Anal. Chem. 1989, 61, 1863-1866
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Development of an Optical Relative Humidity Sensor. Cobalt Chloride Optical Absorbency Sensor Study Francisca Boltinghouse* and Kenneth Abel
The ABEL Company, SR 774 Box 192-A, Pembroke, Virginia 24136
Cellulose Impregnated wlth cobalt chlorlde was investigated for use In quantitative relative humidity (RH) measurements. Two cellulose substrates were tested: (a) unmodlfled cellulose chromatography paper and (b) the same paper after acetylatlon. Decreaslng the amount of cobalt chlorlde In the cellulose matrix decreases both hysteresis effects and the effective RH range over whlch the salt/substrate combhation can be used for RH measurement. Acetylatlon of the cellulose extends the effectlve RH range, Increases reproduclbllity, and decreases hysteresis effects.
INTRODUCTION Relative humidity can be measured by using various techniques including infrared spectrometry ( I ) , wet/dry thermometers (Z),chilled mirror dew point depression (3),and a variety of sensors based on various electrical properties hygroscopic materials. Some inexpensive hygrometers use the extension of a natural or synthetic fiber, the length of which is proportional to the surrounding water vapor concentration. The most common sensors are those that measure the change in capacitance of dielectric material or the change in resistance of a conductive material as a function of relative humidity. A novel detector for moisture in gases employing a quartz crystal coated with hydroscopic material has been described in which weight changes are measured to determine water gain or loss ( 4 ) . A recent report describes a sensor based on a poly(tetratluoroethy1ene) (PTFE)film in which the impedance of the grafted film decreases as the humidity increases ( 5 ) . Since Winkler (6) first noted a blue solution when dissolving the anhydrous salt of cobalt chloride in absolute ethanol and reported a color change when water is added, colorimetric sensors have been suggested using metal salt hydrates of cobalt, copper, and vanadium (7). Cobalt chloride has six states of hydration and exhibits progressive color changes with corresponding changes in hydration state. Recent reports indicate that with excess exposure to moisture cobalt chloride can exist in even higher states of hydration (8) although no further color change can be noted. As the initially anhydrous cobalt salt bonds with each water molecule, it exhibits a color change from blue to a fully hydrated pink (9). Figure 1 illustrates the transmission absorbing spectra of fully hydrated and anhydrous cobalt chloride on filter paper. The scattering/absorbency spectrum, corrected for substrates’ reflectance, is essentially identical. From a practical standpoint, measurement of the change in intensity of light reflected from a sensor element surface is easier to develop into an optrode than is measurement of the change in intensity of light transmitted through a sensor element. Both absorbency measurements and reflectance measurements were taken. The results were equivalent except a t relative humidity values from 90 to 100% where water absorption into the cellulose substrate results in increased transparency, which in turn reduces the reflectedlscattered *Author to whom correspondence and reprint requests should be addressed.
signal in a nonreproducible manner. Because the absorbency experiments were more comprehensive, the majority of the data reported herein was taken from experiments using the transmission mode of optical absorbency. In the earliest literature reports, cobalt chloride was utilized primarily as a qualitative indicator of the presence or absence of moisture, an application for which i t is still used in determining the residual capacity of solid desiccants. In recent years, attemts to turn the principle into a quantitative method for monitoring relative humidity have appeared in the literature. An optical waveguide humidity detector employing a cobalt chloride-gelatin film on silica optical fibers has been reported (10). An even more recent report describes the application of a porous optical fiber segment impregnated with cobalt chloride to measure relative humidity with the goal of developing a fiber-optic RH sensor (11). This recent work has apparently been prompted by the many reports of fiber-optic sensors being developed for measuring a variety of physical and/or chemical variables. Fiber-optic sensors have been reported for measuring ammonia (12),pH (131,COZ (141, 0 2 (15),temperature (16,17),and pressure (18). In spite of the advances in fiber-optic-sensor development at the laboratory level, only the fiber-optics temperature probe appears to have been successfully commercialized. The work described herein is part of an ongoing investigation into the possible application of optical methods for quantitative moisture and relative humidity sensor systems. This report is based on two methods: optical absorbency and reflectance of cellulose-based substrates impregnated with cobalt chloride as a function of relative humidity.
