1,1,3,3-Tetraethylurea as a solvent for conductance studies

Journal of Solution Chemistry, Vol. 4, No. 7, 1975 1,1,3,3-Tetraethylurea as a Solvent for Conductance Studies Barbara J. Barker, 1'3 Joseph Rosenfarb, 2 and Elizabeth Z. Riesen 1 Received February 13, 1975 1,1,3,3-Tetraethylurea (TEU), a solvent of relatively low dielectric constant, 14.4, was evaluated as a medium for conductance studies of a series of alkali metal salts and organic acids. Conductance data for KSCN, NaSCN, NaI, NaCI04, and NaBPh4 in TEU were analyzed by the Fuoss-Shedlovsky, Fuoss-Onsager, and expanded Pitts equations. The behavior of these salts in TEU Was compared with that of the same salt series in the previously studied tetramethylurea. Tetraethylurea also was used successfully as a solvent for conductometric titrations of several carboxylic acids and phenols. KEY W O R D S : Tetraethylurea; conductance; solvation; conductometric titrations. 1. I N T R O D U C T I O N Recently one of the few urea derivatives which is a liquid at room temperature, 1,1,3,3-tetramethylurea (TMU), was studied quite thoroughly as a solvent for electrochemical investigations and analytical determinations/1-7/Several of these studies revealed that T M U is a rather unique electrolytic solvent. 1,1,3,3-Tetraethylurea (TEU) also is a urea derivative which is a liquid at room temperature, but, by contrast, there have been no similar reports of its use as a nonaqueous medium. The present investigation, which represents the initial use of TEU as an electrolytic solvent, consists of both fundamental and applied conductance studies. Physical properties of the solvent were determined, the conductance 1 Department of Chemistry, Hope College, Holland, Michigan 49423. 2 Department of Chemistry, University of Cincinnati, Cincinnati, Ohio 45221. 3 To whom correspondence should be addressed. 571 9 1975 Plenum Publishing Corporation, 227 West 17th Street, N e w York, N.Y. 1001 I. N o part o f this publication m a y be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, microfilming, recording, or otherwise, without written permission o f the publisher. 572 Barker, Rosenfarb, and Riesen behavior of a series of salts in TEU was elucidated, and a few carboxylic acids and phenols were analyzed by conductometric titrations. Conductance data for the salts were analyzed by both the older Shedlovsky<a,9~ and Fuoss-Onsager<~~ methods and by the relatively recent Fern~indez-Prini (12~ expanded form of the Pitts <la'x~ equation. Included in the present study is an evaluation by the Pitts method of former conductance data for the same salts in TMU. Comparisons between the behavior of these electrolytes in tetraethylurea and tetramethylurea also are discussed. 2, E X P E R I M E N T A L SECTION 2.1. Solvent Purification and Physical Properties Tetraethylurea (TEU), obtained from the R.S.A. Corporation (Ardsley, New York) was purified by vacuum distillation. Approximately 2.8 liters of solvent was refluxed over granular barium oxide at 28-30~ for 18-20 h; about 2.5 liters of solvent was collected over a period of 36 h. The collected fraction, which became very pale yellow within several days, was redistilled under the same conditions. Approximately 2.3 liters of purified solvent was obtained from the redistillation. This solvent, which remained colorless for months, was stored and dispensed under a nitrogen atmosphere. Unlike TMU, which is somewhat hygroscopic and slowly increases in conductance during storage, TEU maintained a very low specific conductance of 7.415.0 x 10-9 ohm-1-cm- 1 for a period of 8-10 weeks. By previously described techniques,(~s) the density, viscosity, and dielectric constant of TEU at 25~ were determined to be 0.9026 + 0.0001 g-m1-1, 2.363 + 0.002 cP, and 14.4 + 0.l (1 MHz), respectively. 