Medium Activity Coefficients of Iodate is not explained by the difference in anion size and charge density on the cations; alone since the chelate salt is more dissociated in PhN02--cC14 mixtures than in MeOH-CC14 mixtures where the chelate salt is thought to form a contact ion pair a5 described above. Therefore i t must be concluded that any specific interaction between the chelate cation and PhP302 may play an important role in decreasing the associa.tion in PhNOa-CC14 mixtures. This is the third explanation. In Table I1 are given the distance a k from Kn, Rm* from Stokes' law corrected for the dipole relaxation3" of solvent molecules, m d the values of the Gilkerson's term calculated with tha: corresponding a k values. The corresponding values of Bu4NBr derived from the recompntation of the data by Ihoss, et al., with the 1965 conductance theory are al.so given for comparison. Judging from the plots in Figure 1, a h was set equal to 4.4 A in both mixtai:es. It i s seen in the table that a larger value of E,,/kT in PhNQ2--CC14 mixtures than in MeOH-CC14 mixtures suggests the specific interaction of the chelate cation with PhN02. 'The hydrodynamic radius, however, is nearly the same in both mixtures. Therefore the interaction between the chelate cation and PhNO2 decreases the ionic association but does not retard the ionic migration. On the other hand, the interaction of the chelate with methyl ethyl ketone which is also dipolar aprotic, has been found to retard though the KA in MEK is approximately the same as that in PhN02-CC14 mixtures a t the same dieiectri,c constant, as seen in Figure 1. We
523
assume that this interaction is a kind of ion-dipole interactions since PhNOz molecules have large dipole h o m e n t (3.99 D). Kay, et u L . , ~reported ~ from the conductance study on alkali metal perchlorates and tetraphenylborides in CH&N that the degree of solvation of large ions is determined predominantly by the dipole moment of the solvent molecules, whereas that of small ions is determined predominantly by the acid-base properties of the solvent molecules. Preliminary measurements of the chelate salt in CH3CN-CC14 mixtures indicated that the salt i s also more dissociated in these mixtures than MeOH-CC14 mixtures. This fact seems to support the above assumption since CH3CN molecules have also large dipole moment (3.37 D). It is seen in Table 11 that R , - value of &r- ion in MeOH-CC14 mixtures is considerably larger than that in PhNO2-CC14 mixtures and still larger than that of C104ion in both mixtures despite the fact that C104- ion is much larger than Br- ion in crystallographic radius. Therefore marked solvation of Br- ion by methanol assumed by Fuoss, et al., seems to be justified. Acknowledgment. The authors would like to express their deep gratitude to Professor Y. Yamamoto of Hiroshima University for his interest and encouragement in this study. (35) R. M. Fuoss. Proc. Nat. Acad. Sci. U. S.,45, E07 (1959). (36) R. L. Kay, B. J. Hales, and G . P Cunningham, J. Phys. Chern., 71, 3925 (1967).
Coefficient of iodate in Methanol, Acetonitrile, and Dimethyl Siilfoxide with Reference to Water I. M. Kolthoff* and M. K. Chantooni, Jr. Schcioi of Chemistry, University of Minnesota, Minneapolis, Minnesota 55455 (Received August 30, 7972)
Values of pKSP(AgI03) at 25" in water (W), methanol (M), acetonitrile (AN), and dimethyl sulfoxide (DMSO) were found equal to 7.5, 12.7, 10.6, and 9.8, respectively. Values of pKSP(KI03) in W, M, and DMSO are 1.6, 7.3, and 7.7, respectively, while those of pWyM(103-),pWyAN(IOa-),and pWpMsOO(I03-) are 3.9, 6.9, and 8.3. respectively. The value of pwys(I03-) is unusually large as compared to corresponding values of nitrate, chloride, and acetate, indicating unusually large solvation of iodate in water as conpsred to that in M, AN, and DMSO. The mean ionic activity coefficient f*(KIO3) is only 0.38 in 0.44 M (saturated) solution in water, indicating some KIO3 association of ions.
