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Ozone chemiluminescence in environmental analysis
ANALYTICA CHIMICA ACE4 EIBVIER
Analytica
Chimica Acta 303 (1995) 127-135
Ozone chemiluminescence in environmental analysis Alexandros M. Mihalatos, Antony C. Calokerinos
*
Universiq ofAthens, Department of Chemistry, Laboratory of Analytical Chemistry, Panepistimiopolis, Zografou, 157 71 Athens, Greece Received 26 September
1994
Abstract The chemiluminescence arising from the reaction of ozone with a plethora of compounds is a powerful analytical tool. The applications related to environmental analysis are reviewed in this paper. The various methods of ozone generation and calibration of ozonizers are also discussed. The design and construction of a gas phase ozone chemiluminometer is presented. The apparatus was used to evaluate the ability of halogenated compounds to deplete ozone. The results indicate that gas phase ozone chemiluminescence can be successfully used for monitoring major environmental problems. Keywords:
Chemiluminescence;
Ozone; Chlorofluoro
compounds;
Halons
1. Introduction Ozone was discovered in 1839 by Christian Friedrich Schonbein who named the new compound after the greek verb “6{w” (to smell). The structure
of the molecule as 0, was established in 1898. Since its discovery, ozone has been used extensively in organic chemistry as an oxidant to produce ozonides. In 1880, Sir Walter N. Hartley observed that ozone absorbs ultraviolet radiation strongly in the region 200-300 nm, with maximum absorption at 254 nm. Shortly after World War II, ozone was detected as an atmospheric pollutant in Los Angeles [l]. The major chemical reaction which produces ozone in the atmosphere is that between atomic and molecular oxygen O,+O+M-tO,+M
(I)
where M is any third body, such as N, or 0, which removes energy and stabilizes ozone. Ozone absorbs UV radiation of longer wavelengths (W-B radiation, 240-300 nm) and breaks down to molecular and atomic oxygen which then consumes ozone
0, + 0 + 20, As a result, a naturally balanced ozone layer is established in the stratosphere which acts as a shield to solar W-B radiation [2]. At altitudes < 10 km, where only radiation with wavelengths > 280 nm is present, ozone as a pollutant can be generated via reaction 1 from atomic oxygen produced from nitrogen dioxide NO, + hv(280-430nm)
l
Corresponding
author.
0003-2670/95/$09.50
0 1995 Elsevier Science B.V. All rights reserved
SSDI 0003-2670(94)00486-2
+ NO + 0
The ozone concentration in unpolluted tropospheric air is in the range of 20-50 ppbv while in
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A.M. Mihalatos, A.C. Calokerinos/Analytica
polluted urban areas it can increase up to 400 ppbv [31. The ozone layer depletion occurs by halogenated hydrocarbons such as chlorofluorocarbons (CFCs) and halons which generate halogen atoms upon UV radiation [4], as shown for CFC-12 (CF,Cl,)
Chimica Acta 303 (1995) 127-135
follow a single mechanism and various CL reactions will be discussed independently. Nitrogen monoxide The reaction has been thoroughly investigated [7]: NO+O,-+NO;
+O,
CF,Cl, + hv + CF,Cl + Cl
NO; + NO, + h v( 600-875
Cl + 0, + Cl0 + 0,
and the kinetic parameters have been studied [8]. The reaction is used for continuously monitoring nitrogen monoxide at pptv levels with linearity over 6 orders of magnitude in polluted atmospheric air [9]. Nitrogen dioxide does not interfere with the measurement and can be determined only after photolytic [lo] or thermal [ll] conversion to nitrogen monoxide.
Cl0 + 0, + Cl + 20, Ozone depletion by halons is more severe than by CFCs since bromine atoms are also present, as simplified for CF,ClBr (Halon-1211) CF,ClBr + hv + CF, + Cl + Br Cl + 0, + Cl0 + 0, c10+0,+c1+20, Br + 0, + BrO + 0, BrO+O,+Br+20, Hence, it is very important to follow the concentration of ozone and study its reactivity towards compounds which are introduced as substitutes to ozone depleting compounds. Chemiluminescence (CL) offers a very sensitive way to measuring ozone and will be reviewed in this paper. 1.1. Chemiluminescence from ozone Ozone is one of the most widely used CL reagents. Since 1896 [S], when the CL induced by the oxidation of ethanol with ozone was observed, it has been shown that nearly all gas-phase reactions of ozone are chemiluminescent [6]. The wavelengths emitted are in the range 250 to 2500 nm. The chemiluminogenie properties of ozone can be attributed to the weak O-O (24.9 kcal mol-‘) bond in ozone which is broken to form stronger 0==0 (119 kcal mol-‘) in molecular oxygen and much stronger C=O (172 kcal mol-‘) bonds in reactions with many organic compounds. The mechanism of ozonolysis has been of tremendous importance in giving a better understanding of the course of the reactions and particularly the nature and the fate of the active oxygen present in the ozonolysis product. The reaction does not appear to
nm)
Ethylene The overall reaction can be written as 2CzH, + 20, + 4CH,=O
+ 0, + hv
( &n,x = 440nm) but the reaction involves many steps [12]. Trimethylethylene and tetramethylethylene generate CL (A,,, = 520 nm) which is 50 times more intense than that from ethylene [13]. Fluoroethenes also chemiluminesce during ozonolysis, such as tetrafluoroethene by forming excited CF, (500-700 nm) [14] and chlorotrifluoroethene by forming excited FCO (375-475 nm) [151. Carbon monoxide The CL reaction [16] CO + 0, --) CO, + 0, + hv( A,,, = 410 nm) is very slow but as the temperature is raised, it becomes faster due to the decomposition of ozone to atomic oxygen which then reacts with carbon monoxide [ 171. Sulphur compounds Ozone reacts selectively with reduced sulphur compounds to generate excited SO, [18]. Thus, hydrogen sulphide and methyl mercaptan generate CL [19] by the following reactions H,S+03+SOz+H,0+hv CH,SH + 0, + CH,OH + SO, + hv
A.M. Mihalatos, AK. Calokerinos/Analytica Chimica Acta 303 (1995) 127-135
Recently, a sulphur chemiluminescence detector has been investigated. The detector incorporates a hydrogen-rich hydrogen/oxygen flame. Air containing sulphur dioxide is introduced into the flame where SO is produced. The flame gases are then mixed with ozone and the chemiluminogenic reaction SO+O,+SO,+0,+hv allows measurement of sulphur dioxide from ca. 1 up to 100 ppmv [20]. Sulphur compounds in petrol can be determined after gas chromatographic separation and conversion to SO which is then measured chemiluminometrically [21].
