peroxytrifluoroacetic acid at room temperature, was oxidized by peroxy-m-chlorobenzoicacid (50). Solutions of diphenylnitroxide were easily prepared from diphenylamine and peroxy-m-chlorobenzoic acid in ether; the reaction has been reported before with peroxybenzoic acid (18). (50) T. J. Delia, M. J. Olsen, and G. B. Brown,J . Org. Chem., 30, 2766 (1965).
ACKNOWLEDGMENT
The author thanks Willie Turner for assistance in preparative RECEIVED for review September 23,1969. Accepted February 11, 1970. Some of the results were presented at the Central Regional Meeting, American Chemical society, firon, Ohio, May 10,1968.
Nuclear Magnetic Resonance Study of Ion-Exchange Resins R. W. Creekmore’ and C. N. Reilley Department of Chemistry, University of North Carolina, Chapel Hill, N . C. 27514 The temperature dependent proton chemical shift difference between the resin phase water and the bulk water (in an aqueous suspension of ion-exchange resins) has been utilized to estimate the hydration number of several counterions in the resin phase. A comparison of these hydration numbers with those obtained for the same counterions in homogeneous halide solutions suggests that some ion association occurs in the resin phase. Further evidence of ion association was obtained in the case where the effect of crosslinking on the 2aNaNMR linewidths for a sodium resin was investigated. The increased line width obtained with increasing crosslinking is attributed to ion association and counterion rotational restriction. The 19F chemical shift of a fluoride resin was also dependent on crosslinking.
A PHYSICAL DESCRIPTION of the interaction of a counterion with its surrounding water structure and with the fixed ionic sites within an ion-exchange resin is of considerable interest and importance in understanding ionic selectivities of these systems. The theories which have been evoked to explain ion selectivities in crosslinked ion-exchange resins may be primarily classified in two groups. In the first, ion selectivities are attributed primarily to differences in counterion hydration while in the second the selectivities are attributed to differences in electrostatic interactions in which both the fixed ion and the solvent are considered to interact with the counterion. Gregor’s theory ( I ) , for example, postulates that selectivity arises primarily because of differences in the partial volumes of the counterions, which are related to the degree of ionic hydration. Thus, the resin has a higher affinity for the least hydrated ion. In contrast to this, Harris and Rice (2,3)have explained ion-exchange selectivities in terms of the electrostatic interaction of a counterion with the fixed ionic site and with the solvent and give, in this sense, a more molecular picture of the resin phase system. The counterions, for example, are viewed as existing in two states: some are located in the close proximity of the fixed ion, while others may be termed as “free.” The ionic selectivities were shown 1 Present address, Marshall Research Laboratories, E. I. du Pont de Nemours, Philadelphia, Pa. 19146.
(1) H. P. Gregor, J. Amer. Chem. SOC.,73,642 (1950). (2) F. E. Harris and S. A. Rice, J . Chem. Phys., 24, 1258 (1956). (3) S. A. Rice and F. E. Harris, 2.Physik. Chem. (Frankfurt), 8, 207 (1956). 570
ANALYTICAL CHEMISTRY, VOL. 42, NO. 6, MAY 1970
to be related to differences in ion pairing. Eisenman ( 4 ) also recognized the importance of electrostatic interactions in governing selectivities, especially in the case of crossovers and affinity reversals. An excellent review of the above theories may be found in Marinsky’s book (5). Because NMR (Nuclear Magnetic Resonance) has been proved quite useful in studying electrolyte solutions (6), it should prove particularly useful in studying ion-exchange resins. In fact, several studies have been reported. Gordon (7) for example, found that NMR could provide a very valuable tool for investigating resin heterogeneity, exchange phenomena, and counterion concentrations. Other workers (8-10) have demonstrated the analytical importance of NMR in characterizing ion exchange resins. The present study extends the earlier work by providing an estimate of the aqueous coordination number of the counterion, obtained from the temperature dependence of the proton chemical shifts of the water; and evidence for ion association between the counterion and the fixed ions, obtained by observing the NMR spectra of the counterion, Na+. Such studies are expected to offer some experimental credence to the above mentioned theories of ion-exchange selectivities. EXPERIMENTAL
