Hydrolysis rate

Objective of the study

The laboratory tests are performed to assess abiotic hydrolytic transformations of chemicals in aquatic systems at pH values normally found in the environment (pH 4 – 9). The experiments are performed to determine:

(i)     the rate of hydrolysis of the test substance as a function of pH, and

(ii)    the identity or nature and rates of formation and decline of hydrolysis products to which organisms may be exposed.

Circumstances under which the study is recommended to be required

The study is required for all outdoor uses, and for indoor uses when environmental exposure is likely to occur, including agricultural premises, in or around farm buildings, barnyards, and beehives.

Principle of the test

Sterile aqueous buffer solutions of different pH values (pH 4, 7 and 9) are treated with the test substance and incubated in the dark under controlled laboratory conditions (at constant temperatures). After appropriate time intervals, these buffer solutions are analyzed for the test substance and for hydrolysis products. With radio-labelled test substance (e.g. 14C), a mass balance can be more easily established.

Test substance

A non labelled or labelled test substance can be used to measure the rate of hydrolysis. Labelled material is generally preferred for studying the pathway of hydrolysis and for establishing mass balance; however, in special cases, labelling may not be absolutely necessary.

Typical endpoints of the study

The following information is recommended to be reported when hydrolysis rate is determined:

• plots  of  concentrations  versus  time  for  the  test  substances  and,  where  appropriate,  for  the hydrolysis products at each pH value and temperature;
• tables  of  results  of  Arrhenius  equation  for  the  temperature  20°C/25° C,  with  pH,  rate  constant [h^-1 or  day^-1];
• half-life  or  DT₅₀,  temperatures  [C]  including  confidence  limits  and the coefficients of correlation (r²) or comparable information;
• proposed pathway of hydrolysis.

Testing guidelines

The following test guidelines may be used for determination of the hydrolysis rate of a pesticide:

• OECD Guidelines for the Testing of Chemicals. Test No. 111: Hydrolysis as a Function of pH
• US-EPA OCSPP Harmonized Test Guidelines. Fate, Transport and Transformation Test Guidelines: OPPTS 835.2120 Hydrolysis