Anal. Chem. 1994,66,85-91
Sulfur Dioxide-Selective Optodes Matthlas Kuratll and Ern0 Pretsch' Department of Organic Chemistty, Swiss Federal Institute of Technology (ETH), Universitatstrasse IS, CH-8092 Zurich, Switzerland
Plasticized poly(viny1 chloride) (PVC) membranes based on limit allowed for SO2 emissions is 250 mg/m3 at a mass flow of 2.5 l ~ g / h . ~ a hydrogen ion-selective chromoionophore optically sense gaseous sulfur dioxide in the presence of humidity. If the At present, SO2 is still determined indirectly. A known membraneadditionally containsa lipophilic alcohol or its matrix amount of sample gas is trapped in an aqueous solution and consists of hydroxylated PVC, it also responds to dry S02. In sulfite (or sulfate) is detected by photometry,"'' chemilueither case, sensitivity and selectivity of the optodes can be minometry,12 cond~ctometry,'~ or am~er0metry.l~ As an improved by incorporating a lipophilic aldehyde into the additional method, the reduction of SO2 followed by ,the membrane phase. The chemical equilibria involved in the detection of sulfur by molecular emission spectrometry has different sensor types are discussed. With octadecyl 4-forbeen proposed.15 The analysis procedure may be automated mylbenzoate (ETH 5444) as the lipophilic aldehyde and by using FIA systems.12v16Direct monitoring of SO2is possible 9-(diethylamino)-5-[(2-octyldecyl)imino]-5~-be~~~~henoxby gas ~hromatography.'~ azine (ETH 5350) as the basic chromoionophore, SO2 For on-line monitoring of S02, e.g. for air pollution or concentrations as low as 4 ppb, are determined. The response process control, it is more advantageous to use sensors than times (tm)of the sensors are in the order of 0.5 min for to perform batch analyses. Several sensors based on solid membranes without aldehyde but 10-20 min for those with electrolytes have been described18-20that, owing to their high aldehyde and a secondary amine as a catalyst. Very good operating temperature, can be applied to process gas control. reproducibilities can be achieved, the relative standard deviation On the other hand, SO2 may be determined from changes the for repeated measurements being 11.2%. gas causes in the dielectric properties of a chemically modified silicon membrane.21 A more selective sensor combines a gaspermeable membrane with an anion-selective electrode.22 A large variety of optical sensors (optodes) has been However, all these systems are sensitive to electronic and described in recent years.' Especially challenging is the electromagnetic interferences. In contrast, optical sensors do development of optical sensing devices that respond to not suffer from such influences. A fiber optic wave guide electrically neutral species, such as humidity,2 ethan01,~ sensor based on a fluorescence-quenching mechanism2' as well ammonia$J or carbon dioxide.6 The present paper reports on as a sensor making use of a reversibly formed colored SO2 a novel type of optode for the determination of gaseous sulfur adduct of a copper complex24 has been developed for dioxide. determining S02. Sulfur dioxide monitoring is essential for air pollution Recently, we described a hydrogen-selective optode memcontrol and industrial applications. In the atmosphere, SO2 brane containing a basic chromoionophore and a lipophilic plays an important role because it is actively involved in aldehyde.25 In that case, the protonation of the chromoionphysicochemical processes such as the formation of aerosols, ophore is influenced by the bisulfite addition reaction and clouds, and acid precipitafions. Swiss legislation allows a 24-h depends on the hydrogen activity of the sample solution. In average concentration of 100 pg/m3 of SO2 in air (1 ppby = 2.62 pg/m3 of S O Z ) ,whereas ~ the USA limit is set at 365 (10) Naumann, R. V.; West, P. W.; Tron, F.; Gaeke, G. C., Jr. AMI. Chem. 1960, pg/m3 of S O Z . Major ~ industrial sources of SO2 emission are 32, 1307-1311. (1 1) Scaringelli, F. P.; Saltzmann, B. E.; Frey, S. A. Anal. Chem. 1967,39, 1709electrochemical generation and non-ferrous smelting, while 1719. natural oxidized sulfur compounds mainly emanate from (12) Al-Tamrah, S. A.; Townshend, A.; Wheatly, A. R. Analyst 1987,112,883886. swamps, oceans, and volcanoes. In Switzerland, the upper * Corresponding author. (1) Janata, J. Anal. Chem. 1990, 62, 33R-44R. (2) Wang, K.; Seiler, K.; Haug, J.-P.; Lehmann, B.; West, S.; Hartmann, K.; Simon, W. Anal. Chem. 1991, 63, 97&974. (3) Seiler, K.; Wang, K.; Kuratli, M.; Simon, W. Anal. Chim. Acta 1991, 244, 151-160.
