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(4) Sawada. T.; Gohshi, Y . ; Abe, C.; Furuya, K. Anal. Chem. 1985, 5 7 , 1743. (5) Clark, 0.M.; Garlick, R. Thermochlm. Acta 1979, 3 4 , 365. (6) Yuyama, S.; Kishl, T.; Hisamatsu, Y. J . Acoust. €miss. 1983, 2 , 71. (7) Droulllard, T. F.; Hamstad, M. A. In The Proceedings of The First I n ternational Conference on Acoustic Emission from Reinforced Plas tlcs, Society of the Plastics Industry, 1983. (8) Brunauer, S.; Emmett, P. H.; Teller, E. J . Am. Chem. SOC.1938, 6 0 , 309. (9) Belchamber, R. M.; Betterldge, D.; Chow, Y. T.; Hawkes. A. G.; Cudby, M. E. A.; Wood, D. G. M. J . Comp. Mater. 1983, 17,420. (10) Belchamber, R. M.; Betteridge, D.; Chow, Y. T.; Lilley, T.; Cudby, M. E. A.; Wood. D. G. M. In The Proceedings of The Flrst International Conference on Acoustic Emission from Relnforced Plastics, Society of the Plastics Industry, 1983. (11) Betterwe, D.; Connors, P. A,; Lilley, T.; Shoko, N. R.; Cudby, M. E. A.; Wood, D. 0.M. Polymer 1983, 2 4 , 1206. (12) Davis, J. C. Statistics and Data Analysis in Geology; Wiiey: London, 1976. (13) Eltzen, D. G.; Wadley, H. N. G. J . Res. Natl. Bur. Stand. (U.S.) 1984, 89, 75. (14) Simmons, J. A.; Wadley, H. N. G. J . Res. Natl. Bur. Stand. ( U S . ) 1984, 89, 55. (15) Moore, W. J. In PbyslcalChemlstry, 5th ed.; Longman (Harlow): Harlow, Essex, UK; Chapter 9.
factor of 2 change in the hydration rate, corresponding to a 10 K increase in temperature, is a little high. CONCLUSIONS Integrated acoustic emission signals have been linked in a quantitative manner to the mass of silica gel reacting, the initial water content, and the particle size. Pattern recognition techniques indicate that the individual AE signals can be related t o distinct physical processes. Analysis of the time intervalsbetween the occurrence of AE signals gives an insight into kinetics of the hydration mechanism that would otherwise have been difficult to determine. From this work, the possibilities and advantages of using acoustic emission as a basis for chemical analysis and process monitoring are clearly apparent, particularly for systems exhibiting phase changes, which tend to be difficult to monitor in real time using conventional techniques.
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LITERATURE CITED (1) Betbridge, D.; Joslin, M. T.; Lilley, T. Anal. Chem. 1981, 53, 1064. (2) van Ooijen, J. A. C.; van Tooren, E.; Reedijk, J. J . Am. Chem. Soc. 1978, 100, 5569. (3) Sawada, T.; Gohshi, Y.; Abe, C.; Furuya, K. Anal. Chem. lS58, 57, 368.
RECEIVED for review November 25,1985. Accepted January 27,1986. We wish to thank the management of The British Petroleum Company plc for permission to publish this work.
Sulfide Determination by N ,A/-Dimethyl-p -phenylenediamine Immobilization in Cationic Exchange Resin Using an Optical Fiber System Adolfo Martinez, M. C r u z Moreno, a n d Carmen Cbmara* Departamento de Qulmica Analltica, Facultad de Ciencias Qutmicas, Uniuersidad Complutense, 28040 Madrid, Spain
A new method for sMMe determlnatlon Is descrlbed In whkh Its reactlon wlth N,N-dhnethyCpphenylenedlamlne lmmoblllzed on a soHd support yleMs methylene blue. The reactlon a m exchange reeln Dowex 50-X8, and takes place on the c llght reflected from the sample Is focused through an optlcal flber system. Because d the lrreverslblllty of the process, an optlcal cell rather than a sensor was chosen to monitor the methylene blue concentration. Up to 0.06 ppm sulfide, a linear callbratlon graph Is obtalned, although strict adherence to conditlons Is requlred to attaln reproducible results.
