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Effects of ultrasonic irradiation in flow-injection systems
Analytica Chimica Acta, 200 (1987) 51-59 Elsevier Science Publishers B.V.. Amsterdam - Printed in The Netherlands
EFFECTS OF ULTRASONIC SYSTEMS
P. LINARES,
IRRADIATION
IN FLOW-INJECTION
F. LAZARO, M. D. LUQUE DE CASTRO**
and M. VALCARCEL*
Department of Analytical Chemistry, Faculty of Sciences, Cbrdoba (Spain)
University of Cbrdoba,
(Received 26th March 1987)
SUMMARY The effects of ultrasonic irradiation on transport and reactions in several flow-injection manifolds are described. The influence of ultrasound on the physical dispersion of the injected plug is considered. Its effects on systems involving homogeneous (catalytic and non-catalytic) and heterogeneous (liquid/liquid extraction, precipitation) reactions are critically evaluated. In general, greater sensitivity is obtained, especially for heterogeneous systems, slow reactions and low analyte concentrations. Finally, practical considerations and potential uses of the combination of ultrasonic irradiation with flow-injection analysis are discussed.
The propagation of ultrasonic waves, characterized by a minimum frequency of 16 kHz, results in rapid fluid movement through compression and rarefaction, and the waves generated give rise to cavitation, i.e., the formation and collapse of microbubbles. Thus, regular temperature and pressure waves are generated which, in general, facilitate and accelerate chemical reactions. Free radicals and ions may be generated, chemical layers are dispersed and the contact between the ingredients of the reaction mixture is dramatically facilitated. Usually, ultrasonic effects are much greater in heterogeneous than in homogeneous chemical systems, because emulsification is favoured and mass and heat transfer in two-phase systems is increased [l] . Organic chemistry has been the discipline exploiting to a greater extent the systematic use of ultrasounds to facilitate, improve and accelerate a large variety of reactions [2,3]. *Miguel Valc&cel has been Professor of Analytical Chemistry of the University of Cordoba (Spain) since 1976. He is currently Head of the Department of Analytical Chemistry. His present research interests are in automatic methods of analysis, with emphasis on continuous-flow systems and advanced fluorimetric techniques. He is co-author of several monographs and textbooks, as well as of over 200 papers. He was appointed President of the Spanish Society of Analytical Chemists in 1985. **Maria Dolores Luque de Castro obtained her Ph.D. degree from Seville University in 1976 and was appointed Assistant Professor of Analytical Chemistry by the University of Grdoba in 1979. She is the co-author of a monograph on flow-injection analysis and of over 100 papers. 0003-2670/87/$03.50
o 1987 Elsevier Science Publishers B.V.
52
The involvement of sound in analytical chemistry occurs in three areas. One is sound generation by light irradiation in photoacoustic spectroscopy [4, 51. A second is the use of acoustic emissions from chemical reactions as analytical signals [6, 71. Recently, an ultrasonic gas-chromatographic detector was reported for the determination of the absolute weight of an unknown analyte without the need for calibration [8]. The application of ultrasound to on-line analysis has been discussed by Asher [9]. The third is the use of ultrasound to enhance one or more of the properties of an analytical system. In a recent paper [lo], three extraction procedures (Soxhlet, alkaline digestion and ultrasonication) were critically compared for removal of petroleum hydrocarbons in sediments. The system based on ultrasound was found to be much faster, though it also resulted in poorer reproducibility. A similar study was reported on the determination of chlorobenzenes in bottom sediments [ll] and on the extraction of thirteen trace elements from atmospheric particulates collected on glass-fibre high-volume sampler filters [ 121. A sonication system has been used for the direct extraction of metals from plants with hydrochloric acid [13]. Recently, an ultrasonic procedure for liberating trihalomethanes from granular activated carbon was described [ 141. An application of growing interest is in the design of interfaces in hybrid techniques, as in reverse-phase liquid chromatography/mass spectrometry [ 151 and liquid chromatography with flame photometric detection [ 161. The sole application of sonication in flow-injection analysis (f.i.a.) so far reported involves the chemiluminescence of aqueous alkaline solutions of luminol containing dissolved oxygen, catalyzed by cobalt at the sub-pg level [17,18]. This paper is concerned with a study of the effects of ultrasonic treatment on various flow-injection systems involving homogeneous or heterogeneous chemical reactions, as well as on the physical dispersion of an injected zone. EXPERIMENTAL
A Pye Unicam SP6-500 single-beam spectrophotometer equipped with a Hellma 17812QS flow-cell (inner volume 18 ~1) and a Perkin-Elmer LS-1 fluorimeter furnished with a 4-~1 flow-cell were used as detector, the chart recorder being a Radiometer REC80. Gilson Minipuls-2 pumps, variablevolume Tecator LlOO-1 valves, Tecator TM1 and II “chemifolds” and teflon tubing of various diameters were used to construct the manifolds, which are described in detail below. The ultrasound source was a Bandelin Sonorex TK52 ultrasonic cleaner (60 kHz, 80 W) in which all reaction coils, etc., were placed; it was fed with water from a Selecta S-382 circulating thermostat bath. All reagents were of analytical-reagent grade.