(4) Ozawa, S.;Hauscr, P. C.; Scilcr, K.;Tan, S.S. S.;Morf, W. E.; Simon, W. Anal. Chem. 1991, 63, 640-644. ( 5 ) West, S.; Ozawa, S.; Seiler, K.; Tan, S. S. S.; Simon, W. Anal. Chem. 1992, 64, 533-540. (6) Ozawa, S., Ph.D. Thesis, ETH No. 9647, Zlrich, 1992.
(7) Swiss Federal Council. Luftreinhalte-VerordnungNo. 814.318.142.1,December 16, 1985. (8) Federal Office of Environment, Forest and Landscape. Die Bedeutung der Immissionsgrenzwerteder Luftreinhalte-Verordnung; Series Environment No. 180; Bern, July 1992. (9) West, P. W.; Gaeke, G. C. Anal. Chem. 1956, 28, 1816-1819.
0003-2700/94/036&0085$04.50/0 0 1993 American Chemical Society
(13) JanBk, J.; Vecera, Z. Mikrochim. Acta 1990, 3, 29-34. (14) Schiavon, G.;Zotti, G.; Toniolo, R.; Bontempelli, G. Analysf 1991,116,797-
801.
(15) Arowolo, T. A.; Cresser, M. S. Talanra 1992, 39, 1471-1478. (16) Gbcs, I.; Ferraroli, R. Anal. Chim. Acfa 1992, 269, 177-185. (17) Haunold, W.; Georgii, H. W.; Ockelmann, G. LC-GC 1992, 10, 872-876. (18) Skaeff, J. M.; Dubreuil, A. A. Sens. Actuators, E 1993, 10, 161-168. (19) Yan, Y.; Shimizu, Y.; Miura, N.; Yamazoe, N. Chem. Lerf. 1992, 635-638. (20) Maruyama, T.Mofer. Sci. Eng.,A 1991, 146, 81-89. (21) Endres, H. E.; Mickle, L. D.; Kbslinger, C.; Drost. S.; Hutter, F. Sens. Actuators, E 1992, 6, 285-288. (22) Meyerhoff, M. E.; Pranitis, D. M.; Yim, H. S.; Chaniotakis, N. A.; Park, S. B. ACS Symp. Ser. 1989,403, 26-43. (23) Sharma, A.; Wolfbeis, 0.S. Proc. SPIE-lnf.SOC.Opf. Eng. 1989,990, 116-
120. 153-159.
(24) Cook, R. L.; Macduff, R. C.; Sammells, A. F. Anal. Chim. Acta 1989, 226, (25) Kuratli, M.; Badertscher, M.; Rusterholz, B.; Simon, W. A M / . Chem., in press.
