stable at pH 7 and 9.

At pH 4 - 6 a test on the hydrolytic properties of potassium hexadecyl hydrogen phosphate was not feasible as the test item partly precipitated in course of incubation.

Additional information

A study was conducted to determine the rate of hydrolysis of potassium hexadecyl hydrogen phosphate at different environmentally relevant pH-values by quantifying the test item concentration after different incubation periods and at different temperatures according to OECD TG 111 and Regulation (EC) No 440/2008 method C.7. In a preliminary test the test item was dissolved in aqueous solutions buffered at pH 4, 7 and 9 and incubated at 50 °C ± 4.5 °C for a maximum of 5 days. No significant reduction of the test item concentration was observed in the samples incubated at pH 7 and 9 after 5 days of incubation (mean recovery > 90 % of the applied concentration). In the incubated samples at pH 4 a significant reduction of the test item concentration was observed after 5 days of incubation in phosphate and citric acid buffer, respectively (mean recovery 63 – 75 % of the applied concentration).

A main test was performed with a commercially available citric acid buffer solution at pH 4. The test item was dissolved and incubated at 18 °C ± 1.3 °C, 50 °C ± 0.6 °C and 60 °C ± 1.8 °C in the dark. Two test item concentrations (approx. 3 mg/L and 30 mg/L) were used. The concentration of the test item was determined after different incubation times. A LC-MS/MS method was used for quantification of the test item. In course of the test and also in course of sample storage the test item precipitated by the formation of white solid components. In order to execute the test at a pH between 4 to 7, test solutions were prepared with a concentration of approx. 3 mg/L and 30 mg/L. As buffer systems citric acid and acetate solutions were used, both at pH-values of 5 and 6. The solutions were stored at 20 °C ± 1.3 °C in the dark over a period of 13 days. In each case a precipitate was observed. With regard to the observations it was not feasible to investigate the hydrolytic properties of potassium hexadecyl hydrogen phosphate at a pH range of 4 - 6. Neither degradation rate nor corresponding half-life values were calculated as the test item was found to be stable at pH 7 and 9. At pH values of 4, 5 and 6 the test item (partly) precipitated in course of incubation, thus, hydrolysis endpoints could not be determined.