129
The UV emission is due to the reactions Aso; +o,+Aso,+o,+o Aso+o+As+o, As+o,+Aso*
+o,
Aso* +AsO+hv Hence, arsenic, antimony, tin and selenium as hydrides can be determined with limits of detection equal to 0.003, 0.2, 0.7 and 2.2 ppb, respectively
Dl. 1.2. Generation of ozone
Nickel carbonyl
Ozone is generated in ozonizers by W radiation or by an electrical discharge of a stream of pure oxygen or air.
When nickel carbonyl is mixed with ozone and purified carbon monoxide, the following reactions occur [22]
Electrical discharge The ozonizer consists of three coaxial glass cylin-
Ni( CO), + 0, -+ NiO + products NiO+CO+Ni+CO, Ni+O,+NiO*
+O,
NiO’+NiO+hv The emission intensity is measured at ca. 500 nm. The reaction allows the measurement of nickel carbony1 in air in the range lo-60 ppbv with a limit of detection equal to 2 ppbv [23].
O,+e+20
Hydrides
When ozone reacts with arsine, chemiluminescence is generated by a complex reaction sequence. The spectrum shows a discrete emission in the UV region and a continuum in the visible region [24]. The visible emission is due to the following simplified reactions AsH, + 0, + HO, + H&O HO,+O,+OH+20, OH + AsH, + H,O + ASH,
Aso; +AsO,+hv
O,+O+M-,O,+M A double ozonizer has also been described which has two central electrodes and a common outer electrode C261. W radiation In this technique, pure oxygen or air is passed through a tube or a bottle and is subjected to W radiation. The photochemical reactions which take place are the following: 02+hv+20
AsH2+0,-+AsO+H,0 Aso+o,-+Aso;
ders with wall thickness ca. 1 mm. The discharge takes place in the space between the inner and middle cylinders. The inner cylinder and the space between the outer and middle cylinders are fiRed with 10% sodium chloride solution and serve as electrodes. Oxygen or air at atmospheric pressure is purged at the gap between the imier and middle cylinders at flow rates up to 5.5 1 min-‘. As the gas flows through the gap, voltage up to 10 kV is applied across the electrodes and the following reactions occur
+o,
O,+O+M-+O,+M Both types of ozonizers generate ozone within a stream of oxygen. Isolation of ozone from oxygen is
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A.M. Mihalatos, A.C. Calokerinos/Analytica
Chimica Acta 303 (1995) 127-135
troublesome and can be achieved by using the differences in freezing points of the two gases ( - 193 and - 183°C for ozone and oxygen, respectively). Ozonized oxygen is not hazardous but the reaction of ozone with many compounds may lead to ozonides or peroxides, many of which are explosively unstable [27,281.
which, in neutral and alkaline conditions, reacts to liberate iodine at a stoichiometry of 1:l 20, + 21- + 2H,O + 40H-+ 20, + I,
1.3. Calibration of ozonizers
and increased yields of iodine occur by the following sequence of reactions 2H0, + 21- + 4H++ 2H,O + 2H0, + I,
The yield of ozone from ozonizers can be determined by various methods but gas phase titration, UV absorption and iodometry are the most commonly used.
In acidic solutions, the following reaction occurs H++O; -+HO, and HO, then reacts with iodide 2H0, + 21- + 2HO; + I,
2H0, + 21- + 2H+ --) 2H,O, + I, 2H,O, + 21- + 20H + 20H- + I, 20H+21-+20H-+I*
Gas phase titration Ozone is “titrated” chemiluminometrically with nitrogen monoxide. A constant flow rate and concentration of ozone is introduced into the CL cell. A standard dilute mixture of nitrogen oxide is then introduced at increasing flow rates into the cell. The emission intensity is linearly dependent to the nitrogen monoxide entering the cell until the equivalence point where the response remains practically constant [29]. W absorption This is a direct method and is based on the measurement of absorbance at 254 nm. Nevertheless, it requires a long path cell (l-5 m) since 1 ppm of ozone absorbs about 3% of a UV beam per meter
[301.
Therefore, a neutral buffered potassium iodide (NBKI) solution should be used for the liberation of iodine at a stoichiometry of 1:l with ozone. The results of the NBKI method compare very well with the W absorption method [35]. The method can also be used by measuring spectrophotometrically the iodine formed, after addition of starch [36]. 1.4. Measurement of ozone by chemiluminescence Ozone can be measured by chemiluminescence either in the gas phase or in solutions. Gas phase chemiluminescence Gas phase chemiluminometric measurement of ozone is very sensitive but requires accurate control of gas flows. A typical schematic diagram of a gas phase chemiluminometer is shown in Fig. 1. The
Iodometry In this technique, ozonized air is bubbled through a solution of potassium iodide 03+2H++21-+02+H,0+I, and the iodine produced is titrated with standard thiosulphate. This procedure is the reference method [31]. In neutral solutions, the stoichiometry between reacting ozone and liberated iodine is 1:l [32] and the method can be used quantitatively for concentrations of ozone as low as 0.1 ppm [33]. The mechanism involves the intermediate formation of ozonide ion, 0, [34]: 20, + 21- + 20; + I,
Fig. 1. Schematic diagram of gas phase chemiluminometer (A: analyte gas, R: reactant gas, PMT: photomultiplier tube, H.V.: high voltage, i/V: current-to-voltage converter, AMP: amplifier).