Ana&ticalChemistry, Vol. 66,No. 1, January 1, 1994 85
the present paper, this same recognition process is shown to be applicable to the detection not only of hydrogen in solution but also of SO2 in the gas phase. THEORY The response of bulk optodes may be described on the basis of chemical equilibria between a liquid membrane and the contacting sample. The corresponding calibration curves are usually fitted with the help of theoretically derived equation^.^^^^' Of the two main types of optodes employed in this study, one is based on poly(viny1 chloride) (PVC) membranes plasticized with o-nitrophenyl octyl ether (oNPOE) and responds to humid SO2 only. The other has a partially hydroxylated copolymer (OH-PVC) instead of the PVC matrix and is shown to respond to dry SO2 as well. In the following, chemical equilibria are proposed for both types, however, because of the complexity of the mechanisms involved, a description of the response functions on the basis of these equilibria is possible for the simpler cases only. Responseof PVC/eNPOE/Chromoionophore Membranes to Humidified Sulfur Dioxide. With a PVC/o-NPOE optode membrane containing a lipophilic H+-selective basic chromoionophore, the product of the partial pressures of SO2 and water vapor in nitrogen may be determined. In a first step, SO2 and water vapor are distributed according to their respective distribution constants, kso2and kH20(equilibria 1 and 2), between the gaseous sample (gas) and the organic kso,
SO,,,
* SO2mem
(1)
k W
H2Ogas
%Omem
given by (7)
where the extraction constant corresponds to
Thus, the optode membrane responds to the product of the partial pressures of SO2 and water. Its sensitivity and selectivitydepend on the distribution coefficientsof the species involved and on equilibrium 3, which is governed by the PKa of the chromoionophore used. Response of OH-PVC/eNPOE/Chromoionophore Membranes to Humidifiedand Dry Sulfur Dioxide. The membrane described above has the drawback that it requires water as a mediator and can thus be employed in the determination of humid SO2 only. If, in addition, a lipophilic alcohol was incorporated into the membrane phase, either with or without a lipophilic aldehyde, the optode was found to respond to dry SO2 as well. In order to introduce a sufficient amount of hydroxyl functions,subsequent membranes were prepared with OH-PVC as the matrix, which, contrary to liquid alcohols, shows the advantage of high lipophilicity and good membrane compatibility. Although humidity still has an influence on the absorbance, its contribution is greatly reduced. The proposed mechanism relies on the assumption that SO2 in the membrane phase attacks the hydroxyl groups of R’OH (alcohol or OH-PVC) to give first the corresponding sulfurous acid monoalkyl ester28which is then deprotonated by the basic chromoionophore:
(2) KCH+R’OS02-
SO2 mcm + R’0I-h” + Cmem CH’,,
+ HSO;,,
CH’,, (3)
liquid membrane (mem), where they react with the chromoionophore (C) to give the corresponding hydrogen sulfite as the equilibrium salt (equilibrium 3) with KcH+Hso~-,~ constant. Ion pair formation can be neglected if o-NPOE is used as a p l a ~ t i c i z e r . ~ ~ Designating the absorbance of the unprotonated and fully protonated chromoionophore at a selected wavelength by A I and Ao, respectively, and that of the measured sample by A, the fraction a of the concentration of unprotonated [C] to the total chromoionophore concentration [CT] is given as (4)
with square brackets referring to concentrations in the membrane phase. With the mass balance for the chromoionophore
and the condition of electroneutrality [CH’] = [HSO;] (6) the inverse form of the response function, P S O ~ H ~=Act) O is (26) Sciler, K.; Simon, W. Anal. Chim. Acta 1992, 266, 73-81, (27) Bakker, E.; Simon, W. A w l . Chem. 1992.64, 1805-1812.
06
Am~I~aIChemIstry, Vol. 66,No. 1, January I, 1994
*
+ R’OSO;,,,
(9)
KCH+RQSO~again denotes the equilibrium constant. On the other hand, the sulfurous acid monoalkyl ester with a second alcohol molecule can form the correspondingsulfurous acid ester. The water molecule thereby produced reacts with a second SO2 molecule and with the chromoionophore, as shown in equilibrium 3. The overall equilibrium with the corresponding constant KcH+Hso~-,~ is given by
+
2S02 mcm + 2R’OHmem C,, R’O(SO)OR’,,,
~CH+HS03-3
+ CH’,,
+ HSOC,,,
(10)
Equilibria 9 and 10 compete with each other. In the former, one equivalent of CH+ arises from one of S02, whereas in the latter, it comes from the double amount of S02. The response function CY = &so2) derived from these equilibria is of the fourth order in a. If the gas sample contains water, the mechanism described by equilibrium 3 has to be considered in addition to equilibria 9 and 10, but then the response function becomes too complex. However, by neglecting the contribution of equilibrium 10, the response function a = ~ ( P S O ~ , P Hcan ~ Obe ) evaluated. It is of the third order in a and has three real solutions, but only one fits the experimental data points (see Results and Discussion, Figures 4 and 5 ) . The equation, obtained with the computer program Mathematica, would fill many pages. ~~
(28) Simon,A,; Paetzold, R. Z . Anorg. Allg. Chem. 1960,303, 53-71.
