J. Chem. SOC., Faraday Trans. I , 1989, 85(3), 479483 Heat of Solution and Electric Conductivity of Electrolytes in Water-Tetrahydrofuran Mixtures Stefania Taniewska-Osinska,* Zygmunt Kozlowski, Boiena Nowicka, Adam Bald and Adam Szejgis Department of Physical Chemistry and Department of Chemial Didactics, University of t d d i , ul. Nowotki 18, 91-416 t o d i , Poland Integral heats of solution of sodium perchlorate (NaCIO,) in water- tetrahydrofuran (THF) mixtures from 0 to 100molY0 THF have been measured. The electric conductivity of NaCIO, solutions in binary solvents containing from 0 to 80 mol % THF have also been measured, as have the conductivities of solutions of sodium chloride (NaCI) and sodium iodide (Nal) in mixtures containing from 0 to 35 mol% THF. Molar electro- conductivities, Walden products and association equilibria constants have been determined.Taking into account ion-pair association, standard enthalpies of solution have been recalculated. Our earlier thermochemical investigations of solutions of electrolytes and of urea in water-tetrahydrofuran (THF) mixtures were limited to the water-rich region.'. The variations of the standard enthalpies of solution us. mixed solvent composition appeared to be similar to analogous behaviour for the solute-water-t-butyl alcohol system. In this paper we present the results of measurements of AH: (NaClO,) over the whole composition range of water-THF mixtures. Taking into' account the low electric permittivity of solution with high tetrahydrofuran concentrations we considered it necessary to examine the effect of ion-pair formation on the standard enthalpies of electrolyte in solution.For this reason the electric conductance was measured for NaClO,, NaCl and NaI solutions in the mixed solvent under investigation. Experimental The purification of the salts NaClO,, NaCl and NaI and of solvent THF has been described earlier.' The enthalpies of solution of NaClO, in water-THF mixtures were measured in a calorimeter also described in ref. (1). The conductivity measurements were performed using a bridge of the type E 3 15A (Mera-Tronik, Poland). A cell similar to that described by Dagett3 was used. The temperature of the thermostat, 298.15 K, was kept constant to within k0.005 K. There have been no investigations of electric conductivity carried out in mixtures with THF contents > 35 mol%, nor have any measurements been made of heats of solution for NaCl and NaI, because of the extremely low solubility of these salts.'~2 Since the ion association is large, it was possible to perform conductometric measurements for NaC10, solutions with satisfactory precision only up to 80 mol % THF.Results and Discussion In order to determine the enthalpy of solution at infinite dilution, i.e. the standard enthalpy of solution, of NaC10, in water-THF mixtures, as well as of NaCl and NaI examined earlier1T2 in the same mixed solvent, the method presented by Barthel et al., 479 17-2480 Solution Heats and Conductivities of Electrolytes in THF-H,O Table 1. Electric conductivty of electrolytes in water-tetrahydrofuran mixtures at 298.15 K THF A0 dAo" K* dK," 6," (mol %) /cm2 mo1-I R-' /cm2 mol-' R-' /dm3 mol-' /dm3 mol-I /cm2 mol-I l2-l nb 4' 1 .o 2.5 5.0 10.0 15.0 20.0 25.0 30.0 35.0 2.5 5.0 15.0 25.0 30.0 35.0 2.5 4.2 10.0 15.0 20.0 25.0 40.0 60.0 80.0 114.46 100.87 84.90 68.18 59.79 54.52 50.01 47.12 44.85 98.85 80.79 58.41 54.96 56.35 57.41 92.78 80.66 58.70 53.86 52.57 53.53 58.87 71.20 93.33 f 0.02 f 0.02 f 0.02 f 0.02 f 0.02 f 0.02 f 0.02 f 0.04 & 0.04 - + 0.02 - + 0.02 & 0.02 f 0.02 - + 0.04 f 0.06 f 0.02 f 0.02 kO.01 f 0.02 f 0.02 fO.O1 f 0.02 fO.01 f 0.02 NaCl - - - 2.6 5.7 10.3 22.2 42.2 73.5 NaI - - - 7.9 16.7 33.8 NaClO, - - - - - I .o 31.3 352 6773 - - - fO.1 f0.1 f0.2 f 0.2 f 0.6 f 0.7 - - - kO.1 - + 0.5 f 1.0 - - - - - f0.I kO.1 + I k 6 f 0.05 f 0.03 f 0.03 +_ 0.04 f 0.04 k 0.04 f 0.04 f 0.08 f 0.08 f 0.05 f 0.05 f 0.03 f 0.04 - + 0.08 fO.10 f 0.05 f 0.05 k 0.03 f 0.05 & 0.04 f 0.0 1 f 0.04 f 0.02 k 0.04 12 8 14 14 14 16 15 15 13 14 12 13 13 12 12 11 13 14 14 10 14 10 16 19 3.69 3.88 4.23 5.02 5.95 7.02 8.21 9.5 1 10.88 3.88 4.23 5.95 8.2 1 9.5 1 10.88 3.88 4.1 1 5.02 5.95 7.02 8.2 1 12.30 18.68 26.8 1 a dAo, d K , and 6, are the standard deviations of Ao, K,, and A, respectively."Number of experimental points. Bjerrum distance parameter. was employed. In this method the relative apparent molal heat content of a solution containing 'free' ions and ion pairs is described by the following expression: where QL(FI) is the relative apparent molal heat content of a solution with 'free' ions, AH: is the enthalpy of association and a is the degree of dissociation.Taking into (2) account one obtains an expression determining the enthalpy of solution at infinite dilution : AH," = AH,"-a@,,(FI)-(l - a ) A H i (3) mL = -AH: = AHF-AHZ where AH," is the integral enthalpy of solution at a molality m. The @ , , ( F I ) values were calculated using equations proposed by Barthel et aL4 and by Wachter et al.' The necessary values of the degree of dissociation were obtained by us from conductometric measurements. Molar conductivities were analysed by means of the Fuoss-Justice equation :6 A = a[A,-~~aora+E(col)ln(col)+ J ( c ~ ) + J $ U ~ ] . (4) Analytical forms of the remaining quantities have been presented elsewhere."