Specific Spectrophotometric Determination of Ozone in the Atmosphere Using 1,2-Di-(4-PyridyI)Ethylene THOMAS R. HAUSER and DANIEL W. BRADLEY Robert A. Tuft Sanitary Engineering Center, 4676 Columbia Parkway, Cincinnati, Ohio
b A new method for the sampling and analysis of ozone in the atmosphere i s described. Atmospheric ozone i s collected in a solution of 1,2-di-(4pyridy1)ethylene in glacial acetic acid. The collected ozone reacts with the 1,2-di-(4-pyridyl)ethyleneto form an ozonide that undergoes cleavage to yield pyridine-4-aldehyde for which a simple spectrophotometric determination was developed. The relationship between the micrograms of pyridine-4-aldehyde generated per microgram of ozone sampled has been determined, so that pyridine-4aldehyde may be used for calibration. Various other oxidizing or reducing substances do not interfere with the method, at least not in the concentrations in which they are found in the atmosphere. The method offers good sensitivity and reproducibility, and excellent stability for delayed analysis after sampling.
A
for a simple, specific, and sensitive analytical method for the determination of ozone in the atmosphere in the presence of ot,her oxidizing and reducing substances. I t has been demonstrated ( 3 ) that the numerous methods presently available for the measurement of ozone, including the widely used pot,assium iodide procedures ( 1 , 4, 6-a), all possess some undesirable quality, such as a lack of specificity, sensitivity, or stability, or have some unwanted feature, such as high cost of equipment, slow instrument response, or unknown stoichiometry. Bravo and Lodge ( 3 ) reported a specific method for the determination of ozone in polluted atmospheres iitilizing the principles of the ozonolysis reaction ( 2 , 5) followed by spectrophotometric analysis of the aldehyde formed (10). The disadvantage of their method for use as a routine analytical procedure is that the very corrosive trifluoroacetic acid and trifluoroacetic acid anhydride are needed for final color development. This investigation was initiated, therefore, t o determine if microgram quantities of ozone could be used in the ozonolysis reaction to generate products that could be easily analyzed by a simple, routine analysis. The substituted alkene selected for KEED EXISTS
study was 1,%di-(Cpyridyl)ethylene. This substituted alkene reacts with microgram quantities of ozone to form an ozonide which, upon hydrolysis, yields pyridine-4-aldehyde. The procedure of Sawicki et al. (9) was modified for simple analysis of the pyridine-4aldehyde formed on ozonization. The method demonstrates very good stability, sensitivity, and specificity for the determination of ozone in the atmosphere and in allied sources such as irradiation chambers. EXPERIMENTAL
Reagents and Apparatus. The various reagents used were prepared on a weight-volume basis (grams per 100 ml. of solution). The absorbing solution was a 0.5% solution of 1,2-di-(4-pyridyl)ethylenein glacial acetic acid. This reagent, as well as the pyridine-Caldehyde needed for calibration, was purchased from K and K Laboratories, Inc., Plainview, N. Y. The color-developing reagent was a 0.2% aqueous solution of 3-methyl-2benzothiazolinone hydrazone hydrochloride (3-MBTH). The 3-MBTH was purchased from the hldrich Chemical Co., Milwaukee, Wis. Air samples were collected in an allglass fritted bubbler of extra coarse porosity similar to Corning Drawing KO. XA-8370 or Ace Glass Drawing NO. 1-1050. Ozone was generated for standardization by General Electric, $-watt, germicidal ozone bulbs (G.E. Cat. KO. G4S-11). h Cary Model 15 ratio recording spectrophotometer with 1.0-cm. cells was employed for all quantitative analyses. Air Sampling. A sampling train was assembled by connecting in series the absorber, a water trap to protect the flow device, a calibrated limiting orifice for air-flow measurement, and an air pump. Fifteen milliliters of the absorbing solution was added to the absorber, and air was drawn through the solution for approximately 0.5 hour at a rate of 0.5 liter per minute. The sampling time may be extended to 2 hours, if necessary, depending upon the concentration of ozone in the air. Analytical Procedure. When sampling,was completed, the volume of the remaining abqorbing solution was accurately measured, and 10 ml. of the solution was pipetted into a test