Response of PVC/dPOE/Chromoionopbore Membranes Contddoga LipophilicAklehyde to HumiMied SulfurDioxide. As known from previous investigation^,^^ the addition of hydrogen to a lipophilic aldehydecan be used as a recognition process:
+ RCHO,,,
HSO;,,
Kd.1
e RCH(OH)SO;,,,
(1 1)
where Kadd.1 is the formation constant of the adduct. In the membrane phase, SO2 can react with the aldehyde only if water is present. If the water originates from the gas sample, the sensor responds to the product of the partial pressures of SO2 and water (cf. equilibria 1-3). The addition of hydrogen to the carbonyl group leads to a loss in the electron delocalization energy of the aromatic aldehyde and therefore to a hypsochromic shift in its UV absorption. This would make it possible to directly observe the recognition process. For several reasons, however, the indirect detection with the help of a basic indicator (chromoionophore) is more favorable(see Results and Discussion). The change in the absorption of the chromoionophore is thus a function of the SO2 concentration. With the mass balance for the aldehyde (total concentration [LT]
[&I
= [RCHO] + [RCH(OH)SO;]
(12)
and the condition of electroneutrality [CH'] = [HSO;]
+ [RCH(OH)SO;]
(13)
the inverse response function for this type of membrane based on equilibria 1-3 and 11 is given by
Pso&o
=
where
Respome of OH-PVC/u-NPOE/Cbromoionophore Membranes Containing a Lipophilic Aldehyde to Humidified and Dry Spltpr Dioxide. With OH-PVC as the matrix, a membrane containing a lipophilic aldehyde besides the chromoionophoreresponds to dry SOz, giving rise to an even more complex response function than in the previous case. The following overall equilibrium is proposed for describing the response mechanism: Kd.2
+ 2R'OHmem + RCHOmem + Cmem R'O(SO)OR',,, + CH',, + RCH(OH)SOCme,
2S02
(15)
with the corresponding adduct formation constant Kadd.2. For measurements of dry SOz, a response function of the fifth order can be derived when considering equilibria 9 and 15. For the determination of humid SOz, taking into account all relevant equilibria, the response function was too complex to be evaluated.
ETH 2439
ETH 5444 Flgurr 1. Constitutions of the chromoionophoresand of the aldehyde.
EXPERIMENTAL SECTION Reagents. Poly(viny1 chloride) (PVC, high molecular weight), bis(2-ethylhexyl) sebacate (DOS),2-nitrophenyloctyl ether (o-NPOE), tetrahydrofuran (THF), 9-(diethylamino)5-[(2-octyldecyl)imino]-5H-benzo[a]phenoxazine (ETH 5350, Selectrophore), and bis(2-ethylhexy1)amine were obtained from Fluka AG (Buchs, Switzerland). Diundecylamine was purchased from EGA-Chemie (Steinheim, Germany). Carbagas (Riimlang, Switzerland) delivered the gases SOZ,SO2 in Nz, COZ,and NOZ. Syntheses. The synthesis of vinyl chloride-vinyl alcohol copolymer (OH-PVC) (ETH 3538) has been described in ref 29, that of 9-(dimethylamino)-5-[(44 16-buty1-2,14-dioxo-3,15-dioxaeicusyl)phenyl)imino]-5H-benzo[a]phenoxazine (ETH 2439) in ref 30, and that of octadecyl4-formylbenzoatein ref 25. For constitutions, see Figure 1. Membrane Preparation. The compositions of the optode membranes are given in Table 1. The membrane components (in total, about 200 mg) were dissolved in 1.8 mL of freshly distilled THF. By means of a spin-on two identical membranes of approximately 3-rm thickness were cast each on a glass plate (Herasil quartz glass, Mdler AG, Ziirich, Switzerland) and then mounted in a specially designed flowthrough cell made from stainless steel instead of plastic and with poly(tetrafluoroethy1ene)gaskets instead of elastomeric O-ring~.~l Apparatus. UV/visible absorbance measurements were performed with a Lambda I1 double-beamspectrophotometer (Perkin-Elmer,Kiisnacht, Switzerland). Sample gases were mixed with a special device.2s ExperimentalProcedures. All gas-phase experimentswere carried out by metering mixtures of the minor components (29) Diirsclen, L. F. J.; Wcgmann,D.; May,K.;Oesch, U.;Simon, W. Anal. Chem. 1988,60, 1455-1458. (30) Bnkkcr, E.; Lerchi, M.; Rosatzin, T.; Rustcrholz, B.; Simon, W. Anal. Chim. Acta 1993, 278, 211-225. (31) Seilcr, K.;Mort, W. E.; Rustcrholz, B.; Simon, W. Anal. Sd.1989,5, 557561.