A.M. Mihalatos, A.C. Calokerinos/Analytica
131
Chin&a Acta 303 (1995) 127-135
Table 1 Analytical methods for the determination of ozone by solution chemiluminescencc Reactant
Comments
Linear range (detection limit)
Ref.
Eosin Y lndingo-5,5’-disulphonate
In the presence of ethylene glycol 0, is bubbled through the aqueous solution of the reactant Continuous flow measurement Rhodamine-B on silica gel Ethanolic solution
0.2-400 ppbv (0.2 ppbv) 0.025-410 ng ml-’ (0.006 ng ml-‘) up to 100 ppmv (0.4 ppbv) up to 0.4 ppm (< 1 X low4 ppm) (0.0003 % v/v)
[451 [46] 1471 [481 [491
Rhodamine-B Rhodamine-B + Gallic acid
analyte and the reactant are introduced into a cell where they mix and radiation is emitted. The pressure of the cell should be slightly lower than atmospheric. If required, wavelength discrimination can be achieved by filters. The geometry of the cell is important. Typical designs include a spherical cell into which the gases enter through separate inlets 1371 and a cylindrical cell into which the gases are introduced through independent tubes and mix at the back of the window through which the radiation is monitored [38]. The most common analytical method for ozone measurement is based on the CL reaction with ethylene [39] which can be used for the measurement of 3 ppbv to 30 ppmv [40]. Solution chemiluminescence The difficulty of gas handling has made solution CL very attractive for the determination of ozone. A plethora of methods has been developed and this area of application is still open to further research (Table 1). 1.5. Measurement of other compounds by ozone chemiluminescence Numerous compounds have been found to exhibit CL when mixed with ozone in aqueous solution,
such as ethanol [41], umbelliferone, [42], acridine [43] and xanthene dyes [44]. Furthermore, various compounds have been determined by ozone CL (Table 2). This paper describes the construction of an apparatus to investigate the reaction of halogenated compounds with ozone, based on the ozone-ethylene CL reaction.
2. Experimental
Apparatus A schematic diagram of the apparatus is presented in Fig. 2. The detector housing included a borosilicate glass reaction chamber positioned in front of the photomultiplier tube (PMT) without any wavelength discriminator. Inside the reaction chamber the outlets of the tubes for ozone and ethylene are properly placed at the bottom of the quartz window for greatest sensitivity. The chamber was backed by a mirror for maximum light collection by the photocathode. The PMT (Hamamatsu 464R, S-5 response) and the chamber were housed in a laboratory-made light-tight unit. The photomultiplier tube was operated at - 1200 V (cathode luminous sensitivity = 95 PA lm- ’ ) supplied by a Heath Universal Power Supply (O-1500 V). The output of the photocathode was fed
Table 2 Analytical methods for the determination of various compounds by ozone CL Analyte
Comments
Linear range (detection limit)
Ref.
Arsine Isoprene Nickel carbonyl Nitrous acid
Generation of arsine from As,O, + NaBH, 1 In air, emission from HCHO ’ _ In air, emission from NiO * In air, sample through solution of ascorbic acid solution to reduce nitrite to NO Thermal conversion to PH, In natural waters, generation of SiH, with LiAlH,
up to 158 ppbv AsH, up to 350 ppbv (400 pptv) up to 100 ppb (0.01 ppb)
1501 1511 1521
0.21-8.5 ppb (0.11 ppb) @ngPml-‘) (0.5 pg Si)
[531 [541 1551
Phosphate Silicate
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A.M. Mihalutos, A.C. Ca~okerinos/Ana~ytica ChimicaActa 303 (1995) 127-135
to a current-to-voltage (I/V) converter based on an RCA CA3140 operation amplifier. Damping was provided by inserting an RC circuit between the converter and the data acquisition card (PCL 718 high performance DAS card) interfaced to an IBM compatible computer (microprocessor: Intel 80386SX at 33 MHz). A program written in C language supervises the whole measurement sequence, undertakes the data acquisition and the presentation of analytical signals. The flow of each gas reagent was controlled by calibrated Matheson flowmeters. All tubes were made from PTFE or glass. All the experiments were performed at room temperature. Reagents Ozone was generated by passing air through a glass reactor with a total volume of 2.5 1. A low pressure mercury lamp was positioned in the reactor. The lamp was controlled by a Beckman hydrogen lamp power supply and the emission spectrum is shown in Fig. 3. The amount of ozone generated was controlled by the air flow rate. Water vapour was
6. A. U. 50 40 30 20 10 II 400
250
300
350
400
450
500
Wavelength,rim Fig. 3. Emission spectrum of low pressure mercury lamp used for generation of ozone (0.35 m Czemy Turner monochromator, 0.050 mm slit width = 0.1 nm, AU: arbitrary units).
removed from air by passing it through a U-tube filled with silica gel. Ethylene was supplied by a gas cylinder. CFCl, (CFC-ll), CF,Cl, (CFC-12) and CHF,Cl (HCFC-22) were donated by Chemical Industries of North Greece (Thessaloniki), CFrClBr (Halon-1211) and CF,Br (Halon-1301) were purchased from a local supplier. Ozone concentrations were measured iodometritally. The NBKI solution contains 2% KI (Fluka) in phosphate buffer (pH 7.00).
