Determination of Formaldehyde in Gas Mixtures by the Chromotropic Acid Method AUBREY P. ALTSHULLER, DAVID L. MILLER, and STANLEY F. SLEVA Division of Air Pollution, Robert A. Toft Sanitary Engineering Cenfer, U. S. Department of Health, Education, and Welfare, Cincinnati 26, Ohio
b The modification of the chromotropic acid method for formaldehyde proposed b y West and Sen has been investigated. With only minor variations, the present study confirms the previous findings on reagent concentrations, color stability of the product, and the stability of the reagent solution. A much more detailed investigation of the possible interference of olefins, alcohols, aldehydes and ketones, aromatic hydrocarbons, phenols, and of nitrogen dioxide has been made. Nitrogen dioxide, most aldehydes and ketones, and straight-chain alcohols do not interfere significantly. The interference of olefins and aromatic hydrocarbons can b e largely eliminated b y the use of appropriate sampling conditions. ,
A
the various reagents proposed for the determination of formaldehyde, chromotropic acid (1,8-dihydroxynaphthalene-3.6-disulfonicacid) (7-10) has much to recommend it in terniq of sensitivity and general utility. This method has also been Ridely used for the determination of methanol by oxidation t o formaldrhyde (4] 5 ) , and the deterniination of unsaturated compounds containing double bonds in ehternal positions by oxidation to both in chemical formaldehyde. etc. (8), and biological systems ( 6 ) . A modification has been reported R hich results in a very stable product of high color intensity that obeys Beer's Ian- over a vride concentration range ( 1 5 ) . This procedure also is both simple and rapid in its application t o the determination of formaldehyde as an atmospheric pollutant or combustion effluent. The interference of sulfites and aldehydes, other than acrolein, has been reported to be negligible (15). Bricker and his con-orkers have investigated the interference of a number of alcohols, ketones, acids, chlorinated compounds, and a few aromatic compounds in their modifications of the chromotropic acid method (7, 9). These workers showed that most substances which may interfere can be eliminated by adding the sample containing the formaldehyde and the possible interferences to chromotropic acid in the absence of sulfuric acid, heating to dryness to drive off the JIOXG
substances which might interfere, then adding the concentrated sulfuric acid and heating for 30 minutes a t 100' C. (9). However, this modification is considerably more complicated than the West and Sen procedure (15). Bricker and Johnson's original method does not involve heating to dryness, but does involve the 30-minute heating. This step is not only time consuming, but also increases the interference problem oning to the long heating in an oxidizing acid. The West and Sen modification involves no heating, but instead depends on the heat of dilution at high acid concentrations t o provide sufficient heat t o produce stable color formation. In the present study the previous findings (15) have been confirmed, with some minor differences in results. Included as possible interferences are representative members of various classes of compounds known t o occur in association with formaldehyde in combustion effluents and in the atmosphere -Le., aldehydes and ketones, alcohols, phenols, hydrocarbons, and nitrogen dioxide. Bricker and his coryorkers were interested in determining formaldehyde in very large excess amounts of other compounds (9). Consequently, a lack of interference at ratios of these various compounds t o formaldehyde of 20.000 to 1 n-ere of importance. However, the available analytical data on combustion sources and the atmosphere indicate that other oxygenated compounds present will eyist a t lolver concentrations than does formaldehyde. Other gaseous substances which may possibly interfere mill rarely exist a t more than 1 to 10 times the formaldehyde concentration. As a result, a negligible interference at a tenfold excess of an individual substance should be sufficient. EXPERIMENTAL
The reagents and solutions were prepared in the manner described previously (15). The liquid substances being tested were prepared in water or methanol to give a concentration 1000 times that of the formaldehyde. About 0.1 ml. of this solution was added to a 10-ml. volumetric flask containing 1 ml. of 1% chromotropic acid reagent; 1 ml. of formaldehyde solution was