Anel)rticel Chefn&try, Vd. 66,No. 1, January 1, 1994
a7
membranes components
I
ETH 535W ETH 2439 ETH 5444O PVC OH-PVCb corresp to ROHc 0-NPOE bis(2-ethylhexyl)amine4 diunde~ylamine~
I1
0.035 M
0.036 M
31.6 wt %
0.066 M 31.4 w t %
66.4 w t %
64.0 w t %
I11
IV
V
VI
0.029 M
0.023 M 0.107 M
0.032 M 0.178 M
0.031 M 0.148 M
31.4wt % 0.573 M 66.4 w t %
32.2 wt % 0.590 M 63.3 wt %
29.1 wt % 0.532 M 62.6wt %
30.0 wt % 0.548 M 62.9wt % 0.037 M
0.036 M
4 Calculated assuming a membrane density of 1 g/mL. ETH 3538. Calculated from the amount of OH-PVC with a molar ratio of the monomers vinyl chloride/vinyl alcohol of 6.4.
10
08 W
y
06
m 0 iT
$
ez;Fo
04 02
00 -12
-10
-8
-6
-4
-2
0
log(Ps@, x P H @ )
Flgure 2. Response functions of PVC/o-NPOE optode membranes to humM SO2. Calculated equilibrium constants: for membrane I (eq 7 (0)), = 1.0 X lo3 M bar? for membrane 11, with aldehyde (eq 14a (O)), = 1.3 X lo3 M bar2and Ka,, = 8.6 X lo3 Mi. The dynamic range of membrane 11, showing a less steep response curve, is larger than that of membrane I.
e*?
(SO2and interfering species) into a flowing stream of nitrogen (&loo% relative humidity) at 24 f 1 "C5 If necessary, concentrated gases were diluted with nitrogen. Total flow rates were 5-6 L/min, and the minor gas components were introduced at flow rates of 2 mL/min to 1 L/min. Calculations. For calculating the sample compositions, ideal behavior was assumed. The error due to mixing the gases was estimated to be between 5 and Experimental data points were fitted by a nonlinear least squares procedure based on theoretical response functions (see Results and Discussion). All calculations were made with Mathematica using a SiliconGraphics Crimson workstation and a Macintosh personal computer. Equilibrium constants were fitted so that the sum of squares of the difference between calculated and measured absorbances, &I,,,- Amcad)2,was minimal. RESULTS AND DISCUSSION Response and Measuring Range. PVC/o-NPOE Membranes. The response of two PVC/o-NPOE optode membranes as a function of the product of the partial pressures of SO2 and water is shown in Figure 2. The curves were calculated using a least squares fit program. The response function for membrane I, containing only 9-(diethylamino) 5- [(2-octyldecyl)imino]-SH-benzo[a]phenoxazine(ETH 5350) as chromoionophore (for constitutions of compounds, see Figure 1, for membrane compositions, Table l), is described by eq 7 with the calculated equilibrium constant = 88
1.O X lo3 M bar2. The calibration curve for membrane 11, which in addition contains the lipophilic aldehyde octadecyl 4-formylbenzoate (ETH 5444), follows eq 14a, the values for and Kadd,l being 1.3 X lo3 M b a r 2 and 8.6 X lo3 M-l, respectively. The excellent fit for the two curves, with relative residual standard deviations of 0.7and 0.4%, and the good agreement between the optimum values of from the two independent measurements on membrane I and membrane I1support the validity of the proposed mechanisms. Near the center of the dynamic range (at a c OS), the sensitivity of membrane 11,as compared with that of membrane I, is increased