2.2. Reagents Sodium perchlorate (Fischer reagent) and sodium and potassium thiocyanates (Merck reagent) were recrystalized from acetone-ether mixtures. Sodium iodide and sodium tetraphenylborate (Matheson, Coleman and Bell) were used without further purification. All salts were ground finely and dried in vacuo for 24 48 h prior to use. Benzoic (Baker Analyzed), picric (Eastman), and p-nitrobenzoic (Baker) acids were recrystallized from acetone-water mixtures, o-Nitrobenzoic acid, 2,4-dinitrophenol, and diphenylguanidine (DPG) (all Eastman chemicals) were recrystallized, respectively, from water, acetone-water, and toluene. All materials were dried prior to use. 1,1,3,3-Tetraethylurea as a Solvent for Conductance Studies 573 2.3. A p p a r a t u s a n d P r o c e d u r e s Most of the apparatus used in the present investigation have been described in detail. (2,1m For the fundamental conductance study, a Model 4666 Leeds and Northrup Dyke-Jones bridge and conductance cells of the flask design of Daggett, Bair, and Kraus ~17~ were used. The cell constants, 0.2229 + 0.0003 and 0.2259 + 0.0002 cm-1, were obtained from the method of Lind, Zwolenik, and Fuoss. (zm A fiber-glass-insulated Sargent S-84805 thermostatic bath assembly filled with light mineral oil and equipped with a Haake stirrer and a 25-W heater maintained a constant temperature of 25.00 +_ 0.005~ An Re-18 Industrial Instruments conductivity bridge and Sargent model S-29870 conductance cells were used for the conductometric titrations. The electrodes were adjusted to obtain a cell constant of approximately 0.15 cm -1. Titrations were performed at room temperature (approximately 25~ Complete details of the procedures used in the present study have been discussed previouslyJ 2,16) 3. R E S U L T S A N D DISCUSSION Conductance data for the salts studied in tetraethylurea were evaluated by the Fuoss-Shedlovsky method, c~'m the Fuoss-Onsager equations for associated and unassociated electrolytes, (1~ and the expanded (12~ Pitts equation. (13'1~ All calculations were performed by using computer programs (19-2~ written in F O R T R A N IV for a Xerox Sigma Six computer systemJ Above the concentration Cm~x, in which C ~ x = 3.2 x 10-TD 3, the laws for dilute solutions of electrolytes no longer are valid(Ira; however, as previously discussed, the expanded Pitts method of analysis requires data from a relatively wide concentration range. (lm Therefore, although C~a~ = 1 X 10 -a M for T E U solutions, the experimental concentration range was considerably extended. The molar concentrations and equivalent conductances of the salts studied in tetraethylurea are presented in Table I. The Fuoss-Shedlovsky method of evaluation yields values of A0 and KA from least-squares analyses of the functions A; vs C and y vs x. The symbols have their usual meaning :Vo = (A + ~C'D/O lla& = l/a0 + - ,~c~D CA&f2KalA=o (1) (2) in which y = 1/AS~ and x = C A S ~ f 2. In the Fuoss-Onsager method of analysis, the three parameters Ao, Ka, and aa are obtained from the equation A = A o - S ( C 7 ) 1/2 + E C 7 log Cy + J C 7 - K a C y A f 2 (3) 4 The computer program for the analysis of data by the expanded Pitts equation was kindly provided by Drs. J. E. Prue and A. D. Pethybridge. 574 Barker, Rosenfarb, and Riesen Table L Equivalent Conductances of Alkali Metal Salts in 1,1,3,3-Tetraethylurea at 25~ i i NaSCN NaI NaC10~ C x 10~ A C x lOs A C x 10'~ A 0.5089 1.953 6.472 12.41 20.04 26.83 31.92 34.20 19.04 14.20 9.880 7.883 6.633 5.958 5.592 5.453 2.764 6.427 14.33 31.30 57.65 75.31 94.78 17.75 15.25 12.69 10.35 8.767 8.141 7.635 1.585 6.206 21.72 39.39 60.55 79.87 19.50 16.64 13.29 11.69 10.59 9.925 NaBPha KSCN c • 104 A C x 104 A 1.910 5.358 12.92 24.22 54.99 97.04 13.56 12.71 11.77 11.03 10.01 9.300 0.8514 2.896 8.929 16.65 26.23 33.26 42.68 20.51 17.30 13.49 11.44 10.05 9.364 8.693 The expanded Pitts equation which is expressed as A = A0 - S ( C v ) ~12 + EC~, log-C~, + J1C~, - J2(C~,) a/2 - K A C v A f 2 (4) yields values o f Ao, KA, and d. In both equations the symbols have their usual meaning, S ~- ~Ao + 13 and E = E~Ao - E2. The physical properties of T E U at 25~ lead to values of 2.916, 53.26, 85.59, and 228.9 for the coefficients ~, /3, E~, and E2, respectively. For unassociated electrolytes ~ , = 1 and K A = 0 ; for associated electrolytes y < 1 and KA > 0. The coefficients J, ./1, and J2 are functions of the solvent physical properties and the ion size parameter. I n the Fuoss-Onsager evaluation, the ion size parameter aa is considered as a " c o n t a c t distance"; in the expanded Pitts analysis, the ion size parameter d is defined as a "distance o f closest a p p r o a c h . " As in numerous other conductance studies, viscosity corrections were neglected in the present evaluation since they