were 208, 606, and 108 mg., respectively. The recovery Of 822 mg., IVhen the loss on the control strips was considered, accounted for 97% of the original mixture. The last two comPounds Were obtained chromatographically and analytically pure without further purification. Other members of the thiopyrophosphate series have also been isolated satisfactorily by this technique (6).
LlTER4TURE CITED
(1) Danielson, C. E., Arkiv Kemi 5 , 173 (1953). (2) Fischer, A., 13ehrens, M., HoPPeSeuler’s 2. phusiol. Chem. 291, 14 (1952). (3) Hagdahl, L., C nnielson, C. E., Nature
174, 1062 (1954). (4) Hams, C. S., Isherwood, I?. A., Ibid., 164, 1: 07 (1949). (5) McIvor, R. A , , McCsrthy, G. D., Grant, G. A, Can. J . Chem. 34,
1819 (1966).
(6) Mitchell, H. IC, Haslrins, F. A., Science 110, 278 (1949). (7) Porter, \v, L., ANAL. CHERI, 23, 412 (1951). (8) \Ton Solms, J., Hclv. Chim. Acta 38, 1127 (1955). (9) Zechmeister, L., Science 113, 35 (1951). RECEIVED for review November 13, 1956. Accepted December 19, 1956. Issued as Defence Research Chemical Laboratories Report No. 199A.
Microdetermination of To1utmediisocyanutes in Atmosphere KALMAN MARCALI Jackson laboratory, E. 1. du Pont de Nemours & Co., Inc., Vfilmingfon, Del.
A rapid, sensitive, colorimetric method for determining toluenediisocyanates in air depends upon hydrolysis of the toluenediisocyanate to the corresponding diamine, diazotization, coupling to N-1-naphthylethylenediamine, and final color measurement at 550 mp. The method i s capable of detecting 0.01 p.p.m. of toluene-2,4diisocyanate. A portable field analytical kit allows the complete analysis to b e performed in 10 to 15 minutes with a sensitivity of 0.02 p.p.m. of toluene-2,4-diisocyanate. As little as 0.03 p.p.m. of 3,3 ‘-diisocyanato4,4’ dimethylcarbanilide may be determined in the presence of toluenediisocyanate. The method depends upon the spectrophotometric measurement at 450 m p of the yellow color formed when the dimethylcarbanilide i s absorbed in an aqueous solution of ethyl Cellosolve containing sodium nitrite and boric acid.
-
A
of toluenediisocyanates in the general field of synthetic chemistry and in the manufacture of adhesives, protective coatings, foams, fluid polymers, and urethane rubbers (25) has become of increasing importance in the last few years. Polyisocyanates such as toluene-2,4-diisocyanate or mixtures of the 2,4- and 2,6-diisocyanate isomers are sold under trade-mark Hylene by du Pont. Preliminary toxicological studies by Zapp (29) have indicated that the maximum allowable concentration of toluenediisocyanate in air may be 0.1 p.p.m. Exposure to higher concentrations may cause serious respiratory disorders. Analytical methods discussed in the PPLICATION
552
ANALYTICAL CHEMISTRY
literature have been developed principally for the determination of isocyanates in relatively concentrated solutions (1, 5, 22, 24, 26). This investigation was undertaken to develop a sensitive and rapid analytical method for the determination of trace quantities of toluenediisocyanate in air. The developed method depends upon the rapid hydrolysi:; of the toluenediisocyanate to the corri:sponding toluenediamine derivative (;8), diazotization of the toluenedianiine (go), and coupling of the stable diazo compound with 11’1-naphthylethylenediamine to produce a reddish blue colw that is nieasured spectrophotometricdly. GENERAL COt.ISIDERATIONS
Air Sampling. Air is drawn through a n all-glasr; bubbler (1.0-mm. impinger opening) containing 15 ml. of the acid absor3er medium. The saniples are taken at a rate of 0.1 cu. foot of air per 3 mil utes by means of a Mine Safety App1i:uices’ hand pump (16) fitted with the adaptor described below. A single bubbler is sufficient to absorb above 95% of the diisocyanates in the air when the concentration is below 2 p.p.m. Above this concentration about 90% of the isc cyanate concentration is recovered. Preparation of Air-Diisocyanate Mixtures. Since toluenediisocyanates are relatively reactive and are readily adsorbed o 1 equipment surfaces, apparatus of simple design is used to prepare air-diisocyanate mixtures. Depending 0‘1the magnitude of isocyanate concentration, two methods of preparation undw continuous dynamic conditions arc employed to test
