178 Analyst, March, 1968, Vol. 93, $q5. 178-185 A Test-paper Method for the Determination of Tolylene Di-isocyanate Vapour in Air* BY D. A. REILLY (Imperial Chemical Industries Limited, Dyestufls Division, Hexagon House, Blackley, Manchester 9) A test-paper has been developed that will enable up to 0-10 p.p.m. v/v of tolylene di-isocyanate (TDI) vapour to be determined in a 6-litre sample of air drawn through a circle of the paper 1 cm in diameter at the rate of 1 litre per minute. In the presence of tolylene di-isocyanate a reddish grey stain is produced, development of which is complete within 15 minutes of the end of sampling. The intensity of the stain is compared with artificial standards made by coating paper with specially formulated pigment suspen sions. The test-papers are produced by treating Whatman No.1 filter-paper with a solution of sodium nitrite, 2-hydroxy-l lH-benzo [a]carbazole-3-car- boxy+anisidide (Brenthol GB), ammonium acetate and diethyl phthalate in methanol. They are stable for at least 3 months when stored in the dark. This method is simpler than existing “wet” methods and can be carried out by less skilled operators. The 2,4- and 2,6-isomers react equally. IN existing methods of determining tolylene di-isocyanate (TDI) in airJ1sa the air sample is drawn through a bubbler containing an acid-trapping medium, in which the tolylene di- isocyanate is hydrolysed to tolylenediamine. This is then diazotised and coupled with N-1-naphthylethylenediamine dihydrochloride to produce a reddish purple solution, which is compared visually with artificial colour standards.Although this approach is sound, a considerable degree of analytical skill is needed that is not always available to industrial users of tolylene di-isocyanate. An alternative simpler method has therefore been sought. It is an established procedure to determine traces of nitrite or nitrogen dioxide by reaction with an aromatic primary amine to produce the corresponding diazonium compound, which then couples either with the same amine or with a second component to produce a colour. An example of this is the Saltzman reagent3 for determining nitrogen dioxide in air, in which the initial diazotisation of sulphanilic acid is followed by coupling in the same solution with N-1-naphthylethylenediamine dihydrochloride.It seemed logical to try and use this type of reaction to determine isocyanate by reaction with a mixture of sodium nitrite and a suitable coupling component. This approach has proved successful. The work des- cribed below was concerned with the choice of reagents, composition of the solutions used to prepare the test-papers, measurement of the sensitivity of the papers towards di-isocyanate, specificity and quantitative reproducibility of the indication, and with the production of artificial colour standards. DESCRIPTION OF THE METHOD REAGENTS AND MATERIALS- Sodium nitrite-Analyt ical-reagent grade. Ammoniwn acetate-Analytical-reagent grade. Brenthol GB (recrystal1ised)-Recrystallise Brenthol GB, 2-hydroxy-l1H-benzo [alcar- bazole-3-carboxy-$-anisidide (see Note l), three times from pyridine containing 20 per cent.of water. Dry at 100” C. MethanoLAnalytical-reagent grade. Diethyl phthalate. Whatman No. 1 Jilter-$aper, 9 and 15-em circles. Monolite Fast Red 2RVS paste-See Note 1. Monastral Fast Blue BVS paste-See Note 1. Monolite Fast Yellow GNVS paste-See Note 1. China clay, S.P.S. grade-Obtainable from English China Clays Ltd. * The subject of British Patent Application No. 34835/67. 0 SAC and the author.REILLY 179 Bzctakon M L 505 latex-See Note 1. Thin matt white card-An example is Zara White Card, 10 x 12 inches, reference 1/4606, obtainable from R. A. Brand Ltd. Bridge Paper Mills, Manchester 6. Test-$afiers-Weigh into a 250-ml round-bottomed flask 0.5 g of recrystallised Bren- tho1 GB. Add a mixture of 35 ml of diethyl phthalate and 115 ml of methanol, both measured carefully with graduated cylinders.Attach a water-cooled reflux condenser, and boil under reflux for 30 minutes. Cool to room temperature, detach the flask and filter its contents through a Whatman No. 1 filter-paper, folded into a cone, into a clean, dry 250-ml measuring cylinder. Wash out the flask with one or two small portions of methanol and use these, in turn, to wash down the filter-paper. Finally, dilute the contents of the cylinder to exactly 150 ml and mix well. Into a conical flask, place 2.0 g of