Trifluoroiodomethane

Phototransformation in air

Link to relevant study record(s)

Reference

Endpoint:

phototransformation in air

Type of information:

experimental study

Adequacy of study:

key study

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

study well documented, meets generally accepted scientific principles, acceptable for assessment

Qualifier:

no guideline followed

Principles of method if other than guideline:

Several photodegradation experiments were conducted in a quartz cell which is transparent to ultraviolet and visible radiation with wavelengths longer than 200 nm.

GLP compliance:

no

Light source:

Xenon lamp

Light spectrum: wavelength in nm:

> 200

Details on light source:

Independent experiments were conducted using either a 1000 watt xenon arc or a 40 watt fluorescent lamp as the photolytic source.

Details on test conditions:

See 'Any other information on materials and methods incl. tables'.

Duration:

24 h

Reference substance:

no

Parameter:

not applicable

Value:

270 nm

Remarks on result:

other: the ultraviolet absorption spectrum of the test substance exhibits a peak which is centered near 270 nm and extends beyond 350 nm.

% Degr.:

23

Sampling time:

24 h

Test condition:

see 'Any other information on results incl. tables'

Remarks on result:

other: 23% reduction of the test substance was observed after 24 hours.

Key result

DT50:

2.674 d

Test condition:

see 'Any other information on results incl. tables'

Remarks on result:

other: extrapolated from the calculated first-order rate constant of 3E-06 cm³ molecule-1 s-1.

Key result

Reaction with:

OH radicals

Rate constant:

0 cm³ molecule-1 s-1

Remarks on result:

other: first-order rate constant

Transformation products:

yes

Remarks:

see 'Identity of transformation products' and 'Any other information on results incl. tables'

No.:

#1

No.:

#2

No.:

#3

The first set of experiments was designed to determine the extent of degradation of the pure agent in the presence of high intensity radiation from the Xe arc lamp. A comparison of the IR spectra measured before an after the irradiation indicate that the net effect of the photolysis was minimal, presumably because the reverse reaction is favored over the reactions leading to the formation of new products. This is consistent with the entries in the NIST kinetic database which indicate that the rate constant for the formation of the test substance from test substance + I is about twice as fast as the corresponding value for the formation of M1 + I2 (Mallard, 1993). The observed extent of degradation, as estimated by least squares analysis over the region between 2230 per cm to 2272 per cm, was only 5.5. The formation of a small amount of M1, however, is apparent from the presence of a sharp peak centered at about 1250 per cm in the difference spectrum. This assignment may be confirmed by comparing the spectrum with the reference spectrum of M1.

 

Additional measurements were performed to determine whether the presence of an inert gas would have an effect on the photolysis of the test substance. In these experiments, a mixture consisting of 8.6 kPa of Ar and 6.7 kpa of the test substance was exposed to the radiation from the Xe arc lamp for 30 min. The extent of degradation was about 4.1 %; which is comparable to the value obtained in the photolysis of the pure agent. The small discrepancy in the extent of degradation (1.4 %) is indicative of the range of values obtained in repetitive experiments and is most likely due to the systematic errors delineated in the previous section. On this basis, it was concluded that the effect of the inert gas was minor. The presence of even a small amount of O2, however, has a dramatic effect on the photodegradation chemistry of the test substance. This is apparent in the comparison of the UV spectra of a mixture containing 6.7 kPa of the test substance and 0.94 kPa of O2 taken before (top) and after (bottom) irradiation for 30 min with the Xe arc lamp. A thick, white smoke formed almost immediately upon applying voltage to the arc, Shortly thereafter, a brown residue was observed to form in the crevices where the windows were joined to the cell. The IR spectra displayed exhibit a significant loss of spectral intensity in the region between 2230 per cm and 2272 per cm, which correlates with depletion of the agent, and a concomitant increase in absorbance in the region between 2300 per cm and 2400 per cm, which is associated with the formation of CO2. At the same time, the presence of I2 was indicated by the appearance of a new feature centered at about 530 nm in the VIS spectrum. The extent of degradation, based on the change in optical density in the region between 2230 per cm and 2272 per cm in the spectra of the original and photolyzed mixtures was approximately 32 %.This result implies that the stoichiometry of the photo-oxidation process is 2.3 mol of the test substance per mole of O2 (i.e., 6.7(.32)/0.94). A ratio of approximately 2:1 is obtained if the 5 % degradation of the agent, which would be expected to occur upon depletion of all of the available O2, is taken into account. The peak centered at 1950 cm per cm in the difference spectrum is attributable to M2, which can react further with H2O to produce M3.

 

The smaller peak at about 1850 per cm was not definitively assigned but it is consistent with the presence of compounds possessing a carbonyl group (C=O). There was an indication in the IR spectrathat ethanol, which was used to clean the cell, was present as a contaminant during some of the photolytic experiments. It is possible, therefore, that this peak is associated with a product of a photochemical reaction involving the ethanol and that is not related to the photodegradation of the agent.

 

Another set of experiments were performed to determine the extent of degradation of the test substance in the presence of radiation from a common fluorescent lamp. The photolysis of a mixture of 6,7 kPa of the test substance and 82 kPa of O2 was monitored for about 5 days. During this period of time, about 28 % of the test substance was transformed into products. The brown residue, which had been observed in the previous photo-oxidation measurements, formed after the first few hours of exposure to the lamp.

