Phosgene from chloroform

Eric Turk
University of California, Los Angeles

Chemical & Engineering News (2 Mar 1998) Vol. 76, No. 9, pp. 6.

The spontaneous generation of dangerous quantities of phosgene (COCl2) from chloroform, preserved with amylene and stored properly in the absence of heat and light, has surprisingly occurred in two brown glass containers in this laboratory. The chloroform was three years old when potentially lethal quantities of phosgene were discovered. Rather than a manufacturer-specific problem, this oxidation is likely associated with alkene preservation and a catalytic contaminant. Given the benign storage conditions, there may thus exist a widely unrecognized potential chemical hazard associated with some lots of alkene-preserved chloroform.

We were first alerted to this problem by the unusual darkening of an aqueous DNA phase during chloroform extraction, associated with an unpleasant musty odor. Four lab personnel were moderately sickened after smelling the chloroform stock. Many DNA experiments had been ruined. Analysis by test strips, Draeger tubes, and ultimately gas chromatography/mass spectrometry, confirmed phosgene. Colorimetric quantitation ["Phosgene and Related Carbonyl Dihalides"; Am. Ind. Hyg. Assoc., 32, 163 (1971)] showed that one bottle contained 1.1% phosgene (7.1 g), with a headspace concentration of 15,000 ppm. The human inhalation concentration-time product producing 50% lethality is a low 570 ppm per minute.

Chloroform/oxygen mixtures are unreactive in the dark up to 290 C, suggesting catalyst-assisted air oxidation in our samples[ Kirk-Othmer Concise Encyclopedia of Chemical Technology]. Possible catalysts include surface iron(III) colorant of inadequately washed brown glass and the alkene preservative if peroxidized. While addition of trace iron(III) and air to chloroform of an older lot doubled the phosgene content overnight to 0.012%, this was more than 10-fold less than the rate observed after simple air exposure of the problematic chloroform, thereby indicating another catalyst.

Prudence suggests that alkene-preserved chloroform bottles be tested for phosgene. Filter paper strips, wetted with 5% diphenylamine, 5% dimethylaminobenzaldehyde, and then dried, turn yellow in phosgene vapor. The use of ethanol-preserved chloroform is desirable because ethanol, unlike alkenes, reacts quickly with phosgene. Furthermore, more ethanol than amylene is added commercially, providing extra protection against oxidation [Ullmann's Encyclopedia of Industrial Chemistry].

The phosgene-containing chloroform samples are being retained, should any investigator wish to pursue the mystery of the catalytic mechanism. Means to render alkene-preserved chloroform safer could then be pursued.

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