sodium nitrite, 0.15 g of ammonium acetate and 3.0 ml of water. Swirl it gently until all of the solid has dissolved and add exactly 100ml of the filtered Brenthol GB solution. Mix well and pour the solution into a 6-inch diameter evaporating dish.With metal forceps, immerse Whatman No. 1 filter-papers, 9 cm in diameter, in the solution, one at a time, for 30 seconds, remove and quickly blot off surplus liquid between several thicknesses of 15-cm diameter filter-paper (at least two thicknesses above and below). Hang them up to dry for 1 hour at room temperature in darkness or subdued light in an atmosphere free from chemical fumes. From each dried circle cut three strips, 5 x 2 cm. Store in a glass bottle, either painted black on the outside or completely covered with black paper. Papers stored in this way are stable for at least 3 months. Coated paper stafidards-Dilute the three pigment pastes with water to 20 per cent. of their original strength by weighing about 50g of each into a 10-oz bottle and adding four times the weight of water, i.e., 4 ml for each gram.Stopper each bottle and mix the contents well. With 10-ml dropping pipettes, measure the volumes of the diluted pigment pastes, shown in Table I, into a 600-ml beaker fitted with a wide paddle-type stirrer. Measure the specified volumes of Butakon ML 505 latex and water in measuring cylinders to the nearest millilitre. Pour the latex into the beaker, rinsing out the cylinder with small volumes of the water. Add the remainder of the water and stir until the mixture is homogeneous. Add the china clay gradually, with stirring. When all of it has been added, stir for a further 30 minutes. Lay a sheet of white card on a sheet of plate-glass and fasten one of the shorter ends down with adhesive tape. Place the well-type paint applicator (see under Apparatus) on the paper card at this end, close to the side, with its slot parallel to the shorter end,with the cut-away portion facing the fastened down end.Transfer, by pipette, 2 ml of the coating paste into the slot of the applicator. Place one hand on each side of the applicator and, with a gentle downward pressure, draw it steadily along the sheet and off the far end, thus spreading a layer of paste 0-003-inch thick on the card. Detach the card from the glass sheet and place it in an oven at 70" to 80" C , until dry, but for not more than 5 minutes. Mount 2.5 x 46cm strips of each of the five colour standards side by side on a card. Punch a hole 7 mm in diameter through the supporting card and the standard.These reagents can be obtained through Sales Offices of Imperial Chemical Industries Limited. NOTE 1. TABLE I COMPOSITION OF MIXTURES FOR HAND-SPREADING OF TDI COLOUR STANDARDS TDI in a 6-litre air sample, equivalent p.p.m. Diluted Monolite Fast Rkd 2 RVS pastk, ml . . 4.0 6.6 13.9 18.9 25.6 Diluted Monastral Fast Blue BVS paste, ml . . 2.8 4.0 7.6 11.0 14.5 Diluted Monolite Fast Yellow GNVS paste, ml 6.2 7.8 17.2 23.8 31.2 Water, ml .. .. .. 220 210 200 186 170 Butakon ML 505 iatex, I$i . . .. .. 64 64 64 64 64 China clay, SPS grade, g . . .. .. 160 160 160 I60 160 v/v .. . . 0.01 0.02 0-04 0-06 0.10 APPARATUS- Test-paper holder-For exposing a circle of paper 1 cm in diameter to the sample stream. Suitable holders are described elsewhere,4Jj the latter referred to being obtainable from Siebe, Gorman Ltd., Davis Road, Chessington, Surrey.180 REILLY : A TEST-PAPER METHOD FOR THE DETERMINATION [Aaalyst, Vol.93 Means of sztctiow-This should be capable of drawing a 5-litre sample of the test atmos- phere through the paper at about 1 litre per minute. The Mine Safety Appliances Company's 4-cylinder hand-cranked pump or a water-filled aspirator6 can be used. If the former is used the flow-rate must be adjusted before use to 1 litre per minute, with a test-paper in the holder, and a flow-meter attached to the inlet of the test-paper holder. Well-type paint applicator-See Fig. 1. 