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Optical Beam Deflection Induced by Enthalpy of a Chemical Reaction and Its Analytical Application Xing-Zheng Wu,'Hiroaki Shindoh, Masaaki Yamada, and Toshiyuki Hobo Department of Industrial Chemistry, Faculty of Technology, Tokyo Metropolitan University, Minami-Ohsawa, Hachioji, Tokyo 192-03, Japan
INTRODUCTION Most physical and chemical processes are accompanied by the enthalpy change, and thus a temperature change or a temperature gradient is induced by the enthalpy change. Thermal analysis is a well-known analytical method, in which temperature change during the physical and chemical processes is determined.' In thermal analysis, samples and a thermistor or thermopile are usually required to be enclosed in a container, the temperature of which is controlled. This requirement is unacceptable in many cases, such as in situ monitoring of the single crystals growing process,2 phase transformation,3 and polymerization reactions,4 since noncontact and noninvasive measurement is desirable in these cases. Therefore it is important to develop a new analytical method with noncontact and noninvasive measurement properties. The optical beam deflection method based on probing a refractive index gradient (mirage effects)sJ3is a well-known noncontact and noninvasive detection technique. This technique has been applied to some chemical and electrochemical systems, such as electrode reactions,7*8complex formation reactions between ethyleneaminetetraaceticacid (EDTA) and metal ions Fe3+ and Ca2+? and diffusion transport of an analyte through an interface between two phaaes,1° since refractive index gradientswere generated by the concentration gradients in these systems. In photothermal spectroscopy, the beam deflection method has also been applied widely, because a refractive index gradient is induced by a temperature gradient, which was produced by the nonradiative relaxaton of photoenergy adsorbed from a powerful excitation light The temperature gradient produced by the enthalpy change of a physical or chemical process can also induce a refractive index gradient, which is expected to be detectable by the deflection of a probe beam. In this work, we have investigated the optical beam deflection effect (1) Wendlandt, W. Thermal Methods of Analysis, 3rded.;Wiley: New York, 1986. (2) Nason, D.; Burger, A. Appl. Phys. Lett. 1991,59, 355C-2. (3) Spaeth, K.; Gross, F.; Heidmann, C. P.; Andres, K. Ber. BunsenGes. Phys. Chem. 1987,91 (9), 909-11. (4) Bowley, H. J.; Gerrard, D. L.; Smith, M. J. C.; Biggin, I. S. Adu. Org. Coat. Sci. Technol. Ser. 1989, 11, 1-6. (5) Boccara, A. C.; Fournier, D.; Badoz, J. Appl. .. Phys. Lett. 1980, 36 (21, 130-2. (6) Murphy, J. C.; Amaodt, L. C. J. Appl. Phys. 1980,51 (9), 4580-8. (7) Pawliszyn, J.; Weber, Michael F.; Dignam, M. J.; Venter, R. D.; Park, Su-Moon Anal. Chem. 1986,58, 236-9. (8) Pawliszyn, J.; Weber, Michael F.; Dignam, M. J.; Venter, R. D.; Park, Su-Moon A d . Chem. 1986,58, 239-42. (9) Pawliszyn, J. Spectrochim. Acta Reu. 1990, 13 (4), 311-54. (10) Pawliszyn, J. Anal. Chem. 1992, 64, 1552-5. (11) Wu, J.; Kitamori, T.;Sawada, T.Appl. Phys. Lett. 1990,57 (l), 22-4. (12) Kitamori, T.;Sawada, T. Spectrochim. Acta Reu. 1991, 14 (4), 275-302. 0003-2700/93/0365-0834$04.00/0
induced by the enthalpy of a chemical reaction and examined the possibility of using it for analytical application. This study is the first demonstration that optical beam deflection induced by the enthalpy of a chemical reaction in the liquid phase can be used for noncontact, noninvasive quantitative analysis.
EXPERIMENTAL SECTION Figure 1 shows the diagram of the experimental setup. An optical cell (1 cm x 1 cm x 5 cm) of conventional absorbance spectroscopy was used as a reaction cell. The neutralization reaction between HC1and NaOH was chosen as amodel reaction. The lower part of the cell was filled with 2 mL of CCh, and the upper part with 0.3 mL of 2 M HC1 solution. A He-Ne laser (output power: 1mW) was used as the probe beam source. The diameter of the probe beam was 0.65 mm. The probe beam was incident upon the CCh phase, and ita deflection was measured by a knife edge and a ph~todiode.~Jl One-half of the probe beam was blocked by the knife edge, another half was positioned to the photodiode by a lens. The distance between the cell and knife edge was about 80 cm. An electrical resistance was connected to the photodiode to convert the photocurrent signal into an electrical voltage signal. Aliquota (50 rL) of NaOH solutions of different concentrationswere dropped onto the upper layer in the reaction cell to react with HCl. The intensity change of the blocked probe beam after the addition of NaOH into HCl solution was recorded.