Ion Exchange Resins. The resins used in this study were Dowex 50W (cation resin) and Dowex 1 (anion resin), both 50-100 mesh unless otherwise specified. The resins were first treated with 6 N HC1, placed in a column, and then treated with a large excess of the desired counterion. The effluent was monitored for the presence of acid and complete conversion was assumed when a negative test occurred, The resins were then washed with copious amounts of deionized water in order to remove residual electrolyte. The resins (4) G. Eisenman, “Membrane Transport and Metabolism,” A. Kleinzeller and A. Kotyk, Ed., Academic Press, New York, N. Y., 1961, pp 163-179. (5) D. Reichenberg, “Ion Exchange,” J. A. Marinsky, Ed., Marcel Dekker, Inc., New York, N. Y., 1966, pp 227-274. (6) J. F. Hinton, and E. S.Amis, Chem. Reu., 67,367 (1967). (7) J. E. Gordon, J. Phys. Chem., 66,1150 (1962). (8) J. P. DeVilliers and J. R. Parrish, J . Polym. Sci. A2, 1331 (1964). (9) R. H. Dinius, M. T. Emerson, and G. R. Choppin, J. Phys. Chem., 67, 1178 (1962). (10) R. H. Dinius and G. R. Choppin, ibid., 68,425 (1963).
were kept in deionized distilled water prior to NMR measurements. Inner Molalities. A portion of the resin suspension was added to a glass tube having a sintered glass disk at one end. The tube was placed in a centrifuge tube, stoppered to avoid water loss by evaporation, and then centrifuged at 2500 rpm for 45 minutes at room temperature. The inner tube was then wiped to remove exterior moisture, weighed, and placed in a vacuum oven to dry at 5 mm pressure and 95 "C over Pro5 for 48 hours. The tube was then reweighed, and the difference assigned to the water content of the resin. The technique is quite similar to that described by Pepper et al. (11). The dry exchange capacity, Qdrr for the resin was determined with the resins in their standard states (Le., H+ and Cl-, respectively) by titration data. The dry exchange capacities determined agreed with those reported on the labels. After normalizing the dry resin weights to their standard states, the molalities were calculated from the dry exchange capacities and the water regains. Nuclear Magnetic Resonance. The proton spectra were obtained on a Varian A-60 equipped with a Varian V-6040 temperature controller. The temperature was determined by measuring the chemical shift of methyl alcohol and ethylene glycol samples, which were calibrated by using a copper constantin thermocouple. The Varian calibration curves were found to be in error by as much as 3 "C on the ethylene glycol sample. The chemical shift us. temperature for the two samples were found to fit the following equations over the range specified : Methyl alcohol (0-30 "C) t ("C) = -103.9 Av (ppm) 186.9 Ethylene glycol (40-80 "C) t ("C) = -99.25 Av (ppm) 189.3 These equations agree within experimental error with those found recently by Van Geet (12). The 23Naand I 9 F results were conducted on a Varian HA100 operating at 26.452 and 94.1 MHz, respectively.
+
+
RESULTS AND DISCUSSION
Factors Influencing Chemical Shift. Past NMR investigations of ion-exchange resins have shown that both the resin phase and bulk phase water may be viewed separately, owing to the slow exchange rate of water between the two phases. Gordon (7) found in the case of a protonated resin, that the shift difference between the external water peak and the internal water peak corresponds with the proton chemical shifts for strong acids reported by Hood and Reilly (13). In addition the chemical shift of the external water peak was found to be the same as that of pure water except for a sniall shift due to bulk susceptibility differences (7); therefore the external water may be regarded as pure water. However, no one has amply demonstrated the close similarities of the shifts found in ion exchangers with those found in homogeneous electrolyte solutions (q.0. Hinton and Amis's review, ref 6). The early work of Shoolery and Alder (14) showed that the proton chemical shift of an electrolyte solution may be expressed as Jobs = m(n+G+ n.&) (1) where m is the molality of the electrolyte, 6+ and 6- are the contribution to the chemical shift, from the cation and anion,
+
(11) K. W. Pepper, D. Reichenberg, and D. K. Hale, J. Chem. SOC. 1952, 3129. (12) A. L. Van Geet, ANAL.CHEM., 40,2221 (1968). (13) G. C . Hood, 0. Redlich, and C. A. Reiliy, J. Chem. Phys., 22, 2067 (1954). (14) J. N. Shoolery and B. Alder, ibid., 23, 805 (1955).