by a factor of about 350 due to the presence of the aldehyde and is even higher (about 1000-fold) at CY = 0.75. The detection limit of about 4 p p b SO2 (10.5 pg/m3) obtained with membrane 11is sufficient for measurements of SO2 concentrations in ambient air of P H ~ O = bar (corresponding toa relative humidity of 98% at 24 "C). Lower detection limits could be achieved by increasing the aldehyde concentration or using a more basic chromoionophore. OH-PVC/o-NPOE Membranes. Membranes based on OH-PVC and ETH 5350 were found to be more sensitive to SO2 than the corresponding ones with PVC, but the experimental setup5 did not allow measurement of extremely low concentrations. Therefore, the less basic chromoionophore 9-(dimethylamino)-5-[( 4 4 16-buty1-2,14-dioxo-3,15-dioxaeicosyl)phenyl)imino] -5H-benzo[a]phenoxazine (ETH 2439, pKa = 9.9 in a PVC/DOS (1:2, w/w) membrane as compared with 13.3 for ETH 535030)was used instead. In general, the measuring range of all the sensors described here can be shifted by choosing chromoionophores of different basicities. The response of the OH-PVC optode membrane I11 with ETH 2439 alone and of the corresponding one with added ETH 5444 (membrane IV) to dry SO2 are shown in Figure 3. The data points for membrane I11 were fitted on the basis of equilibria 1, 9, and 10, calculating the following products of the equilibrium constants: ksolKcH+Rtoso2-= 3.1 b a r 1 and ( ~ S O J ~ K C H + H S=O0.7 ~ - , ~bar2. This clearly shows that equilibrium 9 dominates in the investigated psoi range. Likewise, calculation of the response function for membrane IV by considering equilibria 1,9, and 15 gave kso&H+R'oso2= 5.4 b a r ' and ( k s ~ ~ ) ~ K ~=d 1.8 a , zbar2. By comparing the results in Figures 2 and 3, the addition of thelipophilic aldehyde to the OH-PVC membrane obviously increases the sensitivity to a lesser degree than in the case of the PVC membrane. This is explained by the fact that the aldehyde only takes part in
Ana~/calChemistry, Vol. 66,No. 1, Januery 1, 1994
(a)
1.0
0.01 0
0
0.8 w
MEMBRANE 111 ETH 2439 OH-PVC/ o-NPOE
0.6
0
0
0
W
MEMBRANE IY ETH 2439 ETH 5444 OH-PVC/O-NPOE
0.4
0.2
8
a:
I I :
. 0
Om
3
8
a
%
0.005
I? a
-0.005
-0.01
0
0.002
0.001
0.0
0.003
pso, -6
-5
-4
-3 'og Pso,
-1
-2
0
Flgure 3. Responsefunctions of OH-PVC/eNPOE optode membranes (b) without and with an aldehyde compound (membranes I11 and IV) to dry SO2. Calculatedequilibriumconstants(seeTheory): for membrane 111 (0),kso2KCH+R~OS0~ = 3.1 bar' and (~SO,)~KCH+HSO,-,~ = 0.7 bar2; for membrane I V (e),kso,KcH+R~oSo,-= 5.4 bar' and ( k ~ o , ) ~ K=~ ~ , 2 1.8 bar2.
0.01
0.005
2
3
G cW n a:
o
-0.005
-0.01
J
'
0
0.005
0.015
0.01
0.02
0.025
H20
Figure 5. ResMuals of the least squares fit of experimental data for membrane III(see also Figure 4), plotted for the dimensions pso, (top) and p ~ f (bottom). i See Results and Discussion. 0.7
-/
o.6] 0.54 1
Figure 4. Three-dimensionalplot of the responsesurface for membrane I11 (OH-PVC/eNPOE, 1:2 (w/w), ETH 2439). Calculated equilibrium constants: kso$cHp = 3.8 M2 bar2, KcH+Hsos-,l= 88 M-l, and kso,K&++Rroso,-= 2.8 bar'.
