Phosgene.

Date:
1939
Catalogue details

Licence: Public Domain Mark

Credit: Phosgene. Source: Wellcome Collection.

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    presence of phosgene can be detected by the test described below. METHODS OF DETECTION One simple method of detecting low concentrations of phosgene is by the use of a test-paper containing diphenylamine and p-dimethylaminobenzaldehyde. The gas produces a yellow or orange stain on the paper and an indication of its presence may be obtained even at concentrations of the order of 1 in 1,000,000 (0.004 mg/1.) by exposing the paper for a few minutes. This test has been adopted as the standard method for the detection of phosgene in industry. It has been made quantitative by drawing known volumes of the atmosphere under test through a definite area of test-paper by means of a hand pump of specified capacity. From the number of strokes of the pump required to produce stains of certain intensity, the concentration present is obtained by reference to a colour chart. In this way, concentrations down to 1 in 1,000,000 (0.004 mg/1.) are readily determined with not more than 85 strokes of the pump. It must be noted, however, that the stains produced are only transient, and that the sampling and comparison of the stains must therefore be carried out as rapidly as possible. The test-papers are sensitive also to hydrogen chloride and chlorine. Traces of these gases can, however, be removed from the sample by drawing it first through a guard tube containing pumice granules impregnated with sodium thiosulphate and sodium iodide. These react with the interfering gases, but allow any phosgene to pass through and react with the test-paper. INSTRUCTIONS FOR CARRYING OUT THE TEST FOR PHOSGENE GENERAL Although the test is simple and accurate, it should preferably be carried out only by a responsible person appointed by the occupier, as for example the person appointed under Regulation 7 of the Chemical Works Regulations, 1922, in the case of entry into vessels. All necessary steps should be taken for the protection of the person carrying out the tests. In determining the concentration of phosgene in a workroom, samples should be taken at a point closely adjacent to the workers, as an average sample for the whole workroom may give misleading figures of the concentrations actually inhaled. In determining the concentration of phosgene in the atmosphere inside an enclosed space prior to the entry of a workman, it may be insufficient to do only one test. When there is any chance of the concentration inside the space rising, no person should enter the space unless he is wearing breathing apparatus as required under Section 27 of the Factories Act, 1937 and Regulation 7 of the Chemical Works Regulations, 1922. APPARATUS The atmosphere being tested shall be sampled by means of a hand exhaust- ing pump with a barrel of approximately 1.25 in. bore and a capacity of 126ml; the inlet end of the pump should be screwed internally to take a spigot screwed 7/16 in. Whit., 14 T.P.I. on 0.437 in. diameter. The pump may be fitted, if desired, with a device for counting the number of strokes made. >