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tube. One milliliter of the colordeveloping reagent was then added, and the mixture was heated in a boiling water bath for 20 minutes and cooled under the water tap. The absorbance was then measured a t 442 mp against a nonaerated blank prepared from absorbing solution that stood in the laboratory for the same length of time as the absorbing solution used in sampling. The concentration of ozone in the absorbing solution (in micrograms ozone per milliliter absorbing solution) was readily calculated from the absorbance-concentration curve described below. Calibration. I t was empirically determined that 1.O pg, of ozone per ml. of absorbing solution generates 2.75 pg. of pyridine-4-aldehyde per ml. of absorbing solution. Therefore, pyridine-4-aldehyde was used to prepare the standard Beer's law curve. Microgram quantities of pyridine-4-aldehyde in glacial acetic acid were carried through the analytical procedure. A very linear relationship was found for 0.0 to 10.0 pg. of pyridine-4-aldehyde per ml. of absorbing solution over an absorbance range of 0.00 to 2.10. This corresponds to 0.0 to 3.65 pg. of ozone per ml. of absorbing solution over the same absorbance range. RESULTS AND DISCUSSION
The first reactions studied were those of ozone with various 1-alkenes to determine if one of the oxygenated fragments formed during ozonolysis was formaldehyde, which could easily be determined colorimetrically. 1Hexene, 1-octene, 1-decene, and 1dodecene mere ozonated, either by themselves or in a variety of solvents including water, methanol, 1-propanol, ethyl acetate, acetic acid, N,N-dimethylformamide, and dimethylsulfoxide. The resultant ozonized mixtures were tested for the presence of formaldehyde. Formaldehyde was found in some of the ozonolysis attempts, but the water insolubility of the alkene? and/or the high degree of color formed in the blank determinations rendered this approach unsuitable as a simple analytical procedure. After further experimentation, 1,2-di-(4pyridy1)ethylene was finally selected as the reagent to be used in the procedure because its acid salt is water soluble and it reacts with microgram quantities Selection of Pyridylethylene.
VOL. 38, NO. 1 1 , OCTOBER 1966
0
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r
7
S'
3 - MBTH
Figure 1.
Probable reaction sequence
of ozone to produce pyridine-4-aldehyde, which also is water soluble and easily determined by a modification of the 3 methyl 2 benzothiazolinone hydrazone procedure (9). Two distinct colors are produced when the pyridine-4-aldehyde is subjected to the originally reported 3-MBTH procedure, Unlike other water-soluble aliphatic aldehydes the pyridine-44dehyde reacts with 3-MBTH to form a yellow product which, like the aliphatic aldehydes, forms a blue color when oxidized by ferric chloride. The first attempts to collect and determine atmospheric concentrations of ozone were made by using an aqueous solution of pyridylethylene and by developing the blue color of the pyridine-4aldehyde formed. The blue color did develop, but the ozone was not efficiently collected by the aqueous pyridylethylene solution. After testing various solvent systems, pyridylethylene in acetic acid was finally selected as the collecting media because ozone was efficiently collected in this reagent. The blue color of the pyridine-4-aldehyde will not develop in acetic acid media; hence, the yellow color was measured without further oxidation. Interferences produced by aliphatic aldehydes present in the air sampled are eliminated because they do not produce the yellow color with 3-MBTH. Proposed Reaction and Spectra Obtained. The probable overall reaction is depicted by the sequence given in Figure 1. l12-Di-(4-pyridyl)ethylene (I) is ozonated in acetic acid media t o form the ozonide intermediate (11), which upon hydrolysis yields pyridine-4-aldehyde (111) , among other products. The pyridine-44dehyde formed is then reacted with 3-MBTH to form the yellow azine (IV). The evidence for this mechanism is based upon the synthesis of the azine (IV) (pyridine-4-aldehyde-3-methyl-2benzothiazolyl azine) for which the carbon, hydrogen, and nitrogen analyses were in close agreement with theoretical values. The wavelength maxima of the azine (IV) (442 mp, E = 26,200) coincided with those of the dye produced in the analytical procedure when measured in the same solvent system
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ANALYTICAL CHEMISTRY
used in the procedure. The visible absorption spectra of the synthesized azine, the yellow compound obtained during analysis, and the blank solution us. acetic acid are shown in Figure 2. The azine was stable in acetic acid for at least 1month. Air Sampling. The absorbing solution contained O.5y0 pyridylethylene in glacial acetic acid. Higher concentrations of the pyridylethylene did not enhance collection efficiency, but did increase the color of the blank. Lower concentrations of pyridylethylene resulted in lower absorbance readings for equal quantities of ozone. The absorbing solution, primarily glacial acetic acid, freezes a t approximately 16' C. and cannot be used for sampling air below this temperature unless either the air stream or the absorber is warmed during sampling.