EXPERIMENTAL SECTION Control of Relative Humidity. A Shimadzu UV-160double-beam recording spectrophotometer with computerized data reduction was used to measure the optical absorbency of the sample. A specially designed sample cell replaced the cell supplied by the manufacturer. The entire cell chamber is fabricated from a block of aluminum, which is attached to a water bath circulator for temperature control. Relative humidity in the test chamber is controlled by mixing dry air and water saturated air streams, using identical, calibrated rotometers and fine control valves to vary the volumetric ratios of the two air streams. The two air streams are mixed within the cell block, passed sequentially through the sample and reference cell, and exhausted through a Rotronic RH sensor (fl% reported reproducibility within the range of 5%-95% RH). The saturated air stream is produced by bubbling dry air through a water chamber built into the cell block to ensure isothermal temperature control. The water chamber incorporates a channel-grooved,highly porous fire brick as an auxiliary wick to fully saturate the wet air stream. Figure 2 is a three-dimensional representation of the cell chamber. The design of the optical path of the UV-160includes matched silicon photodetectors located close to the sample. This feature makes it particularly useful for translucent or turbid samples having high scattering coefficients: The design provides excellent results in transmission absorbency of highly absorbing cellulose paper substrates, allowing reproducible data acquisition across the visible spectrum. For the reflectance studies, a single-beamsystem was fabricated utilizing a Beckman DU quartz prism monochromator, a 12-V tungsten halogen light source operating at 6 V from a constant
0003-2700/89/0361-1863$01.50/00 1989 American Chemical Society
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line, anhydrous: broken line, hexahydrate. voltage regulator, and an amplified silicon diode as the reflected light detector. The stability of this single-beam system was sufficient to allow noise-free, reproducible readings to four decimal places. Figure 3 is a schematic of this system. Acetylated Substrate Preparation. Whatman Chr2 cellulose filter paper (Whatman Labsales, Hillsboro, OR) was immersed at room temperature in distilled water for 5 min to initiate swelling of the cellulcse structure; it was then immersed in a second mixture of 30% pyridine and 70% water (by volume) for another 5 min. This was followed by immersion in a third solution of 70% pyridine and 30% water for an additional 5 min. The fourth and final solution consisted of 70% pyridine and 30% acetic anhydride. The cellulose paper was left for 24 h in the final solution and then dried in a HEPA-filtered laminar flow hood prior to treatment with cobalt chloride solution. For comparison, Whatman Chr2 without acetylation was used directly as received from the manufacturer. Sample Preparation. The cobalt chloride was dissolved in methanol at the following concentrations: 5, 10, 25, 50, and 75 mg/mL. Each substrate was soaked in a specified solution for 1-2 min. For each sample treated with the CoC1, solutions, a second was treated with pure methanol for use in the reference beam of the spectrophotometer. All samples were allowed to dry in a clean laminar flow hood for a minimum of 1 h. Each sample was screened at room temperature by measuring its optical absorbency spectrum at 0 and at 95% RH prior to extensive data
collection. Table I outlines the cobalt chloride paper concentration in grams of solution per gram of paper (g,/gp) for both substrates, the weight of the paper, and the paper thickness. Triplicate sets of data were measured at 10% RH increments both from 0 to 80% RH and from 80 to 0% R H for each sample. The UV-160 was operated in the double-beam kinetic mode, which allowed determination of response times for each incremental RH step. The RH steps were not changed until the kinetic mode response curve showed no more than a 1% change in total step response over a 30-min time period. All optical absorbency and reflectance measurements were taken at the incident wavelength of 675 nm corresponding to the maximum absorbency for the concentrations of interest.
RESULTS Unmodified Cellulose Results. The concentration of cobalt chloride deposited within the cellulosic substrates has a significant effect on the optical properties as a function of relative humidity. Samples prepared from the following cobalt chloride/methanol solutions were tested thoroughly: 5, 10, 25, 50, and 75 mg/mL. Higher concentrations (150 and 300 mg/mL) were briefly examined, but did not provide useful responses and were not tested further. The cobalt chloride paper concentrations that covered the maximum percent relative humidity range were between 0.180 and 0.130 g,/g,. Although this concentration range provides the greatest usable percent relative humidity range, it exhibits poor reproducibility. At and above these cobalt chloride concentration levels,
representation of RH control and test block: A, humidity generating chamber; B, dry/wet air mixing junction: C, two of four quartz optical windows.
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Flgure 4. Effects of cobail chloride concentration and hysteresis effectson untreated cellulose. (A, top panel) a. 0.180 gJgp; c. 0.073 e. 0.036. (8)b. 0.130 gJg,; d. 0.046.
it appears that crystals of the salt precipitate into the cellulose matrix, a fanor that results in distinct steps between hydration statea becoming evident in the response curve with each step exhibiting a different hysteresis effect.