53 RESULTS
AND DISCUSSION
The procedures used entailed parallel studies in the presence of ultrasound to compare critically the results obtained. Influence
of ultrasound
and absence
on dispersion
A single-channel flow-injection configuration was used through which a lo-* M sodium borate solution circulated and into which a dye (0.0025% bromocresol green in lo-* M borate) was injected. The influence of flowinjection variables (flow rate, reactor length, injected volume, inner diameter of the tubing and coil diameter) and other variables (viscosity, temperature) was systematically studied. In general, ultrasound decreased the dye dispersion slightly (930%). The effects are plotted in Fig. 1. The dependence of the signal on the reactor length can be explained as follows. The residence time increases with reactor length, so that the time of exposure to the ultrasound and hence the radial dispersion are increased. When the length is great, there is already much axial dispersion so the effect of ultrasound is less pronounced. For small injected volumes, the effect of ultrasound on dispersion is smaller because the degree of dilution of the plug is higher; with large injected volumes, the dilution is so small that the positive effect of ultrasound is more evident. As increased flow rates decrease the residence time and hence the time of exposure of the zone to sonication, the effect ultimately decreases with the increase in flow rate. From the observation
Flaw
IO
rife
20
ml man-’ Q-e-
He*, ,oi Lo”,th
200
4bo
600
cm
100
200
300
,uL
.. l”JPCll0”
i”n*r
Volume
dllmrfer
02
04
06
amm
-.
Fig. 1. Plots of the percentage increase in the analytical signal (100 (A,, -A)/A) vs. flow rate (o), reactor length (o), injected volume (*), and inner diameter (a) obtained in the study of the influence of sonication on the dispersion of a dye. (A, absorbance in the absence of ultrasound; A,,, in its presence.)
54
(Fig. 1) that ultrasound decreases dispersion at increasing reactor tubing diameter, it can be inferred that sonication exerts a positive influence on radial dispersion thus limiting linear dispersion so that its enhancing effect on the peak height increases with increasing diameter. A similar effect was observed on increasing the coil diameter. For viscosity values (carrier and sample) below 0.1 and above 0.95 cp, respectively, the effect of sonication on the system is almost nil; yet, for intermediate values, an increase in the signal of about 20% is observed in the presence of ultrasound. Increasing the temperature between 10 and 50°C shows a gradual increase in the enhancing effect of sonication from 2% to >15%. The effect on the dispersion in single-bead-string reactors of various lengths was studied by using the same single-channel configuration. Sonication had no effect on the peak height, probably because the effective diameter of the reactor was very small (this is in agreement with the change in the sonication effect with the diameter of the open reactor tubing). Finally, the effect of ultrasound on dispersion in configurations with splitting and confluence points along channels with the same or different lengths [ 191 was studied by applying this effect to one or both channels. No appreciable differences were observed with or without ultrasound. The same behaviour was observed in a cyclic configuration [20]. Influence on homogeneous reactions The application of sonication to aqueous solutions increases the oxidizing rate [l] . To study the influence on an uncatalyzed homogeneous reaction in a flow-injection system, the complex formation between cobalt and salicylaldehyde thiosemicarbazone (SAT) was selected because this requires the prior oxidation of cobalt(I1) to cobalt(II1) by dissolved oxygen [19]. The configuration used is shown in Fig.2A. This system yields an almost linear relationship between reaction rate and pH between 4.5 and 6.0. Within this pH interval, the effect of sonication at low pH values is noticeably greater than at high values, the rate being 37% greater at pH 4.5 but only 6% greater at pH 6.0, as shown in Fig. 3A. Likewise, the effect is greater, the smaller the injected volume. However, reactor length has little effect. Lengthening the residence time of the reactant zone in the system increases the effect of sonication, i.e., increasing the flow rate decreases the effect (Fig. 3B). The calibration graph obtained under the most favourable conditions with sonication (for 2.0-20.0 pg ml-‘) had the equation, absorbance = 0.025 + 0.0248 [Co(II)] (in E.cgml-‘) with r2 = 0.999. This provides 44% more sensitivity than the calibration equation obtained without sonication: for 2-40.0 pg ml-‘, absorbance = 0.0091 + 0.0172 [Co(II)] with r2 = 0.998. Some of the increase arises because the sample plug undergoes increased radial dispersion, as described for the dye. This increase results in a 28% increase of the analytical signal (calculated from the peak height obtained by injecting a dye under the same working conditions with and without ultrasound. Thus, the
55 (A) Co(II).