added, and the resulting mixture was diluted to 10 ml. with concentrated sulfuric acid. The sulfuric acid concentration was maintained near 86%. The absorbances were measured on a Cary hlodel 11 spectrophotometer. Vapor mixtures containing formaldehyde mere generated from a diffusion cell (1) partially filled with formalin solution. Air was passed through the diffusion cell at 0.5 t o 1 liter per minute for 15 minutes to attain near equilibrium conditions, then the formaldehyde vapor was diverted into a collection train of impingers or bubblers. In the portion of the work involving possible interference by olefins or nitrogen dioxide, one of the olefins or nitrogen dioxide was used to fill the reservoir of an apparatus from n hich the gas was ejected into the air stream a t a known and constant rate by a motor-driven Teflon plunger. In later work, Mylar bags (3) mere filled with formaldehyde from the diffusion cell, then the other component was vaporized into the bag by injection into an evacuated glass cylinder and flushing into the bag. RESULTS
Stability of Formaldehyde Solutions. A formaldehyde solution was reacted with freshly prepared chromotropic acid reagent. The absorbances of the reaction product read a t 580 mp a t various time intervals indicated that the formaldehyde solutions are usable for 1 or 2 days after preparation but not longer. Stability of Chromotropic Acid Solutions. A freshly prepared formaldehyde solution was reacted with chromotropic acid reagent a t various time intervals after preparation. Absorbance values (also read a t 580 mp) indicate that it is not advisable to use a batch of chromotropic acid solution for more than a few months. Recalibration against a standard formaldehyde solution is advisable several times during this period. Stability of Color of Reaction Product. After reaction of a 22-day-old chromotropic acid solution with a formaldehyde solution, the absorbance of the product a t 580 mp was followed for a period of 8 days. A less complete series of measurements was made using both freshly prepared chromotropic acid and formaldehyde solutions. VOL. 33, NO. 4, APRIL 1961
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Table I. Collection Efficiencies of Various Solutions for Formaldehyde
Water
0.57 0.75 0.61 0.53 0.58 0.53
0.16 0.14 0.13 0.07 0.09 0.10
78 84 82 -~ 88 86 84
0.98 0.93 1.03
0.07 0.10 0.07
93 90 94
0.1% Chromotropic acid in sulfuric acid 0.58 0.46 1.96 2.07 0.88
0.02 0.00 0.01 0.03 0.01
97 100 99 99 99
1% Aqueous so-
dium bisulfite
Table II. Analysis of Nitrogen Dioxide-Formaldehyde Mixtures
% of Concn. of Calcd. Components, P.P.M. FormalFormal- Nitrogen Abdehyde dehyde dioxide sorbance Content 4 0.97 4 40 1.01 104 200 1. 13a 200 1000 1.21b 107 240 1.35" 240 200 1.40b 104 b
Average of two determinations. Average of three determinations.
It appears that a small but definite increase in absorbance occurs for a number of days. The increase amounts to less than 3% during the course of one day and less than 10% during an 8day period. These results represent a really remarkable stability for this reaction product compared t o the stabilities usually observed for colorimetric products. Kevertheless, the calibration curves obtained on fresh solutions should not be used if the determinations must be postponed for more than 24 hours. Reagent Concentration. Tests conducted show t h a t the reaction is insensitive to the reagent concentration for sulfuric acid concentration above 86%, which agrees with previous findings (16). Dilution of Reaction Product. After formation of the colored product, the solution was then diluted to various concentrations using the reagent blank. Calculated and observed absorbance values indicated that Beer's lam is followed. Heating of Reaction Product. Bricker and Johnson heated the reac622
ANALYTICAL CHEMISTRY
