J. CHEM. SOC. FARADAY TRANS., 1994, 90(4), 579-582 579 Ultrasonic Velocities and lsentropic Compressibilities of some Tetraalkylammonium and Copper(1) Salts in Acetonitrile and Benzonitrile Jasbir Singh, Taranjit Kaur, Vazid Ali and Dip Singh Gill* Department of Chemistry, Panjab University, Chandigarh- 160014,India Ultrasonic velocities and densities of Bu,NBPh,, Pr,NBPh,, Et,NBPh,, Bu,NCIO,, Pr,NCIO,, Et4NCI0,, Bu,NI, CuCIO, .4CH,CN and [Cu(DMPhen),]CIO, in acetonitrile and benzonitile have been measured at 298, 308 and 318 K. The isentropic compressibilities (K~)and apparent molal compressibilities (K~)of various electrolytes have been evaluated. The limiting apparent molal compressibilities (K:) for all electrolytes have been obtained by extrapolation of the plots of IC+vs.square root of molality (m’I2)and split into limiting ionic compressibilities (I&). IC:* is positive in benzonitrile except for Et,N+ and is negative in acetonitrile except for Bu,N+, Ph,B-and Pr,N+. The results indicate stronger solvation of ions in acetonitrile than in benzonitrile. Compressibility data are usually needed for predicting the Results and Discussion pressure dependence of equilibrium properties of electrolyte solutions. Precise compressibility data of electrolytes in non- The ultrasonic velocities (u) as a function of the molality of aqueous solvents are rare1*2 and for copper(r) salts they are the electrolytes in AN and BN are shown in Fig. 1 and Fig. 2. completely lacking. In order to investigate the compressibility The ultrasonic velocities vary linearly with molality.In AN behaviour of some tetraalkylammonium and copper(1) salts in acetonitrile (AN) and benzonitrile (BN), we have undertaken ultrasonic velocity and density measurements of solutions of tetrabutylammonium tetraphenylborate (Bu,NBPh,), tetra-I propylammonium tetraphenylborate (Pr,NBPh,), tetraethyl-1296 ammonium tetraphenylborate (Et,NBPh,), tetrabutyl-ammonium perchlorate (Bu,NClO,), tetrapropylammonium 1292 perchlorate (Pr,NClO,), tetraethylammonium perchlorate (Et,NClO,), tetrabutylammonium iodide (Bu,NI), copper(r) 1288 perchlorate tetraacetonitrile (CuCIO, .4CH3CN) and bis( 2,9-dimet h yl-1,lO-phenant hroline)copper(i) perchlorate 1284 ([Cu(DMPhen),]ClO,) at 298,308 and 318 K. ~~1280 0 0.05 0.10 0.15 0.20 0.25 0.30 Experimental 1259 Ultrasonic velocity measurements were carried out at 2 MHz r 1254 Iwith an ultrasonic time intervalometer (model UTI-101) from fn Innovative Instruments, Hyderabad using a pulse-echo E 1250 \ 5overlap technique.Density measurements were made using a 1246precision densimeter (Anton Paar model DMA-60 with exter- nal measuring cell-602). The absolute accuracy of the sound 1242velocity measurements as reported before3 was better than 2 parts in lo4 and for density measurements it was & 1 x lo-’ 1238g cm-3. The ultrasonic velocities in pure AN at 298, 308 and 0 0.05 0.10 0.15 0.20 0.25 0.30 r 1318 K were 1280.8, 1239.6 and 1200.4 m s-l and in BN were 1418.0, 1382.5 and 1349.1 m s-’.The densities of AN at the 1219 corresponding temperatures were 0.776 851, 0.765 813 and 0.752717 g cmP3 and of BN were 1.00034, 0.991 683 and 1215 0.982945 g cm-3. A comparison of our all values with others 1211cannot be made, but the sound velocity in pure AN and its density at 298 K are in good agreement with the literature 1207 values. q4 AN (99% pure, E. Merck, India) and BN (Puriss, >99% 1203 GC, Fluka Chemika) were purified as reported The sources, grades and methods of preparation/purification of all 1199 1 0 0.05 0.10 0.15 0.20 0.25 0.30 the electrolytes used are given previously.6-8 A range of con- m/mol kg-’ centrations of the electrolytes in AN and BN was produced Fig. 1 Ultrasonic velocity us.molality in AN at (a) 298 K, (b)308 Kby diluting stock solutions of appropriate concentrations. In and (c) 318 K. *, Bu,NBPh,; 0, Pr,NBPh,; 0,Et,NBPh,; x,all cases the measurements were repeated to obtain repro- Bu4NC10,; V, Pr,NClO,; X, Et,NClO,; Q, Bu,NI; 0,ducible results. CuClO, .4CH ,CN ;A, Cu(DMPhen),ClO, . 