Collection and Estimation of Traces of Formaldehyde in Air d
F. H. GOLDMAN AND HERhIAN YAGODA Division of Industrial Hygiene, National Institute of Health, Bethesda, Md.
F
ORMALDEHYDE is one of the common contaminants of the industrial environment. I t s presence in the air constitutes a potential health hazard. Maximal allowable concentrations for its presence in air continue to be formulated and this, of course, necessitates accurate methods for its estimation. I n general, small amounts of gaseous contaminants in the atmosphere are most efficiently collected for analysis by bubbling through a medium that reacts with the vapor to form a nonvolatile compound. This fixation mechanism prevents the expulsion of the gas during the sampling by the aerating action of the unabsorbed constituents. As is well known, formaldehyde gas reacts rapidly with aqueous solutions of sodium bisulfite to form the nonvolatile sodium formaldehyde-bisulfite compound : HCHO
+ SaHS08-,H2C,OH
'
OS02xa
This derivative is stable in slightly acid and neutral solutions and can be decomposed only when the solution is made distinctly alkaline. These properties , permit the direct estimation of small amounts of formaldehyde by destroying excess bisulfite with iodine a t p H 6 to 7, and subsequently liberating the sulfite combined as sulfoxylate by proper adjustment of the hydrogen-ion concentration. The direct titration of this dissociated bisulfite with a standard iodine solution affords a n accurate measure of the formaldehyde collected, which is independent of change in the bisulfite-ion concentration caused by atmospheric oxidation of the collection medium. Analytical methods for different aldehydes based on the formation of sulfoxylate have been described by Clausen (S), Donnally ( 4 ) ,Edwards (j), and Friedemann (6). These methods were devised chiefly for the determination of the acetaldehyde-bisulfite derivative formed in the assay of lactic acid. The principal difficulty in the titration of the formaldehyde sulfoxylate is its slow rate of decomposition in slightly alkaline solutions. While readily decomposed by caustic solutions, the latter interfere with the iodometric determination of the liberated sulfite. Investigation revealed that the formaldehyde complex could be hydrolyzed instantaneously at room temperature by the addition of a solution of sodium carbonate and sodium acetate, and that the resulting solution could be titrated to a stable stoichiometric end point. The formation of bisulfite addition compounds is a characteristic reaction of aldehydes and ketones. A method based on this reaction is not subject to interference by sulfur dioxide, alcohol, the halogen gases, and volatile organic acids which interfere with the determination of formaldehyde using Schiff's reagent ( I ) . The method, as proposed, assays total aldehyde vapor. In the examination of plant atmospheres for toxic constituents, the identity of the vapors is usually available to the analyst from knowledge of the manufacturing procesq. On rare occasions n here the formaldehyde is accompanied by acetone, a separation can be effected i bicarbonate. The acetone derivative
composed in media containing a low concentration of sodium bicarbonate, whereas the formaldehyde-bisulfite is dissociated only when the solution is macle alkaline with sodium carbonate.