tion product for 30 minutes in their modification of the chromotropic acid method (7). West and Sen claimed that no edernal heating was necessary in their procedure (16),which was confirmed by tests conducted here. Heating of the reaction product for 15 and 30 minutes resulted in absorbances of 2.81 and 2.82, compared to an absorbance of 2.79 with no external heating. Only a very slight increase in absorbance will result. Absorption Spectrum of Reaction Product. I n addition to the absorption-band a t 580 mp, the reaction product forms two other bands which have absorption maxima a t 480 and 370 mp. The absorption band a t 480 mp is slightly more than half as intense as the band a t 580 mp in fresh solutions. dfter several days the absorption band a t 480 mp, which increases in intensity more rapidly than the band a t 580 mp, is 0.65 as intense as the band a t 580 mp. The band which appears in the ultraviolet is about as intense as the 580-mp band. Collection Efficiency for Formaldehyde. A series of measurements was made to determine the collection efficiency of water, 1% aqueous sodium bisulfite solution, and a sulfuric acid solution of 0.1% chromotropic acid for formaldehyde. A few measurements also were made on the collection of formaldehyde in concentrated sulfuric acid. The formaldehyde was obtained in the proper concentration range by passing air a t 100 or 500 ml. per minute through a diffusion cell. The gas stream was then passed through two impingers in series. The temperature of the collection solution was 25' =t 2" C. The results are given in Table I. Only the general level of formaldehyde used is known in the runs using diffusion cells containing formalin solutions. Consequently, a material balance cannot be made with an accuracy that would be useful. I n addition to the runs listed in Table I, a few runs were made with three collectors. The rapid decrease in formaldehyde content from first to second to third colIector is excellent evidence for the inappreciable loss of formaldehyde through the entire collection system. The ability of 0.1% chromotropic acid in sulfuric acid to collect formaldehyde is outstanding. Very little formaldehyde manages t o pass through the first impinger. These results indicate that a single impinger will collect most of the formaldehyde in the gas stream if the chromotropic acid reagent is used. Sulfuric acid alone is almost as effective a collecting medium as the 0.1% chromotropic acid in sulfuric acid, but it has several disadvantages in the over-all problems of analysis. Aqueous bisulfite solutions a t either
room or ice-water temperatures will collect formaldehyde with very high efficiency, if an appropriate fritted-glass bubbler is used. While the use of bubblers may be satisfactory with aqueous collection media, the added collection efficiency of chromotropic acid solutions for interferences can be undesirable. This problem will be discussed in detail later in this investigation. Nitrogen Dioxide - Formaldehyde Mixtures. Three vapor mixtures of nitrogen dioxide and formaldehyde were prepared and the resulting formaldehyde concentration determined (Table 11). Vapor mixtures containing formaldehyde alone a t the same concentration in air were analyzed alternately n i t h the nitrogen dioxideformaldehyde mixtures. The results indicate a slightly higher absorbance for formaldehyde in the presence of nitrogen dioxide than without it. The small difference indicates no appreciable interferences by nitrogen dioxide with the formaldehyde analysis. But, if the nitrogen dioxide is allowed contact with a phenolic-type reagent such as chromotropic acid for very long periods of time, the interference of the nitrogen dioxide can become very significant. I n one additional determination, 50 pg. of nitrogen dioxide was collected in 0.1% chromotropic acid-sulfuric acid medium, then 9 pg. of formaldehyde was added. This solution analyzed a t 95% of the formaldehyde content found when the 9 pg. of formaldehyde was added in the absence of the nitrogen dioxide. Olefin-Formaldehyde Mixtures. A detailed study was made of 1-butene, isobutylene, trans-2-buteneJ and 1,3butadiene vapor mixtures rTith formaldehyde in air. I n addition, data were obtained on ethylene, propylene, 1-hexene, and 2-methyl-1,3-butadiene vapor mixed n i t h formaldehyde in air. Formaldehyde a-ould be expected to react directly with olefins via the Prins reaction in sulfuric acid medium ( 2 ) . However, the great importance of appropriate collection conditions in minimizing such interferences is clearly shown in Table 111. The previously discussed work on collection efficiency, along with the control measurements made on formaldehyde alone, all show high collection efficiencies for formaldehyde even in impingers a t moderately high flow rates. d marked decrease in olefin interference is obtained by using impingers at flow rates around 1 liter per minute (Table 111)) apparently because of reduced collection efficiency for olefins under these sampling conditions. Ethylene and propylene a t 10 to 1 excess over formaldehyde reduced the observed formaldehyde concentration by only 5 to 10%. -4 fifteen- to twentyfold excess of 2-methyl-1.3-buta-
