Specific Spectrofluorometric Determination of Atmospheric Ozone Using 2-Di phenylacetyl-l,3-l ndandione-1-H ydrazone Denys Amos Australian Defence Scientific Service, Defence Standards Laboratories, Maribyrnong, Victoria 3032, Australia
A method is described for the determination of atmospheric ozone. Ozone is collected in a solution of 1,2di-(4-pyridyl)ethylene in chloroform giving pyridine-4aldehyde which is estimated spectrofluorometrically 2-diphenylacetyl-1,3-indandione-l-hydrozone. using Pyridine-4-aldehyde may be used for calibration because the relationship between it and ozone in chloroform has been determined. Other atmospheric contaminants do not interfere with the method. The method is extremely sensitive and can be used to estimate concentrations as low as 0.02 ppm ozone.
A SENSITIVE METHOD is required for the determination of atmospheric ozone at levels below the threshold limit value (TLV) of 0.1 ppm (I). At present the most widely used procedure is the potassium iodide method, but this has the disadvantage of being affected by other oxidizing agents. Bravo and Lodge (2) have used the reaction of ozone with an olefin to form an ozonide which can be subsequently decomposed to an aldehyde as a basis for an analytical method. This idea has been further developed by Hauser and Bradley ( 3 ) who eliminated the use of corrosive trifluoroacetic acid and anhydride. They used dipyridylethylene to react with ozone in glacial acetic acid; the ozonide hydrolyzed to pyridine-4aldehyde which was determined spectrophotometrically using 3-methyl-2-benzothiazolinonehydrazone hydrochloride as a color-developing agent. The method is insufficiently sensitive, particularly in cold climates when addition of water to lower the freezing point of the absorbing solution also depresses sensitivity. Braun and Mosher (4) described a reagent for carbonyl compounds, 2-diphenylacetyl-1,3-indandione-l-hydrazone(2DIH) whose carbonyl derivatives are strongly fluorescent. We have used 2-DIH in chloroform for analysis of the pyridine-4-aldehyde formed on ozonization to develop a specific method for ozone which is capable of determining it in the region of the TLV-i.e., below 0.1 ppm.
for standardization by an OREC 0300 ozone test chamber, An Aminco-Bowman spectrophotofluorometer with 10-mm cells was used for all fluorometric analyses. Air Sampling. The sampling system consisted of the absorber, a calibrated rotameter for air flow measurement and an air pump. The bubbler was immersed in an ice/salt bath at -18 OC in a Dewar flask and temperature did not vary during the sampling period. More consistent results were obtained by freezing out water vapor in the sampled air by passing it (before sampling) through a glass coil immersed in the ice/salt bath at - 18 "C. Analytical Procedure. Ozone was absorbed in 5 ml of 1,2-di-(4-pyridyl)-ethylene solution, and when sampling was completed the volume of the absorbing solution was adjusted to 5 ml. One milliliter of the solution was pipetted into a 5-ml volumetric flask, 1 ml of developing agent and 5 p1 of concentrated hydrochloric acid (density 1.180) were added. The flask was stoppered and shaken, then heated in a thermostatted water bath at 65 f 1 "C for 30 min. After cooling, the solution volume was brought up to 5 ml with chloroform. The intensity of fluorescence was measured at 536 nm; the excitation wavelength being 468 nm. The concentration of ozone in the absorbing solutions was calculated from the fluorescence-concentration curve described below. Calibration. Since ozone reacts with dipyridylethylene to form pyridine+aldehyde, the relationship between intensity of fluorescence and concentration of pyridine-4-aldehyde was investigated by passing microgram quantities of pyridine4-aldehyde through the analytical procedure. The relationship was investigated for 0.0 to 12.0 pg of pyridine-4-aldehyde per ml of absorbing solution. Various concentrations of ozone in air were determined by the analytical procedure and simultaneously analyzed using an OREC Model MSA-3 Ozone Analyzer. In this way we established the quantitative relationship between ozone and pyridine-4-aldehyde via intensity of fluorescence. Empirically we found that 1.0 pg of ozone liberates 2.9 pg of pyridine-4-aldehyde from 1,2-di(4pyridy1)ethylene in chloroform solution.