HAc/At
38gl-‘SA/O
11 6g
0 2M
UM
tlNaCl ml mm-’
(Cl Hz& 1U4M OLN
DPKH HCl ml
mln-’
Fig. 2. Flow-injection configurations used for the study of: (A) an uncatalyzed homogeneous reaction; (B) a heterogeneous redox reaction; (C) a heterogeneous catalyzed reaction. SAT, SA, NNEA, and DPKH denote salicylaldehyde thiosemicarbazone, sulphanilamide, N-(I-naphthyl)-ethylenediamine dihydrochloride and 2,2’-dipyridylketone hydrazone, respectively; B is the thermostatted ultrasonic bath. All teflon tubes had an inner diameter of 0.5 mm unless stated otherwise.
effect of ultrasound on the reaction rate is to increase the signal by 16%; this is rather small, as is usual for uncatalyzed reactions. Tests were also conducted on a homogeneous catalyzed reaction, namely, the copper-catalyzed oxidation of 2,2’dipyridylketone hydrazone by hydrogen peroxide [21]. The reaction yield increased by 200-300% when the reaction tubing was ultrasonically irradiated, especially at small catalyst concentrations (a few ng ml-‘). This effect introduces interesting possibilities. Influence on heterogeneous reactions As stated above, those chemical systems involving interfaces are the most favourably influenced by ultrasound. Several types of flow-injection system in which the interface is permanently in the system (e.g., a redox or catalytic reactor) or is formed during the dynamic process (turbidimetry or liquid/ liquid extraction) were studied.
57 TABLE 1 Enhancement of the yield of a copperized-cadmium nitrite, by using ultrasound Reactor length (cm)
0.5 1.0 2.0
redox reactor for reducing nitrate to
Reduction yield (%)a
Enhancement
Without ultrasound
With ultrasound
11.6 47.5 73.0
39.3 86.0 97.4
aCalculated by comparing peak heights for equimolar concentrations
3.4 1.8 1.3 of nitrate and nitrite.
traces of oxidant. The configuration used is shown in Fig. 2C. A study of variables similar to that done for the redox reactor was carried out; the influence of sonication was similar to that described for nitrate determination. The calibration graphs for hydrogen peroxide in the range 5 X lo-‘-3 X lo4 M in the presence of sonication and in its absence showed a sensitivity enhancement of 43% when ultrasound was used. The effect of ultrasonic radiation on a flow-injection system involving continuous precipitation and turbidimetric measurements was considered by using classical chemical systems for the determination of anions. The features of the determination of sulphate by injection of the sample into an aqueous stream merging with a barium chloride solution in hydrochloric medium and in the presence of polyvinyl alcohol, were not appreciably modified when the reactor (300 cm long, flow rate 1.8 ml min-‘) was subjected to ultrasonic radiation. However, in the determination of oxalate by injecting the sample into an aqueous solution of calcium chloride in an ammoniacal medium, sonication of the reactor (200 cm long, flow rate 1.0 ml min-‘) increased the turbidimetric signal by about 20%. The different behaviour of these continuous systems on ultrasonic irradiation may be due to the different initial particle sizes of the precipitates, which are quite large for barium sulphate but much smaller for calcium oxalate. The coagulating effect of sonication [l] would therefore be more advantageous in the latter case. One aspect of interest in the use of continuous liquid/liquid extraction without phase separation in f .i.a. involves continuous measurements made on emulsions, which normally are only possible by fluorimetry. Two approaches have been described in the literature. Kina et al. [24] in 1978 indicated the possibility of determining potassium, which was injected with an anionic fluorophore into an organic solution of a crown ether. Recently, however, Memon and Worsfold [25] proposed the use of microemulsions for the determination of analytes; this has significant advantages over the use of normal emulsions. Here, preliminary experiments were done to demonstrate the viability of ultrasound in a flow-injection design with two immiscible phases and direct measurement of the fluorescence intensity of the emulsified