Introdluction On the basis of the tetraphenylborate assumption we reportlcd in a previous paper1 values of medium activity coefficients Wysi oi a number of cations and anions with reference to water (W) in the solvents (Sj, methanol (M), acetonitrile (AN), and dimethyl sulfoxide (DMSO). In unpublished work we found from the solubility of silver iodate in W and M, pW*yM(I03) of the order of 3.9, a value much larger than that of any other anion studied. In the
present paper we have made more accurate estimates of KSP(AgI03j and also have determined KSP(K103) in the above solvents, pWyS(K+) being known.l The large value of p W p ( I 0 3 - ) is discussed in relation to the structure of the ion. The solubility of potassium iodate in AN was found to be too small to allow an accurate determination of its ~ K s P . (1) I. M. Kolthoff and M. K . Chantooni, Jr.. J. Phys. Chern.. 76, 2024 (1972). ?he Journal of Physical Chemistry, Voi. 77, No. 4, 7973
I . M. Kolthoff and M. K. Chantooni,Jr.
52
The values of p K q A g I 0 3 ) in W, M, AN, and DMSO were estimated potentiometrically with the silver wire and silver I silver iodate electrodes in saturated silver iodate solutions containing tetraethylammonium iodate. To check the value of (pKaP(AgI03))~a chemical exchange method involving chloride was used. Values of pKSP(KI03) were found potentiometricplly in water with the cation sensitive electrode in suitable cells without liquid junction (cells I[ and 11), and in DMSO in a cell with liquid junction (cell 111). Independent estimates of and DMSO were made from the total solubility and/Qr conductivity of the saturated solutions. Activity coefficients were calculated from the partially extended Debye-Huckel equation taking & = 3 A for K + 2 and 4.5 A for IO3 - when p < 0.02.
Experimenki Section Chemicals. Methanol,3 a ~ e t o n i t r i l eand , ~ dimethyl sulfoxide5 werie purified as described elsewhere. Tetraethylammonium. iodate was prepared by neutralization of aqueous t e t r a e t h y l a r n ~ ~ o n ~ hydroxide um with Merck Reagent Grade iodic acid, evaporating the solution to dryness, and recrystallizing the solid from ethyl acetate. Assay by iodometric titration showed 99.3% purity. Tetraethylammonium chloride was a product prepared by F. G . Thomar; in this laboratory.* Potassium chloride was Baker's Analyzed eagent Grade. Silver chloride and iodate were prepared in the conventional way. All salts were dried in vu.cuo a t 70" for 3 hr. Potentiometry. The silver 1 silver chloride electrode was prepared by the method of Brown and McInnes7 by anodization of a silver plated platinum wire in 0.1 M HC1 while the silver I silver iodate electrode was fabricated from a silver plated platinum wire by anodizing for 20 min a t 1 mA/cm2 in 0.05 M potassium iodate by a method similar to that for silver/silver bromide electrodes.8 It had a light t a n color and was prepared daily. The potentiometric cell and 0.01 A4 AgNOaIAg reference electrodes (both halfcells containing the saline solvent) were those described previously5' for paM measurements with the glass electrode. A Beclrniaxi No. 39137 cationic glass electrode was used for u(K + ) measurements. It was calibrated in 0.010.6 M aqueous p assium chloride solutions using values by Bates, et a1.,I0 on the basis of ionic was also calibrated in 1 X 10-4-1 X M potassium perchlorate solutions in DMSO. All emf measurements were made on a Corning Model 10 pH meter. Other Operatiorts. Conductivity measurements were made as described p r e v i ~ u s l y A . ~ conductivity cell having a cell cons,tant of 16.40 was used in the determination of the conductivity of saturated potassium iodate in water. It was calibrated in 0,1 and 1.0 demal potassium chloride solutions of known specific conductivity.11 Exchange experiments betwee.n tetraethylammonium chloride and silver iodate in methanol were performed in a similar way as those involving tetrapheaylborate and bsomide.l2 esults Mobility of t h e lodate Ion. Conductance data of tetraethylammonium iodate in AN, M, and DMSO are presented in 'Table I. The observed slopes of the A us. c1/2 plots and theoretical Onsager slopes are 230 and 241, respectively, in M , 460 and 368 in AN, and 57 and 54.1 in The Journal of Physical Chemistry, Vol. 77, No. 4, 1973
TABLE I: Conductivity of Tetraethylammonium lodate in Various Solventsa
M
DMSO
AN
-
C(Et4N 103). 1 0 3 ~
a
A
C(Et4N l o 3 ) , 103 M
C(E14NIO3). 103 M
A
h
1.88 3.45 5.90 12.4
98
Ao(Et4NIQ3)
= 107 i 2 in M, 186 f 3 in AN, and 38.2 & 0.5 in DMSO.