RESULTS AND DISCUSSION For the experimental setup shown in Figure 1,heat created by the neutralization reaction enthalpy will transfer from the water phase in the upper part of the cell to the CC1,. The temperature in the CC4 is higher in the area near the water/ C C 4 interface and lower a t the bottom of the cell. This temperature gradient generates a refractive index gradient in the CC1, phase, in which the refractive index is larger at the cell bottom than the upper part of the CC&phase. When a probe beam passes through the refractive index gradient, as shown in Figure 2, it will bend toward the bottom of the cell. It should be noted that for an endothermic reaction, the probe beam will bend toward the up side since the refractive index gradient is just opposite to the above situation. The beam deflection of the probe beam, as shown in Figure 2, can be expressed aa fol10ws:~~~ S = (L/n)(dn/dT)(dT/dz) (1) where n is the refractive index of CCh, L is the interaction pathway between the probe beam and the temperature gradient dT/dz, and dn/dT is the temperature coefficient of the refractive index. This beam deflection S can be detected by the light beam position detector consisting of a knife edge and a photodiode as shown in Figure 1. The maximum temperature gradient (dT/dz) induced by enthalpy of the neutralization reaction was expected to be in 0 1993 American Chemical Society
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Flgwo 2. Illustration of the optical beam deflection induced by the enthalpy of neutralization reaction between HCi and NaOH. 2N HCI
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Figure 3. Typical decay curve of probe beam deflection signal induced by neutralization heat.
the reaction medium, the water phase. Therefore the strongest beam deflection effect should be observed when the probe beam passes through the reaction medium. However, convection of the added NaOH solution in the water phase also creates an unstable and complicated refractive index which interferes with the measurement of the refractive index gradient created by the reaction enthalpy. In order to measure the deflection signal induced by the neutralization heat, the probe beam is arranged to pass through the CCl, phase as shown in Figure 1. The reason for choosing CCl, is that it has a large dn/dT value.13 First, the beam deflection due to enthalpy of a chemical reaction is confirmed to be detectable by the setup shown in Figure 1. When 50 pL of 2 N HC1 or HzO is added into 2 N HClsolution, no beam deflection can be detected by the setup. However, after addition of NaOH, the probe beam is first deflected toward the cell bottom and then returns to its original position. A typical signal decay curve is shown in Figure 3. The neutralization reaction is an instantaneous reaction, with a diffusion-controlledreaction rate. The slow decay of the probe beam deflection signal shown in Figure 3 is due to the slow heat conduction process. Theoretically, the decay curve of the probe beam deflection signal is a convolution of (13)Mladen, F.; Chieu, D. T.J. Phys. Chem. 1991, 95, 6688-96.
the reaction rate and the heat conductionfunction. Therefore, it should be possible to obtain information about the reaction rate or heat conduction function from the probe beam deflection signal decay curve. Next, experimental conditions which affect the deflection signal are investigated. The distance between probe beam and the CC4/Hz0 interface can greatly affect the probe beam deflection signal. It is expected that the nearer to the CCW H 2 0interface the probe beam passes, the stronger the signal should be. However, noise due to the disturbance of the interface caused by the addition of NaOH solution is also most severe at the interface. The optimum distance between the CC4/Hz0 interface and the probe beam is about 1mm, at which the best signal to noise (S/N) ratio was obtained. The volume of the 2 N HC1 solution in the upper part of the reaction cell also has an influence on the signal, since the neutralization reaction occurs at the upper part of HCl solution. The smaller the volume of the 2 N HC1 solution is, the larger the temperature gradient in the CC4 phase will be. For the cell used in the experiment, the smallest volume of the 2 N HC1 solution needed to form a stable CCWHCl interface is 0.3 mL. Under above experimental conditions, the relationship between the probe beam deflection signal (peak height in Figure 3) and the absolute amount of NaOH added into the HC1 solution is investigated; the results are shown in Figure 4. A good lineariltywas obtained between the deflectionsignal and the amount of NaOH in the range of 5 X 1V-5 X 10-5 mol. This is because the temperature gradient dT/dz is proportional to the reaction enthalpy which, in turn, is proportional to the amount of the reagents used. A t concentrations lower than 5 X 104 mol, signal to noise ratio (S/ N) is smaller than 3, and thus has no good linearilty between the beam deflectionsignal and the amounts of NaOH. Under present experimental conditions,the absolute detection limit for NaOH is about 5 pmol (S/N = 3); the corresponding enthalpy change in the water phase is calculated to be 67 mcal from the neutralization heat and the amount of NaOH. This detection limit can be lowered by designing a new type of reaction cell in which the disturbance of the interface is minimized and the maximum temperature gradient is induced at the spot where the probe beam passes through. The dynamic range of the calibration curve shown in Figure 4 will be improved by using a position sensor which can detect larger beam deflections. It is also possible to improve the sensitivity of the method by employing a multipath approach for the probe beam in the cell5 and using the optical arrangements proposed by Pawliszyn.9 This method was found to be also applicable for other chemical reactions, such as the typical oxidation-reduction reaction between KMn04 and KzCz04. The applications of this method for other chemical reaction systems, the im-
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provementa on the experimental conditions, and the combination of the method with the flow injection technique will be reported later.
will also be useful for exploring surface and interface phenomena.
In summary, it was demonstrated for the first time that probe beam deflection induced by the enthalpy change of a chemical reaction in the liquid phase can be used for quantitative analysis. Compared with the conventional thermal analysis, the advantages of this method are ita contactlessnese and simple experimental setup. In addition, it can also be used for remote as well as a local analysis because of the use of a laser probe beam in the method. This technique
The authors wish to express their thanks to Professor T. Sawada, Dr. T. Kitamori, and A. Harata of the University of Tokyo, for their helpful discussions. The present work was partially supported by a Grant-in Aid for Scientific Research No. 087oooO from the Ministry of Education, Science, and Culture of Japan.
ACKNOWLEDGMENT
RECEIVED for review October 5, 1992. Accepted December 15, 1992.