Method of Analysis REAGENTS.1 per cent sodium bisulfite, SaHS03. 0.1 N iodine solution. This reagent need not be standardized. 0.01 N iodine, prepared by dilution of 0.1 N iodine solution. This solution must be standardized daily by titration against 0.05 N sodium thiosulfate. 1 per cent starch solution. Sodium carbonate buffer prepared by dissolving 80 grams of sodium carbonate in about 500 ml. of water, adding 20 ml. of glacial acetic acid, and diluting to 1 liter. METHOD. The air is drawn through a midget impinger (8) containing 10 ml. of 1 per cent sodium bisulfite solution at a rate of 1 to 3 liters per minute. At the termination of the sampling the bisulfite solution and washings are transferred to a 300-ml. cone flask. When sampling a t 28 liters per minute the large impinger (8) is filled with 100 ml. of bisulfite solution and a 10-ml. aliquot is removed for analysis. The solution is titrated with the 0.1 N iodine solution to a dark blue end point, using 1 rnl. of starch as an indicator. Excess iodine is destroyed by the addition of 1 or 2 drops of 0.05 N sodium thiosulfate, and the solution is equilibrated to a faint blue color by the dropwise addition of the 0.01 N iodine solution. The excess inorganic bisulfite is thereby completely oxidized to sulfate, and the solution is noJ7 ready for assay of the formaldehyde-bisulfite compound. Twenty-five milliliters of the sodium carbonate reagent are added and the liberated sulfite is titrated with 0.01 N iodine solution to a faint blue end point. If acetone is present, the solution is treated with 2 ml. of 5 per cent sodium bicarbonate after the destruction of the free inorganic sulfite in the collection medium. The sulfite liberated by the bicarbonate is removed by addition of 0.01 Ai iodine solution and the pH is finally adjusted by means of the sodium carbonate solution for the final titration of the formaldehyde-bisulfite. When the volume of solution used for the titration is less than 1 ml. it is desirable to run a blank on 10 ml. of 1 per cent sodium bisulfite. This blank correction does not exceed 0.10 ml. and is negligible in most assayd. One milliliter of 0.01 N iodine solution is equivalent to 0.15 mg. of formaldehyde. The accuracy of the method was tested by adding measured volumes of a standardized formaldehyde solution to 10-ml. portions of 1 per cent sodium sulfite and analyzing the mixture by the described procedure. The standard formaldehyde solution preparel by diluting the 37 per cent commercial solution to a concentration of about 1 mg. per ml. was assayed by the method of Romijn ( 7 ) . The results shown in Table I exhibit good agreement between the formaldehyde taken and recovered.
By using a microburet reading to 0.01 ml. and delivering droplets of about 0.03 ml. i t is possible to establish the two end points with a maximum error of 0.06 ml. and this introduces a possible error of 1 per cent in the volumetric estimation of 1 mg. of formaldehyde. With smaller quantities of formaldehyde, the percentage error increases rapidly, and it is desirable to sample a sufficient volume of air t,o provide at least 0.5 mg. of formaldehyde for the analysis. I n the case of atmospheres that contain formaldehyde concentrations of about 20 parts per million, approximately 20 liters of air must be sampled for each 10 ml. of bisulfite solution in the impinger. The formation of the formaldehyde-bisulfite is not interfered with by the concomitant presence of small amounts of methyl alcohol, bromine, or acetic acid, lvhich may on occasion be collected in the sodium bisulfite solution. This was tested by adding the substances shown in Tnhle I1 to 10 ml. of 1 per cent sodium bisulfite containing 1.03 ing. of
377
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
378 TABLEI.
-4N.ILYSES
Formaldehyde Taken
(UTIONS
F
Difference
Me.
Me.
0.021 0.053 0.106 0.216 0.426 0.638 1.064
f0.003 +0.005 f0.004 4-0.004 1-0.006 -0.013 +0.021 -0,006 -0.03
1.276
1.49 2.13 3.19 5.32
-0.09 -0.18 -0.08
TABLE 11. EFFECT OF OTHER VAPORS OS ESTIX4TION OF FORNALDEHYDE Formaldehyde Taken
Me. 1.03 1.03 1.03 1.03 1.03
-Foreign Mp.
Vapor-
Vol. 15, No. 6
in the formaldehyde vapor concentration. The 95 per cent absorption in 1 per cent sodium bisulfite is in good agreement with the value of 93.6 per cent reported by Barnes and Speicher (2) using 1.25 per cent potassium hydroxide as the collection medium, and the same air flow of 28 liters per minute.
Summary Formaldehyde vapors can be collected quantitatively in a single impinger tube containing 1 per cent sodium bisulfite. The resultant solution is analyzed volumetrically by titrating the formaldehyde-bisulfite by means of a standard iodine solution. The simplicity of the procedure provides a convenient method for field work.
Formaldehyde Recorel ed
MQ. None None
1.02 1.04 1.05 20 Methyl 8!Cohol 50 Acetic acid 1.05 10 Bromine 1.02 50 Bromine 1.00 1.03 1.01 10 Acetone5 1.03 a Acetone-bisulfite dissociated by means of sodium bicarbonate, prior t u hydrolysis of formaldehyde sulfoxylate.
formaldehyde and analyzing the mixtures by the proposed method. I n the absence of other aldehydes the use of bisulfite for the collection medium affords a simple and rapid method for the estimation of the formaldehyde.
