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Hydrolysis

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No half-life for hydrolysis is available for methylal. Experiments showed that methylal hydrolysis is temperature- and pH-dependant. Methylal is stable at pH 2.5 to 9.0 at 37°C for 24 hours and at pH 4.02 to 8.98 at 25°C for 1 year. Hydrolysis occured at pH <2.5 at 37°C and at pH<4.02 at 25°C.

Key value for chemical safety assessment

Additional information

No half-life for hydrolysis value is given in the literature but methylal is stable towards bases and hydrolysis readily in presence of acids to generate aldehydes / ketals. These properties are reported in the National Library of Medicines’s Hazardous Substance Data Base file that refers to Morrison & Boyd (1973) and Clayton & Clayton (1982).

Rate constants for the aqueous acid hydrolysis (HCl 0.104 N) of methylal have been determined by Stanonis et al. (1972) as temperature dependant. Pseudo first-order rate constants (k1) are 2.71 x 10-6at 25°C, 5.32 x 10-6at 30°C and 1.14 x 10-5at 35°C. Second-order rate constants (k2) are 2.61 x 10-5at 25°C, 5.13 x 10-5at 30°C and 1.10 x 10-4at 35°C.  

Salomaa (1961) reports the following rate coefficients for hydrolysis of methylal in dilute hydrochloric acid solutions (0.1-0.15 M) at 25°C: a 105k of 2.50 L mol-1s-1and a k(1 M acid)/k(dil.acid)of 2.15.

Poon et al. (2000) stated that methylal (99% purity, purchased from Aldrich Chemical Co) was stable at pH 2.5, 3.0, 5.0, 7.0 and 9.0 at 37°C for at least 48 hours but was unstable at lower pH of 1.0, 1.3, 1.7, 2.0. The hydrolysis produced methanol and formaldehyde in a ratio of approximatively 2 to 1. The rate of hydrolysis increased with decreasing pH.

Formaldehyde concentration was determined in methylal samples (99.5% purity, Lambiotte, Belgium) using an HPLC method (based on derivatization with dinitrophenylhydrazine) and UV detection with a detection limit down to 0.1 ppm and a relative standard deviation below 5%. The pH of methylal samples was adjusted to 3.50, 4.02, 5.00, 6.02, 7.00, 8.02 and 8.98. Formaldehyde concentrations were determined at initial time, after 43 and 90 days, after 7 months and after 1 year of storage at 25°C. At more formaldehyde concentration was measured in methylal sample stored at 4 and 25°C at initial time, after 2, 4, 7 and 20 days.

Methylal was stable at 25°C at pH 4.02 to 8.98 for at least 1 year: formaldehyde concentration did not exceed 0.58 ppm. Methylal was unstable at pH 3.50 with formaldehyde concentration that evolved from 3.57 to 403 ppm in 1 year (112.89% formaldehyde concentration increase in 1 year). It is important to note that at pH3.50, conservation of the sample at 4°C significantly reduces the formation rate of formaldehyde showing a formaldehyde concentration increase respectively of 23.36 and 5.07% within 20 days at 25 and 4°C (David, 1996).

Morrison RT & Boyd RN (1973).Organic Chemistry 3rd ed.Boston,.Allyn & Baun Inc.,p. 642.

Clayton GD & Clayton FE (eds.) (1982). Patty's Industrial Hygiene and Toxicology: Volume 2A, 2B, 2C: Toxicology. 3rd ed.: John Wiley Sons, p. 2657.

Salomaa P (1961). Differenciation of structural effects in the acid-catalysed hydrolysis of acetals of formaldehyde.Ann. Acad. Scient. Fennicae 103, 3-22. 

Stanonis DJ, King WD and Vail SL (1972). Influence of chain length on the rate of hydrolysis of polyoxymethylene ethers. J. Appl. Polymer Sci., 16(6): 1447-1456.