58
zone. A merging-zones configuration was utilized for the simultaneous injection of microlitre volumes of aqueous solution of aluminium (0.5 pg ml-‘) and a 0.5% 8quinolinol solution in chloroform (60 and 30 ~1, respectively) into a 1 :l water/ethanol carrier (flow rate = 0.6 + 0.6 = 1.2 ml min-‘). Single-bead-string reactor (100 cm) was incorporated after the confluence point and placed in the ultrasonic bath. The signals obtained were enhanced by 35-loo%, depending on the analyte concentration, when ultrasound was applied. DISCUSSION
To demonstrate the possibilities of the use of ultrasonic radiation in analytical continuous-flow systems involving homogeneous or heterogeneous reactions, different procedures previously described for applications of conventional f.i.a. were investigated, and the results obtained with and without the use of ultrasound were critically compared for the same manifolds and reagents. From these studies, it can be concluded that in most of the systems studied the analytical features are more or less enhanced, particularly when interfaces or catalyzed reactions are involved. In general, the effect of sonication is more marked when detection takes place during the initial development of the reaction on which the measurements are based. The results obtained for the large variety of systems considered in this paper allow the following practical conclusions to be drawn. To minimize problems related to the effect of ultrasonics on laboratory personnel, it is advisable to use a simple electronic timer synchronizing injection with the start of the ultrasonic system, which must act only during the residence time of the plug in the reactor. To distinguish the effect of sonication from that of the increase in temperature which it causes in the thermostat bath, thermal control by recirculation is advisable, especially in fundamental studies. The use of temperatures above 30” C in these systems give rise to bubble formation, which makes measurements difficult; thus, prior degassing of sample and carrier is mandatory. The use of an ultrasonic cleaning bath is inadvisable for analytical purposes, owing to the lack of stability of the radiation and the difficulty involved in controlling the radiation frequency and intensity. The application of ultrasonic radiation to the transport and reaction zone of a flow-injection system has a number of prospective applications of great interest. These include ultratrace determinations of catalysts by homogeneous chemical reactions, enhancement of the yield of redox and catalytic reactors and development of continuous liquid/liquid extraction without phase separation based on fluorimetric detection. Other possibilities are in the design of continuous solid(sample)/liquid extraction which would be of interest in the automation of processes such as the determination of essential elements in soil, and trace metals in vegetable material and the desorption of pollutants from “filtration” systems.
59
REFERENCES 1 D. Bremner, Chem. Br., 22 (1986) 633. 2 P. Boudjouk, Nachr. Chem. Tech., 32 (1983) 798. 3 S. Toma and V. Kaliska, Chem. Listy, 79 (1985) 578. 4M. J. Adams, J. G. Highfield and G. F. Kirkbright, Anal. Chem., 49 (1977) 1850. 5M. J. Adams, G. F. Kirkbright and K. R. Memon, Anal. Chem., 51 (1979) 508. 6 D. Betteridge, M. T. Joshn and T. Lilley, Anal. Chem., 53 (1981) 1064. 7 M. T. Joslin, Anal. Proc., (1982) 330. 8 K. J. Skogerboe and E. S. Yeung, Anal. Chem., 56 (1984) 2684. 9R. C. Asher, Anal. Proc., (1985) 180. 10 S. Sporstdl, R. G. Lichtenthaler and F. Oreld, Anal. Chim. Acta, 169 (1985) 343. 