diene over formaldehyde reduced the Table 111. Analysis of Formaldehyde-Olefin Vapor Mixtures observed formaldehyde concentration by 15 or 2OYc when sampling into chroyo of Calcd Concentration. P.P.M. Formalniotropic acid solution in impingers dehyde Collection Flow Rate, Formalwas done a t a moderate flow rate. Content Devicea M,/iUin. Olefin dehyde Olefin The use of \ d e r or a 1% aqueous 73b Bubbler ]-Butene 100 10 120 sodium bisulfite solution as a collecting 83 Bubbler 100 7 60 solution reduces, but does not comBOb Impinger 200 8 60 pletely eliminate the olefin interference. 92c Impinger 500 3 24 09 Impinger 1000 2 12 This is shown by results obtained for ethylene, propylene, 2-methyl-l,3-butaBubbler 100 29c trans-2-Butene 12 120 Impinger 200 80 4 diene, and 1-hexene-formaldehyde mix300 Impinger 500 84c 24 2.5 tures. The 10 to 207, losses of formalImpinger 500 54 120 2.5 dehyde in sulfuric acid containing the 46" Bubbler 100 Isobutylene 15 120 reagent are reduced to :Lbout 5% by the 86c Impinger 100 15 120 use of aqueous collection solutions. Impinger 500 91c 3 24 Aqueous collection m d i a have a de95. 1.5 12 Impinger 1000 cided disadvantage for atmospheric 37d Impinger 100 concentrations, since 1 ml. of aqueous 92b Impinger 200 99 Impinger 1000 solution must be diluted to 10 ml. with chromotropic acid in sulfuric acid to a Collection medium, 20 ml. of 0.1% chromotropic acid in sulfuric acid. maintain the acidity a t the proper level. b Average of three determinations. c hverage of two determinations. Greater dilution of the sulfuric acid will d Average of six determinations. reduce the over-all sensitivity and reproducibility of the method. I n automobile exhaust analysis, the fornialdehyde concentration is high enough to Acrolein will react with the chromoHowever, higher alcohols in liquid mixpermit the use of an aqueous collecting tropic acid to give several per cent tures with formaldehyde reduce the medium without difficulty. interference with an equal amount of absorbance a t 580 mp appreciably. The use of sulfuric acid alone as a formaldehyde. Acrolein forms a prodThe absorbances of mixtures of about collection medium is not satisfactory. uct with a maximum in the 450-mp 40 pg. of formaldehyde with tenfold Olefins and formaldehyde appear to excesses of 2-propanol, 1-butanol, 2range. Since formaldehyde is usually react very appreciably in this medium. in considerable excess over any of the methyl-2-propanol, 1-pentanol, 1At a 10 to 1 ratio of 1-hexene to formalother individual aldehydes in combusheptanol, or cyclohesanol, respectively, dehyde, only about 40% of the formalwere as follows: 1.87, 2.15, 0.38, 2.13, tion gases (11) and the atmosphere ( I S ) , dehyde present in the vapor mixture the interference from other aldehydes is 2.17, and 0.68. The formaldehyde could be detected when sulfuric acid not significant. solution alone gave an absorbance of was used for collection. At a 15 or 2.52. consequently, all of the higher A number of ketones including ace20 to 1 ratio of 2-methyl-l,3-butadiene tone, 2-butanonel 3-pentanone) 3-methalcohols a t tenfold excess reduce the to formaldehyde. less than 5% of the yl-2-butanone, 4-hesanone, and 3-heptaapparent concentration of formaldeformaldehyde present in the vapor could none also were reacted in milligram hyde appreciably, and the branched be detected. ConsequentIy, sulfuric amounts with chromotropic acid rechain alcohols are particularly reactive. acid not containing chromotropic acid These results are analogous t o those obagent. These materials reacted to give reagent is not recommended as a colabsorbance less than 0.1% that of a n tained with the olefins, and it is reasonlection medium. equal amount of formaldehyde a t 580 able to assume that both the olefins and These results show that the use of mp. Consequently, ketones will not the alcohols react with the formalappropriate collection devices and soluinterfere to any appreciable extent with dehyde by much the same mechanism. tions and even moderate flow rates ~ 1 1 the analysis of formaldehyde in comYo attempt was made t o study the