Colorimetric Determination of Carbonyl Compounds in Automotive Exhaust as 2,4-Dinitrophenylhydrazones Louis J. Papa Jackson Laboratory. Organic Chemicals Department. E. I. D u Pont de Nemours & Co., Wilmington, Del. 19898
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A colorimetric technique was developed for determining the total molar concentration of carbonyls in a mixture of their 2,4-dinitrophenylhydrazones (DNPH). The method is based o n the measurement of the colored species formed when alkali is added to a solution of the DNPH's in a 7 0 % pyridine medium which stabilizes the colored product. The minimum detectable quantity is 0.5 X mole and the reproducibility, determined on D N P H standard solutions. is = 1 % relative. The method is applied to the analysis of carbonyls in vapor mixtures and automotive exhaust via collection of the D N P H derivatives in scrubbers. The recovery of the carbonyls by this technique is 98 to 106% and the reproducibility is * 3 % relative.
C
arbonyl compounds-Le.. aldehydes and ketonesare important constituents of automotive exhaust because several of these compounds are known to be toxic. odorous, and are believed to be a cause of eye irritation from photochemical smog. The 2,4-dinitrophenylhydrazine ( D N P H ) procedure reported by Iddles and Jackson ( 1934), and subsequently applied to flame combustion products (Malmberg, 1954) and automotive exhaust (Oberdorfer, 1 9 6 7 ) , appears to offer a good procedure for separating the carbonyls from the bulk exhaust. The gravimetric procedure reported by Oberdorfer (1967) determined the total carbonyl content on an "as formaldehyde" basis. This procedure is time-consuming and inherently inaccurate because the total can only be calculated using the molecular weight of a single carbonyl compound. The molecular weight of the derivatives varies significantly from formaldehyde (210) to tolualdehyde ( 3 0 2 ) . Higher molecular weight species such as bis-dicarbony1 derivatives will, of course. contribute more serious errors. Alternatively an average molecular weight can be used to make the calculation but this depends o n a prior knowledge of the composition. Thus, a better method for determining the total carbonyl content in the derivative mixture was desired. Methods based on direct ultraviolet measurements of the DNPH's have been reported (Lawrence, 1965; Toren and Heinrich. 1955). but it was found that the benzaldehyde. tolualdehyde. and formaldehyde derivatives have substantially different A,,, and e values than the remaining carbonyl derivatives. and these are three of the most important carbonyl compounds in automotive exhaust. Thus, such a method would suffer the same type of limitation as does the gravimetric procedure. Lappin and Clark (1951) reported a colorimetric method for carbonyls based on the addition of alkali to the D N P H derivatives to form a colored resonant ion. These authors reported that the colored product was stable and that a wide variety of carbonyl compounds had an absorp-
tion maximum (480 mp) and molar absorptivity. 6 . which are very nearly independent of structure. Mendelowitz and Riley (1953) reported that the A,,,;,,is not 480 mP and furthermore that it varies with the nature of the carbonyl compound. Pool and Klose (1951 j , Toren and Heinrich ( 1955). Lohman (1958), and Parsons (1966) reported that the color is not stable but fades rather rapidly. The present study has substantiated the work reported by these latter authors-Le.. ,\,llil, is structure-dependent and the final color is unstable under the conditions reported by Lappin and Clark ( 195 1 ) . The formaldehyde derivative is the least stable and decomposes by first-order kinetics 1 minute at room temperature. Howuith a half-life of ever, by changing the solvent medium to make it 70% in pyridine. the final color has been sufficiently stabilized to form the basis of a good method. Also, the c values. at the selected wavelength, of all the carbonyls tested are almost the same. allowing a determination of total carbonyl. The reproducibility of this modified method u a s 1 % relative as determined \\ith prepared standards. The minimum mole. detectable quantity is 0.5 X
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Experinientnl Apparatus. A Beckman Model B spectrophotometer o r equivalent is used to make absorbance measurements. Reagents. The D N P H derivatives are prepared by the procedure of Shriner. Fuson. et al. ( 1 9 5 6 ) . The compounds are recrystallized to a constant melting point. The pyridine and methanol are reagent grade materials purchased from Mallinckrodt and Baker & Adamson, respectively. The methylene chloride ( d u Pont) is commercial grade. These solvents are used as purchased. The D N P H reagent is prepared according to Oberdorfer ( 1 9 6 7 ) . The n-hexane and D N P H reagent are "cleaned" by equilibrating equal volumes together for 2 minutes and passing the ri-hexane through an activated alumina column to remove any D N P H derivatives. The alkaline solution is prepared by dissolving 10 grams of potassium hydroxide (Mallinckrodt analytical reagent) in 20 ml. of distilled water and diluting to 100 nil. with methanol. Calibration Procedure. The D N P H derivative( s ) is dissolved in a mixed solvent of methylene chloride-methanol ( 1 to 1 ) . An appropriate aliquot of this solution is diluted with methanol to obtain a final concentration of D N P H derivatives between 0.05 and 3.0 X 10-7 mole per ml. Then 2.0 ml. of the diluted solution and 7.0 ml. of pyridine are pipeted into a 10-ml. volumetric flask. The solution is diluted to 10 ml. with the alkaline solution. the sample is thoroughly mixed. and the absorbance determined on the spectrophotometer at 440 m p using I-cm. cells. The sample should be read within 3 minutes. Exhaust Sampling. The D N P H derivatives are obtained and isolated from vapor samples and exhaust via a procedure similar to Oberdorfer's ( 1967)-i.e., trio scrubbers Volume 3, Number 4, April 1969 397
in series. The precipitates are first transferred to filter funnels and the methanol (-10 ml.) used to start the dissolution from the filter funnel is first used to wash the scrubber units. Successive 10-ml. volumes of methylene chloride are used to complete the transfer and dissolution of the derivatives. The mother liquor ( D N P H reagent) is extracted twice with 10-ml. volumes of n-hexane. The hexane washings and methylene chloride-methanol solution of the derivatives are combined in a 100-ml. volumetric flask and the mixture is diluted to the mark with methanol. This solution is appropriately aliquoted to obtain a concentration within the calibration range and run through the procedure described above (calibration procedure). Results and Discirssion
The modification of Lappin and Clark’s (1951) original technique is the change in solvent medium. The presence of 70% pyridine stabilizes the final colored species (for at least 15 minutes) for all of the carbonyl derivatives tested except that of formaldehyde. This derivative still decomposes in this medium but at the reduced rate of -0.3% relative per minute. Thus, no significant losses are incurred if the sample is read soon after the alkali is added. As the concentration of pyridine in the solvent mixture is
Table I.