11 F. I. Onuska and K. A. Terry, Anal. Chem., 57 (1985) 801. 12 S. L. Harper, J. F. Walling, D. M. Holland and L. J. Pranger, Anal. Chem., 55 (1983) 1553. 13 D. M. Kumina, A. V. Karyakin and I. F. Gribovskaya, Zh. Anal. Khim., 40 (1985) 30. 14 K. T. Alben and J. H. Kaczmarczyk, Anal. Chem., 58 (1986) 1817. 15 R. G. Christensen, E. White, V. S. Meiselman and H. S. Hertz, J. Chromatogr., 271 (1983) 61. 16 J. F. Karnicky, L. T. Zitelli and S. van der Wal, Anal. Chem., 59 (1987) 327. 17 M. Yamada and S. Suzuki, Chem. Lett., (1983) 783. 18T. Komatsu, M. Ohira, M. Yamada and S. Suzuki, Bull. Chem. SOC. Jpn., 59 (1986) 1849. 19 A. Fernandez, M. D. Luque de Castro and M. Valcarcel, Anal. Chem., 56 (1984) 1146. 20A. Rios, M. D. Luque de Castro and M. Valcarcel, Anal. Chem., 57 (1985) 1803. 21 F. Lbzaro, M. D. Luque de Castro and M. Valcarcel, Anal. Chim. Acta, 165 (1984) 177. 22 R. C. Schothorst, 0. 0. Schmitz and G. den Boef, Anal. Chim. Acta, 179 (1986) 299 (and references therein). 23 M. F. Gine, H. Bergamin F?, E. A. G. Zagatto and B. F. Reis, Anal. Chim. Acta, 114 (1980) 191: 24 K. Kina, J. Shiraishi and N. Ishibashi, Talanta, 25 (1978) 295. 25 M. H. Memon and P. J. Worsfold, Anal. Chim. Acta, 183 (1986) 179.
EFFECTS OF ULTRASONIC SYSTEMS
P. LINARES,
IRRADIATION
IN FLOW-INJECTION
F. LAZARO, M. D. LUQUE DE CASTRO**
and M. VALCARCEL*
Department of Analytical Chemistry, Faculty of Sciences, Cbrdoba (Spain)
University of Cbrdoba,
(Received 26th March 1987)
SUMMARY The effects of ultrasonic irradiation on transport and reactions in several flow-injection manifolds are described. The influence of ultrasound on the physical dispersion of the injected plug is considered. Its effects on systems involving homogeneous (catalytic and non-catalytic) and heterogeneous (liquid/liquid extraction, precipitation) reactions are critically evaluated. In general, greater sensitivity is obtained, especially for heterogeneous systems, slow reactions and low analyte concentrations. Finally, practical considerations and potential uses of the combination of ultrasonic irradiation with flow-injection analysis are discussed.
The propagation of ultrasonic waves, characterized by a minimum frequency of 16 kHz, results in rapid fluid movement through compression and rarefaction, and the waves generated give rise to cavitation, i.e., the formation and collapse of microbubbles. Thus, regular temperature and pressure waves are generated which, in general, facilitate and accelerate chemical reactions. Free radicals and ions may be generated, chemical layers are dispersed and the contact between the ingredients of the reaction mixture is dramatically facilitated. Usually, ultrasonic effects are much greater in heterogeneous than in homogeneous chemical systems, because emulsification is favoured and mass and heat transfer in two-phase systems is increased [l] . Organic chemistry has been the discipline exploiting to a greater extent the systematic use of ultrasounds to facilitate, improve and accelerate a large variety of reactions [2,3]. *Miguel Valc&cel has been Professor of Analytical Chemistry of the University of Cordoba (Spain) since 1976. He is currently Head of the Department of Analytical Chemistry. His present research interests are in automatic methods of analysis, with emphasis on continuous-flow systems and advanced fluorimetric techniques. He is co-author of several monographs and textbooks, as well as of over 200 papers. He was appointed President of the Spanish Society of Analytical Chemists in 1985. **Maria Dolores Luque de Castro obtained her Ph.D. degree from Seville University in 1976 and was appointed Assistant Professor of Analytical Chemistry by the University of Grdoba in 1979. She is the co-author of a monograph on flow-injection analysis and of over 100 papers. 0003-2670/87/$03.50
o 1987 Elsevier Science Publishers B.V.