reduce the olefin interference with bustion or atmospheric gases by the collection of vapor mixtures of alcohols formaldehyde analysis down to quite present method. and formaldehyde. The available data acceptable levels. This is particularly These results were confirmed by the indicate that the quantities of alcohols, true in automobile exhaust or atmosreaction of liquid mixtures containing particularly those heavier than ethyl pheric gases, in which the ratio of the individual aldehydes or ketones and alcohol, in automobile exhaust or in the more reactive olefins to the formalformaldehyde with chromotropic acid atmosphere must be quite small comdehyde may be less than 6 to 1. It is reagent. The other aldehydes or kepared with the formaldehyde conceneven possible that the atmospheric tones were in approximately tenfold tration. The total concentration of concentration of formaldehyde will excess over the formaldehyde. No alcohols in cruise exhaust is only about appreciably exceed the concentration of significant change in the absorbance a t one fifth of the formaldehyde concenfour-carbon and higher olefins. 580 mp occurred compared to the abtration (11). Consequently, the interAldehyde- or Ketone-Formaldesorbance obtained by the use of an equal ference, even assuming efficient collechyde Mixtures. il number of aldeamount of formaldehyde alone. tion of alcohols, should not be significant. hydes including acetaldehyde, proAlcohol-Formaldehyde Mixtures. Aromatic Hydrocarbon-Formaldepanal, butanal, acrolein, 2-butenal, Individual alcohols in milligram hyde Mixtures. The possible interand 2-methyl-2-butenal were reacted amounts did not react directly with ference of aromatic hydrocarbons with directly in milligram amounts with chromotropic acid reagent to give the determination of formaldehyde the chromotropic acid. The saturated absorbances even O.lyo as high as when i t is collected directly into the aldehydes and 2-methyl-2-butenal ret h a t obtained with a n equal amount chromotropic acid reagent solution acted to give less than one-hundredth of formaldehyde. Methanol in ten also has been investigated. The the absorbance a t 580 m p of an equal thousandfold excess in mixtures with formaldehyde might react with the amount of formaldehyde. 2-Butenal formaldehyde appear to cause no aromatic hydrocarbon in the sulfuric significant change in absorbance comgives 1 or 2% of the response shown by acid, reducing the formaldehyde availpared t o pure formaldehyde solutions. an equal amount of formaldehyde. able for reaction with the chromoVOL. 33, NO. 4, APRIL 1961
623
tropic acid, thus causing a reduction in apparent formaldehyde concentration. Vapor mixtures containing formaldehyde and benzene, toluene, p-xylene, o-xylene, or m-xylene were prepared. The aromatic hydrocarbon mas added so as to give hydrocarbon concentrations in excess of the formaldehyde concentration by factors ranging from about 2 to 14. The formaldehyde concentration [vas in the 1 to 5 p.p.m. range. Several liters of each of these vapor mixtures was passed at a flow rate of 1 liter per minute through an impinger containing the chromotropic acid reagent. At about eight- and twelvefold excesses of benzene, the apparent formaldehyde concentration was 80% of the actual concentration. When eight-, teii-, and twelvefold excesses of toluene \$-ere used, the formaldehyde concentration measured averaged about 70% of the actual value. At about a fourteenfold excess of each of the xylenes, only 30 to 50% of the actual formaldehyde concentration \vas found. At two- to threefold excesses of toluene, p-xylene, o-xylene, or m-xylene, the apparent formaldehyde concentration n a s 80 to 90% of the actual concentration. The present investigation proves that aromatic hydrocarbons interfere significantly when present in concentrations in excess of the formaldehyde concentration. The aromatic hydrocarbons appear to be rather efficiently retained in the chromotropic acidsulfuric acid collection medium even at moderately high flow rates through inipingers. The interference of aromatic hydrocarbons nith the analysis of formaldehyde also was investigated when water was used as the collecting medium. The flow rate used was 0.5 liter per minute. The formaldehyde concentration was in the 5 to 10 p.p.m. by S o interference ocvolume range. curred when a mixture containing formaldehyde and either a five- or
Table IV.