3. Results and discussion 3.1. Preliminary work
Pm
The experimental apparatus was designed in order to investigate the ability of halogenated compounds to deplete ozone. Therefore, the apparatus and the experimental parameters were optimized in order to follow the reduction of ozone CL by halogenated compounds. Various designs of ozonizers and CL cells were evaluated, the apparatus used for this work is shown in Fig. 2. The analytical signal depends on gas flow rates, pressure of cell and PMT voltage, which were optimized. 3.2. Effect of pressure
Fig. 2. Schematic diagram of ozone chemiluminometer and cell for monitoring the depletion of ozone by halogenated compounds.
As expected, the pressure in the cell plays an important role on the sensitivity of the measurement (Fig. 4) mainly due to reduction of quenching by the presence of nitrogen and oxygen molecules within
133
A.M. Mihalatos, A.C. Calokerinos/Analytica ChimicaActa 303 (1995) 127-135 R. E. I.
R. E. I.
J 0
J
150
250
350
450
Pressure,
550
650
0.1
Ethylene
750
mmHg
0.2
0.3
0.4
flow rate,
0.6
0.8
0.7
0.8
mmol/s
Fig. 5. Effect of ethylene flow on the emission intensity from (1) 1100, (2) 750, (3) 500, (4) 300 and (5) 220 ppmv of ozone.
Fig. 4. Effect of pressure of cell on the emission intensity from (1) 2400, (2) 2700 and (3) 3000 ppmv of ozone (R.E.I.: relative emission intensity).
the chemiluminescing gas mixture. All further measurements were carried out at 720 mmI-Ig. 3.3. Effect of flow rate of ethylene The effect of flow rate of ethylene on the emission intensity from various concentrations of ozone is presented in Fig. 5. All further work was carried out by supplying 0.05 mmol s-l to the cell since higher flow rates affect the pressure within the cell and reduce reproducibility.
ppmv] - 1.32 (r = 0.993, n = 61, but another linear portion also extends from 500 to 2300 ppmv ozone with lower sensitivity. In order to investigate the ability of a compound to deplete ozone, the compound was mixed with air prior to introduction into the ozonizer (Fig. 2). The
SIN
1M
1
3.4. Effect of PMT uoltage The effect of signal-to-noise ratio of the PMT is shown in Fig. 6; - 1200 V was chosen as optimum.
3.5. Depletion of ozone by halogenated compounds Under the optimized conditions, the calibration graph is linear within the range 50.0-500 ppmv of ozone: REI (rel. emission intensity) = 0.212[0,,
0
zca
4M
ooo
mo
Iwo
l200
14m
PMT voltage, V Fig. 6. Effect of PMT voltage on the signal-to-noise
ratio.
134
A.M. Mihalatos, A.C. Calokerinos/Analytica h
Chimica Acta 303 (1995) 127-135
u.
R.E. I. I 100 -
;;-q --4.
30
---~.-
-
60
-
40
-
20
-
4
20 . 10 . 0
00 3
I 10
20
30
Tqmin
Fig. 7. Reduction of ozone emission intensity (1) without halogenated compound and when (2) 1.08 (3) 3.15 and (4) 4.10 mm01 S -’ of CFsBr (Halon 1301) are also introduced into the ozonizer.
setup is the simplest possible perfectly stirred reactor but has the limitation that it can only detect depletion of ozone by halogenated compounds which decompose to chlorine and/or bromine atoms at 254 nm. A constant flow of ozone is generated within the ozonizer and the corresponding CL signals are measured (Fig. 7). When the halogenated compound is also introduced into the gas mixture entering into the ozonizer, the output is decreased due to ozone depletion (Fig. 7). The %decrease of emission intensity (%DEI) is linearly related to the flow (mmol s-t> of the given halogenated compound which is introduced into the ozonizer, as shown in Fig. 8 for CFC-11 and CFC-12. The slope of the %DEI vs. mmol s-l line is directly proportional to the ability of the compound to deplete ozone under the given experimental conditions. The observation was verified for all compounds examined and the results are summarized in Table 3. By plotting the oxygen depletion potential (ODP) values with the slope of the %decrease of
0
I
0
0.2
0.4
1
I
0.8
0.8
I
1
1.2
CFC, mmole/s Fig. 8. % Reduction of emission intensity as a function of flow of (1) CFC-12 (CFaCI,) and (2) CFC-11 (CFCI,)
emission intensity, a straight line is obtained: slope = 97.0 ODP + 14.55 (n = 5).
4. Conclusions The vast knowledge of ozone chemiluminescence found within the literature has been introduced into the area of evaluation of the depletion of ozone by halogenated compounds. The experimental apparatus is further modified and new procedures are continuously evaluated in order to establish ozone chemilu-
Table 3 %Decrease of emission intensity (%DEI) and ODP values for the halogenated compounds examined Compound
ODP a
Halon(CFsBr) Halon(CF&IBr) CFC-11 (CFCI,) CFC-12 (CF,Cl,) HCFC-22 (CHF,Cl)
10 4 1 1 0.01
Regression line of %DEI vs. mmol s- ’ of halogenated compound Slope
Intercept
Range, mm01 s-r
rMb
781 332.6 136.4 82.4 7.61
4.58 11.1 3.40 0.047 8.04
0.040-0.11 0.10-0.25 0.10-0.70 0.10-1.2 1.0-7.5
0.994 (11) 0.998 (6) 0.998 (13) 0.998 (9) 0.996 (11)
’ Ozone depletion potential 1561. b Correlation coefficient (number of measurements).
J 1.4
A.M. Mihalatos, A.C. Calokerinos/Analytica Chimica Acta 303 (1995) 127-135
minescence as a tool for examining the behaviour of substitutes of halogenated compounds towards ozone. Work on the correlation of experimental parameters with the ability of a compound to deplete ozone is currently extended to a wider range of compounds.