TABLE111. COMPARATIVE EFFICIENCY OF SODIUM BISULFITE MEDIAFOR THE COLLECTION O F FORMALDEHYDE
AND WATER .4S
Formaldehyde Concentration P. p . m. 7 21 101 7 20 78 7 14
Air Flow
L./min. 1
1 1 3 3 3 28 28 28
Collected by 1st Bubbler 1% NaHSOs Water
%
“0
100 97 98 96
77 79 87 68 74 81 69 72
41
97 98
95 95
95
76
Literature Cited Efficiency of Bisulfite as a Collection Medium The retentive power of sodium bisulfite was studied by blonring air through dilute solutions of formaldehyde and passing the vapors, freed from spray by means of a glass wool plug, through a train of two impingers. Both impingers contained sodium bisulfite, and in each case a similar experiment was conducted in which the vapor was first bubbled through water and then through a second impinger containing 1 per cent sodium bisulfite solution. After analysis of the four solutions it was possible to compare the relative collecting efficiencies of water and the bisulfite solution under similar conditions of air flow and formaldehyde vapor concentration. Preliminary tests in which known amounts of dilute formaldehyde solutions were completely volatilized by a stream of dry warm air and collected in bisulfite revealed that the vapor was recovered quantitatively, and that a t a flow of 3 liters per minute, 97 per cent was caught in the first impinger. On the basis of these experiments, it is safe to calculate the efficiency as the ratio of the weight of formaldehyde collected in the first impinger to the total weight found in the train of collecting tubes. Reproducible rates of air flow were secured in the case of the slow speed measurements with the aid of a large Mariotte flask. It was observed that formaldehyde vapor was strongly absorbed by rubber tubing. To avoid loss of formaldehyde from this source, impingers were assembled for the testing work that have the entrance and exit tubes in a straight line with abutting joints. This permits glass-to-glass contact between adjoining impingers, using little or no exposed rubber surface. The results of these experiments (Table 111) show that a t slow rates of air flow (1 to 3 liters per minute) about 98 per cent of the formaldehyde is collected in a single tube of bisulfite solution, and that even a t a flow of 28 liters per minute the 95 per cent recovery makes the use of a singIe impinger tube permissible. Under the same conditions water retains between 69 and 87 per cent of the formaldehyde vapor. The results also reveal that the collection efficiency increases with increase
(1) Ackerbauer,
C. F., and Lebowich, R. J., J . Lab. Clin, N e d . , 28,
372 (1942). (2) Barnes, E. C., and Speicher, H. W., J . Ind. Hug. To+.,24, 10 (1942). (3) Clausen, S. W., J . B i d . Chem., 52, 263 (1922). (4) Donnally, L. H., IND. ENG.CHEM.,ANAL.ED.,5, 91 (1933). (5) Edwards, H. T., J . Bid. Chem., 125, 571 (1938). (6) Friedemann, T. E., Contonio, M.,and Shaffer, P. A,, Ibid., 73 335 (1927). (7) Romijn, G., 2.anal. Chem., 36,19 (1897). (8) Jacobs, M. B., “Analytical Chemistry of Industrial Poisons. Hazards and Solvents”, New York, Interscience Publishers 1941.
Furfural (Correspondence) SIR: The article on furfural by Ira J. Duncan [IND.EXG. CHEM.,ANAL.ED.,15, 162 (1943)l brought to mind some work I once did on preparation of furfural from ground corncobs. I also observed the decomposition of furfural by hot dilute acid and reasoned that considerable furfural is probably destroyed during the hydrolysis of the pentosans before it is liberated from the hot acid. If the concentration of furfural in the acid can be reduced, yield should increase. One way t o accomplish a reduction in furfural concentration in the reaction mixture is by the addition of sodium chloride. When large quantities of salt were added to the acid, it was found that the yield of furfural could be increased from about 10 per cent t o about 15 per cent of the weight of the cobs, indicating a considerable destruction of furfural in ordinary hydrolysis. Perhaps some use could be made of this technique in analytical determinations. VANDERVEERVOORHEES Standard Oil Co. (Indiana) Chicago, Ill.