A
-0.4
-0.3
-0.2
-0.1
0
tO.l
ppm
B
H
J
!
I
I
I
I
I
respectively. Although their measurements were not corrected for changes in bulk susceptibility, other investigators such as Hindman (15)have proved the validity of Equation 1. Because an ion-exchange resin may, as a first approximation, be thought of as a quasihomogeneous electrolyte solution (16), the above equation should apply. In Figure 1, a comparison is made between the molar cationic chemical shifts found in homogeneous metal halide solutions and the molar shifts found for the Dowex 5OW-8X resins having the same counterion (the resinate chemical shift (6-) was arbitrarily set equal to zero). The chemical shifts in the resin system were simply the chemical shift difference between that for the resin phase water (interior water) and that for the bulk water (exterior water.) Fortunately, ion exchange resins are spherical, thereby minimizing the need for susceptibility corrections (17). Absolute comparisons are not possible because the contribution of the resinate ion to the observed shifts is not known precisely; however, it can be seen from Figure 1 that the shifts in the resin system closely parallel those found for the homogeneous electrolytes. Ions which shift the water signal upfield and are normally classified as structure breakers are seen to behave in like manner when placed in an ion exchanger, The same is true for structure formers which shift the water signal downfield both in homogeneous and in the resin phase system. It is also interesting to note that the same chemical shift order of the ions is found in both systems. If the cation and anion remain of the same kind, the observed shift should be proportional to the concentration according to Equation 1. This proportionality is illustrated in Figure 2 where the chemical shifts in a sodium and in a protonated resin are plotted against their inner molalities. DeVilliers and Parrish (8) also found a linear relationship between inner molality and the chemical shift in the resins they studied. (15) J. C . Hindman, J . Chem. Phys., 36, 1000 (1962). (16) W. C. Bauman, and J. Eichorn, J . Amer. Chem. Soc., 69, 2830 (1947). (17) K. Frei and H. J. Bernstein, J. Chem. Phys., 37, 1891 (1962). ANALYTICAL CHEMISTRY, VOL. 42, NO. 6, MAY 1970
571
'"I/
2\
2.0
15
1.o
0.5
2x
R SO3- Na'
a5
P Pm Figure 2. Chemical shift A us. the inner molalities, m, for Dowex 50W resins with two different counterions. The crosslinking is indicated by the
Estimation of Primary Hydration Numbers of Counterions. The chemical shifts of the ion exchange resins investigated were found to be temperature dependent. Figure 3 illustrates the effect of temperature on the two kinds of water. Note that as the temperature is increased the two water peaks in the magnesium resin move further apart. It is, therefore, clear that the effect observed is not due to exchange as was thought earlier by Dinius et al. (9) but to differences in the effect of temperature on the chemical shift of external and internal water. The same effect has been demonstrated in the case of homogeneous electrolyte solutions (18-21), except in this case the temperature dependent shifts for pure water and for the water of the electrolyte solutions were not simultaneously measured. Information concerning the hydration of ions has been obtained from studying the temperature dependency of the observed proton shifts of an electrolyte solution (18-21). The rationale behind the determined hydration number is as follows : the resonance signal observed for an electrolyte solution reflects a weighted average environment for the water molecules. For example, a water molecule can exist in a normal water lattice, or it can exist in the hydration sphere of an ion. This first order approximation may then be expressed as
where 6,,,, is the observed chemical shift corrected for changes in bulk susceptibility, xb and X,are the mole fractions of bulk and solvation water, respectively, and Bb and 6, are the chemical shifts of bulk and solvation water, respectively. By taking the temperature differential of Equation 2 and assuming that (dSJdt) = 0 and that bulk water in an electrolyte solution behaves like pure water, then dL/dt
=
(1
- Xs)d8H,o/dt
(3)
Because X, = hrnJ55.55, where h is the total effective hydration -
(18) E. R. Malinowski, P. S. Knapp, and B. Feuer, J. Chem. Phys., 45, 4274 (1966). (19) E. R. Malinowski and P. S.Knapp, ibid., 48,4989 (1968). (20) P. S. Knapp, R. 0. Waite, and E. R. Malinowski, ibid., 49, 5459 (1968). (21) R. W. Creekmore and C. N. Reilley, J. Phys. Chem., 73, 1963 (1969).
