IONIC ABSOCIATION OF POTASSIUM PERCHLORATE
2923
Ionic Association of Potassium Perchlorate in Sulfolane-Water Mixtures
from Conductance Measurements at 250
.D'Aprano," I. D. Donato,la and R. Palombolb Institute of Physical Chemistry, University of Palermo, Palermo, Italy
(Received January 84, 10'7.g)
The ion pair association of potassium perchlorate in water-sulfolane mixtures has been computed by conductame measurements at 25". Deviations from previous results in pure solvents are observed. An interpretation involving the water structure modification by sulfolane takes into account the observed behavior of ion pair association.
Introduction Over the past years a number of workers have made extensive investigations concerning the effects of the dielectric constant, the nature of ions, and the structure of the solvent on the ionic association. The main purpose of these studies has been to gain a further knowledge on the structure of the electrolyte species in solution. Among the different experimental techniques suitable for these investigations, conductance has been thc most used, the conductometric method for determination of association constants being able to give exact experimental results within 0.01%. Owing to the limitation of the Conductance theory2 most of the published work has been made in solvents or solvent mixtures in the dielectric range below 30, where the association constant for the formation of electrostatic ion pair becomes detectable by the conductance equaThe lack of jnformation about ionic association in solvents of higher dielectric constant has been recently eliminated by Fuoss with a new theoretical treatment of conductance theory. Recent studies on ionic association of alkali perchlora,tes in pure solvents, covering the dielectric constant range betn een 80 and 30, have given experimental evidence of the large effects of solvent structure and ion solvation on the ion-pairing process. In order to get a better understanding of the problem and to investigate the postulated6 effect of water structure modification on the ionic association, the conductance of potassium perchlorate in water-sulfolane mixtures has been measured a t 25'. The results are given and discussed in the present paper. $r5
Experimental Sei &on Reagent grade sulfolane (tetrahydrothiophene 1,ldioxide) was distilled twice under reduced pressure (bp 130" (6.5")). The middle fraction (60-70%) of each distillation was used. Conductivity water ( K O = 1-2 X ohm-' cm-l) was purified as previously described.? Water-solfolane mixtures were made up by weight
and corrected to vacuum. For each solvent mixture used for conductance work, the physical properties were determined at 25". Densities were measured in a Sprengel-type picnometer calibrated with distilled water. Viscosities were measured in a Ubbehlode viscometer. The flow time for water was 471.3 sec reproducible to 0.1 or 0.2 sec. Dielectric constants were measured on the General Radio 761 CSI capacitance bridge at 1 MHz using the stainless-steel cell previously described.R The physical properties of water-sulfolane mixtures are summarized in Table I,9 where w is the weight per cent of water, D the dielectric constants, d(g/cm3) the densities, 10011 the viscosities in cP. Reagent grade potassium perchlorate was recrystallized from conductivity water and dried by heating under vacuum a t 150". Conductance measurements were carried out by the concentration method using the bridge, bath, and electrical apparatus previously described.l0 All measurements were carried out a t 25.00 f 0.003'. Two erlenmeyer-type conductance (1) (a) Postdoctoral Research Fellow CNR, Contract No. 69.00345/ 115/630, 1970. (b) Part of the results presented in this paper have been included in the thesis presented by R. Palombo t o obtain the degree of "Docteur de Specialite" in the University of Montpellier, France. (2) (a) R. M. Fuoss and L. Onsager, J. Phys. Chem., 61, 668 (1957); (b) R. M. Fuoss, J . Amer. Chem. SOC.,81, 2659 (1959). (3) R. M. Fuoss and K. L. Hsia, Proc. Acad. Sei. U . S., 57, 1550 (1967). (4) A. D'Aprano, J. Phys. Chem., 75, 3290 (1971). (5) A. D'Aprano, ibid., 76, 2920 (1972). (6) A. D'Aprano and R. M . Fuoss, J . Amer, Chem. Xoe., 91, 211 (1969). (7) A. D'Aprano, R k . Sci., 34(7), 433 (1964). (8) J. Lind, Jr., and R. M. Fuoss, J. Phys. C h m . , 65, 999 (1961). (9) Tables I and I1 will appear following these pages in the microfilm edition of this volume of the journal. Single copies may be obtained from the Business Operations Office, Books and Journals Division, American Chemical Society, 1155 Sixteenth St., X.W., Washington, D. C. 20036, by referring to code number JPC-722923. Remit check or money order for $3.00 for photocopy or $2.00 for microfiche. (10) I?. Accascina, A. D'Aprano, and R. M. FUQSS, J . Amer. Chem. SOC.,81, 1058 (1959).