were estimated to be small. In the Shedlovsky y - x m e t h o d o f analysis, linear relationships were obtained only in the lower concentration regions ( < 15-20 x 10-4 M). Since only a few data points were obtained in these regions, the y - x m e t h o d o f 1 , 1 , 3 , 3 - T e t r a e t h y l u r e a as a Solvent for C o n d u c t a n c e Studies 575 evaluation was unsatisfactory for determining conductance parameters in the present study. In the Shedlovsky A~ plot (Fig. 1), increasing curvature indicates increasing association. Therefore, in TEU the order of increasing association is NaSCN > KSCN > NaI > NaCIO~ > NaBPh4. This same trend in association behavior was found for these salts in TMU<2.~>; however, because of its lower dielectric constant, the extent of association in TEU is much greater than in tetramethylurea. Initial Ao values used in the Fuoss-Onsager equations were those obtained from the Shedlovsky A~-C method of analysis. The conductance parameters obtained from the Fuoss-Onsager evaluation of unweighted data for the salts are presented in Table II. For the associated salts the use of unweighted data led to a smaller standard deviation aA for individual conductance values than did data weighted by C7. As seen from Table II, with NaI and KSCN as examples, when data at concentrations greater than approximately 35 • 10 -~ M were included, the aA values for salts in TEU increased considerably. Extensive association of all salts except sodium tetraphenylborate is indicated by the large KA values. The Ao values obtained from the Fuoss-Onsager evaluation then were used as initial values in the expanded Pitts equation. As previously discussed} 21,22~ in this method of analysis several procedures exist for the determination of conductance parameters. A value of d can be assigned, and corresponding values of Ao and KA can be calculated; or a range of d values can be selected, and values of A0, KA, and d corresponding to the minimum o NaCIO 4 24 o Nal o NaBPh,4 ~ Na.SCN 9 KSCN /~7 10 I I o 22 I C X 10 4 | | 66 8a Fig. 1. Shedlovsky.A'o-C plots for alkali metal salts in tetraethylurea. 576 Barker, Rosenfarb, and Riesen < b o < ~ t r ~ 0 r c~ 0 ~ 9 0 0 0 o ~4 0 o o ,o 0 • N~ ~ N g 9 X o 9g z ZZ ZZM ~ ZZZZ~ .~ 2 1 , 1 , 3 , 3 - T e t r a e t h y l u r e a as a Solvent for C o n d u c t a n c e Studies 577 standard deviation ~rA can be chosen as the "best-fit" set of parameters for the system. This latter procedure was used in the present study. Conductance parameters were calculated at each 0.5/~ throughout the range of d values 3.0-10.0/~. The expanded Pitts conductance parameters presented in Table II correspond to the "best-fit" d values (i.e., those d values which yielded minimum erA). In this evaluation the upper concentration which led to tolerable aA values also was approximately 35 • 10 -4 M. Although the precision of the results could have been improved if more data points at lower concentration were included in the analyses, an indication of the reliability of the measurements is given by the consistency of the ratio A0(TMU)/A0(TEU). Using the best-fit expanded Pitts values for KSCN, NaSCN, NaI, NaC104, and NaBPh4, this ratio is, respectively, 2.05, 2.03, 1.94, 2.02, and 1.99. Some salts which are fairly soluble in TMU, e.g., KCIO4, KI, and NaNO3, are almost insoluble in tetraethylurea. Although sodium iodide is soluble in TEU, its solutions rapidly change from colorless to an intense dark yellow. This behavior is similar to that of iodide salts in TMU and other nonaqueous solvents. ~1"2,4~ Generally, the limiting equivalent conductances in a series of alkali metal ions increase as the crystallographic radii of the cations increase. As the crystallographic radii increase, the effective size of cations decreases because of decreasing solvation. However, as in tetramethylurea,~2~NaSCN has a higher A0 value than KSCN in tetraethylurea. In only several other solvents--dimethylacetamide,c23) dimethylpropionamide,c24~ and dimethylbutyramide,~25~--have sodium salts had A0 values about 0.5 unit higher than those for corresponding potassium salts. Possible assumptions for this anomalous behavior have been discussed previously32'~ For the conductometric titrations, diphenylguanidine was chosen as the titrant since it is a conveniently