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Environmental fate & pathways

Hydrolysis

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Description of key information

The hydrolytic half-life of CAS# 756-13-8 is ca. 2.5 minutes. However, the rate of hydrolysis may be limited by dissolution of the chemical into liquid water.

Key value for chemical safety assessment

Additional information

The aqueous hydrolysis CAS# 756-13-8 was assessed in five studies.  In the key study, concentrations of CAS# 756-13-8 and the water-soluble hydrolysis product perfluoropropionic acid (PFPA, CAS# 422-64-0) were monitored using LC/MS.  The volatile hydrolysis product, 2-H-heptafluoropropane, was not assessed.  CAS# 756-13-8 could not be detected in samples representing time zero under any of the test conditions (pH 1.2, 5.0, 7.0, or 9.0; temperature 25 °C or 37 °C).  The maximum time delay between adding test substance, mixing and analysis was approximately five minutes for these samples.  Using this delay as a maximum decay time, the minimum pseudo-first-order hydrolysis rate constant was 0.28 s-1and the maximum half-life was 2.5 minutes.  These values are considered a boundary for hydrolysis, and the reaction may occur more rapidly.  This study was conducted in accordance with test guidelines and GLP standards as promulgated by USEPA.  In the first supporting study, hydrolysis of CAS# 756-13-8 was monitored over time by 19F nuclear magnetic resonance (NMR) spectroscopy.  The parent compound, PFPA and 2-H-heptafluoropropane were identified, and no other chemicals were present.  The study demonstrates conclusively that PFPA and 2-H-heptafluoropropane are the sole hydrolysis products of CAS# 756-13-8.  In the second supporting study, hydrolysis of CAS# 756-13-8 at 100 mg/L (nominal concentration, no co-solvent) was assessed by monitoring concentrations of PFPA by HPLC/UV absorbance and 2-H-heptafluoropropane by GC/FID.  CAS# 756-13-8 was fully converted to hydrolysis products after one day based on PFPA formation.  However, the degradation kinetics were substantially slower than in the key study, with ca. 20% hydrolysis at one hour and ca. 60% hydrolysis after six hours, based on PFPA concentration.  Mass balance for 2-H-heptafluoropropane was not fully quantitative (recovery was ca. 85% of theoretical yield).  Incomplete recovery may have been due to high volatility of 2-H-heptafluoropropane, which is a gas at room temperature.  Given the structure of PFPA and 2-H-heptafluoropropane, that no further hydrolysis products were observed in these experiments is the expected result.  In the third supporting study, extent of CAS# 756-13-8 hydrolysis was monitored by acidification of the water phase of a two-phase system.  The study assumed that acidification was due entirely to formation and dissociation of PFPA.   Assuming that the concentration the test substance which was truly dissolved during later phases of the experiment was 1 ppm and that the dominant mechanism at all pH was base catalysis, the authors concluded that the pseudo-first order rate constant for hydrolysis at pH 5.6 was approximately 8000/hr.  While this rate is considerably faster than any other report, the authors stressed that this result was an approximation, to be used as an input value for an analysis making a conservative estimate of the impact of hydrolysis on fate.  The fourth supporting study was conducted at 25 °C using two pH values, 5.6 and 8.5.  The hydrolytic half-life was determined by 19F-NMR.  Half-lives were 1.0 hour at pH 5.6 and 0.6 h at pH 8.5. 

 

The key study was performed in accord with U.S. EPA (OPPTS) 835.2110 (Jan 1998), and in compliance with USEPA GLP standards (Final Rule, published 17 Aug, 1989).  The key study is classified as acceptable and satisfies the guideline requirements for testing hydrolysis as a function of pH, as supported by direct evidence in the first supporting study that PFPA is the sole soluble hydrolysis product of CAS# 756-13-8.  The first supporting study was not conducted in accord with established test guidelines or principles of Good Laboratory Practice.  However, the nature of the study was exploratory so no established guideline may be expected.  The study was performed carefully and was based on sound methodology for purposes of identification of hydrolysis products.  This study was not designed as a kinetic study and reaction conditions did not permit an assessment of hydrolysis rate despite the provision of time-course data.  The second supporting study was also not performed in accord with established test guidelines or GLP standards.  The study report is poorly documented, and at several points it is not clear how the study was executed.  In addition, hydrolysis was measured at a high concentration of 100 mg/L.  Solvation of CAS# 756-13-8 was not guaranteed, and therefore the rate of hydrolysis is likely to have been limited by the kinetics of dissolution.  While the second supporting study provides additional evidence for the identity of soluble and volatile hydrolysis products, the reliability of the hydrolysis data cannot be addressed.  The third supporting study provided essentially no details on the experiments as conducted, and therefore cannot be assessed as to its reliability.  The fourth supporting study provides a summary result only but appears to have been done according to scientifically sound methodology.  However, no indication of mixing is provided, and hydrolysis may again be limited by the kinetics of dissolution.