Outside air at 20' F. was succensfully smpled without freezing when the absorber wm located in our laboratory at 72' F. The freezing point can be effectively reduced by the addition of a small amount of water. Fifteen milliliters of absorbing solution containing 1.0 or 2.0 ml. of water (solutions containing 6.7 and 13.3% water) will freeze at approximately 8' and 0' C., respectively. The addition of water to the absorbing solution has a detrimental effect on final color development probably because of decreased collection efficiency. A comparison between the absorbing solution prescribed and absorbing solutions containing increased amounts of water to reduce the freezing point showed a corresponding decrease in absorbance as the water content of the absorbing solution was increased. From an initial absorbance reading of 1.00, the absorbance was reduced to 0.86, 0.77, 0.69, 0.63, and 0.58 for absorbing solution containing 10, 20, 30, 40, and soy0water, respectively. When the prescribed absorbing solution wm used, the overall collection efficiency for the first bubbler connected in series was at least 98% when it was determined by dividing the amount of ozone found in the first absorber by the total amount of ozone found in three absorbers connected in series. No differences in collection efficiency were noted when the porosity of the frit used in the absorber was changed from extra coarse to coarse (Corning designation). When an im-
X,mr Figure 2.
Visible absorption spectra; X max, 442 mp
5 X 10" azine (----I 2.96 pg. OSper ml. absorbing solution Blank solution VI. acetic acid (-.-*-)
[-I
pinger was substituted for the fritted bubbler, collection efficiency was reduced substantially During experimental development of the procedure, a simple ozone generator, consisting of two ozone bulbs mounted inside a glass pipe equipped with sampling ports at either end, was used as the source of ozone. The bulbs could be used either together or independently depending upon the quantity of ozone desired. This generator was simply removed from the sampling train for air sampling. Fifteen milliliters of absorbing solution was used during sampling because approximately 4 ml. evaporated over a >hour sampling period at 70’ F., and about 10 ml. was required to keep the frit used during the development of the procedure adequately covered. Larger or smaller volumes of absorbing solution can be used, depending upon the absorber used or the sampling time desired. A water trap was connected between the absorber and the air-metering device to protect the device from the evrtporated acetic acid fumes. The air-flow measuring device could probably be any device capable of measuring and regulating air flows. For this procedure, the air flow through numerous syringe needles was measured, and the needles were used as precalibrated limiting orifices. Sampling rates of up to 0.6 liter per minute were tested, and the collection efficiency remained relatively constant over this range. Analytical Procedure. Once ozone is collected in the absorbing solution, i t remains stable for at least 4 days; this was determined by repeated analysis of a large volume of ozonized absorbing solution over a 4-day period. The method, therefore, can be applied to the field analysis of atmospheric ozone in field studies, inasmuch as sampling and analysis need not be performed in the same day. Very little, if any, difference in absorbance was noted between blanks prepared from nonaerated absorbing solution and blanks prepared from the contents of a second bubbler connected in series during sampling. Therefore, nonaerated absorbing solution may be used to prepare the blank for analysis, although this solution should be permitted to stand the same length of time as the absorbing solution used for sampling because of a slight increase in blank coloration with time. I t is recommended that fresh absorbing reagent be prepared at least every 2 weeks. The effect of heating the ozonized absorbing solution prior to the addition of 3-MBTH was investigated. Preheating in a boiling water bath for 30 minutes gave approximately a 25y0 increase in final absorbance when compared to an equivalent solution that was not preheated. This increase in absorbance a
was probably due either to the increased ozonolysis of the pyridylethylene by unreacted ozone present in the absorbing solution, or to more complete hydrolysis of the oronide formed during sampling. The preheating step was not incorporated into the final procedure because with it the Beer’s law curve was not exactly linear a t low concentrations of ozone, and the extra color was lost from samples that were allowed to stand for 2 days after sampling and prior to analysis. Optimum absorbance readings resulted when the concentration of 3MBTH was 0.270. Concentrations less than 0.1% resulted in lower absorbance readings, while concentrations of 0.3% and higher resulted in only slightly lower absorbance readings plus the development of more color in the blank solution. After the addition of 3-MBTH to the ozonized absorbing solution, heating enhanced final color development. A 20-minute heating time in a boiling water bath gave optimum color development, Heating less than 15 minutes decreased color development, while heating up to 40 minutes did not increase or decrease absorbance. Once the yellow color was formed, no loss in absorbance was noted over a period of 1 hour. Hence, the color is easily stable enough for spectrophotometric analysis. Calibration. The method was calibrated