Fuel
No.0
3 .i
fifteenfold excess of o-xylene or ethylbenzene was analyzed for formaldehyde. A series of vapor mixtures containing formaldehyde and a fifteenfold excess of benzene, toluene, pxylene, or m-xylene also were passed through water and analyzed for formaldehyde content. The average decrease in observed formaldehyde content was only 10%. These measurements indicate that the interference of aromatic hydrocarbons can be largely eliminated by sampling into aqueous solution and then reacting with chromotropic acid reagent. Phenol-Formaldehyde Mixtures. Since phenols react JTith formaldehyde t o form polymeric products in acidic media, interference might be anticipated in the analysis of phenolformaldehyde vapor mixtures by the chromotropic acid reagent. Oning t o the solubility of phenols in water, the interference of phenols should occur n h e n sampling into either sulfuric acid or aqueous collecting solutions. Vapor mixtures of formaldehyde nith phenol or p-cresol were collected in chromotropic acid. The flow rates used were either 200 or 500 ml. per minute. The ratio of phenolic compound to formaldehyde ranged from 8: 1 to 2: I on a weight basis in the mixtures. The formaldehyde was present in the mixtures in the 2 to 4 p.p.m. by volume range. Even a t an 8 to 1 ratio the observed formaldehyde concentration was only 10 to 207, less than the actual concentration. At 2 to 1 ratio of p-cresol to formaldehyde, no loss of formaldehyde mas observed. Vapor mixtures of formaldehyde with phenol, p-cresol, o-cresol, or m-cresol were collected a t 1 liter per minute in water. The ratio of phenolic compound t o formaldehyde was in the 3 : l to 1 : l range on a weight basis. The formaldehyde mas present in these vapor mixtures in the 5 to 20 p.p.m. by volume range. The decreases in formaldehyde concentration TT hen compared with the actual concentration
Formaldehyde Analyses of Diluted, Irradiated Automobile Exhaust FormaldehydeC Flow Rate Total Gas Total pg. in P.P.M. ?*ll./.llinute Vol., L. Absorbanceb of Formaldehyde by Vol. 30 0.88 17.0 0.44 500 23.1 0.63 1000 30 1.21 20 0.72 13.8 0.56 1000 1nnn fin 2.17 42.7 0.58 _... .. ~. 18.7 0.50 1000 30 0.98 21.9 1.15 0.59 1000 30 ~
Fluorescent indicator absorption analyses in volume per cent of fuels. Fuel KO.3: saturates, 74.1; olefins, 13.9; aromatics, 39.0. Fuel S o . 5 : saturates, 38.8; olefins, 22.9; aromatics, 38.3. * Measured in 1-cm. optical path length cells. c Assuming vapor density for monomeric formaldehyde of 1.23 grams per liter at 25" C. and 760 mm. a
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ANALYTICAL CHEMISTRY
ranged from 5 to 30y0 and averaged a lOy0 decrease. Only a limited amount of data is available on phenols in combustion sources and the atmosphere (14, 16). These data indicate that the total phenols actually would exist a t lower concentrations than the formaldehyde in automobile exhaust (14) and incineration gases (16) and in the atmosphere (14). Since the data obtained in the present investigation shon ed small losses of formaldehyde even in the presence of excess phenols. in practice the interference of phenols should be small to negligible. Analysis for Formaldehyde in Diluted, Irradiated Automobile Exhaust. Gas samples nere passed from a dynamic irradiation chamber facility (12) through an impinger train a t flow rates of or 1 liter per minute. Each impinger contained 10 nil. of the 0.1% chromotropic acid reagent in sulfuric acid. After collection, 0.5 nil. of nater was added to ensure sufficient heating, but no appreciable change in color intensity folloncd. Evidently, sufficient heating occurs even 11hen atmospheric samples a t moderate relative humidities are passed through the chromotropic acid reagent. The analyses listed in Table IV nere obtained on different da! s. using soniewhat varying engine conditions, and different irradiation periods. Cons?quently, they are meant primarily to illustrate the applicability of the method to samples from a synthetic atmosphere presumably equivalent to polluted atmospheres existing on the Kest coast of the United States. The values measured are reasonably consistent, and certainly show that appreciable concentrations of formaldehyde edsted in the irradiated gases. In most localities the formaldehyde concentrations probably would be almost an order of magnitude loner. DISCUSSION
