Tetrapotassium pyrophosphate

Hydrolysis

Link to relevant study record(s)

Reference

Endpoint:

hydrolysis

Type of information:

experimental study

Adequacy of study:

key study

Study period:

Testing was conducted between 02 October 2009 and 04 February 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:

Date of GLP inspection: 15 September 2009 Date of Signature on GLP certificate: 26 November 2009

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 solutions were 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 3.0 x 10-2 g/l pyrophosphate anion in the four buffer solutions.
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/Tier 1
Sample solutions at pH 4, 7 and 9 were maintained at 50.0 ± 0.5°C for a period of at least 120 hours.

Tier 2
Results from the Preliminary test/Tier 1 showed it was necessary to undertake further testing at pH 4, with solutions being maintained at 50.0 ± 0.5°C, 60.0 ± 0.5°C and 70.0 ± 0.5°C.

Testing at pH 1.2
Results from the Preliminary test/Tier 1 at pH 4 showed it was necessary to undertake further testing at pH 1.2, with solutions being maintained at 37.0 ± 0.5°C.

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

The concentration of pyrophosphate anion in the sample solutions was determined by ion chromatography (IC).

Samples
An aliquot of each sample solution was analysed without further treatment.

Sample blanks
pH 1.2 buffer solution.
pH 4 buffer solution.
pH 7 buffer solution.
pH 9 buffer solution.

Standards
Duplicate standard solutions of sodium pyrophosphate decahydrate (Sigma-Aldrich, purity: 101.9 %) were prepared in reverse osmosis water at a nominal concentration of 30 mg/l*.

Standard blank
Reverse osmosis water.

Buffers:

See attached buffer details.

Details on test conditions:

Refer to details on sampling and analytical methods.

Duration:

264.5 h

pH:

4

Initial conc. measured:

0.031 g/L

Duration:

216 h

pH:

4

Initial conc. measured:

0.031 g/L

Duration:

144 h

pH:

4

Initial conc. measured:

0.031 g/L

Duration:

78 h

pH:

4

Initial conc. measured:

0.031 g/L

Duration:

120 h

pH:

7

Initial conc. measured:

0.03 g/L

Duration:

120 h

pH:

9

Initial conc. measured:

0.03 g/L

Duration:

29.5 h

pH:

1.2

Initial conc. measured:

0.026 g/L

Number of replicates:

Refer to details on sampling and analytical methods.

Positive controls:

no

Negative controls:

no

Preliminary study:

Results
Please see attached results.

Discussion
The kinetics of the hydrolysis has been determined to be consistent with that of a pseudo-first order reaction, since the graphs of log10 concentration versus time were considered to be straight lines.
No significant peaks were observed at the approximate retention time of the test material on analysis of any matrix blank solutions.
Results for the preliminary test indicate that the rate of hydrolysis increases with a decrease in pH.

Test performance:

Validation
The linearity of the detector response with respect to concentration was assessed over the nominal concentration range of 0 to 125 mg/l*. This was satisfactory with a correlation coefficient of 1.000 being obtained. This work was performed as part of Harlan Laboratories Project Number 2920/0050.
* as the pyrophosphate anion

Transformation products:

yes

No.:

#1

No.:

#2

Details on hydrolysis and appearance of transformation product(s):

Identification of Hydrolysis Products

Since pyrophosphates are generally prepared by heating their respective orthophosphates, the product of hydrolysis was expected to be orthophosphate. This expectation was confirmed since decreases in size of the pyrophosphate peak coincided with increases in size of the orthophosphate peak (at a retention time of approximately 2.3 minutes).

The mechanism of the pyrophosphate hydrolysis reaction will be as follows:

(P2O7)4- (+ H2O) ---------> 2 (PO4)3-

However, due to the pKa’s of orthophosphoric acid* at approximately 2.1, 7.2 and 11.9, the hydrolysis product at final solution pH’s of 1.2 and 4 will be orthophosphoric acid and monopotassium dihydrogen orthophosphate, respectively.

* from Albert and Serjeant, Ionisation Constants of Acids and Bases, A Laboratory Manual, 1962

pH:

4

Temp.:

50 °C

Hydrolysis rate constant:

0.002 h-1

DT50:

282 h

Type:

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

Remarks on result:

other: r = -0.999

pH:

4

Temp.:

50 °C

Hydrolysis rate constant:

0.003 h-1

DT50:

268 h

Type:

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

Remarks on result:

other: r = -0.998

pH:

4

Temp.:

60 °C

Hydrolysis rate constant:

0.01 h-1

DT50:

70.3 h

Type:

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

Remarks on result:

other: r = -1.000

pH:

4

Temp.:

70 °C

Hydrolysis rate constant:

0.036 h-1

DT50:

19.2 h

Type:

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

Remarks on result:

other: r = -1.000

pH:

7

Temp.:

25 °C

DT50:

> 1 yr

Type:

other: Test item is stable hydrolytically at pH 7

pH:

9

Temp.:

25 °C

DT50:

> 1 yr

Type:

other: Test item is stable hydrolytically at pH 9.

pH:

1.2

Temp.:

37 °C

Hydrolysis rate constant:

0.027 h-1

DT50:

26.1 h

Type:

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

Remarks on result:

other: r = -0.997

Details on results:

See attached results.

See attached results.

Validity criteria fulfilled:

yes

Conclusions:

The estimated half-life at 25°C of the test material at pH 4, 7 and 9 is greater than 1 year.
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:

Method

The determination 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 .

Conclusion

The estimated rate constants and half-lives at 25°C of the test material are shown in the following table:

pH	Estimated rate constant (hr^-1)	Estimated half-life
4	5.48 x 10^-5	527 days
7	-	>1 year
9	-	>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 26.1 hours.

Description of key information

A number of studies exist to assess the hydrolysis of tetrapotassium pyrophosphate or analogous substances ( sodium pyrophosphates) 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 Na+ and 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; 527 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 26.1 hours. The substance was shown to following the following mechanism of hydrolysis:
Pyrophosphate anion + water  →  2 x orthophosphate anion 
In reality and due to the pKa’s of orthophosphoric acid and the nature of the phosphate ion it is likely that in natural waters the hydrolysis product will be either orthophosphoric acid and monopotassium dihydrogen orthophosphate (which may also dissociate to phosphate ions and sodium ions). 
Results for the preliminary test indicate that the rate of hydrolysis increases with a decrease in pH.
The additional supporting literature provides data to show that the rate of hydrolysis in natural waters is far greater than in distilled water. Pyrophosphates will not persist in natural waters. Biotic degradation and assimilation by algae and/or microorganisms will occur at a faster rate than hydrolysis in distilled water. The breakdown products of such reactions are the ubiquitous orthophosphate anion. 

Additional information

The hydrolytic half-life was determined for tetrapotassium pyrophosphate (EC No: 230 -785 -7). The ionic substance was assumed to undergo dissociation in aqueous solutions (resulting in potassium cations and pyrophosphate anions) therefore the cation was assumed to have a negligible influence on the rate of hydrolysis of the 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:

- disodium dihydrogenpyrophosphate

- trisodium hydrogen diphosphate

- tetrasodium pyrophosphate

- calcium dihydrogenpyrophosphate

- dicalcium pyrophosphate

- tetrairon tris(pyrophosphate)

- copper (II) pyrophosphate

- magnesium pyrophosphate

- magnesium hydrogen pyrophosphate

 

No further testing is considered necessary.