The Journal of Physical Chemistry, Vol. 7'6, No. 20, i97'2
2924
1.1
-
A____
Figure 1 . Log K A os. ( l / D T ) X lo6 for KCIOI in (0)pure solvents and (a) water-sulfolane mixtures.
Figure 2. hV/V100 us. mole fractJion of sulfolane water-sulfolan 3 mixtri "es at 25".
(N2) iri
cells were u3ed. Cell constants 1.6698 f 0.0002 and 7.0897 f 0.0805 were determined as previously described. l 1 The meamred conductance of potassium perchlorate in sulfolane-water mixtures are summarized in Table TI9 where A is the equivalent conductance(ohm-' cm2 equiv-') eind c %heconcentration (equiv/l.). The values in pure w.atcr4 and in pure sulfolane12are omitted since they were previously published. The expednnental data reported in Table I1 were andyzed by a new conductance equation4 -i--
A.
__ sc'/2T'/2
%" E C Y log Jrcy
CY
+
+ J z ~ ~ /-.~ KACyf'A y ~ / ~
The Journal of Ph~/sicalChemistry, Vol. 76, N o . BO, 1972
(1)
quation 1, containincluding terms of the order cS". ing the four unknown Ao, K A ,J , and J z , has been programmed for an electronic computer to find Ao, K A , and 6 parameters. Full details concerning the new equation and the search program have been reported elsewh~re.~We are indebted to FUOSB for making the Fortran computer program availreble to us. The results of the computer analysis are surnmerized in the form of conductance perameters in Table III where the symbols have the usual meaning. A11 data are at 2.5' except in the case of pure sulfolane which has a melting point of 28.45O.I8 In this case the data are a t 30" From an inspeclion of Table BI we note a minimum In the association of polsssium perchlorate in water-sulfolane mixtures. This behavior i s much more evident in Figure 1 where log K A for KCICr, in pure solvents5 and in water-sulfola,nc! mixtures 1.9 plotted against l / D T . The minimum, already found by other ~ o r k e r s ~ ~ ~ * - - ' ~ in several. mixed solvents, demonstrates once more that the bulk dielectric constant is not the only factor involved in the association process, hut other factors, strongly dependent on the nature kznd composition of solvent, must be considered. This meanb taking into (11) J. E. Lind, Jr., J. Zwolenick, and R. M. Fuosa, J . Amer. Chem. Sac., 81, 1557 (1959). (12) B. Fernhndez-Prini and J. E. Prue, Trana. Parurlayj Sue., 62, 1257 (1966). (13) R. Garnsey and J. E. Prue, ibid., 64, 1206 (1968). (14) Y. H. Inami, H. K. Bobenseh, and 9.B, Ramsey, J~ Amar, Chem. Sac., 83, 4745 (1961). (15) F. Conti, P. Delogu, and G . Pistola, J . Phus. Ckem., 72, 1396
(1968). (16) F. Conti and G. Pistoia, ibid., 72, 2245 (1968).