Acknowledgements This work was supported by the Commission of the European Communities within the frame of the ENVIRONMENT programme.
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Analytica
Chimica Acta 303 (1995) 127-135
Ozone chemiluminescence in environmental analysis Alexandros M. Mihalatos, Antony C. Calokerinos
*
Universiq ofAthens, Department of Chemistry, Laboratory of Analytical Chemistry, Panepistimiopolis, Zografou, 157 71 Athens, Greece Received 26 September
1994
Abstract The chemiluminescence arising from the reaction of ozone with a plethora of compounds is a powerful analytical tool. The applications related to environmental analysis are reviewed in this paper. The various methods of ozone generation and calibration of ozonizers are also discussed. The design and construction of a gas phase ozone chemiluminometer is presented. The apparatus was used to evaluate the ability of halogenated compounds to deplete ozone. The results indicate that gas phase ozone chemiluminescence can be successfully used for monitoring major environmental problems. Keywords:
Chemiluminescence;
Ozone; Chlorofluoro
compounds;
Halons
1. Introduction Ozone was discovered in 1839 by Christian Friedrich Schonbein who named the new compound after the greek verb “6{w” (to smell). The structure
of the molecule as 0, was established in 1898. Since its discovery, ozone has been used extensively in organic chemistry as an oxidant to produce ozonides. In 1880, Sir Walter N. Hartley observed that ozone absorbs ultraviolet radiation strongly in the region 200-300 nm, with maximum absorption at 254 nm. Shortly after World War II, ozone was detected as an atmospheric pollutant in Los Angeles [l]. The major chemical reaction which produces ozone in the atmosphere is that between atomic and molecular oxygen O,+O+M-tO,+M
(I)
where M is any third body, such as N, or 0, which removes energy and stabilizes ozone. Ozone absorbs UV radiation of longer wavelengths (W-B radiation, 240-300 nm) and breaks down to molecular and atomic oxygen which then consumes ozone
0, + 0 + 20, As a result, a naturally balanced ozone layer is established in the stratosphere which acts as a shield to solar W-B radiation [2]. At altitudes < 10 km, where only radiation with wavelengths > 280 nm is present, ozone as a pollutant can be generated via reaction 1 from atomic oxygen produced from nitrogen dioxide NO, + hv(280-430nm)
l
Corresponding
author.
0003-2670/95/$09.50
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SSDI 0003-2670(94)00486-2
+ NO + 0
The ozone concentration in unpolluted tropospheric air is in the range of 20-50 ppbv while in
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A.M. Mihalatos, A.C. Calokerinos/Analytica
polluted urban areas it can increase up to 400 ppbv [31. The ozone layer depletion occurs by halogenated hydrocarbons such as chlorofluorocarbons (CFCs) and halons which generate halogen atoms upon UV radiation [4], as shown for CFC-12 (CF,Cl,)
Chimica Acta 303 (1995) 127-135
follow a single mechanism and various CL reactions will be discussed independently. Nitrogen monoxide The reaction has been thoroughly investigated [7]: NO+O,-+NO;
+O,
CF,Cl, + hv + CF,Cl + Cl
NO; + NO, + h v( 600-875
Cl + 0, + Cl0 + 0,
and the kinetic parameters have been studied [8]. The reaction is used for continuously monitoring nitrogen monoxide at pptv levels with linearity over 6 orders of magnitude in polluted atmospheric air [9]. Nitrogen dioxide does not interfere with the measurement and can be determined only after photolytic [lo] or thermal [ll] conversion to nitrogen monoxide.
Cl0 + 0, + Cl + 20, Ozone depletion by halons is more severe than by CFCs since bromine atoms are also present, as simplified for CF,ClBr (Halon-1211) CF,ClBr + hv + CF, + Cl + Br Cl + 0, + Cl0 + 0, c10+0,+c1+20, Br + 0, + BrO + 0, BrO+O,+Br+20, Hence, it is very important to follow the concentration of ozone and study its reactivity towards compounds which are introduced as substitutes to ozone depleting compounds. Chemiluminescence (CL) offers a very sensitive way to measuring ozone and will be reviewed in this paper. 1.1. Chemiluminescence from ozone Ozone is one of the most widely used CL reagents. Since 1896 [S], when the CL induced by the oxidation of ethanol with ozone was observed, it has been shown that nearly all gas-phase reactions of ozone are chemiluminescent [6]. The wavelengths emitted are in the range 250 to 2500 nm. The chemiluminogenie properties of ozone can be attributed to the weak O-O (24.9 kcal mol-‘) bond in ozone which is broken to form stronger 0==0 (119 kcal mol-‘) in molecular oxygen and much stronger C=O (172 kcal mol-‘) bonds in reactions with many organic compounds. The mechanism of ozonolysis has been of tremendous importance in giving a better understanding of the course of the reactions and particularly the nature and the fate of the active oxygen present in the ozonolysis product. The reaction does not appear to
nm)
Ethylene The overall reaction can be written as 2CzH, + 20, + 4CH,=O
+ 0, + hv
( &n,x = 440nm) but the reaction involves many steps [12]. Trimethylethylene and tetramethylethylene generate CL (A,,, = 520 nm) which is 50 times more intense than that from ethylene [13]. Fluoroethenes also chemiluminesce during ozonolysis, such as tetrafluoroethene by forming excited CF, (500-700 nm) [14] and chlorotrifluoroethene by forming excited FCO (375-475 nm) [151. Carbon monoxide The CL reaction [16] CO + 0, --) CO, + 0, + hv( A,,, = 410 nm) is very slow but as the temperature is raised, it becomes faster due to the decomposition of ozone to atomic oxygen which then reacts with carbon monoxide [ 171. Sulphur compounds Ozone reacts selectively with reduced sulphur compounds to generate excited SO, [18]. Thus, hydrogen sulphide and methyl mercaptan generate CL [19] by the following reactions H,S+03+SOz+H,0+hv CH,SH + 0, + CH,OH + SO, + hv
A.M. Mihalatos, AK. Calokerinos/Analytica Chimica Acta 303 (1995) 127-135
Recently, a sulphur chemiluminescence detector has been investigated. The detector incorporates a hydrogen-rich hydrogen/oxygen flame. Air containing sulphur dioxide is introduced into the flame where SO is produced. The flame gases are then mixed with ozone and the chemiluminogenic reaction SO+O,+SO,+0,+hv allows measurement of sulphur dioxide from ca. 1 up to 100 ppmv [20]. Sulphur compounds in petrol can be determined after gas chromatographic separation and conversion to SO which is then measured chemiluminometrically [21].