the accuracy of the spectrophotometric method. METHODA. This method is used for preparing synthetic samples in the range of 2 to 3 p.p.m. toluenediisocyanate based on a 0.1 cu. foot of air sample. A known weight of toluene-2,4-diisocyanate is volatilized into the absorber medium by directly bubbling a given volume of dry air through a weighed sample of toluenediisocyanate a t a known temperature. During an 8-hour standardization period, the micrograms of sample delivered per 100 ml. of dry air per minute can be duplicated with less than 1% relative deviation. METHODB. This method is used in the range of 0.1 to 0.5 p.p.m. of toluenediisocyanate. An appropriate volume of pure, scrupulously dried air is continuously mixed under dynamic conditions of gas flow with a second stream of known air-diisocyanate composition (Method A). Air samples (0.1 to 0.2 cu. foot) of known toluenediisocyanate content from this mixing system are bubbled through the absorber medium and subsequently analyzed. The gas mixer assembly consists of a 6-foot, 1/4-inch outside diameter coiled copper tube fitted with pressure and volume regulators. For quantitative toluenediisocyanate recovery, the apparatus should be allowed to condition continuously 60 to 72 hours with the airtoluenediisocyanate mixture to be used for sample preparation. Calculations for Sample Analysis. At 25’ C., 760-mm. pressure, and with a 10% safety factor
?V X 131.0 X 1.1 V p.p.m. of toluenediisocyanate in air
where
V
total sample volume expressed in milliliters 1V = weight of toluenediisocyanate expressed in micrograms present in sample 8. =
Apparatus. A Beckman RIodel B spectrophotometer and accessories are supplied by Beclrinail Instruments, Inc., South Pasadena, Calif. However, any reliable photoelectric colorimeter or spectrophotometer may be used. Cells, 1-em. and &em. matched quartz cells. Impingers, graduated, all-glass, flask and nozzle assembly, Catalog No. CT-42513, Catalog 7-B, Mine Safety Applianccs Co., Pittsburgh 8, Pa. Pump, hand, air sampling, Catalog No. CT-144475 or CT-14502, Catalog 7-B, Mine Safety Appliances Co., Pittsburgh 8, Pa. This instrument should be fitted with a capillary adapter to control the rate of air sampling (Figure 2). Fabricate the adapter from capillary tubing (approximately 7 mm. in outside diameter and 0.95 mm. in inside diameter) cut to about 10 em. in length as required to deliver a sample of 0.1 cu. foot in 3 minutes a t a vacuum of 8 to 10 inches of water indicated on the pump vacuum gage. The adapter is attached to the delivery side of the pump. Capillary adapters may be calibrated to deliver the desired volume of air with a mean deviation of 0.002 cu. foot. Reagents. Analytical grade. Distill solvents and toluenediisocyanate standards prior t o use t o eliminate impurities.
SPECTROPHOTOMETRIC DETERMINATION TO LUENEDllSOCY ANATE
OF
Reagents. Sodium nitrite solution. Dissolve 3.0 grams of sodium nitrite and 5.0 grams of sodium bromide in about 80 ml. of water. Adjust the volume to 100 ml. Sulfamic acid solution, 10% w./v. The dry acid, HS03.NH2, is available from E. I. du Pont de Nemours & Co., Wilmington, Del. N-1-Naphthylethylenediamine dihydrochloride. CloHJVHCH&H2NH2. 2HC1 is available from Icodak Co.. Rochester, N. Y. Eastman No. 4835. Ai-1-Naphthylethylenediamine solution. Dissolve 50 mg. in about 25 ml. of mater. Add 1 ml. of concentrated hydrochloric acid and dilute to 50 ml. with water. Absorber medium. Aqueous solution, 0.4N with acetic and hydrochloric acids. To approximately 600 ml. of water add 35 ml. of concentrated hydrochloric acid (11.7N) and 22 ml. of glacial acetic acid (17.6N). Dilute the solution to 1 liter. Fifteen milliliters is used in each impinger flask. Acetic acid, glacial, 17.GN (99.7y0 minimum). Hydrochloric acid, 11.7NJ specific gravity = 1.1923 a t GO" F.