-"'S. Taniewska-Osinska et al. 48 1 C 10 0 6 L g I 2 -10 2 2 -2 0 -3c -\ THF (mol %) 1 1 I 1 I I I I I 1 THF (molOr0) 20 40 60 80 100 Fig.1. Standard enthalpies of solution, AH,", of electrolytes in water-tetrahydrofuran mixtures at 298.15 K. (-) A% obtained by use of eqn (3) for a = 1 ; (---) A% taking into account the effect of ionic association; 0 and ., NaI; 0 and 0 , NaCIO,; A and A, NaCl. The degree of dissociation and the association constant KA were calculated from the relationship I -a -- - KA a2CY: where y+ - is the activity coefficient and A cfat Iny, = - 1 + BqchB where A and B are coefficients of the Debye-Hiickel equation. According to Justice's' suggestions, in our work we adopted the Debye-Huckel closest-approach parameter a, equal to the Bjerrum distance q (q = e2/2&kBT). In this way eqn (4) becomes a diparametric equation, and was resolved by a least-squares rnethod.'*-l2 The values obtained of the parameters A.and KA and their standard errors dAo and 6KA are included in table 1. As can be seen from these data, ionic association was not observed, in the water-rich solutions, up to 10 mol % THF for NaCl and 20 mol O h482 Solution Heats and Conductivities of Electrolytes in THF-H,O 20 40 60 80 Q4 THF (mol%) Fig. 2. Walden product A, 17 of electrolytes in water-tetrahydrofuran mixtures at 293.15 K, A, NaCl ; 0, NaI;O, NaCIO,; A, for solutions of electrolytes in water from ref. (14); 17, our values, ref. (1 5) and (16). THF for NaI and NaC10,. In the systems with higher THF contents ionic association becomes significant, with the KA values increasing with an increase in THF content.In order to obtain true values AH," we used eqn (3). To solve this equation it is sufficient to know the experimental values of AH? [this work and ref. (1) and (2)] and the ion-pair formation equilibrium constants for NaClO,, NaCl and NaI in water-tetrahydrofuran mixtures. Eqn (3) was solved by a least-squares method which simultaneously permits one to calculate the values AH," and AH:. In fig. 1 standard enthalpies of solution AH," of electrolytes obtained by the above mentioned method are plotted along with those directly extrapolated from calorimetric data. The plots of AH," us. mol% THF both taking into account ionic association and neglecting it are of similar character. In all cases ionic association diminishes the enthalpies of solution.The plot of the standard enthalpy of solution of NaC10, passes through a minimum in tetrahydrofuran-rich solutions, suggesting a change in the interactions or structure of the mixture. This function resembles an analogous dependence in electrolyte- water-butanol systems.13 The Walden product (fig. 2) for solutions of the three investigated electrolytes in water-tetrahydrofuran mixtures was calculated. The only conclusion we were able to draw from it is that there is difference in behaviour for NaCl solutions with regard to NaI or NaClO, solutions. The same conclusion results from calorimetric data (fig. 1). Financial support for this work from the CPBP-01 .15 programme is acknowledged. References 1 S. Taniewska-Osinska, B. Piestrzyriska and R.togwinienko, Can. J. Chem., 1980, 58, 1584. 2 S. Taniewska-Osinska and B. Nowicka, Thermochim. Acta, 1987, 115, 129. 3 H. M. Dagett, E. J. Bair and C. A. Kraus, J . Am. Chem. SOC., 1951, 73, 799. 4 J. Barthel, H. J. Gores, G. Schmeer and R. Wachter, Non-Aqueous Electrolyte Solutions in Chemistry 5 R. Wachter and K. Riederer, Pure Appl. Chem., 1981, 53, 1301. and Modern Technology, Topics in Current Chemistry (Springer, Berlin, 1983), vol. I l l , p. 49.S. Taniewska-Osinska et al. 483 6 J . C. Justice, Electrochim. Acta, 1971, 16, 701. 7 R. M. Fuoss and K. L. Hsia, Proc. Natl Acad. Sci. USA, 1967, 57, 1550. 8 R. Fernandez-Prini, Trans. Faraday SOC., 1968, 64, 2146. 9 R. M. Fuoss and F. Accascina, Electrolytic Conductance, (Interscience, New York, 1959), p. 195. 10 J. Barthel, Angew. Chem., 1968, 80, 253, (Angew. Chem. Int. Ed. Engl., 1968, 7 , 260). 11 J. C. Justice, R. Bury and C. Treiner, J. Chem. Phys., 1968, 65, 1708. 12 J. Barthel, J. C. Justice and R. Wachter, 2. Phys. Chem., N.F., 1973, 84, 100. 13 S. Taniewska-Osinska and H. Piekarski, J . Solutn Chem., 1978, 7 , 12. 14 J. L. Hawes and R. L. Kay, J. Phys. Chem., 1965,69, 2420. 15 B. Nowicka, A. Kacperska, J. Barczynska, A. Bald and S. Taniewska-Osinska, J . Chem. Soc., Furaduy 16 S. Taniewska-Osinska and B. Nowicka, unpublished data. Trans. I, in press. Paper 71001 501 ; Received 5th October, 1987