 

In the final experiment, the valve on the cell containing 6.7 kPa of the test substance was opened to the atmosphere and allowed to equilibrate. The valve was closed after a few minutes and the resulting mixture of the test substance and air was placed under the fluorescent lamp for approximately 24 hours. The idea was to simulate as closely as possible the environment inside of a building during a release of the agent. A comparison of these spectra indicate a 23 % reduction in optical density in the region between 2230 per cm and 2272 per cm which is attributable to the loss of the test substance, and a concomitant increase in optical density in the region between 2300 per cm and 2400 per cm which is due to the formation of CO2. The first order rate constant obtained from the observation of a reduction of 23 % in the concentration of agent in 24 hours is 3.0E-06/s. Not surprisingly, this is of the same order of magnitude as the value which was calculated for the rate constant of the photolysis of the test substance in direct sunlight (Nyden, 1994). It is then useful to estimate the extent of degradation of the test substance and to assess the potential hazard to occupants should an accidental discharge take place. The magnitude of the threat to public safety will vary significantly depending on the ventilation and lighting in the vicinity of the release. Thus, for example, the rupture of a cylinder of the test substance in a well-lit storage area will pose a more serious threat than a controlled release of the same agent into a dark dry bay.

Validity criteria fulfilled:

not applicable

Conclusions:

The test substance photolyzes in the presence of radiation from common fluorescent lights with a rate constant which is comparable to that in photolysis in direct sunlight. The potential byproducts of this process include M2 and M3. The high level of toxicity associated with these compounds merits their inclusion when considering the impacts of accidental releases of the test substance in well-lit, occupied spaces. However, the experiments performed as part of this study indicate that the indoor rate of photolytic decomposition is quite low, and a calculation based on conservative estimates further indicates that the resulting concentrations of the most hazardous degradation products (i.e., M3 and M2) would also be expected to be quite low. Based on the results, the half-life time was determined to be 2.67 days, derived from a calculated first-order rate constant of 3E-06 cm³ molecule-1 s-1.

Executive summary:

In this article, the photodegradation of the test substance was investigated in a series of experiments. No standardized guideline was followed and the experiments were not performed in compliance with GLP. The experiments were conducted in a quartz cell which is transparent to ultraviolet and visible radiation with wavelengths longer than 200 nm. The cell was cylindrical in shape with a diameter of 5 cm. Samples containing approximately 6.7 kPa of the agent, either by itself or mixed with a variety of other gases, were irradiated for 30 min or 24 hours with the high intensity Xe arc lamp. The products of the photolysis of the test substance were characterised by UV, visible and infrared light spectroscopy. Based on the findings, the following could be concluded: the test substance photolyzes in the presence of radiation from common fluorescent lights with a rate constant which is comparable to that in photolysis in direct sunlight. The potential byproducts of this process include M2 and M3. The high level of toxicity associated with these compounds merits their inclusion when considering the impacts of accidental releases of the test substance in well-lit, occupied spaces. However, the experiments performed as part of this study indicate that the indoor rate of photolytic decomposition is quite low, and a calculation based on conservative estimates further indicates that the resulting concentrations of the most hazardous degradation products (i.e., M3 and M2) would also be expected to be quite low. Based on the results, the half-life time was determined to be 2.67 days, derived from a calculated first-order rate constant of 3E-06 cm³ molecule-1 s-1.

Description of key information

The half-life time was determined to be 2.67 days, derived from a calculated first-order rate constant of 3E-06 cm³ molecule-1 s-1.

In a review article on the test substance (NAS Review, 2004), a comparable half-life time value of 1.15 days is presented. This is used as supporting information.

Key value for chemical safety assessment

Half-life in air:

2.67 d

Degradation rate constant with OH radicals:

0 cm³ molecule-1 s-1

Additional information

In this article, the photodegradation of the test substance was investigated in a series of experiments. No standardized guideline was followed and the experiments were not performed in compliance with GLP. The experiments were conducted in a quartz cell which is transparent to ultraviolet and visible radiation with wavelengths longer than 200 nm. The cell was cylindrical in shape with a diameter of 5 cm. Samples containing approximately 6.7 kPa of the agent, either by itself or mixed with a variety of other gases, were irradiated for 30 min or 24 hours with the high intensity Xe arc lamp. The products of the photolysis of the test substance were characterised by UV, visible and infrared light spectroscopy. Based on the findings, the following could be concluded: the test substance photolyzes in the presence of radiation from common fluorescent lights with a rate constant which is comparable to that in photolysis in direct sunlight. The potential byproducts of this process include COF2 and HF. The high level of toxicity associated with these compounds merits their inclusion when considering the impacts of accidental releases of the test substance in well-lit, occupied spaces. However, the experiments performed as part of this study indicate that the indoor rate of photolytic decomposition is quite low, and a calculation based on conservative estimates further indicates that the resulting concentrations of the most hazardous degradation products (i.e., HF and COF2) would also be expected to be quite low. Based on the results, the half-life time was determined to be 2.67 days, derived from a calculated first-order rate constant of 3E-06 cm³ molecule-1 s-1.