The use of optical chemical sensors involving reactive substances immobilized on a solid support is becoming more widespread (1).This is because interaction with an analyte under these conditions offers several attractive features: (1) heightened sensitivity due to accumulation of products in a small volume, (2) greater reagent stability and ease of transport compared to the corresponding solutions, and (3) possibility of using the same solid reagent several times (2). In this paper the behavior of NJV-dimethyl-p-phenylenediamine immobilized on cationic Dowex 50W-X8resin is described. The optical changes of the immobilized reagent in contact with trace amounts of sulfide are detected as diffuse reflectance measurements using an optical fiber system. The reaction, as originally studied in aqueous solution (3), was found to be almost specific for hydrogen sulfide. 0003-2700/86/0358-1877$01.50/0
EXPERIMENTAL SECTION Apparatus. Monochromatic light from a tungsten lamp is conducted through a bundle of eight optical fibers to the optical cell where it is reflected by the sample and focused along another optical bundle into the detector. The two optical bundles form a single cable where they enter the optical cell. A Unicam SP600 spectrophotometer attached to a Hitachi Perkin-Elmer 139 photomultiplier with its power supply was used. The optical cell construction is shown in Figure 1. It consists of two bundles of eight poly(methy1 methacrylate) optical fibers (0.78 mm2 active surface area, 64O acceptance cone, and 1.496 refraction index) in a glass tube, all of which is inserted in a plastic tube and held in place by means of two plastic sleeves. The end of the sensitive tip is introduced into a plastic sample holder containing the solid phase to be measured. The desigwof this capsule allows loading with equal amounts of resin for each reading. Optical cable connections are made by means of special attachments for optical fibers (RS-399-385). The connectors at both fiber and sensitive tip ends are gently rubbed down with sandpaper and cleaned with a commercial polish to ensure good light transmission. Reagents. Chemicals of analytical reagent grade and distilled-deionized water were used for the preparation of all solutions. Sulfide solutions M) were prepared in recently boiled distilled water, used immediately after preparation, and titrated iodometrically. Ammonium ferric sulfate solution (10-l M) was acidified with concentrated sulfuric acid to pH 1. Solid Reagent Phase. Two grams of Dowex 50W-X8 cation exchange resin was added to 50 mL of 0.2% N,N-dimethyl-pphenylenediamine dihydrochloride. After 15 h, the aqueous supernatant solution was decanted and the resin washed with 0 1986 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 58, NO. 8, JULY 1986
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Flgure 2. Spectral characteristics: (I) lamp emission spectrum, (11) immobilized reagent spectrum, (111, IV, V) methylene blue spectrum
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corresponding to 0.5, 1.0, and 5.0 ppm sulfide.
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Figure 1. Diagram of optical cell distilled water. The oven-dried resin (70 O C , 3 h) was triturated in an agate mortar until it passed through a 0.160-mm sieve. Procedure. The calibration curve points were obtained as follows: 0.2 g of the immobilized reagent and 10 mL of 1M H$O4 were placed in a 15-mL closed glass vial, and 0.1 mL of 0.1 M NH4Fe(S04)2 and different aliquots of sulfide solutions (containing from 1.0 to 9.0 pg) were added with the aid of a syringe through a rubber septum. Upon completion of the reaction, the mixture was filtered through a grade 4 porous glass filter and the solid washed with distilled water. In order to minimize differences in resin moisture, the immobilized methylene blue was kept in distilled water until the reflectance at 690 nm was measured 2-3 h after preparing the samples. RESULTS AND CONCLUSIONS Reagent Immobilization. In order to select the optimum dihydrosolid support, N,N-dimethyl-p-phenylenediamine chloride and its fiial reaction product (methylene blue) were tested with the following ion exchange materials: (1)Dowex 1-X10 anion exchange resin, (2) Dowex 50W-X8 cationic exchange resin, and (3) styrene-divinylbenzene 2 % copolymer. Three identical series of aqueous methylene blue solutions, covering a lo-' to 10 ppm range, were prepared; one type of support was added to each and the reflectance measured. After 24 h the styrene-divinylbenzene copolymer and the anionic exchange resin displayed a pale blue color, while the supernatants were an intense blue. However, in contrast, the intense blue color of the cationic exchange resin, unalterable by washing with water, along with the colorless supernatant, demonstrated that considerable adsorption of methylene blue had taken place on this support. Color intensity was a function
of methylene blue concentration. The same procedure was carried out with the reagent N,N-dimethyl-p-phenylenediaminedihydrochloride. Once again the lightest colored supernatant and most intensely colored support were those of the cationic exchange resin, indicating the greatest degree of adsorption. After the resins were washed with deionized water, two or three drops of concentrated HCl, a small crystal of Na2S.9H20, and, finally, several drops of dilute Fe(II1) solution were added. Dowex 1x10 and styrene supports remained unaltered and the supernatants stayed blue, while the cationic exchange resin turned an intense blue indicating that the reaction between immobilized N,N-dimethyl-p-phenylenediamine and sulfide, in the presence of Fe(III), yielded methylene blue in the solid phase. Spectral Characteristics. The spectra were obtained by placing the treated resins in the optical cell capsule and recording the reflected light intensity for each wavelength. The reflectance spectra of the resin-immobilized reagent and methylene blue at sulfide concentrations of 0.5, 1.0, and 5.0 ppm are shown in Figure 2. As can be seen, 690 nm is the optimum wavelength for measurements. The presence of sulfide in the samples reduces the intensity of diffuse reflected light at 690 nm. Reaction Time of Resin and Analyte. Compared to the same process in homogeneous conditions, the methylene blue reaction studied shows longer constant response periods and equilibrium is reached more slowly, due to a mass transfer from solution to solid phase. Thus, the reaction time between immobilized reagent and analyte was studied by plotting reaction time against reflectances for different immobilized reagent concentrations. The slope of the calibration curves was found to be a function of reaction time and reagent concentration on the resin, making it essential to use a constant analyte-resin reaction time (the same applies for treated resin preparation). pH Influence. Many authors have reported the formation of methylene blue to be pH dependent (4) with faster reaction rates of higher acidity, which would indicate that a strong acid medium is more desirable. Unfortunately, since the work reported here was carried out using cationic exchange resin,
ANALYTICAL CHEMISTRY, VOL. 58, NO. 8, JULY 1986
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Flgure 3. Effect of H,SO, concentration on desorption of methylene
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