Toluene-2,4-diisocyanatc, distilled. Boiling point 133.0" C. a t 20 inin. of mercury. Procedure. AIR SAMPLEAXALYSIS. Glass impingers are used to secure appropriate air samples as described above. After sampling is complete, loosen the inner bubbler tube, gently tap the tube against the side of thebubbler cylinder to remove adhering absorber solution, and finally remove the inner bubbler completely. To a clean bubbler cylinder add 15 ml. of acidified absorber medium and carry this blank through all steps of the sample analysis. T o each bubbler cylinder add 0.5 ml. of 3% sodium nitrite solution, gently agitate, and sllo\T the solution t o stand. After about 90 seconds add 1 ml. of 10% sulfamic acid solution, agitate, and allow the solution to stand about 2 minutes to destroy all the excess nitrous acid present. Add 1 ml. of 0.1% N naphthylethylenediamine solution. Agitate and allow the color t o develop. After about 5 minutes color development is complete. A reddish blue color indicates the presence of toluenediisocyanate in the original air sample. Add mater t o adjust the final volume to 20 ml. in the bubbler cylinder. Use the blank solution to adjust the spectrophotometer to 100% transmittance and determine the % transmittance of the sample a t 550 mfi in a 1-cm. or 5-em. quartz cell. From a previously prepared calibration curve read the micrograms of toluenediisocyanate corresponding t o the sample transmittance and calculate the parts per million toluenediisocyanate in the sample as indicated above. CALIBRATION.Standard Toluene2,4-diisocyanate, Solution A. Carefully weigh 200 to 255 mg. of pure toluene-2,4-diisocyanate into 6GO ml. of glacial acetic acid. Agitate the solution to dissolve the diisocyanate. Immediately dilute t o 1liter n-ith miter in a glass-stoppered volumetric flask. This solution should be used within 15 minutes after final dilution to prepare solution B. STANDARD TOLUENC-2,4-DIISOCY.4NATE, SOLUTION B. Transfer an appropriate aliquot of Solution A to a glassstoppered 1-liter volumetric flask to contain 2.2 mg. (2200 y) of toluene-2,4diisocyanate. Add a sufficient volume of 8.8N acetic acid so that when solution B is diluted to 1000 nil. it will be 0.6N with respect to total acetic acid. Dilute the volume t o one liter with water. One milliliter of solution B contains 0.0022 mg. (2.2 y) of toluene-2,4-diisocyanate. To a series of eight graduated absorber cylinders, add 5 ml. of 1.2N hydrochloric acid. Add 10.0, 9.5, 9.0, 8.0, 7.0, 6.0, 5.0, and 0.0 ml. of 0.6N acetic acid. Next add 0.0, 0.5, 1.0, 2.0, 3.0, 4.0, 5.0, and 10.0 ml. of solution B, respectively, containing 0.0, 1.1, 2.2, 4.4, 6.6, 8.8, 11.0 and 22.0 y of toluene2,4-diisocyanate. The solution which contains no diisocyanate is a blank for the standardization of the spectrophotometer. T o each cylinder add 0.5 ml. of 3y0 sodium nitrite solution and continue as indicated above under Air
Sample Analysis. Finally, .prepare a calibration curve by plotting transmittance a t 550 mp against micrograms of toluene-2,4-diisocyanate on semilogarithmic paper. Experimental. A number of variables were investigated during process development. ABSORBIKG RIEDIUM. A dilute aqueous solution 0.4N in hydrochloric and in acetic acid;; is an efficient absorber medium for absorbing diisocyanates in the 0 to 25 p.p.m. range. The acetic acid is used as solvent for toluene-2,4-diisocyanate during the standardization of the method. Under the suggested conditions of the analysis, interference from the formation of carbamyl chloride (7), urea (18), and amide (17) has not been detected. An organic solvent such as dioxane tends to peroxidize readily during air sampling to cause 60 to 80% final color destruction. Aqueous sodium hydroxide (0.1 to 0.5iV) is also an effective absorber medium' (28). A disadvantage is that the base must be neutralized with hydrochloric acid before continuing diazotization. ACETICACID COXCENTRATION. Quantitative recovery of toluene-2,il-diisocyanate is obtained when the acetic acid concentration in the absorber medium is 1N or less during diazotization and color development. Higher concentrations of acetic acid tend to prevent full color development-for example, in. the presence of 6 N acetic acid only about 81% of the toluenediisocyanate is recovered. SODIUM NITRITE CONCENTRATION. A final nitrous acid concentration of 0.008 to 0.017iLI produces quantitative isocyanate recovery. When diazotization occurs in solutions containing 0.004 ill