SO, CONCENTRATION MEMBRANE ETH 2439 ETH 5444
1
N 1061 ppmv
/Mr- 288
OH-PVC /o-NPOE
pa 0.48 0.3m 0.2 0.1 -
equilibrium 15, but not in 9. The sensitivity of membrane IV can be further enhanced by using higher concentrationsof the aldehyde relative to the chromoi~nophore~~ or by choosing a more basic indicator.30 The response of the above OH-PVC membranes to dry SO2 is influenced by humidity. This is evidenced by the threedimensional plot of the calibration surface for the aldehydefree membrane I11 in Figure 4, representing experimental data of three series of measurements. During the first series, pso2was varied between 10-6.O and 1C 2 a 0 bar a n d p ~between ,~ and bar. During the second and third series,pso, was kept nearly constant at about and 10-2.8 bar, respectively, while varyingpH20between 1 and 10-1-7bar. At SO2 concentrations of 200 and 1600 ppmv, the relative absorbance decreased by about 12and 1696,respectively, when was increased from 0 to 10-1-54bar (24 f 1 "C). In Figure 5, the correspondingresiduals are plotted separately for the dimensions pso2and pH20. The distribution of those forpS02indicates a systematic error. This could be due to the fact that, for deriving a simplified response function, equilibrium 10 was neglected. Notwithstanding, its contribution l p 3 e 8
C 3 a 0
0.0I
I
I
I
I
I
I
I
I
to the response must be small, since the residuals forpso2vary by only about 1 4 % of the total pso2range. The influenceof humidity on the response is not very large. When the optode membrane IV is exposed to increasing SO2 concentrationsin humidified nitrogen, the absorbanceof the protonated form of the chromoionophore ETH 2439 at 666 nm increases (Figure 6). Since a! =fiS02,pH20) for this membrane could not be evaluated (see Theory), the threedimensional plot of the calibration surface (Figure 7) was approximated on the basis of the response function for membrane I11 (cf. Figure 4). The influence of humidity was found to be similar in both cases. Interferences. For applications in environmental studies, selectivities with respect to C02 and NO, are of primary Ana&tkalChemistry, Vol. 66, No. 1, January 1, 1994
89
100 ppm,S02
33 min
TIME
-re
1
Flgwe7. ~~mensionalplotoftheresponseswfaceformembrane I V (OH-PVC/o=NpoE,ETH2439, ETH5444), similartothat of membrane I11 without aldehyde (cf. Figure 4), but not calculated on the basis of chemical equilibria (see Results and Discussion).
interest. The optodemembranesdescribedhere showvirtually no response to C02. When exposing them to 50 vol % COz in nitrogen of 50% relative humidity, the change in a was less . than 1%. The sensitivity of the membranes toward NO, could not be quantified due to the instability of the chromoionophores. It is known that NO, attacks the amine groups of Nile blue to give the corresponding nitroso compounds.32 When exposing HO-PVC/o-NPOE membranesto 10ppm, of NO, (expressed as NO2) during 20 min, the absorptionof ETH 2439 decreased by about 13%. The effect could not be reversed by flushing with humid or dry nitrogen, but only after the membrane was brought into contact with 0.02 M KOH. Investigations on replacing the Nile blue derivatives by other chromoionophores are in progress. Recently it has been shown that by incorporating the lipophilic aldehyde to the membrane phase the selectivity of an analogous hydrogen-sensitiveoptode is considerably improved.25 Since the response mechanisms of the two systems are similar, the improvement of the selectivity is expected to be in the same range for the present system. Response Time. The kinetics of the bisulfite addition reaction, which is catalyzed by oxygen and nitrogen bases,33 is pH-dependent. In aqueous solutions, minimum reaction rates have been observed at pH 3-4.34 The response time of optodes without aldehyde was found to be much shorter than that of membranes with ETH 5444. For example, if membrane I11 (OH-PVC/o-NPOEIETH 2439) was brought into contact with a sample containing 100 ppm, of SO2 @so2= 10-4 bar), it showed a response time t90% of 0.5 min, subsequently requiring 3 min in a SO2-free sample to reach equilibriumagain. The correspondingresponse times for membrane IV (OH-PVC/o-NPOEIETH 2439/ETH 5444) are 21 and 95 min, respectively. Obviously, the addition of hydrogen to the aldehyde is the rate-limiting step. Since amines are known to catalyze this reaction, additional (32) March, J. Advanced Organic Chemistry, 3rd ed.;Wiley & Sons: New York, 1985. (33) Young, P. R.; Jencks, W. P. J. Am. Chem. Soc. 1977,99,12061214. (34) Young, P. R.; Jencks, W. P. J. Am. Chem. Soc. 1979,101,3288-3292.