572
ANALYTICAL CHEMISTRY, VOL. 42, NO. 6, MAY 1970
L
(RSOj), M i t
Figure 3. The effect of temperature on the proton chemical shifts for a sodium and magnesium Dowex 5OW-8X resin. The two peaks correspond to the interior and exterior water, the latter being indicated by an arrow Each division represents 10 Hz number and m is the molality, then
(4) In the case of ion-exchange resins, the numerator of Equation 4 was determined by measuring the shift between external and internal water (which will be referred to as A) at various temperatures. The validity of Equation 4 and the justification of some of the assumptions necessary in the derivation have been discussed previously (18-21). As can be seen in Figure 3, the intensities of the two water peaks for the magnesium resin change over the temperature range observed; this effect is not observed in the case of the sodium resin. The line width of the interior water broadens as the temperature is decreased, thus changing the intensity ratio of the two peaks. The observed broadening may be due to the exchange of free water with hydration water at a rate insufficient to narrow the observed line, yet fast enough so that only one peak is observed for interior water. The broadening may also be due to the large difference in relaxation exhibited by free and hydration waters. This explanation would be especially true for ions which are tightly bound to the resin. The above effect was also found in the case of an aluminum resin. The effect was not observed for those resins containing weakly hydrated ions. The broadening effect should not, however, alter the chemical shifts measured. In Table I, dA/dt values for both cation and anion resins are given with their calculated hydration numbers. The temperature dependent shift of water, d&,o/dt was taken as 0.00958 ppm/OC. as reported in our previous paper (21). In the case of both cation and anion resins, it is assumed that the fixed ions because of their large size to charge ratio are not hydrated; therefore, the hydration numbers determined for the various resins should reflect only the hydration of counterions.
Table I. A. Hydration Numbers of Various Cation Resins, Dowex 50W-X8(50-100 Mesh Resins) Counter ion Molality dAldT h H+ 4.4 0.00242 & 0.00011 2.ga Na+ 5.4 0.00261 & 0.00008 2.9 Kf 6.3 0.00297 i 0.00011 2.7 Rb+ 5.9 0.00278 i 0.00010 2.6 cs+ 5.8 0.00318 f 0.00004 3.2 Mg++ 2.65 0.00305 z!= 0.00015 6.7
E
a a
B. Hydration Numbers for Anion Exchange Resins, Dowex 1X8 (50-100 Mesh) Br5.8 0.00127 i 0.00023 1.3 I8.9 0.00178 i 0.00003 1.2 a Corrected to include per cent of acidic protons.
Table 11. Hydration Numbers of Electrolytes Determined by Temperature Variation Method Electrolyte Molality h HC1 ... 3.4" NaCl 3.08 4.6< NaBr 3.01 4.46 NaC104 2.98 3.OC Na (p-toluene 1.56 3.5d sulfonate) 2.47 3.0d 2.73 2.gb 3.29 2.gd KF 2.79 4.4d KC1 2.99 4. 6b RbCl 3.20 4.0b CSCl 3.02 3.96 2.92 8.2b MgCh (CHdrNC1 2.66 (0.6-1 . O ) b (CHahNBr 2.92 1. O b a Taken from the work of Knapp et al. ( 2 0 ) . b Taken from the work of Creekmore and Reilley ( 2 1 ) . c Values reported by both Malinowski et a / . (18, 20) and Creekmore and Reilley ( 2 1 ) . Values that have not been previously reported in the literature. The same technique was used here as reported in Creekmore and Reilley (21).
