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time. These sensors have worked in the laboratory for more than 3 months without any change in sensitivity or precision. Fiber-optic chemical sensors based on continuously released dyes have been prepared by trapping reagents into an EVA polymer matrix. This technique is simple and straightforward. It overcomes the limitation of using only reversible indicating systems to prepare fiber-optic sensors. This approach should allow optical sensors to be used in remote sensing of groundwater or hazardous environments by linking a spectrophotometer to the remote environment through optical fibers. Although we have developed a pH sensor to prove the feasibility of this technique, the more fundamental goal was to prove the suitability of this type of continuous sensor as a means of releasing reagents capable of detecting other chemicals or metal ions. Finally, the use of continuous release reagents should have other analytical applications such as in flow injection analysis and chromatographic detection.
preparing the polymer matrix, and Ms. Christiane Munkholm and Professor Rudolf Seitz for helpful discussions. Registry No. (EVA) copolymer, 24937-78-8; HPTS (trisodium salt), 6358-69-6; SR-640, 60311-02-6.
ACKNOWLEDGMENT We wish to thank Dr. Fred Milanovich for his assistance in designing and constructing the optical systems, Professor Robert Langer (MIT) and Ms. Liz Albert for their help with
RECEIVED for review July 28,1988. Accepted October 26,1988.
LITERATURE CITED Angel, S. M. Spectroscopy 1987, 2 , 38-48. Hliilard. L. A. Anal. R o c . 1985, 22, 210-224. Bolsde, G.; Blanc, F.; Perez, J. J. Talanta lg88, 35. 75-82. Wolfbeis, 0.S. Pure Appl. Chem. 1987, 59. 663-672. Seitz, W. R. Anal. Chem. W84, 56,16A-34A. Seitz, W. R. CRC Crit. Rev. Anal. Chem. 1988, 19, 135-173. Rhine, W. D.; Hsleh, 0. S. T.; Langer, R. J . Pharm. Sci. 1980, 69, 265-270. Langer, R. Meth. Enzymol. 1981, 73,57-75. Langer. R. CHEMTECH 1982, February, 98-105. Munkholm, C.; Walt, D. R.; Mllanovich, F. P.; Klalner, S. M. Anal. Chem. 1988, 58,1427-1430. Zhang, 2.; Seitz, W. R. Anal. Chim. Acta W84, 160, 47-55. Offenbacher, H.; Wolfbeis, 0. S.; Fiirllnger, E. Sens. Actuators 1986, 9 . 73-84.
This work was supported by a grant from the Environmental Protection Agency through Tufts Center for Environmental Management.
Conductometric Titrations of Polyprotic Acids in Nonaqueous Mixed Solvents. Effects of Temperature and Composition of the Solvent Mixture Giancarlo Franchini, Andrea Marchetti, Carlo Preti, Lorenso Tassi, and Giuseppe Tosi*
Department of Chemistry, University of Modena, Via G. Campi 183, 41100 Modena, Italy
The effects of temperature and composltlon of the solvent mixtures on the shape and on the analytical recoveries of conductometric tltratlons of polyprotlc aclds were studled for the solvent system 2-methoxyethanoVethane-1,2-dloi operating at -10, 25, 50, and 75 ‘C. The experimental evidence indicates that the N- or chair-shape of the tltratlon curves depends on the solvent physicochemical propertles, on the temperature, and on the acld solute to be tltrated; in particular the Influence of the distance between the carboxylic groups Is discussed. Titrations performed In 2-methoxyethanoi are associated in general wlth N-shaped curves, which however turn to chalr-shaped as the temperature Increases. Ethane1,2-dloi always produces chair-shaped curves as a consequence of its more dlssoclating ability toward the tltratlon formed adducts. The study of the tltrations of phthalic acld In some mixtures of the above solvents exhibited behaviors consistent wlth previous observations regarding the exlstence of a partlcuiar “ilmltlng mlxture” whlch separates the solvent system under study Into two well-defined groups (0 I x . ~ < “limiting ~ ~ mixture” ~ ~and “limiting , ~ mixture” ~ ~< x ~ I1). The ~ feaslblilty~ of the resolution ~ of ~acid mixtures has been demonstrated by tltrating cltrlc acld in the above solvents.
INTRODUCTION In the last years alcohols have received good attention as solvents for acid-base determinations, although not many 0003-2700/S9/0361-0177$01.50/0
conductometric studies appeared about their mixtures (1-3). The technique of the titrimetric determination of weak electrolytes in nonaqueous media can provide useful information about the formation of ion pairs or other ionic aggregates during the titration (1,2); in particular for conductance measurements, a careful choice of the solvent system can enable the differentiation of single functions present either in different molecular units ar in the same molecule. In our previous works (4-7) conductometric titrations of monoprotic and polyprotic acids, phenols, and aromatic nitro derivatives in pure 2-methoxyethanol (Gliem) and ethane1,2-diol (Gliet) using N,”-diphenylguanidine (DPG) titrant solutions in the above solvents were presented; the choice of solvents and titrant was suggested by their physical and chemical properties (4-8). In a recent paper (8) we reported the results obtained by studying the effects of the solvent properties and of the temperature on the conductometric titrations of monoprotic acids and phenols. In the same work a detailed study was done on the titrations of picric acid at various temperatures in a series of binary mixtures of Gliem and Gliet covering the 13-49 dielectric constant range. of our studies deals with the , The present ~ development ~ investigation of the effects of the above-mentioned factors on the titration curves of polyprotic acidic species. The shapes of the titration curves of bivalent acids and bases in nonaqueous media depend on temperature, solute, solvent, and titrant used and it is clear that the form of the titration curve is important for practical purposes. For certain bivalent compounds N-shaped curves can be obtained in some solvents, 0 1989 American Chemical Society
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showing a minimum of conductance of the solution corresponding to the second equivalent point, whereas no minimum and chair-shaped curves are observed in other solvents; some bivalent acids give chair-shaped curves under all conditions. A classificationof the two groups was made (9,lO)on the basis of the K,/K2 ratio in water (>lo0 N-shaped and