52
The involvement of sound in analytical chemistry occurs in three areas. One is sound generation by light irradiation in photoacoustic spectroscopy [4, 51. A second is the use of acoustic emissions from chemical reactions as analytical signals [6, 71. Recently, an ultrasonic gas-chromatographic detector was reported for the determination of the absolute weight of an unknown analyte without the need for calibration [8]. The application of ultrasound to on-line analysis has been discussed by Asher [9]. The third is the use of ultrasound to enhance one or more of the properties of an analytical system. In a recent paper [lo], three extraction procedures (Soxhlet, alkaline digestion and ultrasonication) were critically compared for removal of petroleum hydrocarbons in sediments. The system based on ultrasound was found to be much faster, though it also resulted in poorer reproducibility. A similar study was reported on the determination of chlorobenzenes in bottom sediments [ll] and on the extraction of thirteen trace elements from atmospheric particulates collected on glass-fibre high-volume sampler filters [ 121. A sonication system has been used for the direct extraction of metals from plants with hydrochloric acid [13]. Recently, an ultrasonic procedure for liberating trihalomethanes from granular activated carbon was described [ 141. An application of growing interest is in the design of interfaces in hybrid techniques, as in reverse-phase liquid chromatography/mass spectrometry [ 151 and liquid chromatography with flame photometric detection [ 161. The sole application of sonication in flow-injection analysis (f.i.a.) so far reported involves the chemiluminescence of aqueous alkaline solutions of luminol containing dissolved oxygen, catalyzed by cobalt at the sub-pg level [17,18]. This paper is concerned with a study of the effects of ultrasonic treatment on various flow-injection systems involving homogeneous or heterogeneous chemical reactions, as well as on the physical dispersion of an injected zone. EXPERIMENTAL
A Pye Unicam SP6-500 single-beam spectrophotometer equipped with a Hellma 17812QS flow-cell (inner volume 18 ~1) and a Perkin-Elmer LS-1 fluorimeter furnished with a 4-~1 flow-cell were used as detector, the chart recorder being a Radiometer REC80. Gilson Minipuls-2 pumps, variablevolume Tecator LlOO-1 valves, Tecator TM1 and II “chemifolds” and teflon tubing of various diameters were used to construct the manifolds, which are described in detail below. The ultrasound source was a Bandelin Sonorex TK52 ultrasonic cleaner (60 kHz, 80 W) in which all reaction coils, etc., were placed; it was fed with water from a Selecta S-382 circulating thermostat bath. All reagents were of analytical-reagent grade.
53 RESULTS
AND DISCUSSION
The procedures used entailed parallel studies in the presence of ultrasound to compare critically the results obtained. Influence
of ultrasound
and absence
on dispersion
A single-channel flow-injection configuration was used through which a lo-* M sodium borate solution circulated and into which a dye (0.0025% bromocresol green in lo-* M borate) was injected. The influence of flowinjection variables (flow rate, reactor length, injected volume, inner diameter of the tubing and coil diameter) and other variables (viscosity, temperature) was systematically studied. In general, ultrasound decreased the dye dispersion slightly (930%). The effects are plotted in Fig. 1. The dependence of the signal on the reactor length can be explained as follows. The residence time increases with reactor length, so that the time of exposure to the ultrasound and hence the radial dispersion are increased. When the length is great, there is already much axial dispersion so the effect of ultrasound is less pronounced. For small injected volumes, the effect of ultrasound on dispersion is smaller because the degree of dilution of the plug is higher; with large injected volumes, the dilution is so small that the positive effect of ultrasound is more evident. As increased flow rates decrease the residence time and hence the time of exposure of the zone to sonication, the effect ultimately decreases with the increase in flow rate. From the observation
Flaw
IO
rife
20
ml man-’ Q-e-
He*, ,oi Lo”,th
200
4bo
600
cm
100
200
300
,uL
.. l”JPCll0”
i”n*r
Volume
dllmrfer
02
04
06
amm
-.
Fig. 1. Plots of the percentage increase in the analytical signal (100 (A,, -A)/A) vs. flow rate (o), reactor length (o), injected volume (*), and inner diameter (a) obtained in the study of the influence of sonication on the dispersion of a dye. (A, absorbance in the absence of ultrasound; A,,, in its presence.)