It is interesting to note that the alkali metal counterions investigated have approximately the same primary hydration number while the hydration number for the magnesium counterion is more than double the values for the alkali metals. By a more direct NMR method, Mgz+ was found to be hydrated in its first coordination shell by six waters (22); therefore, the number obtained by this method is quite reasonable The larger value for CsT is probably due to experimental error since A approaches zero in this case. Comparison of Resin Hydration Numbers with Hydration Numbers of Simple Electrolytes. Table I1 lists the hydration numbers previously obtained for several electrolytes containing the same resin counterions studied here. Once again the same trends found in the homogeneous solutions are also apparent in the quasihomogeneous ion-exchange resins. It appears that the alkali metal ions are hydrated to the same extent with the notable exception of a slight decrease in the determined hydration number in the series from sodium to cesium in the homogeneous electrolyte solutions. Also, the magnesium hydration number is found to hold about the (22) N. A. Matwiyoff andH. Taube, J . Amer. Chem. SOC.,90, 2796 (1968).
OC Figure 4. The effect of temperature on the proton chemical shift for Dowex 50W resins having various crosslinkings(%) same relationship with the alkali metal hydration numbers in both the homogeneous system and the ion exchange system. The anion exchange resins studied indicated that the primary hydration number is approximately one, a value which is consistent with the studies of (CH&NCl and (CH3)?NBrwhich range from 0.6 to 1.0. It is generally accepted that the hydration of anions is less than that of cations. The hydration numbers for the cation resins are obviously lower than those found for the alkali metal halides solutions. The values found for the alkali metal resins counterions are more in line with the values which have been previously reported for NaC104 and Na(ptoluenesu1fonate). The decreased hydration number in the case of NaC104 and Na(ptoluenesulfonate) could not be wholly attributable to a change in the anion hydration but suggested some association between the anion and the cation. This idea is consistant with the 23Na relaxation times obtained in homogeneous solution (23) for these compounds which also suggested some type of interaction between the anion and the cation. This interaction might be attributed to collision complexes between sodium and the C104- and p-toluenesulfonate anions. The value for Mgz+in the resin phase is also lower than the value found for MgC12. Here, the lower hydration number found for MgZ+ in the resin may be attributed to the differences between the C1- hydration (0.6-1.0) and the resinate hydration ( h -0). Effect of Crosslinking on Hydration. The effect of crosslinking on the determined hydration number also was studied. Figure 4 gives a plot of A us. temperature for several crosslinkings of a Dowex 50W type resin, each in the sodium cycle. If hydration is not a function of concentration, then all four lines should intersect at the same point (where Sb = as). It can be seen that the 2% resin deviates significantly from the others. The slopes of these lines along with the calculated hydration numbers are listed in Table 111. The hydration number appears to increase as the resin phase becomes more dilute, thus approaching a more ideal solution. The same hydration trend was also found for the monomeric analog, Na(ptoluenesulfonate), in homogeneous solution. These results appear to be consistent with the idea that, in these systems, ion pairing occurs at concentration greater than 1 molar. The (23) M. Eisenstadt and H. L. Friedman, J . Chem. Phys., 44, 1407 (1966). ANALYTICAL CHEMISTRY, VOL. 42, NO. 6, MAY 1970
573
~~~
~
~~~
Table 111. Hydration Numbers for Sodium Counterion as a Function of Crosslinking DVB, Molality dA/dT h 25 2.3 0 00140 3.5 4b 3.2 0.00169 3.0 84 5.4 0.00266 2.9 12c 7.0 0.00341 2.8 a Dowex 50W 50-100 mesh. Dowex 50W 20-50 mesh. Dowex 50W 100-200 mesh.