Sampling directly into chromotropic acid in sulfuric acid offers three major advantages. The sensitivity is 10 times higher than if the formaldehyde is collected into aqueous solution and thrn diluted 9 to 1 11-ith chromotropic acid solution. The procedure is extremely simple, involving only a single reagent, no dilutions or additions of other components, and no heating or prolonged reaction periods before analysis. The collection efficiency of a sulfuric acid solution of chromatotropic acid is so high that only a single impinger is required. -4 disadvantage of the procedure is the likelihood of interference in a sulfuric acid solvent medium. A possible second disadvantage lies in the
increased care needed in handling a strong acid during the sampling period. I n sampling for formaldehyde in combustion gases a pair of impingers in series containing an aqueous collecting medium may be the sampling procedure of choice. Passing 30 liters of gas containing 0.5 p.p.m. of formaldehyde through chromotropic arid solution results in a n absorbance of approximately 1.0 (Table IV). If an absorbance of 0.2 is considered satisfactory for quantitative analysis, this intensity can be obtained by collecting the formaldehyde from 60 liters of gas containing 0.5 p.p.m. of formaldehyde into an aqueous medium. Below the 0.5 p.1i.m. by volume level of formaldehyde it is necessary t o use a very large gas volume and long sampling periods to obtain sufficient formaldehyde for analj-siq. blternatively, if 3 ml. of aqueous solution is added to 27 ml. of chromotropic acid reagent solution, enough solution volume is available to use a spectrophotometer cell of eithw d- or 10-cm. optical path length. It is in the analysis of combustion gascls for formaldehyde by direct collection irito chromotropic acid that the interference problem is the most critical. This is particularly the situation in automobile exhaust. The total olefin contcnt of automobile exhaust may exceed the formaldehyde concentration by a factor of 8 or 10. However, the resultb of the present investigation indicate that this olefin content need not constitute a serious interference. The aromatic hydrocarbons in automobile exhaust likewise may exceed the formaldehyde content by a factor of 5 or 10. This quantity of aromatic hydrocarbons will constitute a serious interference. Consequently, sampling into aqueous solution is the preferred
procedure. Any olefin or aromatic hydrocarbon collected in this way will still interfere when the chromotropic acid solution is added, but the collection efficiency of water for hydrocarbon is much lower than that of sulfuric acid. Phenols and higher molecular weight alcohols if present in excess of the formaldehyde concentration will interfere somewhat in both aqueousand sulfuric acid-type collecting media. However, the available data indicate that both of these classes of compounds occur in much lower concentrations than formaldehyde in most combustion sources and in the atmosphere. It is in the atmospheric sampling situation that sampling directly into chromotropic acid is most advantageous. Formaldehyde is unlikely to exceed a few tenths of a part per million by volume in the atmosphere. I n smoggy atmospheres the small interference from olefins is reduced