Detection and Estimation of Low Concentrations of Aldehyde in Air ERNEST E. HUGHES and SHARON G. LIAS National Bureau o f Standards, Washington 25, D. ,Aldehyde is adsorbed from an air stream on purified silica gel. The gel is then treated with a solution of pphenylenediamine and hydrogen peroxide. Oxidation of the p-phenylenediamine is catalyzed by aldehydes and the depth of color produced on the gel by the oxidation products is a measure of the quantity of aldehyde adsorbed. The method is simple and requires little equipment. Quantities as low as mole % can b e easily detected. The method can b e readily adapted for field use.
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are among the principal organic pollutants in the atmosphere ( 1 ) . Because aldehydes may plav a part in reactions in polluted air, there is an interest in their detection a t low concentrations. The most successful procedures now used in monitoring atmospheric aldehydes are based on the Goldman-Yagoda bisulfite method (3), which is precise and accurate but requires considerable time and equipment. The method is impractical for the detection of very low concentrations, because the volume of gas required is prohibitively large. The procedure described in this paper is not as accurate as the bisulfite method, but it is simpler and moFe convenient for detecting aldehydes a t very low concentrations. The method is ideally suited for the semiquantitative measurement of concentrations of aldehyde as low as 0.1 part per billion (p.p.b.) by volume. Aldehydes increase the rate a t which phenylamines are oxidized by hydrogen peroxide. This effect was first described by Woker ( 5 ) . Feigl made use of the effect in the oxidation of pphenylenediamine by hydrogen peroxide as a spot test for aldehydes (a). The oxidation product is a black precipitate known as Bandron ski’s base. The test described is an adaptation of this reaction to the determination of aldehyde a t low concentrations in gases. The aldehyde is concentrated from the gas stream on a column of silica gel which is subsequently treated with a mixture of p-phenylenediamine and hydrogen peroxide. The oxidation product forms on the surface of the gel and the depth of the color of the stain is
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an indication of the amount of aldehyde adsorbed on the gel. REAGENTS
A saturated solution of p-phenylenediamine in water is mixed with 3y0 hydrogen peroxide in the ratio of 1 to 2 immediately before use. Only about 3 ml., enough for one test, should be mixed a t one time. The silica gel was prepared according to the directions outlined by Shepherd ( 4 ) . However, any clean gel, free of the tan color due t o oxides of iron, should be satisfactory. The gel should be protected from contamination by mater vapor and aldehyde. A granular size between 30 and 50 mesh is best.
LDEHI’DES
PROCEDURE
The silica gel on which the aldehyde is adsorbed is contained in glass tubes having an internal diameter of about 5 mm. and a length of 7 em. The tubes are packed by inserting a wad of cotton in one end and filling them about half full of gel. KOpacking is plac,ed on the gel a t the inlet end. The tubes are fixed in a vertical position and the air sample is drawn through the gel with a syringe or pump. The remainder of the tube is then filled with clean silica gel and is closed with a second cotton plug. The tube is maintained in the vertical position during this operation and care is exercised to avoid disturbing the exposed gel while adding the unexposed gel. The gel added after exposure serves as a filter to remove the dark oxidation products which are always present in the developing reagent. Without this filter, it is impossible to distinguish between the color of the reagent impurities and the color of the oxidation product. The tube is then dipped into the developing reagent, inlet end down, and the liquid rises through the silica gel by capillary action. A dark band caused by the impurities in the reagent appears a t the inlet end of the tube, and, in the case of a positive test for aldehyde, a second dark band appears a t the boundary of the two layers of gel. GAS MIXTURES
The gas mixtures used for evaluating the method contained acetaldehyde or formaldehyde. Low concentrations were obtained by successively diluting mixtures having known high concentretions of the aldehyde, The method
for preparing gns iiiixtures is described in detail by Shepherd (4). The concentration of aldehyde in each mixture was determined by analysis until the concentrations n-ere below the limit of detection. Beyond this limit, the concentrations were calculated from dilution ratios. The mixtures were contained in stainless steel cylinders. KOevidence of any significant loss of aldehyde through adsorption or reaction was noted over the concentration range covered by the independent analyses. COMPARISON STANDARDS
Aldehydes speed the oxidation of p phenylenediamine, but the reaction proceeds a t a measurable rate in the absence of aldehyde. In the presence of aldehyde, the rate of reaction is directly proportional to aldehyde concentration. The rate of formation of the oxidation product in several liquid reaction mixtures was follon-ed by measuring the transmittance at 425 inp a t various times during the reaction. I t was found that at any fised time from the beginning of the reaction, the amount of oxidation product formed is proportional to the aldehyde present. For this reason, exposed tubes must be compared after the same time interval. A convenient interval is 30 seconds, as the tubes darken rapidly, and after about a minute are too dark to read. Because the tubes must be conipared a very short time after exposure, it is impractical to compare them with tubes exposed to known concentrations of aldehyde. However, simple comparison standards can be made by filling the tubes with silica gel dyed to simulate the appearance of exposed tubes. The difference between tubes esposed to quantities of aldehyde a half order of magnitude different in n eight is substantial. Consequently, the standards need not be identical to exposed tubes, but can simply approximate their appearance. The band a t the junction of the gel layers in a tube esposed to aldehyde varies from a pale purple-bronn to B deep purple-brown or black. The filtering gel layer a t the inlet end of the tube and the gel beyond the dark band a t the junction are both stsined a light purplebrown. To approximate this appearance, tubes were packed with dyed VOL. 32, NO. 6. MAY 1960
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Table
Mixture
I.
