Nitrapyrin

Hydrolysis

Description of key information

pH 5: T½ = 186 h; pH 7 (1): T½ = 233 h; pH 7 (2): T½ = 173 h; pH 9: T½ = 129 h (At 25 ± 1 °C); EPA Guideline Subdivision N 161-1; Peterson & Carpenter (1988).

Key value for chemical safety assessment

Half-life for hydrolysis:

178 h

at the temperature of:

25 °C

Additional information

Several studies are available to assess the hydrolysis of the test material.

 

In the first study, reported by Peterson & Carpenter (1988), the hydrolysis of 14C-test material was studied at 25 ± 1 °C in sterile buffered water in solutions of pH 5, 7, and 9. The buffer solutions were sodium acetate/acetic acid (pH 5), N-2-hydroxyethylpiperazine-N'-2-ethane sulfonic acid/KOH (HEPES, pH 7 (1)), tris(hydroxyethyl)aminoethane/HCl (Tris, pH 7 (2)), and boric acid/sodium borate (pH 9).

First-order kinetics was used to describe the rate of hydrolysis. Half-life values and corresponding rate constants of 186 hours (0.00373 hour^-1), 233 hours (0.00298 hour^-1), 173 hours (0.00401 hour^-1), and 129 hours (0.00538 hour^-1), were obtained for the pH 5, pH 7 (1), pH 7 (2), and pH 9 systems, respectively.

An average half-life value of 178 hours (rate constant 0.0388 hour^-1) was obtained when data gathered from all solutions through the first 20 days of the study were considered.

Only one hydrolysis product was formed under these test conditions. This product was identified as 6-chloro-2-picolinic acid (6-CPA) in all the buffer systems studied by gas chromatography/mass spectrometry after isolation (by high performance liquid chromatography) and chemical derivatisation.

The study was conducted under GLP conditions and in accordance with a standardised guideline; it was therefore assigned a reliability score of 1. Since this was the only available study available which had been conducted to a standardised method, it is considered to be the key study.

The following additional studies are included as supporting information.

 

In the second study, reported by Meikle (1974), the hydrolysis of the test material was studied at 35 °C and at three values of pH (5.1, 7.1, and 8.0) using phosphate buffers, ionic strength 0.005 M.

The hydrolysis rate (first-order) of test material, in buffered, distilled water (0.15 ppm, 6.3 x 10^-7 M) was determined for the pH range 5.1 to 8.0. The reaction was found not to be pH-dependent.

The test material in aqueous solution is hydrolysed at a moderate rate, half-life 40 hours at 35 °C. Only a single product of hydrolysis, 6-CPA, was formed.

Since the study pre-dated GLP study, but was conducted to sound scientific principles, it was assigned a reliability score of 2.

 

In the third study, reported by Laskowski & Regoli (1971), a hydrolysis study was conducted in phosphate buffers over the pH range 3.1 to 8.4, and a second study undertaken to determine the effect of temperature on the rate of hydrolysis over the range 25 to 45 °C.

The study investigating the effect of pH on hydrolysis of the test material was conducted at 35 °C in phosphate buffers of constant ionic strength at pH values of 3.1, 4.5, 5.1, 6.0, 7.0 and 8.4, which is the range most often encountered in agricultural soils. The second study was undertaken to determine the effect of temperature on the rate of hydrolysis and was carried out at temperatures of 25, 35 and 45 °C. Gas chromatography was used as the analytical tool for both studies.

Rates of hydrolysis were not affected by pH, but they were approximately doubled for every 5 °C rise in temperature. In the pH study, the half-life was calculated to be 2.0 days at 35 °C at all pH values tested.

In the temperature study, the calculated half-life values arising from the assumption of first-order kinetics and linear regression of the data were 7.7, 2.0, and 0.54 days at 25, 35, and 45°C respectively. Extrapolation to temperatures below those studied in the experiment yielded half-life values of 162 and 73 days at 5 and 10°C, respectively.

This study shows that the test material is unstable in water solution; therefore, the basic tendency of this compound will be never to accumulate in the environment or be found as a contaminant of water.

Again, the study pre-dated GLP study, but was conducted to sound scientific principles, and was assigned a reliability score of 2.

 

The next study was reported in published literature by Meikle et al. (1978). The paper reported on the hydrolysis and photolysis rates of the test material in dilute aqueous solution.

The hydrolysis rate of the test material, in buffered, distilled water followed simple first-order kinetics over the concentration range, 6.2 x 10^-7 to 8.7 x 10 ^-5 M. There was only a single product from the reaction, 6-CPA. The rate of the reaction decreased with increasing buffer concentration at 35 °C (M buffer concentration-half-life): 0.005 M - 1.7 days, 0.02 M - 2.0 days, 0.067 M - 4.0 days. The hydrolysis rate was independent of pH over the range, 3.2 to 8.4. Additional data were obtained for rates at 25 and 45 °C. The activation energy for the reaction under these conditions was 25.0 kcal/mole.

Photolysis of test material at 25 °C in 0.005 M phosphate buffers at pH 5.1, 7.1, 8.0, and in natural water also followed simple first-order kinetics over the test material concentration range 7.1 x 10 ^-6 to 7.5 x 10 ^-6 M. Again, there was no observable pH effect on the rate over the pH range investigated, nor was there a rate enhancement in the natural water. The half-life of the reaction under these conditions was 0.5 days.

The products of this reaction were two identifiable radioactive photolysis products, 6-chloropicolinic acid (6-CPA) and 6-hydroxypicolinic acid (6-OHPA), along with polar unknowns. Simulation of the set of sequential reactions using determined first order rate constants at 25 °C and a starting concentration of 1.7 ppm predicts that nitrapyrin will be half gone in 0.5 days. The non- GLP study was conducted to sound scientific principles with a sufficient level of detail to assess the quality of the relevant results; it was assigned a reliability score of 2.

 

The final study was reported in published literature by Hendrickson & Keeney (1978). The effect of various materials and environmental factors on the hydrolysis of the test material was evaluated. The hydrolysis of the test material was investigated through determination of the rate of Cl^- liberation. The procedure allowed determination of Cl^- from as little as 0.5 mg/L test material and permitted analysis of up to 10 samples per hour.

The method must be-restricted to experiments where background Cl^- can be either removed by leaching or corrected for by use of control samples.

Temperature was the most important factor affecting test material hydrolysis and the process was not biologically-mediated. The rate of hydrolysis was not affected by pH in the range 2.7 - 11.9, or by sorption on colloidal surfaces providing the surfaces remained in contact with the solution. Hydrolysis was slower in a soil at field moisture capacity than at saturation.

The findings also showed that experiments conducted in containers with lipophilic components (e.g, rubber or polyethylene) may be suspect due to sorption of test material.

The study conducted to sound scientific principles with a sufficient level of detail to assess the quality of the relevant results; it was assigned a reliability score of 2.