5 r Section on AA ' 3 ,_ I Fig. 1. Well-type paint applicator: all measurements are in inches. Material is of stainless steel, smooth finish PROCEDURE- Insert a test-paper in the holder in an atmosphere free from TDI, attach to the source of suction and draw 5 litres of the test atmosphere through the paper at a rate of about 1 litre per minute.If it is not convenient to have the source of suction near to the sampling point, an extension tube can be used, but this must be placed between the paper holder and the source of suction. On no account must any rubber tubing be attached to the inlet of the test-paper holder. Remove the paper, mark it for identification purposes and place it in a box or envelope. Fifteen minutes after the end of sampling, compare the stain with the five colour standards by holding it under the hole in each, in turn, and in contact with the card, so that the standard surrounds the stain.View the card at arm's length with the window or other source of light behind one. Assign the appropriate concentration to the stain, interpolating, if necessary, a value between two of the standards. EXPERIMENTAL PRODUCTION OF KNOWN ATMOSPHERIC CONCENTRATIONS OF TOLYLENE DI-ISOCYANATE (TD1)- Pure nitrogen was bubbled at a measured flow-rate through TDI contained in a sintered-plate gas washing bottle. This was diluted with a second metered stream of pure nitrogen. A portion of this diluted mixture was metered into a stream of moistened compressed air to produce the final working concentration, which could be varied over a wide range by suitably chosen changes in the gas flows. Some typical conditions for the aspiration method were as follows.Nitrogen bubbled at 250 ml per minute through TDI at 20" C was diluted to 5 litres per minute; 1 litre per minute of this diluted stream was further diluted to 25 litres per minute. The TDI concentration produced was 0.033 p.p.m. v/v. Nitrogen bubbled at 50 ml per minute through TDI at 20" C was diluted to 5 litres per minute; 1 litre per minute of this diluted stream was further diluted to 25 litres per minute. The TDI concentration was 0.014 p.p.m. v/v. A second approach was to inject a dilute solution of TDI in carbon disulphide or cycle- hexane at a known slow rate into a metered stream of moistened compressed air by using a Palmer slow injection apparatus6 When a 0.051 per cent. v/v solution of TDI in carbon di- sulphide was injected at 3.02 p1 per minute into an air flow of 10 litres per minute, the con- centration of TDI produced was 0.013 p.p.m.v/v. With the same solution strength and In the earlier part of the work an aspiration method was used.March, 1968) OF TOLYLENE DI-ISOCYANATE VAPOUR IN AIR 181 injection rate, but with an air flow of 8 litres per minute, the TDI concentration produced was 0.019 p.p.m. v/v. This was the preferred method as it gave steadier concentrations. With neither method was it possible to calculate in advance the concentration of TDI to be produced. The concentrations were always less than expected in the range 0.005 to 0.10 p.p.m. v/v. This was thought to be caused by the hydrolysis of TDI in the moist air and possibly also to the absorption on surfaces of the apparatus, which was all-glass.The desired concentrations were therefore obtained by empirical adjustment of the operating conditions. They were checked by an unpublished method, a variant of one published for the determination of di-isocyanatodiphenylmethane in air,6 in which the test atmosphere was drawn through an aqueous solution of sodium acetate and hydrochloric acid. The hydrolysis product was coupled with p-nitrodiazobenzene, and the colour produced extracted into chloroform and measured with a spectrophotometer. CHOICE OF COUPLING AGENT- In each instance Whatman No. 1 filter-paper was dipped for 30 seconds in the treatment solution and then hung to dry for 1 hour at room temperature. The treatment solutions were prepared by dissolving 2 g of sodium nitrite in a saturated solution of the coupling agent in a mixture of 10 ml of diethyl phthalate and 90 ml of methanol.Each paper was tested by drawing 1 litre of air containing about 0-1 p.p.m. v/v of TDI through a 1-cm diameter circle of the test-paper in 1 minute. The intensities of the stains produced by the different papers were compared visually. The following compounds were tested in this way : 3-hydroxy-2-naphthoic acid, 2-hydroxy- 1-naphthoic acid, disodium 2-naphthol-3,6-disulphonate (R-salt) and nineteen compounds from the I.C.I. Brenthol range. Three of the Brenthol range gave the most intense colours, Brenthol AS (2-hydroxy-3-naphthanilide), Brenthol MN (2-hydroxy-3-naphtha-m-nitro- anilide) and Brenthol GB (2-hydroxy-l lfl-benzo [a]carbazole-3-carboxy-~-anisidide). The first two gave pink stains and the third a reddish grey.Brenthol GB was chosen for further development. METHOD OF ASSESSING STAIN INTENSITY BY USING COLOURED PAPER STANDARDS- A