EXPERIMENTAL
RESULTS AND DISCUSSION
Reagents and Apparatus. The absorbing solution was a 0.25% w/v solution of 1,2-di-(4-pyridyl)ethylene in chloroform. The developing reagent was a 2.5 X 104M solution in chloroform of 2-diphenyl-l,3-indandione-l-hydrazone. The chloroform used in both the absorbing and developing solutions was purified by distillation from granular calcium chloride to remove ethanol, followed by refluxing for 30 min over 2,4-dinitrophenyl hydrazine to remove phosgene and compounds containing carbonyl groups. After distillation from DNP, the chloroform was dried finally by passing through a silica gel column and stored over anhydrous potassium carbonate in a dark bottle. Air samples were collected in a fritted-glass bubbler of extra coarse porosity (Corning grade). Ozone was generated
The possible overall reaction is depicted in Figure 1 ; the 2-DIH is shown as the resonance-stabilized enolic form ( 4 ) . 1,2-Di-(4-pyridyl)ethylene is ozonized in chloroform forming as ozonide (11) which is decomposed by hydrolysis and reduction to pyridine-4-aldehyde (111). The pyridine-4-aldehyde reacts with the hydrazone, 2-DIH, forming the conjugated azine (IV). This reaction is acid catalyzed. The azine (IV) has been synthesized from pyridine-4-aldehyde, and its fluorescence spectrum (Table I) coincides with that produced during the analytical procedure. 2-DIH itself fluoresces, the excitation maximum being 430 nm and the emission maximum 520 nm. Reactions between 2-DIH and various aldehydes and ketones in chloroform were studied to see whether, by changing the chromophore structure, particularly the conjugation, it would be possible to alter the position of the excitation and emission maxima. Acetone, benzophenone, anisaldehyde, benzaldehyde, and pyridine-4aldehyde were examined as ozonolysis products. Table I shows the excitation and emission maxima of the products of
(1) Industrial Hygiene Digest, 32, No. 10 VI (1968). (2) H. A. Bravo and J. P. Lodge, ANAL.CHEM., 36,671 (1964). (3) T. R. Hauser and D. W. Bradley, ibid.,38, 1529 (1966). (4) R. A. Braun and W. A. Mosher, J . Amer. Chem. SOC.,80,3048 (1958). 842
ANALYTICAL CHEMISTRY, VOL. 42, NO. 8, JULY 1970
Table I. Excitation and Emission Maxima of Carbonyl Derivatives of 2-DlH Wavelength, nm Derivative Excitation Emission
Acetone Benzophenone Benzaldehyde Anisaldehvde Pyridine-4-aldehyde 2-DIH
425 465 467 470 470 430
520 527 525 525 537 520
-
L
I
I
J
HYOROPEROXIDE
p "%
reaction with 2-DIH. The spectrum of the pyridine-4aldehyde derivative shows the largest deviation from 2-DIH, in particular the emission maximum of 537 nm will permit less interference from the color-developing agent, 2-DIH. Consequently 1,2-di-(4-pyridyl)-ethylene was chosen as the absorbing reagent. After testing various solvents as collecting, media chloroform was chosen since the reaction between 2-DIH and pyridine-4aldehyde proceeded best in this solvent and solvent loss from bubblers was not too great. In alcohols such as propanol and butanol no fluorescence was observed except at high concentrations of pyridine-4-aldehyde and such solvents would be unsuitable to detect low ozone concentrations. In glacial acetic acid as used by Hauser and Bradley ( j ) , no fluorescence developed with 2-DIH. I n protic solvents, the fluorescence appears to be quenched and solvents containing hydroxyl groups are not satisfactory. Air Sampling. The absorbing solution contained 0.25% w/v dipyridyl ethylene in chloroform. Higher concentrations did not improve collection efficiency but increased the blank fluorescence. Lower concentrations gave lower fluorescence readings for the same level of ozone. The absorbing media, chloroform, can be used for sampling at low temperatures. Solvent loss was minimized and absorption was more efficient when the absorbing solution was maintained at -18 "C in an ice/salt bath. Using chloroform at -18 "C, collection of ozone was efficient; when 2 bubblers were used in series, no ozone was detected in the second and one bubbler was considered adequate. Collection efficiency was not impaired when the porosity of the bubbler frit was changed from extra coarse (Corning) to coarse, The reaction between pyridine-4-aldehyde and 2-DIH is water-sensitive; to obtain consistent readings for a given ozone concentration, atmospheric moisture must be removed. We found this could be done conveniently by freezing out water in a glass coil immersed in the ice/salt bath. Five milliliters of absorbing solution were used during a 20min sampling period and about 1.5 ml evaporated during this time. This was the minimum volume necessary to keep the frit covered during sampling. Larger volumes could be used for higher concentrations of ozone. Analytical Procedure. After ozone is absorbed by dipyridylethylene, the ozonide remains stable for at least 48 hours if the sample is kept dark to avoid photo-oxidation of the chloroform. No difference was noted between blanks prepared from absorbing solution and those prepared from ozonefree air, and consequently the absorbing solution was used to prepare the analytical blank. We recommend that fresh absorbing reagent be prepared every 2 weeks and 2-DIH solution every week and that the solutions be kept cold and in the dark. The effects of temperature and time on the reaction between pyridine-4-aldehyde and 2-DIH were investigated. Heating