further, since the formaldehyde present is formed in part by the consumption of olefins by photooxidation reactions. Fern data are available to estimate the ratio of aromatic hydrocarbon t o formaldehyde in smoggy atmospheres. I n situations similar to the irradiation conditions resulting in the formaldehyde concentrations reported in Table IV, that ratio is unlikely t o exceed 2 to 1, even without further reduction in the aromatic hydrocarbon content by photo-oxidation reactions. The small amount of laboratory data available indicates that xylenes may disappear by photo-okidation reactions about as rapidly as olefins such as 1-butene or I-pentene ( I S ) . The considerations discussed above suggest that only a small interference by aromatic hydrocarbons and olefins nil1 occur in the analysis for formalde-
hyde in smoggy atmospheres by the direct collection in chromotropic acid solution. I n other gas mixtures, and particularly those Y,ith large amounts of aromatic hydrocarbons, collection into aqueous solution is recommended. LITERATURE CITED
(1) Altshuller, A. P., Cohen, I. R., ANAL. CHEM.32, 801 (1960). (2) Arundale, E., hlikeska, L. X.,Cheva. Revs. 51, 505 (1952). (3) Baker, R. A., Doerr, R. C., Intern. J . A i r Poll. 2, 142 (1959). (4)Beyer, G., J . Assoc. Ofic. d g r . Chemists 34, 745 (1951). (5) BOOS,R. x., -4N.kL. CHEN. 20, 964
(1958). (6) Boyd, J. &I., Logan, 11. A,, J . Biol Chem. 146, 279 (1942). (7) Bricker, C. E., Johnson, H. R.,IND. ENG.CHEM., ANAL.ED. 17, 400 (1945). (8) Bricker, C. E., Roberts, K. H., A S A L . CHEM.21, 1331 (1949). (91 Bricker, C. E., Vail, -4.H., Ibid., 22, (20 (1050). (10) Eegriwe, 2. anal. Chem. 110, 22 (1937). (11) Ellis, C., Burn, R. W., “Oxygenated Compounds in Automobile Exhaust Gases as Shown by Chemical Analysis,” Bureau of Mines Rept., 1961. (12) Rose, A. H., Brandt, C. S., J . A i r Poll. Control Assoc. 10, 331 (1960). (13) Schuck, E. A., Doyle, G. J., “Photooxidation of Hydrocarbons in Mixtures Containing Oxides of Sitrogen and Sulfur Dioxide,” Rept. No. 29, Air Pollution Foundation, San Marino, Calif., October 1959. (14) Smith, R. G. MacEwen, J. D., Barrow, R. E., A m . Ind. Hyg. Assoc. J . 20, 149 (1959). (15) West, P. W., Sen, B., 2. anal. Chena. 153, 177 (1956). (16) Yocom, J. E., Hein, G. >I., Selson, H. W., J . A i r Poll. Control Assoc. 6 , 84 (1956). RECEIVED for review September 21, 1960. Accepted December 12, 1960. Division of R7ater and Waste Chemistry, 138th hleeting, ACS, Kew York, S. Y., September 1960.
Colorimetric Assay for Diosgenin and Related a Lompounds I
S. C. SLACK and W. J. MADER Research Deparfment, Ciba Pharmaceutical Products Inc., Summit,
b A rapid assay for the diosgenin content of crude diosgenin is based upon the chromogen produced when the sapogenin is treated with perchloric acid. The specificity of the reaction and its application to structurally related compounds are discussed. The assay of crude diosgenin by the colorimetric method and by a lengthy chromatographic procedure are compared.
S
N. J.
have a prominent role in the synthesis of steroidal hormones. I n this respect, diosgenin is used as the starting material in the preparation of testosterone and methyl testosterone. Rhizomes of the plant genus, Dioscorea, yield diosgenin and its stereoisomer, yamogenin, the concentration being dependent upon the species of the plant. BPOGENINS
Crude diosgenin is obtained by extracting the ground rhizomes after acid hydrolysis. The extract is purified and the material obtained is composed of diosgenin, yamogenin, trace amounts of gentrogenin and correlogenin (31, and resinous material of unknown structure. Steroidal sapogenins hare been identified by the absorption spectra of their sulfuric acid chromogens (1, 4). VOL. 33, NO. 4, APRIL 1961
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