54
(Fig. 1) that ultrasound decreases dispersion at increasing reactor tubing diameter, it can be inferred that sonication exerts a positive influence on radial dispersion thus limiting linear dispersion so that its enhancing effect on the peak height increases with increasing diameter. A similar effect was observed on increasing the coil diameter. For viscosity values (carrier and sample) below 0.1 and above 0.95 cp, respectively, the effect of sonication on the system is almost nil; yet, for intermediate values, an increase in the signal of about 20% is observed in the presence of ultrasound. Increasing the temperature between 10 and 50°C shows a gradual increase in the enhancing effect of sonication from 2% to >15%. The effect on the dispersion in single-bead-string reactors of various lengths was studied by using the same single-channel configuration. Sonication had no effect on the peak height, probably because the effective diameter of the reactor was very small (this is in agreement with the change in the sonication effect with the diameter of the open reactor tubing). Finally, the effect of ultrasound on dispersion in configurations with splitting and confluence points along channels with the same or different lengths [ 191 was studied by applying this effect to one or both channels. No appreciable differences were observed with or without ultrasound. The same behaviour was observed in a cyclic configuration [20]. Influence on homogeneous reactions The application of sonication to aqueous solutions increases the oxidizing rate [l] . To study the influence on an uncatalyzed homogeneous reaction in a flow-injection system, the complex formation between cobalt and salicylaldehyde thiosemicarbazone (SAT) was selected because this requires the prior oxidation of cobalt(I1) to cobalt(II1) by dissolved oxygen [19]. The configuration used is shown in Fig.2A. This system yields an almost linear relationship between reaction rate and pH between 4.5 and 6.0. Within this pH interval, the effect of sonication at low pH values is noticeably greater than at high values, the rate being 37% greater at pH 4.5 but only 6% greater at pH 6.0, as shown in Fig. 3A. Likewise, the effect is greater, the smaller the injected volume. However, reactor length has little effect. Lengthening the residence time of the reactant zone in the system increases the effect of sonication, i.e., increasing the flow rate decreases the effect (Fig. 3B). The calibration graph obtained under the most favourable conditions with sonication (for 2.0-20.0 pg ml-‘) had the equation, absorbance = 0.025 + 0.0248 [Co(II)] (in E.cgml-‘) with r2 = 0.999. This provides 44% more sensitivity than the calibration equation obtained without sonication: for 2-40.0 pg ml-‘, absorbance = 0.0091 + 0.0172 [Co(II)] with r2 = 0.998. Some of the increase arises because the sample plug undergoes increased radial dispersion, as described for the dye. This increase results in a 28% increase of the analytical signal (calculated from the peak height obtained by injecting a dye under the same working conditions with and without ultrasound. Thus, the
55 (A) Co(II).
HAc/At
38gl-‘SA/O
11 6g
0 2M
UM
tlNaCl ml mm-’
(Cl Hz& 1U4M OLN
DPKH HCl ml
mln-’
Fig. 2. Flow-injection configurations used for the study of: (A) an uncatalyzed homogeneous reaction; (B) a heterogeneous redox reaction; (C) a heterogeneous catalyzed reaction. SAT, SA, NNEA, and DPKH denote salicylaldehyde thiosemicarbazone, sulphanilamide, N-(I-naphthyl)-ethylenediamine dihydrochloride and 2,2’-dipyridylketone hydrazone, respectively; B is the thermostatted ultrasonic bath. All teflon tubes had an inner diameter of 0.5 mm unless stated otherwise.
effect of ultrasound on the reaction rate is to increase the signal by 16%; this is rather small, as is usual for uncatalyzed reactions. Tests were also conducted on a homogeneous catalyzed reaction, namely, the copper-catalyzed oxidation of 2,2’dipyridylketone hydrazone by hydrogen peroxide [21]. The reaction yield increased by 200-300% when the reaction tubing was ultrasonically irradiated, especially at small catalyst concentrations (a few ng ml-‘). This effect introduces interesting possibilities. Influence on heterogeneous reactions As stated above, those chemical systems involving interfaces are the most favourably influenced by ultrasound. Several types of flow-injection system in which the interface is permanently in the system (e.g., a redox or catalytic reactor) or is formed during the dynamic process (turbidimetry or liquid/ liquid extraction) were studied.