series of papers of graded depth of colour was prepared, the shades of which closely matched those given by TDI on test-papers. Each coloured paper was prepared by dipping a 9-cm circle of Whatman No. 1 filter-paper in a solution of mixed dyestuff for 30 seconds, blotting quickly between several thicknesses of filter-paper and hanging up to dry at room temperature. The mixed dyestuff solutions were made from freshly prepared 0.2 per cent. w/v stock solutions of the following dyestuffs in distilled water: Chlorazol black PB 150, Durazol brown G and Chlorazol yellow G 200.Before use, equal volumes of the Chlorazol black and Durazol brown stock solutions were mixed to form a solution containing 0.2 per cent. w/v of “mixed dye.” The working solutions used to prepare the coloured paper standards were made by mixing an aliquot of the “mixed dye” solution plus 2.0 ml of the yellow solution and diluting with distilled water to 100ml. The shade of each coloured paper standard was characterised numerically by the percentage of “mixed dye” in the solution used to prepare it. To facilitate comparisons of test stains with the range of coloured paper standards, a hole, 7 mm in diameter, was punched in each of the latter with a number 3 cork borer. A test stain was placed under the hole in each standard in turn, until a match was obtained when the two were held at arm’s length, the operator’s back being towards the light.COMPOSITION OF THE SOLUTION USED TO IMPREGNATE THE TEST-PAPERS- Bmzthol GB-As received, this gave a rather dark coloured solution in methanol, there- fore, it was recrystallised three times from aqueous pyridine and dried at 100” C. A saturated solution was prepared by refluxing 0.5 g of recrystallised Brenthol GB with the appropriate volume of diethyl phthalate diluted to 150 ml with analytical-reagent grade methanol for 30 minutes, cooling to room temperature and filtering into a 250-ml measuring cylinder. The solid on the filter-paper was washed with small volumes of methanol until the volume of filtrate and washings was 150ml; 100ml of this solution was mixed with the solution of sodium nitrite, ammonium acetate and water (see below) to give the final solution used to impregnate the test-papers. Several potential coupling agents were examined.182 REILLY: A TEST-PAPER METHOD FOR THE DETERMINATION [Analyst, Vol.93 Sodium aitrite and ammonium acetate-Preliminary tests suggested that by increasing the sodium nitrite content of the impregnation solution deeper stains would be produced at a given TDI concentration, but 2.0 per cent. w/v was about the greatest concentration that could be tolerated without leaving scaly deposits on the surface of the test-papers. The addition of 0.15 g of ammonium acetate to each 100 ml gave a test-paper with an appreciably paler background colour and did not appear to affect its sensitivity towards TDI.The sodium nitrite and ammonium acetate were first dissolved in 3 ml of water before adding the Brenthol GB solution, as direct dissolution in the latter was slow. DiethyZ fihthaZate-This was found to cause a substantial increase in the rate of trapping of TDI by the test-paper. Thirty millilitres per 150ml of Brenthol GB solution was the most that could be used without producing a paper that was too translucent for stains caused by TDI to be seen readily. By using this proportion and hanging the papers up to dry, without blotting after impregnation, the stains obtained with TDI were rather mottled, as though the reagent was unevenly distributed on the surface of the paper. Much more even stains were obtained with papers that had been blotted between several thicknesses of filter-paper.To compensate for the slight loss of sensitivity that was caused by blotting, the volume of diethyl phthalate in each 150 ml of Brenthol GB solution was increased to 35 ml without making the papers translucent. TIME REQUIRED FOR THE DEVELOPMENT OF STAINS- Five-litre air samples containing TDI were drawn through 1-cm diameter circles of test-paper at 1 litre per minute. The stains produced were matched, at intervals, after the end of sampling with coloured paper standards prepared as described above. Three prepara- tions of TDI were used, the 2,4- and 2,6-isomers and an 80 + 20 mixture of the two. 2,4-TDI-Concentrations were not checked. Results are shown in Table 11. TABLE I1 RATE OF COLOUR DEVELOPMENT BY TEST-PAPERS WITH 2,PTDI Test No... 0 . 