90 Ana&ticalChemistry, Vol. 66, No. 1, January 1, 1994
8. Response of membranes I V (top) and V (bottom: cf. Table 1) to stepwise SO2 concentration changes between 0 and 100 ppm, of SO2. The presence of 20 mol % of diundecylamine(relathre to the chromoionophore)as a catalyst shortensthe responsetime by a factor of 3.
membranes were prepared containing a lipophilic amine. As shown in Figure 8, the response was much faster when 20 mol % of diundecylamine (relative to the aldehyde) was incorporated into membrane IV, thus obtaining membrane V with tw%values of 5 and 12min for 100ppm, of SO2 and humidified nitrogen, respectively. The use of other amines was less satisfactory. Although the response times were considerably shortened by the presence of bis(2-ethylhexy1)amine in membrane VI, they rapidly increasedduring the measurements because the amine easily evaporated. On the other hand, the more lipophilic dioctadecylaminewas not sufficiently soluble in the membrane phase. In general, the primary and tertiary amines were catalytically less active than the secondary compounds. Considering the solubilityand catalytic activity of the amine additives tested, diundecylamine proved to be the most suitable. At first glance, it might be surprising that the protonation of the chromoionophoreis not more influencedby the presence of an aliphatic amine in the optode membrane. In a polar solvent, diundecylamine is more basic than ETH 2439 (cf. PKa = 11.3 for dib~tylamine~~ and 7.7 for ETH 2439:O both at 25 O C in water). By varying the relative amounts of diundecylamine and ETH 2439 in the OH-PVC/o-NPOE membrane, it was estimated that the PKa of the former is only about 0.2-0.3 units higher than that of the latter. The smaller difference in basicity is ascribed to the possibility of charge delocalization in ETH 2439, which causes the charged protonated chromoionophore in the membrane phase to be better stabilized than the aliphatic ammonium ion. Since the two PKa values are so similar, there is always enough catalytically active free amine present. However, when exposing a SO2-selective membrane to 100 ppm, of SO2 in 98% relative humidity, its sensitivity is expected to decrease by about 10%if 30 mol % of diundecylamine and 300 mol 96 of aldehyde (both relativeto the chromoionophore)are added. Of course, if a more basic chromoionophore is used, the loss in sensitivity will be smaller. Short-TermRepeatabilityand Stability. Repeatability tests were carried out with optode membrane V, which contains diundecylamine as a catalyst. Repeated exposure to samples of 0 and 100 ppm, of S02, having a relative humidity of 9896, (35) Girault-Vexlearschi, G. Bull. Soc. Chim. Fr. 1956, 589.
yielded a remarkably high reproducibility of the absorbance measured at,,A = 524 nm (see also Figure 8, bottom curve). The mean absorbancevaluesand standard deviations obtained from five concentration step changes were 0.3496 f 0.0034 and 0.2002f 0.0024for 0 and 100 ppm, of S02, respectively, with relative standard deviations of 1.O-1.2%. Obviously, optode membranes containing a lipophilized benzaldehyde derivative reversibly repond to S02. When kept under N2, T H F solutions of the membrane compounds and freshly prepared membranes can be stored for several weeks. In the presence of light and oxygen, decomposition of the chromoionophore and oxidation of the aldehyde may take place, although no such effects have been observed during 1 month at least.
CONCLUSIONS Several SO2-sensitive optodes are described. With plasticized PVC membranes containing a basic chromoionophore, humid SO2 may be determined on the basis of acid-base interactions, the response being a function of the product of the SO2 and water partial pressures. By simultaneously using an optical humidity sen so^,^ psol could be monitored as well. With OH-PVC as the matrix, the membranes also respond to dry S02. Selectivity and sensitivity of the optodes are
improved by incorporating a lipophilic aldehyde into the membranes. Since, in this case, the bisulfite addition reaction involved is rate-limiting, response times become much longer. They are shortened by additionally incorporating a lipophilic secondary amine. The detection limits of the optodes described are sufficient for monitoring environmental samples. Future work will concentrate on replacing the Nile blue derivatives used as chromoionophores, since they are instable in the presence of NO,. This is not expected with charged (acidic) chromoionophores. One might even envisage SO2:selective optodes without a chromoionophore,monitoring the UV/vis spectrum of the aldehyde added. As ETH 5444 used in this work absorbs around 250 nm, where interferences from other membrane or sample components may take place, the synthesis of aldehydes absorbing in the visible is planned.
ACKNOWLEDGMENT The authors thank Dr. D. Wegmann for careful reading of the manuscript and for helpful suggestions. This work was partly supported by the Swiss National Science Foundation and by Ciba-Corning Diagnostics Corp. Recelved for review July 20, 1993. Accepted October 7, 1993.e Abstract published in Advance ACS Absfracfs, November 15, 1993.
Ana&tical Chemisby, Vol. 66, No. 1, January 1, 1994
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