molar Absorptivities, €, of DNPH Derivatives at 440 m p Compound (as DNPH) t X 10.4 (Liter Mole-’ Cm:’)
Formaldehyde Benzaldehyde p-Tolualdehyde Acetaldehyde Propanal Acetone Acrolein 1-Butanal 2-Butanone ’-Butanal Crotonaldehyde 2-Methylbutanal 3-Methylbutanal 1-Pentanal 1 -Heptanal ?-Ethylhexanal Glutaraldehyde
2.12 2.11 2.07 2.43 1.32 2.22 2.39 2.20 2.23 2.25 2.64 2.36 2.36 2.24 2.45 2.43 4.54
f
Table 11. Carbonyl Recoveries from Vapor Mixtures Total % Recovered Carbonyl Without With Present, Sample ‘‘
A B C
Moles
x
104
back extraction
back extraction
92 97 96
99 105 103
2.9 18 3.227 3.550
’’ Legend: A = 90% formaldehyde, 2% 1-butanal, 2% 1-pentanal, 3 % benz-
aldehyde, 3% tolualdehyde. B = 80% formaldehyde, 3% 1-butanal, 3 % 1-pentanal, 7 % benzaldehyde, 7 % tolualdehyde. C = 70% formaldehyde, 4% 1-propanal, 3 % 1-butanal, 3% 1pentanal, 10% benzaldehyde, 10% tolualdehyde.
398 Environmental Science & Technology
reduced. by replacement with methanol. the decomposition rate is accelerated. This modified method circumvents the problems of the ultraviolet and gravimetric techniques4.e.. dissimilar E values and molecular weights, respectively. The absorbance measurements are made at 440 mv.the A,,,;,,for the formaldehyde derivative, which is the most prominent and thought to be the most important carbonyl present in exhaust. The c values of the other carbonyl compounds tested are all similar to that of the formaldehyde derivative at the selected wavelength (Table I ) . In exhaust analysis the differences in values shown in Table I should cause only a small positive error due to the predominance of formaldehyde. benzaldehyde. and tolualdehyde. The bisD N P H derivatives of 1.2-dicarbonyls (osazones) form blue colors (,\,,,,,, - 600 mp) and are significantly less responsive at the selected wavelength. However. these components are thought to be present at very minor concentrations in exhaust and thus would cause only a small negative error. The analysis takes approximately 30 minutes’ work-up time after complete reaction of derivatives (precipitation) has occurred. This represents a substantial savings in time over the gravimetric procedure which requires several hours (usually overnight) to evaporate the solvent and obtain a constant weight. The technique has been applied, with success. to the determination of total carbonyls in synthetic vapor samples and in exhaust. The values obtained on exhaust from standard vehicles have ranged between 40 and 150 p.p.m. The recoveries on synthesized vapor samples have ranged from 98 to 106% and typical examples are summarized in Table 11. This table also shows the recoveries (as formaldehyde) obtained with and uithout the use of a back extraction with n-hexane to remove the dissolved derivatives. It is recommended that the back extraction be used routinely because this guards against the loss of the more soluble derivatives and accidental losses due to insufficient cooling of the reagentprecipitate mixture. This method has been tested on many standards both as liquid solutions of the D N P H derivatives for calibration purposes and as vapor mixtures for checking the recovery and reproducibility through the entire procedure. The reproducibility of the calibration solutions is 1% relative and the reproducibility of the vapor mixtures (through the entire procedure) is 3 9 relative. Litercctirre Cited Iddles. H. A.. Jackson, C. E . , Ind. Eng. Chem. Annl. E d . 6. 454 ( 1 9 3 4 ) . Lappin. G. R., Clark, L. C., Anal. Chem. 23, 541 ( 1951). Lawrence, R. C.. Nature 205, 1313 (1965). Lohman. F. H., Anal. Chem. 30, 972 (1958). Malmberg, E. W., J . A m . Chem. SOC.76, 980 (1954). Mendelowitz, A., Riley, J. P., Analyst 78, 704 ( 1 9 5 3 ) . Oberdorfer, P. E.. Paper 670123, Society of Automotive Engineers Meeting, Detroit, Mich., January 9-1 3, 1967. Parsons, A . M.. Analyst 91, 297 (1966). Pool, M . F., Klose, A. A., J . A m . Oil Chemists’ SOC.28, 215 (1951). Shriner. R. L., Fuson, R. C., Curtin. D . Y . .“The Systematic Identification of Organic Compounds.” 4th ed.. p. 219. Wiley, New York. 1956. Toren. P. E., Heinrich, B. J.. Anal. Chem. 27, 1986 (1955). Received f o r review July 1 I , 1968. Accepted January 24, 1969. Research & Development Division Publication N o . 432.