EFFECT OF Hrm PRESSURE ON AQUEOUS EuSOa Table 1x1 : Derived Constants
n
A0
KA
d
78.3b 78.16 77 I75
L40.73 rt 0 . 1 138.78rt0.03 135.19zk0.04
3.3 3.3
77.15
130.19kQ.09
75.20 12.37 65.20 64.90 60 10
118.14f0.1 98.25 k 0.05 63.63rtO.06 63.0510.08 46.48i-0.03 31.165 :F- 0.008
0.98 i 0.04 0.99 f 0.01 0.93 =t0.02 0.79 i 0.09 (0.41f0.24) (0.19 I 0.03) ( 0 . 0 rt 0.07)
I
54 1.2 ~
49.40 43 I33" is
3.5 3.8
0.01 0.005 0.008
0.03 0.09 0.01
0.05 0.08 0.05
(0.12i0.10)
(0.34 5 0.05) 1.64 rt 0.04 2E..287'=k0.004 3.95 rt 0.01 10.760 rt 0.001 9.86 I 0 . 0 3
rT
6.2 5.2 3.9
0.02 0.04 0.07
Values at 30°, see ref 12.
account the influence on the association of the specific ion-solvent interactions connected t o the solvent strucSure.
I n order t o investigate the dependence of ionic solvation on the solvent composition, the volumetric behavior of water-sulfolane mixtures was derived from the density values given in Table I. 'The results are shown in Figure 2 where the molar volume exccss in per cent [(AV/V)lOO] is plotted against the mole fraction of sulfolane ( N z ) . As can be seen, a negative volumc excess is found as sulfolane is added to pure water. Assuming that the increased ~ ~ c k density ~ n g in the water-rich mixtures enhances the preferential solvation of ions by water, the observed decrease of ~ ~ ~ ~ association of potassium perchlorate can he ~ considering the increased screening effect 01 the d v a tion shell on the ion-ion interaction. A further comment should be made, finally, about the postulated increase of ionic solvation caused by the water structure modification. If such a process occurs, a decrease OC ionic mobility must be observed in the water-rich region. The behavior of Walden product shown in Figure 3 seems t o confirm, &o from the hydrodynamic point of view, the above ~ ~ ~ s ~ ~
igh Pressure on the Formation of Aqueous EuSO,+ at 25" by Clarence F. Hale and F. H. Spedding" Ames Laboratory, U , S. Atomic Energy Commission and Department of Chemistry, Iowa State University, Ames, Iowa 60010 (Received March 30,1973) Publication costs assisted by Ames Laboratory, Iowa State University, Ames, Iowa
The formation of aqueous EuS04' was studied a t various pressures from atmospheric to 2040 atm a t 25" using uv absorption spectrophotometry. Data were gathered a t a constant ionic strength of 0.046 m and as a function of ionic strength from 0.010 to 0.046 m. The formation constants were found to be independent of wavelength from 240 to 250 mh. The plots of log K and log K O vs. pressure were found to exhibit distinct quadratic behavior. For the infinitely dilute solution, AVO of formation was calculated to be 25.6 and 12.0 ml/mol ab atmospheric pressure and 2040 atm, respectively. The large AVO value a t atmospheric pressure when compared to those from similar studies and, from theories on ion-pair formation provides overwhelming ~ ~ +calculated to evidence that EuS04+ is an inner-sphere complex in dilute aqueous solution. j Y ' ~ ~ s was be -44 ml/mol a t 1 atm and 25'. I
Introduction An earlier investigation from this laboratory employed differential uv absorption spectrophotometry' to study the association of aqueous E ~ 3 -and t s042-ions as a function of dilute ionic strength, temperature, and wavelength. Analysis of the resulting AH" and AS" data provided strong evidence that EuSO4-t- in dilute aqueous solution at 25" existJsprimarily in the innersphere form; i.e., the ions are in mutual contact with no solvent molecules separating them as in t,he Case for
the outer-sphere type. Since the change in the partial molal volumes, A v o , for E u S O ~ +formation should also be m x d i v e t o the type of complex formed,2it was decided t o make a similar investigation of this system as a function of pressure. Other studies made on the formation of aqueous com(1) C. F. Hale and F. H. Spedding, J . Phv5. Chem., 76, 188 (1972). Pressure Physics and Chemistry,,7 vel. 2, (2) R.s. Bradley, Academic Press, New Y o r k , N. Y . , 1963, PP 131-162.
The Journal of PJ~ysicalChemistru, Vol. 76. A-0. 20,1972