129
The UV emission is due to the reactions Aso; +o,+Aso,+o,+o Aso+o+As+o, As+o,+Aso*
+o,
Aso* +AsO+hv Hence, arsenic, antimony, tin and selenium as hydrides can be determined with limits of detection equal to 0.003, 0.2, 0.7 and 2.2 ppb, respectively
Dl. 1.2. Generation of ozone
Nickel carbonyl
Ozone is generated in ozonizers by W radiation or by an electrical discharge of a stream of pure oxygen or air.
When nickel carbonyl is mixed with ozone and purified carbon monoxide, the following reactions occur [22]
Electrical discharge The ozonizer consists of three coaxial glass cylin-
Ni( CO), + 0, -+ NiO + products NiO+CO+Ni+CO, Ni+O,+NiO*
+O,
NiO’+NiO+hv The emission intensity is measured at ca. 500 nm. The reaction allows the measurement of nickel carbony1 in air in the range lo-60 ppbv with a limit of detection equal to 2 ppbv [23].
O,+e+20
Hydrides
When ozone reacts with arsine, chemiluminescence is generated by a complex reaction sequence. The spectrum shows a discrete emission in the UV region and a continuum in the visible region [24]. The visible emission is due to the following simplified reactions AsH, + 0, + HO, + H&O HO,+O,+OH+20, OH + AsH, + H,O + ASH,
Aso; +AsO,+hv
O,+O+M-,O,+M A double ozonizer has also been described which has two central electrodes and a common outer electrode C261. W radiation In this technique, pure oxygen or air is passed through a tube or a bottle and is subjected to W radiation. The photochemical reactions which take place are the following: 02+hv+20
AsH2+0,-+AsO+H,0 Aso+o,-+Aso;
ders with wall thickness ca. 1 mm. The discharge takes place in the space between the inner and middle cylinders. The inner cylinder and the space between the outer and middle cylinders are fiRed with 10% sodium chloride solution and serve as electrodes. Oxygen or air at atmospheric pressure is purged at the gap between the imier and middle cylinders at flow rates up to 5.5 1 min-‘. As the gas flows through the gap, voltage up to 10 kV is applied across the electrodes and the following reactions occur
+o,
O,+O+M-+O,+M Both types of ozonizers generate ozone within a stream of oxygen. Isolation of ozone from oxygen is
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A.M. Mihalatos, A.C. Calokerinos/Analytica
Chimica Acta 303 (1995) 127-135
troublesome and can be achieved by using the differences in freezing points of the two gases ( - 193 and - 183°C for ozone and oxygen, respectively). Ozonized oxygen is not hazardous but the reaction of ozone with many compounds may lead to ozonides or peroxides, many of which are explosively unstable [27,281.
which, in neutral and alkaline conditions, reacts to liberate iodine at a stoichiometry of 1:l 20, + 21- + 2H,O + 40H-+ 20, + I,
1.3. Calibration of ozonizers
and increased yields of iodine occur by the following sequence of reactions 2H0, + 21- + 4H++ 2H,O + 2H0, + I,
The yield of ozone from ozonizers can be determined by various methods but gas phase titration, UV absorption and iodometry are the most commonly used.
In acidic solutions, the following reaction occurs H++O; -+HO, and HO, then reacts with iodide 2H0, + 21- + 2HO; + I,
2H0, + 21- + 2H+ --) 2H,O, + I, 2H,O, + 21- + 20H + 20H- + I, 20H+21-+20H-+I*
Gas phase titration Ozone is “titrated” chemiluminometrically with nitrogen monoxide. A constant flow rate and concentration of ozone is introduced into the CL cell. A standard dilute mixture of nitrogen oxide is then introduced at increasing flow rates into the cell. The emission intensity is linearly dependent to the nitrogen monoxide entering the cell until the equivalence point where the response remains practically constant [29]. W absorption This is a direct method and is based on the measurement of absorbance at 254 nm. Nevertheless, it requires a long path cell (l-5 m) since 1 ppm of ozone absorbs about 3% of a UV beam per meter
[301.
Therefore, a neutral buffered potassium iodide (NBKI) solution should be used for the liberation of iodine at a stoichiometry of 1:l with ozone. The results of the NBKI method compare very well with the W absorption method [35]. The method can also be used by measuring spectrophotometrically the iodine formed, after addition of starch [36]. 1.4. Measurement of ozone by chemiluminescence Ozone can be measured by chemiluminescence either in the gas phase or in solutions. Gas phase chemiluminescence Gas phase chemiluminometric measurement of ozone is very sensitive but requires accurate control of gas flows. A typical schematic diagram of a gas phase chemiluminometer is shown in Fig. 1. The
Iodometry In this technique, ozonized air is bubbled through a solution of potassium iodide 03+2H++21-+02+H,0+I, and the iodine produced is titrated with standard thiosulphate. This procedure is the reference method [31]. In neutral solutions, the stoichiometry between reacting ozone and liberated iodine is 1:l [32] and the method can be used quantitatively for concentrations of ozone as low as 0.1 ppm [33]. The mechanism involves the intermediate formation of ozonide ion, 0, [34]: 20, + 21- + 20; + I,
Fig. 1. Schematic diagram of gas phase chemiluminometer (A: analyte gas, R: reactant gas, PMT: photomultiplier tube, H.V.: high voltage, i/V: current-to-voltage converter, AMP: amplifier).