580 1440 1436 1432 1428 1424 1420 1416 0 0.1 0.2 0.3 0.4 1380 I II 0 0.1 0.2 0.3 0.4 rnlmol kg-Fig. 2 Ultrasonic velocity us. molality in BN at (a) 298 K, (b)308 K and (c)3 18 K. Symbols as in Fig. 1. the ultrasonic velocities are smaller than those in BN. The sound velocities of tetraalkylammonium tetraphenylborates are larger than those of the corresponding perchlorates and Bu,NI in both solvents.The ultrasonic velocity of CuC10,.4CH3CN is smaller and that of [Cu(DMPhen),]ClO, almost comparable to those of tetra- alkylammonium salts in AN and BN. The ultrasonic velo- cities decrease with increasing temperature. The isentropic compressibility of each electrolyte was cal- culated from the ultrasonic velocity and density (p) using the equation : KS = 1/(U2P) (1) Fig. 3 and 4 show K~ as a function of molality. In contrast to the ultrasonic velocities, the isentropic compressibilities decrease non-linearly with increasing salt concentration and increase with temperature. The apparent molal volumes, V, and apparent molal isen- tropic compressibilities, K+ of all the electrolytes at 298 K have been calculated using the equations and (3) J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 79 1 I 78 77 76 75 1 I J 0 0.05 0.10 0.15 0.20 0.25 0.30 85 r I Z 83n 0I 0381 79 0 0.05 0.10 0.15 0.20 0.25 0.30 93 1 92 91 90 89 80 87 86 0 0.05 0.10 0.15 0.20 0.25 0.30 rn/mol kg-Fig. 3 Isentropic compressibility us. molality in AN at (a) 298 K, (b) 308 K and (c) 318 K. Symbols as in Fig. 1. where m is the molality, M is the molar mass (g mol-I) of the solute, K,(sin) and xS(1) are the isentropic compressibilities of the solution and pure solvent and &in) and p(1) are the densities (g cm-3) of the solution and the pure solvent. The limiting apparent molal compressibilities (@ of the electrolytes were obtained from the extrapolation of the linear plots of K, us.m112 by the least-squares method using the equation: K, = K$ +-A,m'I2 (4) Plots of K, vs. m112are shown in Fig. 5. The extrapolated IC:and A, values of eqn. (4) are reported in Table 1. The maximum uncertainty in the IC~values is f3 x lo-, cm3 mol -bar -I. Table 1 shows that K$ values in AN are negative for Et,NBPh,, Pr,NClO, , Et,NCIO, , CuClO, -4CH3CN and [Cu(DMPhen),]ClO, while for the other electrolytes they are positive. In BN K: values are positive for all the solutes. Our K: value for Et,NClO,, -91 x lo-, cm3 mol-' bar-', in AN is in good agreement with the value, -92.4 x lo-, an3 mol-' bar-', reported by Davidson et al.' Table 2 lists the pairwise differences of K$ values among a series of tetraalkylammonium salts to verify the additivity rule.The difference between appropriate pairs of electrolytes does not exceed 3 x lo-, m3mol-' bar-', which is within the limit of our experimental error. This verifies the additivity rule of compressibilities (K: values) in AN and BN. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 49.8 I 1 0 0.1 0.2 0.3 0-4 r 1 52.8 52.4 52.0 I b 51.8 n 51.2 0 7 50.8 '50.4 1 50.0 1 Id 0 0.1 0.2 0.3 0.4 55.8 55.4 55.0 54.6 54.2 53.8 -53.4 There is no reliable and direct method of breaking K: values into ionic components. Some approaches have been The method' already used in AN is based upon K~(P~,B-) 0. This approach is not appropriate as the = Ph,B-ion is large (0.535 nm)," even larger than the Bu,N+ ion (0.50 nm)I3 and therefore its compressibility contribution cannot be taken as zero.Millero" has split the partial molar volumes of electrolytes into ionic components using Ph,AsBPh, as a reference electrolyte on the basis of v:Ph4 As + )/'gh4 B -) = $Ph4 As + )/':Ph4 B -) A similar model based on Bu,NBPh, as a reference electro- 58 1 150 100 50 0 -50 --100 n 7--150 I I I 1 I 1E I 0.1 0.2 0.3 0.4 0.5 0.6 m *g 200 I2--. Yi 100 0 -1 00 I I I I Fig. 5 Apparent molal isentropic compressibility vs. square root of molality at 298 K in (a) acetonitrile and (b) benzonitrile. Symbols as in Fig. I. lyte for splitting the viscosity B coefficients of electrolytes into the contributions from individual ions was suggested by Gill and Sharma.', This model has been used in the present work.The K:* values obtained in