1 2 3 - Time after end of samp- ling, minutes.. .. 6 10 16 6 10 16 6 10 15 20 “Mixed dye” correspond- ing to colour standard matched, percent. . . 0.050 0.065 0-065 0.095 0.100 0.100 0-040 0.050 0.060 0-060 2,6-TDI and the 80 + 20 mixture-Concentrations were checked by the method referred to above. Results are shown in Tables I11 and IV. TABLE I11 RATE OF COLOUR DEVELOPMENT BY TEST-PAPERS WITH 2,6-TDI 0.019 0.048 0.048 Time after end of Test No. TDI, p.p.m. sampling, minutes 1 0.019 5 10 16 20 6 10 16 20 90 6 10 15 20 25 5 10 15 20 “Mixed dye” corresponding to 0.025 0-040 0.045 0*060 0.025 0.040 0.045 0.050 0.055 0.040 0-065 0.080 0-085 0.100 0.035 0.060 0.080 0.095 colour standard matched, per cent.March, 19681 OF TOLYLENE DI-ISOCYANATE VAPOUR IN AIR TABLE IV RATE OF COLOUR DEVELOPMENT BY TEST-PAPERS WITH MIXED TDI 183 Time after end of Test No.TDI, p.p.m. sampling, minutes 1 0.039 5 10 15 65 2 0.020 3 0.031 4 0.055 6 10 16 20 175 5 10 15 25 5 10 15 20 26 "Mixed dye" corresponding to colour standard matched, per cent. 0.080 0.080 0.080 0.085 0.030 0.040 0.040 0.040 0.045 0.055 0.065 0-065 0.066 0.065 0.080 0.095 0.095 0.096 It was thus evident that with 2,4-TDI development of the test stain was complete in 15 minutes, whereas with the 2,6-isomer, 20 minutes, or more, were necessary. The effect of this slightly slower colour development with the 2,6-isomer was not apparent with the 80 + 20 mixture. Colour development was again complete in 15 minutes and the stain appeared to be stable for at least 3 hours.Qualitative inspection of test stains that had been stored in the dark suggested that the stains did not fade for several days. EFFECT OF TOLYLENEDIAMINE- To check the possibility of interference from any tolylenediamine that might be present with the TDI, atmospheric concentrations of 2,4-tolylenediamine were prepared by passing either 1 or 2 litres of air per minute through a bubbler containing the amine held just above its melting-point of 99" C , and diluting this to 10 litres per minute with clean air, the combined stream being passed through a silica-wool plug before sampling. Concentrations were checked by Marcali's method.2 Five-litre samples were drawn through test-papers at 1 litre per minute. At the two concentrations, 0-07 and 0.10 p.p.m.v/v, no visible stains were produced on the test-papers. EFFECT OF OTHER ISOCYANATES- Qualitative tests with other industrially used isocyanates have shown that 4,4'-di-iso- cyanatodiphenylmethane (MDI), 1,5-di-isocyanatonaphthalene (NDI) and 4,4'-di-isocyanato- 3,3'-dimethylbiphenyl (TODI) give stains with the test-paper similar to that given by TDI. Hexamethylene di-isocyanate gives no stain. SIZE OF AIR SAMPLE AND SPEED OF SAMPLING- Five litres was chosen as the sample volume because it gave a good range of readily distinguishable stains over the range 0.01 to 0.10 p.p.m. v/v of TDI. Smaller volumes gave rather pale stains in the region of the threshold limit value of 0.02 p.p.m. v/v.' Larger volumes made it difficult to distinguish between the stains given by the higher concentrations in this range.The effect of variations in sampling speed for a given total sampling volume was studied; 5-litre samples were drawn through test-papers at different rates. The TDI concentrations were held as steady as possible during each series of tests but were not checked independently. Stain intensities were measured, as before, by visual comparison with coloured paper standards 15 minutes after the end of sampling, the results being given in Table V. Stain intensity was thus largely independent of sampling rate in the range 0.5 to 2.0 litres per minute.184 REILLY: A TEST-PAPER METHOD FOR THE DETERMINATION [A%at?ySt, VOl. 93 TABLE V EFFECT OF VARYING RATES OF SAMPLING ON STAIN INTENSITY “Mixed dye” corresponding to colour standard matched, per cent. 0-6 likes per 1.0 litres per 2.0 litres per 3-0 litres per 4.0 lities per Test No.minute minute minute minute minute 1 - 0.070 0.060 - 0.065 2 - 0.036 0.030 0.035 - 3 0.060 0.046 0.060 - 0.035 4 0*100 0.080 0.090 - - 0.070 0.040 0.046 STORAGE STABILITY OF TEST-PAPERS- On exposure to bright sunlight the unused test-papers, which are initially pale yellow, rapidly acquire an orange - brown colour. Although they are still sensitive to TDI, it becomes impossible to make accurate judgments of stain intensities by comparison with the coloured paper standards. If the test-papers are stored in a glass