Figure 1. Proposed reaction sequence
at 65 "C for 30 min gave optimum fluorescence; longer heating did not increase or decrease fluorescence. Acid has a marked effect on the reaction; while small quantities are necessary to catalyze the formation of the azine, increasing quantities of concentrated HCl depress the development of fluorescence and also pose problems in that insoluble pyridine hydrochlorides are produced. Five microliters of concentrated HC1 per 1 ml of absorbing solution gives optimum fluorescence without producing interfering quantities of pyridine hydrochloride. As the reaction takes place in an aprotic solvent the ozonide (11), should be capable either of hydrolysis to give 1 mole of pyridine-4-aldehyde and 1 mole of hydroperoxide; or of reduction to 2 moles of aldehyde. Examination of the yield of pyridine-4-aldehyde showed that 1 mole of ozone gives 1.3 moles of aldehyde suggesting that the dipyridyl ethylene is acting as an oxygen acceptor and is reducing some of the hydroperoxide. We attempted to increase the yield of aldehyde by reduction of the ozonide and investigated reagents known to be effective in reducing ozonides. These were trimethylphosphite (5), triphenylphosphine (6), and dimethyl sulfide (7). None increased the yield of aldehyde. Calibration. The method was calibrated by comparing it with the neutral potassium iodide procedure for ozone using a n OREC model MSA-3 Ozone Analyzer. The ozone-enriched air stream was split and simultaneously analyzed by 2-DIH and potassium iodide procedures. The amount of ozone found with the potassium iodide was used as standard and related to the intensity of fluorescence observed from the dipyridyl ethylene procedure. Using standardized solutions, pyridine-4-aldehyde was carried through the analytical procedure and the relationship between aldehyde concentration and fluorescence intensity determined. A linear relationship was found up to 0.0 to 5.0 pg ml-1 pyridine-4-aldehyde. Relating ozone concentration to pyridine-4-aldehyde concentration oia the intensity of fluorescence, we found that 1.0 pg of ozone corresponded to 2.9 pg of pyridine-Caldehyde, i.e., 1 mole of ozone generates 1.3 moles of aldehyde. Sensitivity and Precision. The dipyridyl ethylene method using 2-DIH as developing agent is very sensitive; using 5 ml of absorbing reagent and sampling 10 liters at 0.5 1. min-I, the lower limit of detection is 0.05 ppm of ozone. By taking a larger aliquot of the absorbing solution, it is possible to detect (5) W. S. Knowles and Q . E. Thompson, J. Org. Chem., 25, 1031 (1 960). (6) J. Carlos and S. Fliszar, Can. J. Chem., 47, 1113 (1969). (7) J. J. Pappas and W. P. Keaveney, Tetrahedron Lett., 36, 4273 (1966). ANALYTICAL CHEMISTRY, VOL. 42, NO, 8, JULY 1970
843
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Table 11. Atmospheric Concentration of Interfering Substances and Their Equivalent Concentration in Absorbing Solution Concentration Usual of pollutant Interfering concentration in absorbing substance in air, ppm solution, pg 1.-1 Dimethyl butene 0.2 0.68 Acrolein 0.1 0.23 Nitrogen dioxide 0.3 0.56 Sulfur dioxide 1 .o 2.62 Hydrogen sulfide 0.5 0.69 Formaldehyde 0.1 0.12
0.02-0.05 ppm. The method can be extended to high atmospheric concentrations of ozone by increasing the volume of the absorbing solution. Thus the method described is some 2 or 3 times more sensitive than the colorimetric method of Hauser and Bradley (3). Twelve different ozone concentrations were analyzed for ozone simultaneously by potassium iodide and 2-DIH, and 4 samples were taken at each level. From an analysis of variance, the overall precision of a single measurement was found
to be ~k0.08pg 0 3 per ml absorbing reagent and the standard error was k0.065 pg per ml. INTERFERENCES
The effect of interfering substances on fluorescence was determined. An ozone-enriched air stream was split in half; one half was sampled by the prescribed procedure, in the other, the absorbing solution contained microgram quantities of the interfering substance. After passing both samples through the analytical procedure, any increase or decrease in fluorescence was noted. The substances tested and the concentrations used are shown in Table 11. The concentration of pollutant in absorbing solution is such as to be equivalent to the amount of pollutant normally found in 10 liters of air. Interferences due to aliphatic aldehydes and ketones are eliminated since the chromophore produced by them fluoresces at 520 nm with excitation at 425 nm. At the concentrations of interfering substances found in air and within the limits of experimental error, none of the substances caused a significant change in fluorescence and thus the method is specific for ozone in the atmosphere.
