Phosgene in ambient air

The following discussion considers the sources of formation of phosgene, and its removal, in the lower atmosphere, a subject which has been recently reviewed by Helas and Wilson [932b]. 

Although phosgene is undoubtedly a photolysis product of a number of commodity chlorinated hydrocarbons, the quantity of phosgene formed in the atmosphere, and the relative importance of the various sources, is difficult to assess [2088b]. Phosgene is likely to be 

Data concerning the environmental distribution of phosgene are not available. 

However, phosgene is a gas at temperatures above about 8 C, and it has a density similar to air it is also relatively stable and it may therefore be expected that phosgene can be transported over considerable distances in the environmental air. From the knowledge of the kinetics of hydrolysis of gaseous phosgene (see also Section 9.10.3.1), carried out in the temperature range of 260-350 C, the rate constant for reaction (3.16)  

Although this hydrolysis study is far removed from conditions relevant for the atmosphere, and interpolation to ambient temperature cannot be regarded as reliable, the half-life of phosgene 

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Phosgene in ambient air, derived from the photooxidation of C2CI4 and C2HCI3 [1886], is discussed in Section 3.7. 

At room temperature and atmospheric pressure, carbonyl difluoride is a colourless gas which is condensable to give a colourless liquid (boiling at -84.6 C at normal atmospheric pressure [1580]) or a white solid (melting at -111.3 C [1580]). It has a pungent odour and, unlike phosgene, is rapidly hydrolysed even in ambient air. The relative molecular mass of COFj is 66.0072, and the density of the gas is estimated to be 2.744 g dm 3 [1764]. The gas is heavier than air, with a specific gravity of 2.34 at 21 C and atmospheric pressure... 

Elemental composition C 12.14%, 0 16.17%, Cl 71.69%. Phosgene can be analyzed by GC using FID or a balogen-specific detector or by GC/MS. Ambient air may be collected in a metal container placed in an argon bath or condensed into any other type cryogenically cooled trap. Alternatively, the air may be collected in a Tedlar bag. The sampled air may be sucked by a condensation mechanism into tbe GC column. 

A variety of stationary phases and chromatographic supports have been employed in analyses for phosgene, depending upon the components and their respective concentrations in the particular system. Some of these are summarized in Table 3.3. A range of columns for the detection of phosgene in air has been evaluated [598] recently, a fused-silica capillary column has detected phosgene at 7 p.p.t. (v/v) in 1 litre of ambient air [98a]. 

Clearly, the preferred method to use for the analysis of phosgene depends upon the particular application to hand. For routine use in the laboratory, for monitoring the ambient air, impregnated paper strips and Dra ger tubes can be recommended as both reliable and easy to use. On the plant, automatic methods for continuous analysis would be appropriate, and one of the electrical or automated spectroscopic techniques would be suitable. For accurate measurement of very low (p.p.b.) concentrations however, a gas chromatographic procedure using one of the special detectors is most suitable. 

It is known that alumina is chlorinated exothermically at above 200° C by contact with halocarbon vapours, and hydrogen chloride, phosgene etc. are produced. It has now been found that a Co/Mo-alumina catalyst will generate a substantial exotherm in contact with vapour of carbon tetrachloride or 1,1,1-trichloroethane at ambient temperature in presence of air. In absence of air, the effect is less intense. Two successive phases appear to be involved first, adsorption raises the temperature of the alumina then reaction, presumably metal-catalysed, sets in with a further exotherm. 

At ambient temperature and pressure, phosgene is a colorless gas which exhibits an irritating and suffocating odor. At low concentrations, it has a characteristic odor like moldy hay. However, the odor threshold of phosgene is higher than its toxic limit and one must remember that the sense of smell fails to detect small concentrations in air. 

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