94
89 82
170.5 165
1.27 2.12 6.35 10.6
149 139
1.98
3.76 9.50 17.5
36
34.1 33.2 30.4
TABLE II: Ionic Mobilities of Iodate and Several Anions in Various Solvents Solvent X0(103-) Xo(C104-) Xo(N03-) xo(Cl-)
W M AN DMSO
4O.Sa 4gb
10lb 23.5b
67.3a 7lC
Xo(l-)
71.4a
76.1a
78.P
76.ga
60.8"
54JC
56.!iC 100.ld 24.7f
62.8g
103.4d d04e 25.2f
ho(Br-)
27.71
9je
1O2.Od 24.3f
aReference 22. bThis work. c J . Prue and P. J. Sherrington, Trans. Faraday Soc., 1795 (1951); ref 13. J. F. Coetzee and G . P. Cunningham, J. Amer. Chem. Soc., 87, 2529 (1965). e P. Walden and E. Birr, Z. Phys. Chem., 144, 269 (1929). f P. Sears, G. Lester, and i. Dawson, J. Phys. Chem., 60, 1433 (1956). g R. E. Jervis, D. Muir, J. P. Butler, and A. Gordon, J. Amer. Chem. SOC., 75, 2855 (1953). M. Barak and H. Hartley, 2. Phys. Chem., 165, 290 (1933).
DMSO, indicating practically complete dissociation in these three solvents in the concentration range studied. Resulting values of Xo(IO3-) in M, AX? and DMSO tabulated in Table I1 were found using Xo(EtaN+) = 58.2 in MI3 and 85 in AN.14 Since Xo(Et4N+) is unavailable in DMSO, a value o f 14.7 was calculated from the relation vX0 = (Fe/1800r)/(R,+ S/D),I4 assuming the constants R,'and S found for Et4N+ in acetonitrile-carbon tetrachloride mixtures14 (equal to 2.748 and 4.20, respectively) to be the same in DMSO. In the above relation D refers to the dielectric constant while the other symbols have their usual significance. Solubility Product of Silver Iodate. Potentiometric data of the emf of cells composed of the silverlsilver iodate electrode in water, M, AN, and DMSO sojutions saturated with silver iodate and containing tetraethylammonium iodate and AgIO.O1 M AgNCh in S as reference electrode presentei-in Table Ie. Inwater and methanol the poJ. Kielland. J. Amer. Chem. SOC.,59, 1675 (1937) I. M. Kolthoff and M. K. Chantooni, Jr.. Anal. Chem., 44, 194 (1972). I . M. Kolthoff, S. Bruckenstein, and M . M. Chantooni, Jr., J. Amer. Chem. SOC., 83,3927 (1961). I . M. Kolthoff, M. K. Chantooni, Jr., and S . Rhowmik. J. Amer. Chem. SOC.,90, 23 (1968). I. M. Kolthoff and F. 6.Thomas, J. Phys. Chem., W,3049 (1965). A. S. Brown and D. A. Mclnnes, J. Amer. Chem. SOC., 57, 1356 (1935). D. J. lves and G. J. Janz, "Reference Electrodes Academic Press, NewYork, N. Y., 1961, p 207. I. M. Kolthoff and M. K. Chantooni, Jr,, J. Amer. Chem. SOC.,87, 4428 (1965). R . G. Bates, B. R . Staples, and R. A. Robinson, Anal. Chem., 42, 867 (1970). G. Jones and B. C. Bradshaw, J. Amer. Chem. Soc., 55, 1780 (1933). I. M . Kolthoff and M. K. Chantooni, Jr., A n d . Chem., 44, 194 (1972). E. C. Evers and A. G. Knox, J. Amer. Chem. SOC.,73, 1739 (1951). D. S . Berns and R. M. Fuoss. J. Amer. Cheri. SOC., 82, 5585 (1960). "
Medium Activity Coefficients of Iodate
525