by comparing i t with the neutral potassium iodide procedure for ozone, The ozone-enriched air stream from the generator was split and simultaneously analyzed for ozone by the pyridylethylene and the potassium iodide procedures. T h e total amount of ozone found with the potassium iodide procedure was used a s the standard and plotted against the corresponding absorbance observed from the pyridylethylene procedure to obtain the Beer’s law relationship of absorbance us. micrograms of ozone per milliliter of absorbing solution. Results showed a linear relationship for at least 0.00 to 3.75 pg. of ozone per ml. of absorbing solution over an absorbance range of 0.00 to 2.16. From this relationship, it was easily determined that 1.74 pg. of ozone would give an absorbance of 1.00 in the procedure, which corresponds to 4.78 pg. of pyridine4-aldehyde carried through the procedure. Therefore, 1.0 pg. of ozone generates 2.75 pg. of pyridine-4-aldehyde, and/or 1 mole of ozone generates 1.24 moles of pyridine-4-aldehyde. This observation that more than 1 mole of the pyridine-4-aldehyde is generated per mole of ozone is consistent with the reported generalizations (2, 5) that an increase in aldehyde formation will result when less than stoichiometric amounts of ozone are used to carry out
the ozonolysis reaction on a dilute solution of the alkene in an aliphatic acid at low temperatures. Precision and Sensitivity. A t least 10 ozone-enriched air streams were split in half and each half was simultaneously analyzed for ozone by the pyridylefhylene procedure. If the entire method is considered (air sampling and analysis), the results of t h e simultaneous analyses demonstrate a precision range of & 5 %. If duplicate analyses were performed on the same air sample after collection of ozone, a precision range of *1’% is noted. The pyridylethylene method exhibits approximately the same sensitivity as the neutral potassium iodide procedure in that it is capable of determining a few parts per hundred million of ozone in air when air is sampled for 0.5 hour at 0.5 liter per minute. Because of the stability of ozone in the collecting reagent, the method could probably be extended to the determination of very low or very high atmospheric concentrations of ozone by either increasing t h e sampling time or diluting the collected sample, respectively. INTERFERENCES
The effect of interfering substances on final color was determined. An ozoneenriched air stream was split in half. One half of the air stream was sampled and analyzed by the prescribed pyridylethylene procedure; simultaneously, the other half was analyzed by the same procedure, except that microgram quantities of the interfering substances were placed in the absorbing solution prior to sampling. The ozone-enriched air stream was pulled through the absorbing solution for 20 minutes at 0.5 liter per minute. Any increase or decrease in absorbance noted in this simultaneous determination was attributed to the interfering substance. The quantity of interfering substance added was determined from its suspected parts-permillion range found in ambient atmospheres, For example, if the concentration of sulfur dioxide in air was 1.0 p.p.m., then 26.2 pg. of sulfur dioxide would either be collected or be passed through the absorbing solution when sampling was continued for 20 minutes at 0.5 liter per minute. Therefore, 26 pg. of sulfur dioxide was added to the absorbing solution. The interfering substances tested at their suspected parts-per-million concentration (in parentheses) in air were the following: sulfur dioxide (1.0), nitrogen dioxide (0.3), hydrogen sulfide (0.5), acrolein (0.1) , formaldehyde (O.l), hydrogen peroxide (0.05), peracetic acid (0.0.5), peroxyacetylnitrate (0.10), di-tcrt-butyl peroxide (0.05), and 1-hexene (0.20). The three organic peroxides tested were chosen to simulate any organic peroxides VOL 38, NO. 1 1 , OCTOBER 1966
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found in air; the hexene w m selected t o simulate any olefinic material that could possibly compete for the collected ozone. The results show that none of the interfering substances tested caused either a positive or negative interference greater than 5.0%. Because this value is within the limits of the precision of the method, none of these substances interfere and the method is specific for ozone in the atmosphere. APPLICATION
The pyridylethylene and the neutral potassium iodide procedures were simultaneously applied to the analysis of ozone in both ambient atmospheres and irradiation chambers. I n controlled atmospheres, where substances that interfere with the potassium iodide procedure were absent, good agreement was obtained between the
methods. I n atmospheres oontrrining substances known to interfere with the potassium iodide procedure, agreement between the methods was not always good, although the pyridylethylene procedure gave fairly reproducible results. This nonagreement was attributed to the erratic results obtained with the potassium iodide procedure. Limited application in the analysis of ambient atmospheres outside the window of our laboratory showed fairly good agreement between the procedures, with the pyridylethylene procedure always giving slightly higher ozone concentrations. A more detailed investigation into the field applicability of the pyridylethylene procedure for air pollution surveys is currently being pursued. LITERATURE CITED
(1)Altshuller, A. P., Schwab, C. M., Bare, M., ANAL.CHEM.31,1987 (1959).
