Methenamine

Hydrolysis

Link to relevant study record(s)

Reference

Endpoint:

hydrolysis

Type of information:

experimental study

Adequacy of study:

key study

Study period:

1980

Reliability:

2 (reliable with restrictions)

Qualifier:

no guideline followed

Principles of method if other than guideline:

Public available literature. No guideline indicated, but to some extent comparable to OECD 111. For details on method see IUCLID5 materials and methods section.

GLP compliance:

not specified

Radiolabelling:

no

Analytical monitoring:

yes

Details on sampling:

At suitable time intervals, one culture tube was removed and cooled to room temperature by quenching in an ice bath. One mL of the internal standard solution (methylparaben) was added and the sample was subjected immediately to GLC analysis. The pH was measured at the beginning and end of each run, and no significant change was found.

Buffers:

All buffer solutions were prepared in deionized, distilled water and were adjusted to an ionic strength of 0.5 with potassium chloride. Buffer solutions in the pH range of 2.0 to 7.4 were prepared by combining various amounts of 0.1 M citric acid and 0.2 M dibasic sodium phosphate stock solutions.

Details on test conditions:

Amounts of methenamine equal to 0.75 mg/mL were weighed and placed in volumetric flasks. At time zero, the methenamine was dissolved in buffer solutions of pH 2.0, 2.4, 3.4, 4.6, 5.1, 5.5, 5.8 or 7.4 preivously equilibrated to 37.5°C. After dilution with the respective buffers, 10.0 mL samples were removed and placed in 16 x 125 mm culture tubes with polytef-lined screw caps. These tubes were placed into a constant temperature water bath.
Hexamethylenetetramine at 0.75 mg/mL was also studied kinetically at 47, 57 and 67 °C in pH 2.0, 5.1 and 7.4 buffers.

Duration:

6 h

Initial conc. measured:

0.75 g/L

Number of replicates:

not indicated

Positive controls:

no

Negative controls:

no

Statistical methods:

Arrhenius plot

Preliminary study:

no preliminary study

Test performance:

good test performance

Transformation products:

yes

No.:

#1

No.:

#2

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

Formation of Formaldehyde and ammonia.

Key result

pH:

2

Temp.:

37.5 °C

Hydrolysis rate constant:

0.43 h-1

DT50:

1.6 h

Type:

(pseudo-)first order (= half-life)

Remarks on result:

other: r2: 0.9978

Key result

pH:

2.4

Temp.:

37.5 °C

Hydrolysis rate constant:

0.33 h-1

DT50:

2.12 h

Type:

(pseudo-)first order (= half-life)

Remarks on result:

other: r2: 0.9848

Key result

pH:

3.4

Temp.:

37.5 °C

Hydrolysis rate constant:

0.22 h-1

DT50:

3.09 h

Type:

(pseudo-)first order (= half-life)

Remarks on result:

other: r2: 0.9920

Key result

pH:

4.6

Temp.:

37.5 °C

Hydrolysis rate constant:

0.19 h-1

DT50:

3.73 h

Type:

(pseudo-)first order (= half-life)

Remarks on result:

other: r2: 0.9993

Key result

pH:

5.1

Temp.:

37.5 °C

Hydrolysis rate constant:

0.08 h-1

DT50:

8.29 h

Type:

(pseudo-)first order (= half-life)

Remarks on result:

other: r2: 0.9882

Key result

pH:

5.5

Temp.:

37.5 °C

Hydrolysis rate constant:

0.06 h-1

DT50:

10.9 h

Type:

(pseudo-)first order (= half-life)

Remarks on result:

other: r2: 0.9935

Key result

pH:

5.8

Temp.:

37.5 °C

Hydrolysis rate constant:

0.05 h-1

DT50:

13.8 h

Type:

(pseudo-)first order (= half-life)

Remarks on result:

other: r2: 0.9935

Key result

pH:

7.4

Temp.:

37.5 °C

Type:

other: not applicable

Remarks on result:

other: No observable degradation over the time interval studied.

Key result

pH:

2

Temp.:

47 °C

Hydrolysis rate constant:

1.15 h-1

Type:

(pseudo-)first order (= half-life)

Remarks on result:

other: r2: 0.9985

Key result

pH:

2

Temp.:

57 °C

Hydrolysis rate constant:

4.19 h-1

Type:

(pseudo-)first order (= half-life)

Remarks on result:

other: r2: 0.9985

Other kinetic parameters:

pH 2 (67°C): k = 11 h-1
pH 5.1 (47°C): k = 0.11 h-1
pH 5.1 (57°C): k = 0.23 h-1
pH 5.1 (67°C): k = 0.43 h-1

Details on results:

The kinetics of degradation also were measured at 47, 57 and 67°C, and the reaction obeyed the Arrhenius relationship. At pH 2.0, the activation energy was calculated to be 23.5 kcal/mole; at pH 5.1, it was 12.0 kcal/mole.

Validity criteria fulfilled:

yes

Conclusions:

The conversion of methenamine is pH dependent.

Executive summary:

The kinetics of degradation of methenamine were studied in citrate-phosphate buffers between pH 2.0 and 7.4 at 37°C. Gas Chromatography was used to monitor the rate of hydrolysis. The conversion rate of methenamine to formaldehyde was found to be pH dependent in the buffers of constant ionic strength, with the reaction half-life decreasing from 13.8 h at pH 5.8 to 1.6 h at pH 2.0. The kinetics of degradation also were measured at 47, 57 and 67°C, and the reaction obeyed the Arrhenius relationship. At pH 2.0, the activation energy was calculated to be 23.5 kcal/mole; at pH5.1, it was 12.0 kcal/mole.

Description of key information

The conversion rate of methenamine to formaldehyde at a constant ionic strength was found to be pH dependent, with a reaction half-life decreasing from 13.8 h at pH 5.8 to 1.6 h at pH 2.0. The value below is given for pH 5.8.

Key value for chemical safety assessment

Half-life for hydrolysis:

15 d

Additional information

The kinetics of degradation of methenamine (Strom, 1980) were studied in citrate-phosphate buffers between pH 2.0 and 7.4 at 37°C. Gas Chromatography was used to monitor the rate of hydrolysis. The conversion rate of methenamine to formaldehyde was found to be pH dependent in the buffers of constant ionic strength, with the reaction half-life decreasing from 13.8 h at pH 5.8 to 1.6 h at pH 2.0. The kinetics of degradation also were measured at 47, 57 and 67°C, and the reaction obeyed the Arrhenius relationship. At pH 2.0, the activation energy was calculated to be 23.5 kcal/mole; at pH 5.1, it was 12.0 kcal/mole.

After the release of the remaining substance from the waste water treatment plant into surface water, methenamine is expected to be further degraded hydrolytically to ammonia and formaldehyde. This needs to be considered especially for acidic receiving waters. Formaldehyde is classified as readily biodegradable (OECD 2002), and hence expected to be degraded by microbial activity. Ammonia is natural widely occurring, e.g. as excretion product of aquatic organisms. Ammonia is expected to volatilise from water to a certain degree. Under aerobic conditions it is transformed by nitrifying bacteria to nitrite and nitrate. As a realistic worst case for the degradation of methenamine a hydrolytical half-life of ~ 10 days is assumed. The half-life for the biodegradation of the hydrolysis products is estimated to be 15 days (according to the TGD, 2003). Hence, as realistic worst-case a half-life of 15 days, corresponding degradation rate constant of 0.046 d^-1 is used for the exposure estimation.