RECEIVED for review November 10, 1969. Accepted April 3, 1970.
Liquid Anion Membrane Electrodes Sensitive to Metal Cation Concentration Giancarlo Scibona, Leda Mantella, and Pier Roberto Danesi Industrial Chemistry Laboratory, Comitato Nazionale per L'Energia Nucleare, Centro di Studi Nucleari Casaccia, Rome, Italy
Liquid membranes formed by organic solutions of long chain alkylammonium salts behave as liquid electrodes sensitive to the anion concentration. Liquid anion membranes can be used as electrodes sensitive and selective to the aqueous concentration of metal cations through their anionic complexes. By means of this new use of the liquid anion membrane, it is possible to develop electrodes that are highly selective with respect to other metals that either do not form complexes in the aqueous solution or form weaker complexes than the metal ion, which anionic complex is the counterion of the alkylammonium radical. Metal concentrations of zinc or palladium are determined by means of liquid membranes formed by benzene solutions of tetrachlorozinc or tetrachloropalladium(ll) salts of alkylammonium.
LONG CHAIN alkylammonium salts dissolved in low dielectric constant solvents are known to behave as liquid anion exchangers. The electrical potential of liquid membrane electrodes (organic solutions of long chain alkylammonium salts interposed between two aqueous electrolyte solutions of suitable composition) have been theoretically described and experimentaly tested for both the cases of monoionic and biionic potentials with ions of the same charge (1-5). (1) J. Sandblom, G. Eisenman, and J. L. Walker, Jr., J. Phys. Chem., 71, 3862, 3871 (1967). ( 2 ) G. Eisenman, ANAL.CHEM., 40,310 (1968). (3) 0. D. Bonner and D. C . Lunney, J. Phys. Chem., 70, 1140 (1966). (4) C. J. Coetzee and H. Freiser, ANAL.CHEM., 40, 2071 (1968). (5) P. R. Danesi, B. Scuppa, and G. Scibona, J. Phys. Chem., in
press. 844
ANALYTICAL CHEMISTRY, VOL. 42, NO. 8, JULY 1970
The diffusion-migration processes in the membrane phase and the exchange properties of the liquid exchangers seem to contribute to the electrical potential of the system. When an anion is strongly preferred by the membrane phase, the organic solution of the corresponding alkylammonium salt can be used as a liquid membrane electrode sensitive to it. Since some metal anion complexes are strongly preferred by the liquid anion exchangers and the concentration of the complexes depends on the metal and on the ligand concentrations, a membrane can be prepared that behaves as a liquid electrode highly selective to metal cations and sensitive to their concentrations (through their anionic complexes). In this paper, the electrochemical properties of a liquid anionic exchanger interposed between solutions containing one or more cations and one complexing anion have been studied. The simple complexing anion and the metal complex anions present in the solution will, of course, contribute (at different extent) to the membrane potential. Liquid membrane electrodes formed by benzene solutions of trilaurylammonium and tetraheptylammonium salts of tetrachlorozinc and tetrachloropalladium(I1) interposed between aqueous solutions containing zinc and palladium(I1) (as cations) and chloride (as anions) will be discussed. EXPERIMENTAL
Reagents and Solutions. Trilaurylamine (TLA) supplied by Rhone and Poulenc and tetraheptylammonium iodide (THAI) supplied by Eastman Kodak have been used as materials for the preparation of the other alkylammonium