nitrous acid, results are about 3% relative lon-. Large excess of nitrous acid has been found by Schwalbe and Hantzsch to be harmful to the stability of diazo solutions (81). SULFARIIC ACIDCONCENTRATION. The destruction of excess nitrous acid prevents an interfering color due to diazotization and self-coupling of the AT-1naphthylethLylenediamine. I n order to ensure complete rapid destruction of excess nitrous acid at least a twofold molar excess of sulfsmic acid is suggested. Apparently the use of urea to destroy nitrous acid has no advantage. TIMEOF DIAZOTIZATION. A diazotization period of a t least 1 minute recovers above 95% of diisocyanate present. A period of half a minute tends to yield only 85 to 88% of diisocyanate. A longer reaction period (15 minutes11 did not increase isocyanate recovery. SODIUME~ROMIDE. Sodium bromide acts as a reaction catalyst. Diazotization performed in the absence of sodium bromide yields results that are as much as 30 to 40tG relative low. Increasing the sodium bromide concentration is of no value. TIME OF' COLOR DEYELOPXEXT. Color develcipment due to the coupling of the diazotized amine from tolueneVOL. 29, NO. 4, APRIL 1957
* 553
diisocyanatc with N-l-naphthylethylenediaminc is complete in 3 minutes a t 26-27" C. in the presence of 0.4iV hydrochloric acid and 0.0002M diamine reagent. Doubling amine concentration tends to accelerate color development by about 30 seconds. COLORSTABILITY.After full color development with samples containing about 0.3 p.p.m. of toluenediisocyanate, no detectable change in transmittance occurs after standing 30 minutes in daylight. After 3 hours, a slight decrease in color equivalent to 0.5% transmittance is found. ABSORBER EFFICIENCY. To test absorber efficiency, two inipingers are used in series on the gas mixer during the analysis of known mixtures. At the suggested sample rate of 0.1 cu. foot of air per 3 minutes and with air containing 0.3 p.p.ni. of toluene-2,4diisocyanate, a single bubbler retains better than 95% of the diisocyanate actually present. At the 25 p.p.m. toluenediisocyanate level, a single bubbler retains about 90% of the diisocyma t e. SPECTRAL CHARACTERISTICS OF REDDISH BLUE COLOR. Figure 1, curve A , shows the spectrum of the color derived from a mixture of 31.7 y of toluene-2,4-diisocyanate and 16.9 y of toluene-2,6-diisocyanate. Curve B is the absorption spectrum of the color obtained with toluene-2,4-diisocyanate alone. The absorption maximum obtained in both cases is a t approximately 550 nip, I n solutions containing only the toluene-2,6-diisocyanateJthe absorption spectrum shifts to 555 to 560 mp. All analytical determinations have been performed a t 550 mp. STANDARD AIR-TOLUEXEDIISOCY.4NaTE A~IXTURIZ PREPARATION. The direct volatilization of toluenediisocyanate into a controlled stream of dry air is acceptable for the preparation of various mixtures of toluenediisocyanate in air. With the use of 10 to 25 ml. of toluenediisocyanate in a fritted-glass bubbler, the loss of toluenediisocganate a t a particular temperature and gas rate is accurately standardized for the production of air-toluenediisocyanate mixtures. For the production of mixtures in the range of 2 p.p.m. of diisocyanate, this technique may be used directly. In the 0.3 p.p.m. of diisocyanate range, a single or two-stage mixer may be employed. A few precautions should be eniphasized when an air mixer is employed. To obtain quantitative toluenediisocyanate recovery, the air used for sample preparation should be thoroughly dried, as with phosphorus pentoxide. When ordinary undried air is used, the toluenediisocyanate recovered is usually 10 to 30% low. This may be due to the formation of toluenediamine, a carbanilide, or some other hydrolysis product that may coat the inside surfaces of the apparatus. Furthermore, the air mixer should be allowed to equilibrate for about 72 hours to overcome adsorption effects. Chilton and Genereaux (9) found that complete gas mixing may be attained in a short tube of narrow diameter. A tube 3 to 6 feet 554
ANALYTICAL CHEMISTRY
.
Figurc? 1 Absorption spectra of diazotized toluenediamines coupled to N-1-naphthylethylenediamine A. 21.7 y of toluene-2,4-diisocyanate and 16.9 y of tolucme-2,6-diisocyanate per 20 ml. B. 47.6 y of toluene 2,4-diisocyanate per 20 ml. -.-cm. cells
long and inch n inner diameter is satisfactory for mixing air and gaseous toluene-2,4-diisocy~nate a t a total gas flow rate of 0.1 to 0.3 cu. foot per minute.