LO
2.0
30
m
-
Figure 6. 23Nalinewidth (WI/J as a function of concentration (molality) for solution of Na(p4oluenesulfonate) The linewidths of 23Naspectra are governed by a quadrupole relaxation mechanism (24) which may be expressed as:
8% DVB
- 40
0
Hz
t
40
Figure 5. The effect of crosslinking (%) on the 23Nalinewidths result would tend to indicate that sodium would possess four waters of hydration in dilute solutions (2 m). Counterion Coordination. If the hydration number determined for NaCl can be properly interpreted as indicating an average coordination of four water molecules for Na+, it would appear from the values of Table I that Na+ in a resin would coordinate three water molecules on the average. This also appears to be the case for the other alkali metal ions investigated. This coordination number may be the result of the sulfonic group occupying one of the coordination sites of sodium, the other three being occupied by waters. This is not to say that there is any covalent nature to the proposed species but that there is coulombic interaction between the fixed ion, RS03-, and the counterion, Na+. This possibility must be considered in view of the high concentration within the resin phase. It is known that the solvation waters of the alkali sec); hence, coordinametal ions are kinetically fast (tion with the anion, although weak, may be expected a t these concentration levels. Counterion Interaction with Fixed Ions. To test the feasibility of such a coordination model, the 23Naline broadening of sodium resins with different crosslinkings was investigated. 574
0
ANALYTICAL CHEMISTRY, VOL. 42, NO. 6, MAY 1970
where q is the electric field gradient, Q is the quadrupole moment, and T~ is the correlation time of the nucleus. If the environment around the ion is nearly spherical, then q is very small; the larger the asymmetry, the larger will be the value of q. Figure 5 illustrates the 23Naline broadening obtained as the crosslinking is increased and, thus, the internal ionic concentrations increased. The results reported here are consistent with the results of Jardetzky and Wertz (25). They found that the 23Na spectra for Dowex Ag-50 X 8 (Na+) was broadened beyond detection, which they rationalized as indicating strong binding of sodium to the resin. No other crosslinkings were reported by them, however. The increase in 23Na linewidth accompanying an increase in crosslinking may be due to several factors: increase in q, increase in T ~ and , increase in q and re. If association of sodium with the fixed ionic group exists, an increase in q should result since the resinate group should have a different electrostatic effect than water. An increase in T~ should also accompany interaction between the fixed ions (which would have a very large T ~ and ) more mobile sodium ions. However, the possibility that the resin matrix itself is responsible for the larger rC cannot be excluded. One can argue, nevertheless, that the resin matrix changes only the macroscopic viscosity and does not contribute to the microscopic viscosity, which should be the observable effect. Gordon (7) has shown that the proton spectra of proton-containing counterions, such as tetramethylammonium ion, are dipolar broadened as the crosslinking is increased, indicating an increase in rc. The effect seen in Figure 5 may, therefore, be attributable to both an increase in T~ and also to an increase inq with higher degrees of crosslinking. The 23Naline broadening found for the monomeric analog, Na(p-toluenesulfonate), can be seen in Figure 6. The same line broadening trend is observed as was found in the resin phase (24) H. G. Hertz, Ber. Bunserzges. Phys., Chem., 67, 311 (1963). (25) 0. Jardetzky and J. E. Wertz, J . Amer. Chem. SOC.,82, 318 (1960).
Table IV.
19F Chemical Shifts for a Dowex 1 Resin QdV"
DVB,
1 2 8
'Z
Qwet'
(megig)
(megig)
2.8 4.3 3.6
0.4 0.8 1.3
ZHzOa
Shift? ( P P ~ )
81.8
-0.49
71.6 45.0
-0.83 -2.24
Values taken from label accompanying purchased resins. * Relative to 3 m KF (external).
crosslinking. The downfield shift with increasing crosslinking (thus increasing interior electrolyte concentation) possibly results from a deshielding of the fluoride ion by some electrostatic association with the fixed ion. At present, however, little is known about the fluoride chemical shifts in homogeneous electrolyte solutions (26, 27), and a more precise interpretation must await further studies and interpretations.