recrystallized, stable, primary standard material. Figure 2 indicates the results of the acid titrations. The conductance curves have been displaced both horizontally and vertically in order that typical results for all investigated acids can be presented clearly and concisely. Volume corrections were applied to all conductance data from which Fig. 2 was prepared. For 2,4-dinitrophenol and benzoic acid, the differences in slope before and after the end point were insufficient for obtaining satisfactory recovery. The recovery from duplicate titrations of all acids in TEU is presented in Table III. The precision of the results is comparable to that generally obtained from titration of organic acids in nonaqueous solvents, c5-7~ Since the promising potential of tetraethylurea as an electrolytic solvent has beea revealed in the present investigation, several future studies would be of interest in this medium. A conductance investigation of the behavior of quaternary am monium salts, including colloidal electrolytes at concentrations 578 Barker, Rosenfarb, and Riesen Ill o , , , c o . .o. . H Z 63 No2c6H4coo / Z O L) VOLUME DPG Fig. 2. Conductometric titration curves of monoprotic organic acids in tetraethylurea. Table III. Conductometric Titrations A c i d s in T e t r a e t h y l u r e a Acid Benzoic acid p-Nitrobenzoic acid o-Nitrobenzoic acid 2,4-Dinitrophenol Picric acid of Organic Millirnoles taken for analysis % Recovery from analysis 0.9875 1.018 0.8095 0.5989 0.6743 0.7228 0.4872 0.6534 0.4548 0.4886 -99.9 98.9 98.9 100.4 -99.4 99.5 1,1,3,3-Tetraethylurea as a Solvent for Conductance Studies 579 b e l o w 3 x 10 -3 M, c o u l d be considered. O f interest w o u l d be the degree o f a s s o c i a t i o n in these systems. T h e c o n d u c t o m e t r i c analyses could be e x t e n d e d to p o l y f u n c t i o n a l acids a n d to acid mixtures. Also, the use o f t e t r a e t h y l u r e a as a solvent for o t h e r electrochemical studies could be investigated. ACKNOWLEDG M ENTS A p p r e c i a t i o n is e x t e n d e d to Drs. J. E. Prue a n d A. D. P e t h y b r i d g e o f T h e University, R e a d i n g , England, who k i n d l y p r o v i d e d the c o m p u t e r p r o g r a m used for the analysis o f d a t a by the e x p a n d e d Pitts equation, a n d to G e o r g e L. W e b e r a n d his staff at the H o p e College c o m p u t e r center for assistance with the c o m p u t e r p r o g r a m s . REFERENCES 1. B. J. Barker and J. A. Caruso, in The Chemistry o f Nonaqueous Solvents, J. J. Lagowski, ed., Vol. 1V (Academic Press, New York, to be published). 2. B. J. Barker and J. A. Caruso, J. Am. Chem. Soc. 93, 1341 (1971). 3. ]3. J. Barker and J. A. Caruso, J. Phys. Chem. 77, 1884 (1973). 4. B. J. Barker and J. A. Caruso, Electrochim. Acta 18, 315 (1973). 5. S. L. Culp and J. A. Caruso, Anal. Chem. 41, 1329 (1969). 6. S. L. Culp and J. A. Caruso, Anal. Chem. 41, 1876 (1969). 7. M. S. Greenberg, B. J. Barker, and J. A. Caruso, Anal. Chim. Acta 54, 159 (1971). 8. T. Shedlovsky, J. Franklin lnst. 225, 739 (1938). 9. R. M. Fuoss and T. Shedlovsky, J. Am. Chem. Soc. 71, 1496 (1949). 10. R. Mo Fuoss and F. Accascina, Electrolytic Conductance (Interscience, New York, 1959). 11. R. M. Fuoss and L. Onsager, J. Phys. Chem. 61, 668 (1957). 12. R. Fern~indez-Prini and J. E. Prue, Z. Physik. Chem. (Leipzig) 228, 373 (1965). 13. E. Pitts, Proc. Roy. Soc. (A) 217, 43 (1953). 14. E. Pitts, B. E. Tabor, and J. Daly, Trans. Faraday Soc. 65, 849 (1969). 15. H. L. Huffman, Jr., and P. G. Sears, J. Solution Chem. 1, 187 (1972). 16. G. A. Schwartz and B. J. Barker, Talanta, manuscript accepted. 17. H. M. Daggett, E. J. Bair, and C. A. Kraus, J. Am. Chem. Soc. 73, 799 (1951). 18. J. E. Lind, Jr., J. J. Zwolenik, and R. M. Fuoss, J. Am. Chem. Soc. 81, 1557 (1959). 19. R. L. Kay, J. Am. Chem. Soc. 82, 2099 (1960). 20. J. L. Hawes and R. L. Kay, J. Phys. Chem. 69, 2420 (1965). 21. B. J. Barker, H. L. Huffman, Jr., and P. G. Sears, J. Phys. Chem. 78, 2689 (1974). 22. B. J. Barker and P. G. Sears, Y. Phys. Chem. 78, 2687 (1974). 23. G. R. Lester, T. A. Gover, and P. G. Sears, J. Phys. Chem. 60, 1076 (1956). 24. E. D. Wilhoit and P. G. Sears, Trans. Ky. Acad. Sci. 17, 123 (1956). 25. H. M. Smiley and P. G. Sears, Trans. Ky. Aead. Sci. 19, 62 (1958).