The lowest concentration (0.042 g./gp on nontreated and 0.036 gJgP on the treated) provides the highest reproducible response. This concentration, however, has a very short nsahle range with most of the change occurring between 0% and 20% RH for the nontreated and between 0% and 40% RH for the acetylated. In fact, samples prepared from each concentration of cobalt chloride/methanol solution have their own response characteristics over specific ranees - of absorhencv and humidities. Figure 4 illustrates the low reproducibility, the shift in RH range with cobalt chloride concentration, and the hysteresis effect a t each concentration on untreated cellulw filter paper. A solid line (open circles) is the test data obtained upon starting a t 0% RH and ending a t approximately 90% R H the dashed line (closed circles) is the test data obtained upon starting a t 90% RH and ending a t 0% RH. The open circles represent the actual data points collected from dry to wet, and the closed circles represent the wet to dry data points. The 97% response times (the time required for 97% of the finalequilibrium value to be obtained following a step change in relative humidity) for 10% RH incremental steps for each concentration starting at 0% RH and progressing to 80% RH were monitored. Response times ranged from 16 to 35 min for increasing RH incremental steps. For the reverse response time were typically longer, ranging from 25 min t o almost 1 h for equilibration. Response times going from wet to dry are appreciably longer than these going from dry to wet, implying that the removal of water from either (or both) the cellulosic matrix or the salt hydrate microcrystals is more difficult than the addition of water vapor to the matrix. ModifiedCellulose Results. The most striking difference between acetylated and untreated cellulose was the decreased scatter and the improved reproducibility of the data. These samples also possessed much shorter response times than did the nonmcdified cellulose samples. In addition, acetylation of the cellulose also results in lower hysteresis effects as compared to unmodified ceuulw, On the acetylated the concentrations that cover the maximum precent relative humiditv ranee lie between 0.173 and 0.115 .e./e....- These concentrations cover the same percent relative humidity range: 15%-70%RH with the higher concentration absorbing more light at each percent RH. Figure 5 illustrates the effect of concentration on R H range, reproducibility, and hysteresis
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Figure 6. Temperature dependency on acetylated ChrP: a, 0 . 1 7 3 g,/g,; b, 0.155; c, 0.061;d, 0.042.
effects. The two higher concentrations exhibited overlapping response characteristics and hysteresis effects; due to the similarity the 0.200 g,/g, concentration data was omitted from the figure. As in Figure 4 the solid line (open circle) represents the dry to wet test data and the dashed line (closed circle) the wet to dry data points. Response times for the acetylated samples were monitored. Reproducibility of response times was excellent: Identical response time values were collected during repetitive testing. Dry to wet equilibration required between 5 and 19 min (depending on the R H step range), while the wet to dry required between 7 and 20 min. Acetylation clearly leads to reduction of response times; however, no clear trends appeared to exist from the data collected. Temperature Dependency. Because of the lower degree of reproducibility obtainable with the unmodified cellulose, temperature dependency studies were made only with samples prepared from acetylated cellulose. There is a substantial shift in response toward higher R H values with increasing temperature, as is illustrated in Figure 6. As observed in the room-temperature studies, the two higher concentrations exhibit quite similar characteristics, and the response characteristics are only shown for the concentrations in the 0.0180-0.0036 g,/g, range. The change in response versus temperature in the linear response RH ranges a t any given percent relative humidity approximates a straight line relationship fitted to the equation y = 0 . 0 3 4 ~ 0.30.