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131
Chin&a Acta 303 (1995) 127-135
Table 1 Analytical methods for the determination of ozone by solution chemiluminescencc Reactant
Comments
Linear range (detection limit)
Ref.
Eosin Y lndingo-5,5’-disulphonate
In the presence of ethylene glycol 0, is bubbled through the aqueous solution of the reactant Continuous flow measurement Rhodamine-B on silica gel Ethanolic solution
0.2-400 ppbv (0.2 ppbv) 0.025-410 ng ml-’ (0.006 ng ml-‘) up to 100 ppmv (0.4 ppbv) up to 0.4 ppm (< 1 X low4 ppm) (0.0003 % v/v)
[451 [46] 1471 [481 [491
Rhodamine-B Rhodamine-B + Gallic acid
analyte and the reactant are introduced into a cell where they mix and radiation is emitted. The pressure of the cell should be slightly lower than atmospheric. If required, wavelength discrimination can be achieved by filters. The geometry of the cell is important. Typical designs include a spherical cell into which the gases enter through separate inlets 1371 and a cylindrical cell into which the gases are introduced through independent tubes and mix at the back of the window through which the radiation is monitored [38]. The most common analytical method for ozone measurement is based on the CL reaction with ethylene [39] which can be used for the measurement of 3 ppbv to 30 ppmv [40]. Solution chemiluminescence The difficulty of gas handling has made solution CL very attractive for the determination of ozone. A plethora of methods has been developed and this area of application is still open to further research (Table 1). 1.5. Measurement of other compounds by ozone chemiluminescence Numerous compounds have been found to exhibit CL when mixed with ozone in aqueous solution,
such as ethanol [41], umbelliferone, [42], acridine [43] and xanthene dyes [44]. Furthermore, various compounds have been determined by ozone CL (Table 2). This paper describes the construction of an apparatus to investigate the reaction of halogenated compounds with ozone, based on the ozone-ethylene CL reaction.
2. Experimental
Apparatus A schematic diagram of the apparatus is presented in Fig. 2. The detector housing included a borosilicate glass reaction chamber positioned in front of the photomultiplier tube (PMT) without any wavelength discriminator. Inside the reaction chamber the outlets of the tubes for ozone and ethylene are properly placed at the bottom of the quartz window for greatest sensitivity. The chamber was backed by a mirror for maximum light collection by the photocathode. The PMT (Hamamatsu 464R, S-5 response) and the chamber were housed in a laboratory-made light-tight unit. The photomultiplier tube was operated at - 1200 V (cathode luminous sensitivity = 95 PA lm- ’ ) supplied by a Heath Universal Power Supply (O-1500 V). The output of the photocathode was fed
Table 2 Analytical methods for the determination of various compounds by ozone CL Analyte
Comments
Linear range (detection limit)
Ref.
Arsine Isoprene Nickel carbonyl Nitrous acid
Generation of arsine from As,O, + NaBH, 1 In air, emission from HCHO ’ _ In air, emission from NiO * In air, sample through solution of ascorbic acid solution to reduce nitrite to NO Thermal conversion to PH, In natural waters, generation of SiH, with LiAlH,
up to 158 ppbv AsH, up to 350 ppbv (400 pptv) up to 100 ppb (0.01 ppb)
1501 1511 1521
0.21-8.5 ppb (0.11 ppb) @ngPml-‘) (0.5 pg Si)
[531 [541 1551
Phosphate Silicate
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to a current-to-voltage (I/V) converter based on an RCA CA3140 operation amplifier. Damping was provided by inserting an RC circuit between the converter and the data acquisition card (PCL 718 high performance DAS card) interfaced to an IBM compatible computer (microprocessor: Intel 80386SX at 33 MHz). A program written in C language supervises the whole measurement sequence, undertakes the data acquisition and the presentation of analytical signals. The flow of each gas reagent was controlled by calibrated Matheson flowmeters. All tubes were made from PTFE or glass. All the experiments were performed at room temperature. Reagents Ozone was generated by passing air through a glass reactor with a total volume of 2.5 1. A low pressure mercury lamp was positioned in the reactor. The lamp was controlled by a Beckman hydrogen lamp power supply and the emission spectrum is shown in Fig. 3. The amount of ozone generated was controlled by the air flow rate. Water vapour was
6. A. U. 50 40 30 20 10 II 400
250
300
350
400
450
500
Wavelength,rim Fig. 3. Emission spectrum of low pressure mercury lamp used for generation of ozone (0.35 m Czemy Turner monochromator, 0.050 mm slit width = 0.1 nm, AU: arbitrary units).
removed from air by passing it through a U-tube filled with silica gel. Ethylene was supplied by a gas cylinder. CFCl, (CFC-ll), CF,Cl, (CFC-12) and CHF,Cl (HCFC-22) were donated by Chemical Industries of North Greece (Thessaloniki), CFrClBr (Halon-1211) and CF,Br (Halon-1301) were purchased from a local supplier. Ozone concentrations were measured iodometritally. The NBKI solution contains 2% KI (Fluka) in phosphate buffer (pH 7.00).