57 TABLE 1 Enhancement of the yield of a copperized-cadmium nitrite, by using ultrasound Reactor length (cm)
0.5 1.0 2.0
redox reactor for reducing nitrate to
Reduction yield (%)a
Enhancement
Without ultrasound
With ultrasound
11.6 47.5 73.0
39.3 86.0 97.4
aCalculated by comparing peak heights for equimolar concentrations
3.4 1.8 1.3 of nitrate and nitrite.
traces of oxidant. The configuration used is shown in Fig. 2C. A study of variables similar to that done for the redox reactor was carried out; the influence of sonication was similar to that described for nitrate determination. The calibration graphs for hydrogen peroxide in the range 5 X lo-‘-3 X lo4 M in the presence of sonication and in its absence showed a sensitivity enhancement of 43% when ultrasound was used. The effect of ultrasonic radiation on a flow-injection system involving continuous precipitation and turbidimetric measurements was considered by using classical chemical systems for the determination of anions. The features of the determination of sulphate by injection of the sample into an aqueous stream merging with a barium chloride solution in hydrochloric medium and in the presence of polyvinyl alcohol, were not appreciably modified when the reactor (300 cm long, flow rate 1.8 ml min-‘) was subjected to ultrasonic radiation. However, in the determination of oxalate by injecting the sample into an aqueous solution of calcium chloride in an ammoniacal medium, sonication of the reactor (200 cm long, flow rate 1.0 ml min-‘) increased the turbidimetric signal by about 20%. The different behaviour of these continuous systems on ultrasonic irradiation may be due to the different initial particle sizes of the precipitates, which are quite large for barium sulphate but much smaller for calcium oxalate. The coagulating effect of sonication [l] would therefore be more advantageous in the latter case. One aspect of interest in the use of continuous liquid/liquid extraction without phase separation in f .i.a. involves continuous measurements made on emulsions, which normally are only possible by fluorimetry. Two approaches have been described in the literature. Kina et al. [24] in 1978 indicated the possibility of determining potassium, which was injected with an anionic fluorophore into an organic solution of a crown ether. Recently, however, Memon and Worsfold [25] proposed the use of microemulsions for the determination of analytes; this has significant advantages over the use of normal emulsions. Here, preliminary experiments were done to demonstrate the viability of ultrasound in a flow-injection design with two immiscible phases and direct measurement of the fluorescence intensity of the emulsified
58
zone. A merging-zones configuration was utilized for the simultaneous injection of microlitre volumes of aqueous solution of aluminium (0.5 pg ml-‘) and a 0.5% 8quinolinol solution in chloroform (60 and 30 ~1, respectively) into a 1 :l water/ethanol carrier (flow rate = 0.6 + 0.6 = 1.2 ml min-‘). Single-bead-string reactor (100 cm) was incorporated after the confluence point and placed in the ultrasonic bath. The signals obtained were enhanced by 35-loo%, depending on the analyte concentration, when ultrasound was applied. DISCUSSION
To demonstrate the possibilities of the use of ultrasonic radiation in analytical continuous-flow systems involving homogeneous or heterogeneous reactions, different procedures previously described for applications of conventional f.i.a. were investigated, and the results obtained with and without the use of ultrasound were critically compared for the same manifolds and reagents. From these studies, it can be concluded that in most of the systems studied the analytical features are more or less enhanced, particularly when interfaces or catalyzed reactions are involved. In general, the effect of sonication is more marked when detection takes place during the initial development of the reaction on which the measurements are based. The results obtained for the large variety of systems considered in this paper allow the following practical conclusions to be drawn. To minimize problems related to the effect of ultrasonics on laboratory personnel, it is advisable to use a simple electronic timer synchronizing injection with the start of the ultrasonic system, which must act only during the residence time of the plug in the reactor. To distinguish the effect of sonication from that of the increase in temperature which it causes in the thermostat bath, thermal control by recirculation is advisable, especially in fundamental studies. The use of temperatures above 30” C in these systems give rise to bubble formation, which makes measurements difficult; thus, prior degassing of sample and carrier is mandatory. The use of an ultrasonic cleaning bath is inadvisable for analytical purposes, owing to the lack of stability of the radiation and the difficulty involved in controlling the radiation frequency and intensity. The application of ultrasonic radiation to the transport and reaction zone of a flow-injection system has a number of prospective applications of great interest. These include ultratrace determinations of catalysts by homogeneous chemical reactions, enhancement of the yield of redox and catalytic reactors and development of continuous liquid/liquid extraction without phase separation based on fluorimetric detection. Other possibilities are in the design of continuous solid(sample)/liquid extraction which would be of interest in the automation of processes such as the determination of essential elements in soil, and trace metals in vegetable material and the desorption of pollutants from “filtration” systems.
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