tentials with the silvw wire electrode were irreproducible, while reproducitale potentials to within f 2 mV, which agree to within :k2 rnV with those of the AgIO3lAg electrode, were obtairield in AN and DMSO. In all solutions in Table I11 tetraethwylammoniumiodate was taken as completely dissociated Under our experimental conditions no indication was obtained of complexation between iodate with silver iodate From the value in water of Kf(Ag(Io.&-)= a(Ag(103)~~-)/a(Ag+)a2(I03-) = 8 X lO3,15 we find in the aqueous solutions in Table 111 a(I03-)/ a(Ag(IO&-) 4 x l.03, taking pKsp(Ag103) = 7.5. From the data in Table I11 the average value of pKSp(AgI03) in water equal to 4.5 i s in good agreement with the values reported by other w o 1 * k e r s ~ 6(7.45 - ~ ~ to 7.52) using various methods, thus attesting to the reliability of the silver I silver iodate electrode. The potentiometric value of pKSP(AgIO3) in methanol was checked by a chemical exchange method involving d v e r iodate and tetraethylammonium chloride. When the initial Concentrations were 7.85 X 10-3 and 1.44 X 10 M , the iodometrically determined concentrations of iodate in the equilibrated solutions were 2.00 >: 10-3 and 3.84 X M , respectively. From the value of pKeP(AgCE) in methanol equal to 13.220 an average value of 12.7 is calculated for pKSP(AgI03), in satisfactory agreement wi?h the potentiometric value of 12.6 in Table III. A value of I pKsp(AgIO3))w equal to 7.55, also in good agreement with the potentiometric value, i s obtained in the present study from the specific conductivity of a saturated solution of silver Iodate in water, 1.53 x ohm-' em-*. To summarize, pKsP(AgI03) has been found in the present study to be 7.5 in W, 12.6 in M, 10.6 in AN, and 9.8 in DMSO. SoELibility Product of PotassiiLm Iodate. The solubility product of potassium iodate was estimated in water from the difference in emf of cells 11 and I without liquid junction.
TABLE Ill: Potentiometric Determination of pKsp(Ag103) in Various Solvents
w
1.15 2.90
4.83 8.70
=i
0.44 M KCl sat. AgCl
EII- Er 0.0591[(pKsP(AgC1) pK*P(Agli83)] +. pK'P(KI03)
K(gU
I1
+ 2 log C(KCl)f*(K@l)]
In cells T and TI the same cation sensitive glass electrode, K(gl), was used. 'The concentration of potassium chloride in cell 11 was q u a l to the total solubility of potassium iodate in water. In this way no large difference in C(K+) in cells I and IT are expected, obviating checking the linear response of the cation glass electrode with pa(M+). With our particular cation glass electrode El = +0.061 V and E11 = 4-0.213 V which combined with the accepted values in water of pKSP(AgC1) = 9.7, pKWAgI03) = 7.5, and the mean ionic activity coefficient f*(KCl), in 0.44 M potassium chloride equal to 0.65 (interpolated from values calculated from hydration theory by Bates, et a1.,1o) yield pKSP(K103) = 1.64. In DMSO pa(K+) was determined in the saturated potassium iodate solution with the cation glass electrode in cell 111 with liquid junction K(gl) I