3. Results and discussion 3.1. Preliminary work
Pm
The experimental apparatus was designed in order to investigate the ability of halogenated compounds to deplete ozone. Therefore, the apparatus and the experimental parameters were optimized in order to follow the reduction of ozone CL by halogenated compounds. Various designs of ozonizers and CL cells were evaluated, the apparatus used for this work is shown in Fig. 2. The analytical signal depends on gas flow rates, pressure of cell and PMT voltage, which were optimized. 3.2. Effect of pressure
Fig. 2. Schematic diagram of ozone chemiluminometer and cell for monitoring the depletion of ozone by halogenated compounds.
As expected, the pressure in the cell plays an important role on the sensitivity of the measurement (Fig. 4) mainly due to reduction of quenching by the presence of nitrogen and oxygen molecules within
133
A.M. Mihalatos, A.C. Calokerinos/Analytica ChimicaActa 303 (1995) 127-135 R. E. I.
R. E. I.
J 0
J
150
250
350
450
Pressure,
550
650
0.1
Ethylene
750
mmHg
0.2
0.3
0.4
flow rate,
0.6
0.8
0.7
0.8
mmol/s
Fig. 5. Effect of ethylene flow on the emission intensity from (1) 1100, (2) 750, (3) 500, (4) 300 and (5) 220 ppmv of ozone.
Fig. 4. Effect of pressure of cell on the emission intensity from (1) 2400, (2) 2700 and (3) 3000 ppmv of ozone (R.E.I.: relative emission intensity).
the chemiluminescing gas mixture. All further measurements were carried out at 720 mmI-Ig. 3.3. Effect of flow rate of ethylene The effect of flow rate of ethylene on the emission intensity from various concentrations of ozone is presented in Fig. 5. All further work was carried out by supplying 0.05 mmol s-l to the cell since higher flow rates affect the pressure within the cell and reduce reproducibility.
ppmv] - 1.32 (r = 0.993, n = 61, but another linear portion also extends from 500 to 2300 ppmv ozone with lower sensitivity. In order to investigate the ability of a compound to deplete ozone, the compound was mixed with air prior to introduction into the ozonizer (Fig. 2). The
SIN
1M
1
3.4. Effect of PMT uoltage The effect of signal-to-noise ratio of the PMT is shown in Fig. 6; - 1200 V was chosen as optimum.
3.5. Depletion of ozone by halogenated compounds Under the optimized conditions, the calibration graph is linear within the range 50.0-500 ppmv of ozone: REI (rel. emission intensity) = 0.212[0,,
0
zca
4M
ooo
mo
Iwo
l200
14m
PMT voltage, V Fig. 6. Effect of PMT voltage on the signal-to-noise
ratio.
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A.M. Mihalatos, A.C. Calokerinos/Analytica h
Chimica Acta 303 (1995) 127-135
u.
R.E. I. I 100 -
;;-q --4.
30
---~.-
-
60
-
40
-
20
-
4
20 . 10 . 0
00 3
I 10
20
30
Tqmin
Fig. 7. Reduction of ozone emission intensity (1) without halogenated compound and when (2) 1.08 (3) 3.15 and (4) 4.10 mm01 S -’ of CFsBr (Halon 1301) are also introduced into the ozonizer.
setup is the simplest possible perfectly stirred reactor but has the limitation that it can only detect depletion of ozone by halogenated compounds which decompose to chlorine and/or bromine atoms at 254 nm. A constant flow of ozone is generated within the ozonizer and the corresponding CL signals are measured (Fig. 7). When the halogenated compound is also introduced into the gas mixture entering into the ozonizer, the output is decreased due to ozone depletion (Fig. 7). The %decrease of emission intensity (%DEI) is linearly related to the flow (mmol s-t> of the given halogenated compound which is introduced into the ozonizer, as shown in Fig. 8 for CFC-11 and CFC-12. The slope of the %DEI vs. mmol s-l line is directly proportional to the ability of the compound to deplete ozone under the given experimental conditions. The observation was verified for all compounds examined and the results are summarized in Table 3. By plotting the oxygen depletion potential (ODP) values with the slope of the %decrease of
0
I
0
0.2
0.4
1
I
0.8
0.8
I
1
1.2
CFC, mmole/s Fig. 8. % Reduction of emission intensity as a function of flow of (1) CFC-12 (CFaCI,) and (2) CFC-11 (CFCI,)
emission intensity, a straight line is obtained: slope = 97.0 ODP + 14.55 (n = 5).
4. Conclusions The vast knowledge of ozone chemiluminescence found within the literature has been introduced into the area of evaluation of the depletion of ozone by halogenated compounds. The experimental apparatus is further modified and new procedures are continuously evaluated in order to establish ozone chemilu-
Table 3 %Decrease of emission intensity (%DEI) and ODP values for the halogenated compounds examined Compound
ODP a
Halon(CFsBr) Halon(CF&IBr) CFC-11 (CFCI,) CFC-12 (CF,Cl,) HCFC-22 (CHF,Cl)
10 4 1 1 0.01
Regression line of %DEI vs. mmol s- ’ of halogenated compound Slope
Intercept
Range, mm01 s-r
rMb
781 332.6 136.4 82.4 7.61
4.58 11.1 3.40 0.047 8.04
0.040-0.11 0.10-0.25 0.10-0.70 0.10-1.2 1.0-7.5
0.994 (11) 0.998 (6) 0.998 (13) 0.998 (9) 0.996 (11)
’ Ozone depletion potential 1561. b Correlation coefficient (number of measurements).
J 1.4
A.M. Mihalatos, A.C. Calokerinos/Analytica Chimica Acta 303 (1995) 127-135
minescence as a tool for examining the behaviour of substitutes of halogenated compounds towards ozone. Work on the correlation of experimental parameters with the ability of a compound to deplete ozone is currently extended to a wider range of compounds.
Acknowledgements This work was supported by the Commission of the European Communities within the frame of the ENVIRONMENT programme.
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