a
studies. These 23Naresults strongly suggest that sodium is associated electrostatically with the resinate ion and that the difference between line widths of the resin and the monomeric analog at comparable concentration levels are chiefly due to differences in re since it is expected that q would be approximately the same in both cases. The NMR resonance of a counterion in an anion exchange resin was also investigated. In Table IV are given the lQF chemical shifts for a fluoride resin (Dowex 1) as a function of
RECEIVED for review September 30,1969. Accepted February 16, 1970. One of the authors (RWC) wishes to acknowledge the financial assistance provided by the University of North Carolina Materials Research Center, Contract SD-100 with the Advanced Research Projects Agency and by National Institutes of Health Grant GM-12598. We would also like to acknowledge the National Science Foundation Grant GP-6880 for the purchase of the Varian HA-100 spectrometer used in these studies. (26) A. Carrington and T. Hines, J . Chem. Phys., 28,727 (1958). ( 2 7 ) R. E. Connick and R. E. Poulsen, J. Phys. Chem., 62, 1002 (1958).
-
Homogeneous Chemiluminescent Measurement of Nitric Oxide with Ozone Implications for Continuous Selective Monitoring of Gaseous Air Pollutants Arthur Fontijn,' Albert0 J. Sabadell, and Richard J. Ronco AeroChem Research Laboratories, Inc., P.O. Box 12, Princeton, N . J . 08540 The reactions of common air pollutants, such as NO, NOz, and CO, with certain second reactants, such as ozone or 0 atoms, are known to result in light emission. Measurements of the emission intensity could be used to determine the concentration of the pollutants. In a detector based on this principle, ambient air and the second reactant would be continuously flowed through and mixed in a reactor under moderate vacuum. After calibration a continuous record of pollutant concentration could be obtained. Specific sensitivity to a given pollutant would be obtained by a suitable choice of the second reactant and a light filter. To demonstrate the feasibility of the method, the detection of NO using 0, has been studied experimentally. A linear response from about 4 ppb (v/v) NO to at least 100 ppm NO is obtained. NOz, C02, CO, CzH4,NH,, SO2,and HzOin concentrations encountered in air quality control do not interfere with NO monitoring. Based on these results and experimental data for other chemiluminescent reactions, conclusions show that homogeneous chemiluminescence monitors can probably also be developed for at least 03, NO,(=NO NO2), and CO.
+
SOMEHOMOGENEOUS GAS phase reactions of common air pollutants, such as NO, NO*, and CO, with certain second reactants, such as ozone or 0 atoms, result in light emission. These reactions have usually been studied in continuous flow reactors. Measurement of the light intensity of the reactions occurring when the pollutants are mixed with a large excess of the second reactant should in principle be a suitable method for continuous monitoring of pollutants. A large 1
To whom all inquiries should be addressed.
excess of second reactant is needed so that its concentration is not measurably affected by the pollutants. A priori calculations based on the published spectral distribution of the light emitted, the rate constants for light emission, and the response characteristics of photomultiplier tubes, indicated that the light intensity would be quite adequate for monitoring of pollutants over the concentration ranges of interest. The suggested use of homogeneous gas phase chemiluminescent reactions for monitoring purposes appears attractive for a number of reasons, particularly the following: the emissions are specific for the pollutant being monitored; suitable choice of a light filter and the second reactant should allow interference-free measurements; the chemiluminescent light intensities from homogeneous gas-phase reactions in continuous flow systems are rather insensitive to changes in surface properties; and a family of chemiluminescence monitors may be constructed, each unit of which is specific for one pollutant, but all of which are similar in operation. The convenience in the operation of monitoring stations of families of instruments with similar manipulation and maintenance requirements would be considerable. A schematic design for a chemiluminescence detector is shown in Figure 1. The air to be monitored and the second reactant, e.g., ozone, enter the reaction vessel through separate inlets. Rapid mixing occurs and a chemiluminescent reaction takes place. A preset flow of the gases is maintained by a mechanical vacuum pump. The pressure in the reaction vessel is typically 1 torr and the size of the vessel, 1 liter. The intensity of the light emitted is measured by a photomultiplier tube and associated read-out devices (current meter ANALYTICAL CHEMISTRY, VOL. 42, NO. 6, MAY 1970
575
