1.8.5.3. Identification of hydrolysis products (Tier 3)
Any major hydrolysis products at least those representing > 10 % of the applied dose should be identified by appropriate analytical methods.
1.8.5.4. Optional tests
Additional tests at pH values other than 4, 7 and 9 may be required for a hydrolytically unstable test substance. For example, for physiological purposes a test under more acidic conditions (e.g. pH 1,2) may be required employing a single physiologically relevant temperature (37
o
C).
2. DATA
The amounts of test substance and of hydrolysis products, if relevant, should be given as % of applied initial concentration and, where appropriate, as mg/L for each sampling interval and for each pH and test temperature. In addition, a mass balance should be given in percentage of the applied initial concentration when labelled test substance has been used.
A graphical presentation of the log-transformed data of the test substance concentrations against time should be reported. Any major hydrolysis products at least those representing ≥ 10 % of the applied dose should be identified and their log-transformed concentrations should also be plotted in the same manner as the parent substance to show their rates of formation and decline.
2.1. TREATMENT OF RESULTS
More accurate determinations of half-lives or DT
50
values should be obtained by applying appropriate kinetic model calculations. The half-life and/or DT
50
values (including confidence limits) should be reported for each pH and temperature together with a description of the model used the order of kinetics and the coefficient of determination (r
2
). If appropriate, the calculations should also be applied to the hydrolysis products.
In the case of rate studies carried out at different temperatures, the pseudo first-order hydrolysis rate constants (k
obs
) should be described as a function of temperature. The calculation should be based on both the separation of k
obs
into rate constants for acid catalysed, neutral, and base catalysed hydrolysis (k
H
, k
neutral
, and k
OH
respectively) and the Arrhenius equation:
[Bild bitte in Originalquelle ansehen]
where A
i
and B
i
are regression constants from the intercept and slope, respectively, of the best fit lines generated from linearly regressing ln k
i
against the reciprocal of the absolute temperature in Kelvin (T). Through the use of the Arrhenius relationships for acid, neutral and base catalysed hydrolysis, pseudo first-order rate constants, and thus half-lives can be calculated for other temperatures for which the direct experimental determination of a rate constant is not practicable (10).
2.2. EVALUATION AND INTERPRETATION OF RESULTS
Most hydrolysis reactions follow apparent first order reaction rates and, therefore, half-lives are independent of the concentration (see equation 4 in Appendix 2). This usually permits the application of laboratory results determined at 10
-2
to 10
-3
M to environmental conditions (≤ 10
-6
M) (10). Several examples of good agreement between rates of hydrolysis measured in both pure and natural waters for a variety of chemicals were reported by Mabey and Mill (11), provided both pH and temperature had been measured.
3. REPORTING
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