Ozone

Hydrolysis

Link to relevant study record(s)

Reference

Reference 1

Endpoint:

hydrolysis

Data waiving:

study technically not feasible

Justification for data waiving:

other:

Reference 2

Endpoint:

hydrolysis

Remarks:

No real hydrolysis but self-decomposition in water.

Type of information:

experimental study

Adequacy of study:

key study

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

other: other: Peer-reviewed well documented publication

Reason / purpose for cross-reference:

reference to same study

Qualifier:

no guideline followed

Principles of method if other than guideline:

Kinetic studies on ozone decomposition in water were performed over a range of temperatures from 10 to 40 °C and pH range from 2.5 to 9.The ozone decomposition chemical reaction was followed by determining the concentration of dissolved ozone by reaction with a buffered potassium iodide solution, measuring the triiodide ions liberated spectrophotometrically at the wavelength of 352 nm.

GLP compliance:

no

Radiolabelling:

no

Analytical monitoring:

yes

Details on sampling:

Not reported.

Buffers:

Stock buffered water: Water was first deionized by ion exchange and then distilled.
A mixture of KH2PO4, and Na2HP04 was used to adjust both the ionic strength (0.15) and the pH of the solutions. The ionic strength was kept constant because this parameter affects ozone decomposition (Gurol and Singer, 1982)

Estimation method (if used):

n.a.

Details on test conditions:

- Reactor: The reactor was a 750-cm³ glass vessel with inlets for bubbling, stirring, sampling, venting, and temperature measurement and was submerged in a bath equipped with stirring, heating, and refrigeration systems, which allowed us to keep the temperature constant within ca. 0.5 °C.
- Agitation speed: 100-700 rpm initially, later only 100 rpm.

Procedure: Stock buffered water (500 cm³) was added to the reactor and then ozonated with an 02-03 stream (see above) for 30 min. About this time, saturation of water was reached, and the gas stream was stopped. Subsequently, the ozone decomposition chemical reaction was followed by determining the concentration of dissolved ozone.

Duration:

30 min

pH:

2.5

Temp.:

10 °C

Remarks:

pH ranged between 2.5 and 9; temnperature range: 10-40 °C

Number of replicates:

-

Positive controls:

not specified

Negative controls:

not specified

Statistical methods:

-

Transformation products:

yes

No.:

#1

No.:

#2

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

At pHs around 9 direct ozone decomposition leads to the formation of hydroxyl radicals (OH·). Hydroxide ion action produces peroxy radicals (HO2·) and hydroxyl radicals.

Key result

pH:

7

Temp.:

20 °C

DT50:

32 min

Type:

not specified

Included below are Figures 2 and 3 which illustrate the time taken for degradation of ozone at a range of pHs and temperatures. The specific numerical values are not provided in the report, however the results indicate that at a pH of 7, increasing temperatures result in more rapid ozone decomposition, and that at a constant temperature of 30 °C, as pH becomes more alkaline the ozone decomposition rate increases.

Validity criteria fulfilled:

not applicable

Remarks:

not to guideline

Conclusions:

Ozone is unstable in water. The half-life in deionized and distilled water, at 20°C and pH 7, was calculated to be circa 2000 seconds (ca. 32 min). In any real water (containing ions and other subtsances) the half-life will be much shorter

Executive summary:

Kinetic studies on ozone decomposition in water were performed over a range of temperatures from 10 to 40 °C and pH range from 2.5 to 9. The ozone decomposition chemical reaction was followed by determining the concentration of dissolved ozone. Ozone decomposition was observed to follow a two-term rate equation supported by a reaction mechanism. The results indicated that at a pH of 7, increasing temperatures result in more rapid ozone decomposition, and that at a constant temperature of 30 °C, as pH becomes more alkaline the ozone decomposition rate increases. Overall, ozone is unstable in water. The half-life in deionized and distilled water, at 20°C and pH 7, was calculated to be circa 2000 seconds (ca. 32 min). In any real water (containing ions and other subtsances) the half-life will be much shorter.

It was shown that at pHs lower than 7, whatever the temperature was, direct ozone decomposition and the initiation step involving the hydroxyl radicals are the main cause of ozone decomposition. At higher pHs, the importance of the peroxy radicals and of the hydroxide ion initiation step increases. Thus, at pHs around 9, the ozone decomposition rate depends on two major contributions: direct ozone decomposition, which leads to the formation of hydroxyl radicals, OH·, and hydroxide ion action, which produces not only peroxy radicals, HO2·, but also hydroxyl radicals. Finally, the nature of the ionic species (carbonate, sulfate, phosphate, etc.) present in the water greatly influences ozone decomposition, inhibiting some of the reactions in the mechanism proposed.

Description of key information

Although ozone has no hydrolysable groups in its chemical structure and is therefore considered not to be prone to hydrolysis, it self-decomposes rapidly in water with a half-life of 32 min. For details on ozone self-decomposition in water please refer to IUCLID section 5.6.

Key value for chemical safety assessment

Half-life for hydrolysis:

32 min

at the temperature of:

20 °C