Comparison of Developed Tubes with Color Standards
Aldeh de Conon., %.P.M. by Volume
2.0 2 0.9 2.0 1 0.12 3 0.12 3 0.12 3 4 0.03 4 0.03 4 0.03 0.12 3 0.054 5 0.12 3 5 0 054 0.005 6 6 0.005 7 0.0014 3 0.12 6 0.005 8 0.0008 8 0.0008 9 0.0001 Volume sampled at 700 ml./min. 1
I
a
VOl. of Sample, Litera0 2.8 2.8 0.7 4.2 2.5 2.1 7.0 5.0 3.75 0.7 1.4 0.35 0.7 3.6 2.8 10.0 0.07 1.4 8.8 2.0 9
Moles Aldehyde
x
108
0.23 0.10 0.057 0 020 0,011 0.010 I
0.0086
0.0066 0.0045 0.0034 0.0032 0.0017 0.0015 0.00073 0.00057 0.00057 0.00032 0.00030 0.00030
0.000066 0.000036
Standard I I I I1 I1 I1 I1 I1 I11 I11 I11 I11 I11 I11
IV IV IV IV IV V V
Propionaldehyde, butyraldehyde, cinnamaldehyde, phenylacetaldehyde, vanillin, and acrolein were tested qualitatively by sampling atmospheres containing small quantities of the aldehyde. Only vanillin failed to give a positive test. The colors produced by these aldehydes in neutral reagent solution are very similar to the colors produced by formaldehyde and acetaldehyde. The quantitative sensitivity of the test to these other aldehydes should be similar to that for formaldehyde and acetaldehyde. The minimum quantities of different aliphatic aldehydes detected by Feigl differed very little, with the exception of propionaldehyde. Our results, however, indicate that the sensitivity of the test to propionaldehyde is actually similar to the sensitivities for other aliphatic aldehydes. The test is less sensitive to aromatic aldehydes by about one order of magnitude. INTERFERENCES
purple gels. A light purple gel was used for the filtering layer and for the gel beyond the band at the junction, while various darker shades and thicknesses of purple gel were used a t the boundary layer. The tubes were then filled with a solution of a reddish brown dye. The color and texture produced in this way closely resembled exposed tubes. Any purple coloring may be used which approximates the color of the gel in the exposed tubes. Purple drawing ink was satisfactory. A solution of alizarol brown was used for the red-brown dye. A series of tubes containing dyed gels of progressively darker color is prepared. Tubes are then exposed to known concentrations of aldehyde, and after 30 seconds, or other time interval, the tubes containing dyed gel are matched with the exposed tubes. A complete set of standards is selected in this way. EXPERIMENTAL RESULTS
Five dyed-gel tubes were prepared. The color of each tube approximated the range of colors corresponding to a concentration interval of about one order of magnitude. Various volumes of several acetaldehyde mixtures were passed through tubes, which were then developed and compared with the standard dyed tubes exactly 30 seconds after the
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ANALYTICAL CHEMISTRY
addition of the reagent. The standard which most closely resembled the exposed tube is shown in the last column of Table I. Approximate limits, expressed in terms of the moles of aldehyde, were set for these five standards. Thus, for a specified volume of sample, any standard represents a fixed concentration range. The median values for the five standard dyed tubes are shown below. Approximate Mole of Aldehyde Standard Represented by Tube Standard Tube I 10-7 I1 10 -8 10-0 I11 IV 10-10 V 10-11 The limits of sensitivity are determined by the number of standard tubes available for comparison. The results shown in Table I indicate the possibility of distinguishing differences in weight of aldehyde as small as one half order of magnitude even when the total quantity of aldehyde is as low as 10-11 mole. The volume of sample required to detect concentrations as small as 0.0001 p.p.m. is less than 10 liters. Flow rates a~ high as 0.7 liter per minute can be used without noticeable loss of aldehyde.
Nitriles, aldehyde ammonia, oximes, and aldehyde bisulfite compounds behave similarly to aldehydes. Oximes and nitriles are much less reactive than aldehydes. Ketones give no’ effect a t all. With the possible exception of nitriles, these interfering substances are not significant air pollutants. All materials in the sampling system ahead of the adsorption tube should be free of aldehyde. Air drawn through certain kinds of plastic tubing contained detectable quantities of aldehyde. Such contamination did not occur when pure gum rubber tubing was used. LITERATURE CITED
(1) Cholak, J., “The Nature of Atmospheric Pollution in a Number of Indus-
trial Communities,” Proceedings 2nd Natl. Air Pollution Symposium, Stanford Research Institute, Los Angeles, Calif., p. 6, 1952. (2) Feigl Fritz, “Spot Tests in Organic Analyds,” 5th ed., pp. 214-15, Elsevier, New York, 1956. (3) Goldman, F. H., Yagoda, H., IND. ENQ. CHEM., ANAL, ED. 15, 377-8 (1943). (4) Shepherd, M., ANAL.CHEM.19,77-81 (1947). (5) Woker, G., Ber. 47, 1024 (1914).
RECEIVED for review November 13, 1959.
Accepted February 25, 1960. Work performed as part of the Air Pollution Program sponsored by the Public Health Service, Department of Health, Education, and Welfare.