Results. The reliability of the diazonietric methcmd mas evaluated a t two levels of toluc~ne-2,4-diisocyanate concentration in air. Samples a t the 0.3-p.p.m. level mme prepared with a continuous air mixer described above. Samples a t the 2-p.p.m. diisocyanate level were prepartd by volatilizing a known quantity of tol~iene-2~4-diisocyanate from a calibrated fritted-glass bubbler into a known colume of air flowing a t a rate of 100 ml. per minute a t 27' 1' C. The analyiical results obtained with the suggested spectrophotometric method are preseried in Table I. To test the procedure with commercially availabIe mixtures just above 0.1 p.p.m., solutions of Hylene T3I (80% toluene2,4-diisocyanate a i d 20% toluene-2,6diisocyanate) and also solutions of Hylene T M 65 (65'3, toluene-2,4-diisocyaiiate and 35% tolu ene-2,6-diisocyanate) mere analyzed by the suggested procedure a t the 0.21 p.p.m. level. The data are presented in Table 11. Discussion. Table I indicates that the suggested c iazometric method yields accurate results with synthetic air mixtures a t both the 0.3- and 2.5p.p.m. toluenediisclcyanate levels. The precision of the method expressed in terms of the confidence interval, L, for average of duplica1,e determinations a t a probability level of %yois 0.022 and 0.26 p.p.m. a t th: 0.3 and 2.5 p.p.m. tol~ene-2~4-diisocyanatelevels, respectively. At the lower level tested, the accuracy of the method is better than 96'% of the quantity of diisocyanate present. At the :'.5-p.p.m. level, as indicated in Table 1, the accuracy of the method is within the confidence interval indicated for p-ecision. As seen from Table 11, although satisfactory recovery is obtained for both Hylene T M and Hylene TM 65 mix-
tures, somewhat lower recovery of total toluenediisocyanate is obtained as toluene-2,6-diisocyanate increased in the mixture. All results presented in Table I1 mere determined with the aid of a standard curve prepared for toluene2,4-diisocyanate. I n any investigation where Hylene Til1 or Hylene TM 65 are being analyzed predominantly, a standard curve should be prepared with the
Table 1. Determination of Toluene2,4-diisocyanate by Spectrophotometry Toluene-2,4-diisocyanate, P.P.M.
Sample PresNo. ent 1 0.32 2 3 4 5 6 7 8
9
10 11
12
13 14 15
z
=
5 si
PresFound ent Found 0.32 2.51 2.46 2.52 0.29 2.33 0.31 2.52 0.29 2.79 0.29 2.43 0.32 2.52 0.29 2.70 0.32 2.75 0.32 0.32 0.33 0.33 0.32 0.32 0.31 0.31 2.56 3 = 0.31 S = 0.014 0.16 L = 0.022 0.26 P = 05% =
mean
i=l N
S = standard deviation
L = f S hp = confidence interval N for average of duplicate determinations ( N = 2) at aprobability level P (4) P = probability level, y', (95%) Tloo-, = "Student's t" as a significance level 100 - p (corresponding to a confidence interval of P = 95yo.) N = number of determinations
Figure 3. Portable chemical kit for analysis of toluenediisocyanate in air Figure 2.
Midget impinger and hand pump
Table II. Determination of Mixtures of Toluene-2.4-diisocyanate and Toluene-2,6-diisocyanote in Solution Hylone T M Hylene TM 65 Present, Found, Present, Found, p.p.m. p.p.m. p.p.m. p.p.m. 0.24 0.22 0.24 0.19
5 =
S = L =
P
=
0.24 0.23 0.23 0.22 0.23 0.23 0.23 0.23 0.23 0.0061 0.010 95%
0.21 0.19 0.19 0.21 0.19 0.20 0.21
0.20 0.01 0.017
appropriate mixture to attain maximum accuracy. At room temperature under the conditions of the analysis, the urea derivative (3,3’-diisocyanato-4-4’-dimethylcarbanilide) that may be formed from toluene-2,4-diisocyanate in the presence of moisture does not give the typical reddish blue color. Aromatic amines such as aniline present in the atmosphere interfere in the analysis by yielding high results. Phenol did not interfere. Potential contaminants present in the atmosphere should be tested individually to determine the extent of interference under the conditions of rapid diazotization employed in the suggested procedure. The sensitivity of the diazometric method may be appreciably increased if a sample size of 1 cubic foot of air and 5-cm. optical cells are employed. PORTABLE FIELD KIT FOR DETERMINATION O F TOLUENEDIISOCYANATE
A method for monitoring the prrs-
ence of minute traces of toluenediisocyanate (