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DISCUSSION AND CONCLUSIONS Studies have been reported on the interaction of the cellulose water vapor over long periods of time (19-21). Cellulose is an imperfect hygroscopic crystalline polymer that readily absorbs water vapor, causing morphological changes within the matrix. Existing theories (22) on the sorption characteristics of cellulose account for at least four major phenomena and related effects. These are (a) the magnitude of sorption, (b) heat effects accompanying sorption, (c) swelling, and (d) hysteresis. I t is generally accepted that the unbounded cellulose hydroxyls act as sorption sites capable of sorbing water with bonding energy greater than that of water. The first water molecules adsorbed a t low relative humidity are principally adsorbed by hydrogen bond formation with hydroyxl groups in the cellulose matrix. This sorption of water causes the structure to swell, and it continues to swell with increassing humidity. The hydrogen bonds of the cellulose break due to
the swelling of the structure and thus increase the number of sorption sites as more water is absorbed. Equilibration time for sorption of water on filter paper was reported to be as long as 14 h at 79% RH (5). Acetylation of cellulose reduces the interaction of water vapor and the cellulose matrix, thereby allowing more rapid equilibration with subsequent reduction in response time. The acetylation reaction, if carried to completion, results in the substitution of the three hydroxyl groups with three acetyl groups for each cellulose unit (24). Low-power microscopy showed that the cellulose acetylated in our laboratory possesses a more grossly porous structure than that of the unmodified cellulose, which may also be a factor in the more rapid equilibration of water vapor through the acetylated matrix. With unmodified cellulose, lowering the concentration of cobalt chloride decreases the usable percent relative humidity response range. Although having decreased usable RH ranges, the lowest concentrations have the highest reproducibility and least hysteresis effect. The acetylation of the cellulose causes a shift in the usable percent relative humidity response range from that observed with unmodified cellulose at the same concentration values. Acetylation not only shifts the percent relative humidity range, but also increases it; this is clearly observed in Figure 5 for the 0.042 g,/g, concentration. The CoC1, on unmodified cellulose possesses a percent relative humidity range that covers from 4% to 20% RH, while the acetylated cellulose at approximately the same concentration possesses a range from 4% to 60% RH. The concentration range from 5 to 25 mg/mL is also the range with minimal hysteresis effect and high reproducibility. The response to relative humidity of CoClz on cellulose is strongly temperature dependent. This dependency, combined with the hysteresis effects a t constant temperature and the limited RH range over which a given concentration of CoC1, will function, does not allow this system to act as a substitute for conventional, nonoptical, full-range RH sensors. Because of the very short range over which a substantial change in optical properties occurs, and because this range can be shifted by changing the concentration of CoClzin the cellulose matrix, the method could find application in optical control of RH in facilities requiring constant relative humidity levels.
LITERATURE CITED Husband, R . M.; Peters, P. J. Tappi 1066, (December), 49. Wexler, A.; Brombacher, G. Natl. Bur. Stand. Circ. (US'.) 1951, No. 512. Davey, F. R. Humidv and Moisture; Ruskin. R. E., Ed.: Reinhold: New York, 1965; Vol 11, pp 571-577. King, W. H., Jr. Res. D e v . 1969, 2 1 , 28-33. Seaki, Y.; Sadaoka, Y.; Ikeuchi, K. Sens. Actuators 1986, 9(2), 125-132. Winkler, C. J. Prakt. Chem. lW4, 91, 209-211. King, W. H., Jr. HumMity and Moisture; Ruskin, R. E.. Ed.; Reinhold: New York, 1965; Vol. 11, pp 578-589. Russell, A. P.; Fletcher, K. S. Anal. Chim. Acta 1965, 170, 209-216. Katzin, L. I.; Ferraro, J. R. J. Am. Chem. SOC. 1952, 7 4 , 2752. Ballantine, D. S.;Wohltjen, H. Anal. Chem. 1086, 58, 2883-2885. Siegel, G. H., Jr. Kritz, D.; Shahriari, M. R.; Zhou, 0.Anal. Chem. 1988, 60, 2317-2320. Giulin, J. F.; Wohlten, H.; Jarvis, N. L. Opt. Len. 1983, 54, 281. Saari, L. A.; Seitz, W. R. Anal. Chem. 1982, 5 4 , 821. Vurek, G. G.: Feustel, P. J.; Severlnghass, J. W. Ann. Biomed. Eng. 1983, 1 7 , 499-509. Peterson, J. I.; Fitzgerald. R. V.; Buckhold, D. K. Anal. Chem. 1984, 56, 62-70. Wickershiem, K. A.; Alves, R. V. I n d . Res. D e v . 1979, 21(12). 82. Cetas, T. C.; Connor. W. G. Med. Phys. 1978, 5 , 79-83. Giallorezni. T. G. Opt. Laser Techno/. 1981. April, 73-77. Bull, H. B. J . Am. Chem. SOC. 1044, 66, 576-5188. Dole, M. J . Chem. Phys. 1948, Aprll, 73-76. Whlte, H. J.. Jr.; Eyring, H. rex. Res. J . 1947, 17, 523. CeUulose Chemistry and Its Application; Albin, F., Ed.; Interscience Publishers: New York, 1965.
RECEIVED for review February 15, 1989. Accepted June 15, 1989. This work was supported by the U.S. Department of Energy under SBIR Contract No. DE-AC01-85ER80299.