bottle in complete darkness, they retain their pale colour and sensitivity to TDI for at least 3 months. REPRODUCIBILITY OF STAIN INDICATION AT DIFFERENT TDI CONCENTRATIONS- Papers from several preparations were tested with known concentrations of TDI in air, as described above, the stains being matched 15 minutes after the end of sampling with the coloured paper standards.The results are shown in graphic form in Fig. 2. The variation is about &20 per cent. up to 0.09 p.p.m. of TDI. 1 / I I I I I I I I I I I I I L 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.10 0.1 I 0.12 013 0.1 TDI in 5-litre air sample, p.p.m. Fig. 2. Reproducibility of test-paper stains with known TDI concentrations: a, test-paper batch 1; 0, test-paper batch 2 ; x, test-paper batch 3; and A test- paper batch 4 PRODUCTION OF PERMANENT COLOUR STANDARDS- The practical use of any test-paper for the quantitative determination of a toxic gas or vapour in air depends on the availability of reproducible, stable colour standards, with which the test stains can be compared.The coloured paper standards described above, although adequate for use in the development of the test-paper, were not permanently stable. After 3 or 4 weeks their shades changed and test stains could no longer be matched. Alterna- tives were, therefore, sought. Attempts to obtain a disc with permanent, glass colour standards were unsuccessful, as were attempts by a commercial printer to match the dyed paper standards. Success was finally achieved by coating thin paper card with aqueous suspensions of china clay and appropriately chosen amounts of red, blue and yellow dispersed pigments of high light fastness, a butadiene - styrene copolymer latex being used as a binder. By trial and error,March, 19681 OF TOLYLENE DI-ISOCYANATE VAPOUR IN AIR 185 five standards were produced matching the stains obtained on the test-papers with 0.01, 0.02, 0.04, 0.06 and 0.10 p.p.m.v/v of TDI in a 5-litre air sample. The pigments used were Monolite Fast Red 2RVS paste, Monastral Fast Blue BVS paste and Monolite Fast Yellow GNVS paste. Full details of the preparation of these standards are given above under Description of the method. Repeated preparations of a given standard were indistinguishable from one another. Their validity was checked by comparison with stains obtained at the appropriate, known TDI concentrations. The coated standard at each concentration corre- sponded to the middle of the small range of variation obtained with individual test-papers.DISCUSSION This work has resulted in the development of a test-paper method, which is simpler to operate than earlier solution methods and, unlike them, is suitable for use by people without formal analytical training, a valuable consideration in many of the situations where TDI is produced and used. Both 2,4- and 2,6-isomers are detected with equal sensitivity, but no response is obtained from 2,4-tolylenediamine. Although the elapsed time (about 21 minutes) for a single determination is longer than with the most rapid of the solution methods1 (about 12 minutes), 15 minutes of this time is taken up with colour development and is available for the operator to take further air samples. By using the test-paper, considerable saving of time will be made in carrying out a multi-point survey of conditions in a plant either malting or using TDI. The test-paper is easy to produce and is stable for at least 3 months, if stored in complete darkness. The coated paper standards can be reproduced without undue difficulty. Single copies of the colour chart will be available on request from Imperial Chemical Industries Limited, Dyestuffs Division. REFERENCES 1. Reilly, D. A., Analyst, 1963, 88, 732. 2. Marcali, K., Analyt. Chem., 1957, 29, 652. 3. Saltzman, B., Ibid., 1954, 26, 1949. 4. Dixon, B. E., Hands, G. C., and Bartlett, A. F. F., Analyst, 1958, 83, 199. 6. “Ministry of Labour, H.M. Factory Inspectorate, Methods for the Detection of Toxic Substances in Air, Booklet No. 8, Phosgene,” H.M. Stationery Office, London, 1961. 6. Hanson, N. W., Reilly, D. A., and Stagg, H. E., Editors, “The Determination of Toxic Substances in Air, A Manual of I.C.I. Practice,” Second Edition, W. Heffer & Sons Ltd., Cambridge, 1965. 7. “Ministry of Labour, Safety, Health and Welfare New Series No. 8, Dusts and Fumes in Factory Atmospheres,” Third Edition, H.M. Stationery Office, London, 1966. Received August 18th, 1967