S., Chem. Rev. 58, 925 (1958). (3) Bravo, H. A., Lodge, J. P., ANAL, CHEM.36, 671 (1964). (4) Byem, D. H., Saltzman, B. E., Am, Ind. Hy Aseoc. J. 19, 251 (1958). ( 5 ) Long, Jr., Chem. R y . 27,437(1940). (6) Public Health Service Publication NO. 999-AP-11, pp. D-1, E-1, USDHEW, R. A. Taft Sanitary Engineering Center, Cincinnati, Ohio
(2) Baily, P.
c.,
(lQR.5 ). \ - - - - I -
(7) Saltzman, B. E., Gilbert, N.,ANAL. CHEM.31, 1914 (1959). (8) Saltzman, B. E., Wartburg, A. F., Ibid., 37, 779 (1965). (9) Sawicki, E., Hauser, T. R., Stanley, T. W., Elbert,. W.. . ANAL. CHEM.33, 93 (1961). (10)Sawicki, E., Stanley, T. W., Hauser, T. R., Chemist-Analyst 47, 31 (1958).
RECEIVEDfor review May 9, 1966. Accepted June 23, 1966. Division of Water, Air, and Waste Chemistry, 152nd Meeting, ACS, New York, N. Y., September 1966. Mention of commercial products does not imply endorsement by the Public Health Service.
Fujiwara Reaction and Determination of Carbon Tetra c hI o ride, ChI o rof o rm, Tetrachloroethane, and Trichloroethylene in Air G. A. LUGG Department of Supply, Defence Standards Laboratories, Australian Defence Scientific Service, Maribyrnong, Victoria, Australia The system pyridine-sodium hydroxide-water has been examined as it applies to the development of color with chlorinated hydrocarbons using the Fujiwara reaction. Methods for the determination of carbon tetrachloride, chloroform, s-tetrachloroethane, and trichloroethylene in air are presented employing both twophase and one-phase procedures. Carbon tetrachloride can be determined if a ketone is present. Absorption studies of the compounds in pyridine have shown that at least 90% of the vapors can be collected in two bubblers. Data are given on the precision, accuracy, and specificity of the methods.
T
Fujiwara reaction (10) is the classical method for determining a large number of halogenated hydrocarbons. It is characterized by the red color developed when the halogen compound is heated with sodium hydroxide and pyridine. Two absorption bands are formed, one at 368 mp, the other initially at about 535 mp. The reaction has been used for determining the concentration in air of carbon tetrachloride (4, 7 , 9, 15, 17, 20, 21, 25), HE
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ANALYTICAL CHEMISTRY
chloroform (7, 12, 15, 17, 18), tetrachloroethane (16, l 7 ) , and trichloroethylene ( 3 , 5 , 1 5 , 1 7 , 2 5 ,ad), and for the determination or detection of these and other polychloro compounds in body fluids and other media. Ross (22) attributed the reaction to compounds containing the general formula RC(halogen)8. Webb, Kay, and Xichol (25) stated that, in general, compounds containing only two halogen atoms per molecule, or a maximum of two on any one carbon atom when more than two halogen atoms per molecule were present, showed much less sensitivity than the compounds having three halogen atoms attached to the same carbon atom. Contradictory statements have appeared on the reaction conditions required for carbon tetrachloride (6, 14, 16). Bromo compounds (25) and iodo compounds (22) have been detected by the reaction, but there are no reports on fluoro compounds. Generally, absorption spectra have been determined using the pyridine layer separated from the caustic layer after development of the color with a two-phase procedure, but some investigators have used solvent to dilute the mixture to avoid the two phases (1 , 11).
Rogers and Kay (21) were the first to use a one-phase procedure of pyridinewater-sodium hydroxide, and this method was subsequently used by other workers (4, 6, 13, 17, 23). The Fujiwara reaction has had numerous modifications: These have involved the solvents, the concentration and relative amount of sodium hydroxide, the time and temperature of heating, the time of standing before absorbance is measured, and the wavelength at which it is measured. Most of the accounts refer to the necessity of adhering strictly to the stated amounts of the reagents and the conditions of heating, but it is not possible to define clearly the optimum conditions for the reaction from the published work on the subject. For this reason an investigation of the method was undertaken, particularly as it applied to the determination of chloroform, carbon tetrachloride, trichloroethylene, and tetrachloroethane in the air. EXPERIMENTAL
Throughout the experimental work, 1-em. cells were used and the volume of pyridine was standardized a t 5 ml., as this is a convenient quantity for use with these cells. All work carried out