this way are listed in Table 3. The most striking features of the ionic K:* values in AN are the large positive values for Bu,N+ and Ph,B- ions, the very small value for Pr,N+ and the negative values for I-, ClO,, Et,N+, Cu+ and [Cu(DMPhen),]+ ions. In benzoni- trile, the ionic K:* values for all ions except Et,N+ are posi- tive. They are relatively large for Bu,N+, Ph,B-and [Cu(DMPhen),]+ and relatively small for Pr4N+, ClO, and Cu'. The large rc:* values arise either from some free space present in the solution between the ions and the solvent mol- ecules or from conformational changes in the structure of the large R,N+ cations. For a large R4N+ ion such as Bu,N+, the large K:* value can be explained due to changes of con- figuration from long chains into a coiled or spherical configu- ration by the application of pressure from the ultrasonic wave. This effect should decrease with decreasing size of the R,N+ cation.This is seen from the results of Table 3 as ionic Table 1 Limiting apparent molal isentropic compressibilities and the slope (A,) of eqn. (4) for some tetraalkylammonium and copper(1) salts in AN and BN at 298 K AN BN electrolyte K$/~O-, cm3 mol-' bar-' 104~~ K$/~O-, cm3 mol-' bar-' 104~, Bu,NBPh, 108 92 138 102 Pr,NBPh, 62 76 90 -14 Et,NBPh, -6 210 70 -228 Bu4NC10, 20 80 81 52 Pr4NC104 -25 -7 30 -30 Et,NCIO, -91 -80 12 -228 Bu,NI 12 112 130 -72 CuClO, * 4CH,CN -40 24 49 -8[Cu(DMPhen),]CIO, -150 230 90 -222 J. CHEM.SOC. FARADAY TRANS., 1994, VOL. 90 Table 2 Test of the additivity rule for limiting apparent molal com- negative or small K$* values for most of the ions in AN pressibilities in AN and BN at 298 K therefore indicate stronger solvation of ions in AN than in BN.K$'~O-~cm3 mol-' bar-' electrolyte pair AN BN The authors are grateful to the CSIR, New Delhi for a research grant under the research scheme 1( 122 1)/9 1 -EMR-I1Bu4NBPh4-Bu4NC104 88 57 and for the award of a senior research fellowship to J.S. and a Pr4NBPh4-Pr,NC10, 87 60 Et,NBPh,-Et,NCIO, 87 58 Research Associateship to V.A.Bu,NBPh,-Pr,NBPh, 46 48 Bu,NC10,-Pr,NC104 45 48 Bu,NBPh,-E t,N BPh4 114 68 BU~NCIO,-E~~NCIO~ 111 69 References Pr,NBPh,-Et,NBPh, 68 20 Pr,NCI0,-Et4NC104 66 18 1 I. Davidson, G. Perron and J. E. Desnoyers, Can. J. Chem., 1981, 59, 2212. 2 M. S. Bakshi, J. Singh, S. K. Bhullar, B. Kaur, S. C. Sharma and I. M. Joshi, Acoustica, 1992, 75, 292. Table 3 Limiting ionic apparent molal isentropic compressibilities 3 D. S. Gill, T. Kaur, H. Kaur, I. M. Joshi and J. Singh, J. Chem. of some ions in AN and BN at 298 K SOC.,Faraday Trans., 1993,89, 1737. 4 J. A. Riddick, W. B. Bunger and T. K. Sakano, Organic Solvents, cm3 mol-' bar-' Physical Properties and Methods of Purijication, Wiley Inter- ~g,/lO-~ science, New York, 4th edn., 1986. ion AN BN 5 D.S. Gill, K. S. Arora, J. Tewari and B. Singh, J. Chem. Soc., Faraday Trans. I, 1988,84,1729. Bu4N+ 48 62 6 D. S. Gill and B. Singh, J. Chem. Soc., Faraday Trans. I, 1988, Pr4N+ 3 11 84,4417. Et,N+ -63 -7 7 D. S. Gill, A. N. Sharma and H. Schneider, J. Chem. SOC., +cu -12 30 Faraday Trans. 1, 1982,78,465. Cu(DMPhen) -64 71 8 D. S. Gill, K. S. Arora, B. Singh, M. S. Bakshi and M. S. Ph,B -60 76 Chauhan, J. Chem. Soc., Faraday Trans., 1991,87, 1159. c10; -28 19 9 R. D. Lisi, S. Milioto and R. E. Verrall, J. Solution Chem., 1990, 1--36 68 19, 665. 10 F. Millero, J. Phys. Chem., 1971, 75, 280. 11 R. Zana, G. Perron and J. E. Desnoyers, J. Solution Chem., 1980, 9, 59. K$* values for the smaller Pr,N+ and Et,N+ ions are much 12 D. S. Gill and M. B. Sekhri, J. Chem. Soc., Faraday Trans. I,lower than that of Bu,N+ in AN and BN. For Ph,B-, the 1982, 78, 119. large Kgi value may also arise due to the change from a loose 13 D. S. Gill, J. Chem. SOC.,Faraday Trans. I, 1981,77, 751. structure into a more compact structure by the application of 14 D. S. Gill and A. N. Sharma, J. Chem. SOC., Faraday Trans. I, pressure from the sound wave. 1982, 78,475. Very small or negative tc$* values usually arise from enhanced structural effects due to the solvation of ions. The Paper 3/05018A; Received 18th August, 1993