Conductances, Densities, and Viscosities of Solutions of Sodium Nitrate in Water and in Dioxane-Water, at 25 "C

Transcription

1 Conductances, Densities, and Viscosities of Solutions of Sodium Nitrate in Water and in Dioxane-Water, at 25 "C ELINOR M. KARTZMARK Department of Chemistry, University of Manitoba, Winnipeg, Manitoba R3T 2N2 Received March 14, 1972 The conductances, densities, and viscosities of solutions of sodium nitrate in water and in 20, 40, 60, 70, and 80 wt. % dioxane-water have beeh measured at 25 OC, from C rr 5 x M to saturation. The data have been fitted to the Fuoss-Onsager equation, yielding values of &, 8, and K,; and in concentrated solutions to the equations of Wishaw-Stokes and Gellings. Apparent molar volumes have been calculated. No appreciable ion association is apparent in solutions whose dioxane content is less than 60% by weight. Passing from water to 80% dioxane, the values of 8 obtained from the Fuoss-Onsager equation increased from 2.74 to 7.60 A, a sharp increase occurring beyond 60%. Les conductances, densites et viscosites des solutions de nitrate de sodium dans l'eau et dans des melanges a 20,40,50,70, et 80% en poids de dioxane et d'eau; ont kt6 mesurees a 25' pour C -- 5 x Mjusqu'a saturation. Les donnees ont ete rajustees a l'tquation de Fuoss-Onsager donnant ainsi les valeurs de A,, a", et K,; et pour les solutions concentrees, aux equations de Wishaw-Stokes et Gellings. Les volumes molaires apparents ont ett calcules. I1 n'y a pas d'association ionique appreciable dans les solutions dont la teneur en dioxane est inferieure A 60% en poids. En passant de l'eau au dioxane a SO%, les valeurs de '?i obtenues a partir de l'equation de Fuoss- Onsager augmentent de 2.74 a 7.60 A, avec un accroissement marque au-deli de 60%. [Traduit par le journal] Canadian Journal of Chemistry, 50, 2845 (1972) Previously in this laboratory, the conductances of solutions of lithium and of sodium chlorates in water and in water-dioxane have been measured, and the degrees of association calculated (1 ). The present paper is a report on a similar study, using sodium nitrate. The only previous work on the conductance of sodium nitrate in aqueous solution, at 25 "C, in the dilute region, is that due to Jones (2). The concentrated region has been investigated by Gellings (3), up to 4.84 M. No previous studies have been made on sodium nitrate in waterdioxane mixed solvents. Conductance measurements in the dilute region yield A, as a function of dielectric constant and values of ii and KA can be calculated using the Fuoss-Onsager equation. The concentrated region is of great interest, compounding as it does the problems of viscosity, association, and solvation. For some salts, the Wishaw-Stokes equation gives a relatively good quantitative representation of the concentrated region but it has been found to be quite inadequate for others (1). This paper reports measurements of density, viscosity, and conductance over the complete concentration range, that is, from 5 x M to saturation in water and in 20, 40, 60, 70, and 80 wt.% dioxane-water. Experimental Purity of Materials Sodium Nitrate Fisher "Certified" sodium nitrate was recrystallized twice from conductance water, dried in an oven at 120 C with periodic grinding. Conductance Water This was prepared by distillation of water which had passed through a mixed resin demineralization column, using a silica still and collecting flasks. Dissolved carbon dioxide was removed by purging with purified nitrogen. Dioxane Fisher "Certified" dioxane was further ~urified bv the method of Hess and Frahm (4). The purified dioxane had a density of g/ml and refractive index of n& = Lind and Fuoss (5) report a density of after a week's refluxing over sodium. Frey and Gilbert (6) report a refractive index of Potassium Chloride This was recrystallized from conductance water, dried and fused in air. Procedure The experimental procedures have been recorded in ref. I ; only minor changes have been made. A Beckmann conductance bridge, Model RC-18 A, was used. All weighings were reduced to vacuum. The temperature was controlled at f OC. The dielectric constants of pure dioxane and 60 and 80 wt. % dioxane were measured with a Sargent Model 5 Oscillometer, thermostated to C. It was calibrated using spectroscopically pure benzene, nitrobenzene, and ethylene dichloride. The dielectric constants of 20 and 40% dioxane were taken from Harned and Owen (7). Viscosities were measured with a Cannon and Fenske

2 CANADIAN JOURNAL OF CHEMISTRY. VOL. 50, 1972 TABLE 1. Densities, viscosities, and conductances of sodium nitrate at 25 "C in water and water-dioxane mixtures; M, using Wishaw-Stokes' equation and Gellings' equation (a) Aqueous solution* *M,, W-S equation: 12, = ; 8 = M,, Gellings'equation: A, = ; 8 = 2.74; z = (b) 20 wt.% dioxane (d = g/ml)* 'M,. W-S equation: A, = 87.20; a"= MI. Gellings' equation: A, = 87.20; a"= 3.06; z = (c) 40 wt.% dioxane (d = g/ml)*

3 KARTZMARK: ON SOLUTIONS OF SODIUM NITRATE TABLE 1 (c) (Concluded). d c (g/ml) tlrd A A A2 *M,, W-S equation: A, = 63.50; 8 = 4.0. M,, Gellings' equation: & = 63.50; 5 = 3.06; z = (8) type viscometer, with a run-time with water of s. The viscosity values for dioxane-water mixtures compared very well with the data of Lind and Fuoss (5). A silica cell of the Shedlovsky type was used for the conductance measurements of the dilute solutions. At the end of each series of measurements with the Shedlovsky cell, the density of the solution was determined; no detectable change was observable from the density of the solvent (i.e. within ). Results The equivalent conductances in the dilute region are to be found in Table A in the Depository of Unpublished Data, together with cal- (d) 60 wt.% dioxane (d = g/ml) d c (g/ml) Vre1 A (e) 80 wt.% dioxane (d = g/ml) culations from the Fuoss-Onsager equation.' The data of Jones which extend from c = 5 x to 0.5 M, range from 3 to 6 mhos lower than the present data and do not fit the Debye- Hiickel slope in the limiting region. In the concentrated region, densities, viscosities, and equivalent conductances were measured and are shown in Table 1, together with the deviation in A calculated using the 'Tables A and B are available, at a nominal charge, From the Depository of Unpublished Data, National Science Library, National Research Council of Canada, Ottawa, Canada, KIA 0S2.

4 2848 CANADIAN JOURNAL OF CHEMISTRY. VOL 50, 1972 equations of Wishaw-Stokes and of Gellings.,,, I I I I I I An extended form of this table, including calculations of the apparent molar volume, is given as Table B in the Depository of Unpublished Data. 100 Discussion Dilute Region A, was determined in all solvents, either by extrapolation of the A us. & plot (for water, 20, and 40% dioxane), or by the best fit of the data with the Fuoss-Onsager equation: [I] A = A, - Sc1I2 + Ec log c + Jc [2] A = A, - Scf l2 + Ec, log ci + Jc, - KAci f 2A where ci = cy, concentration of free ions, and KA = (1 - y)/(cy2f 2), the association constant. The activity coefficient, f, was calculated from the Debye-Hiickel equation For solvents containing 0,20, and 40% dioxane, no ionic association was found; eq. 1 was thus used. Following the procedure of Fuoss and Accascina (9), a preliminary graphical determination of A, allowed calculation of S and E, from which A' =A + Sc1I2 - EC log c was calculated. A' was plotted us. c, yielding a straight line whose slope is J. From the value for J, the parameter ii was determined. The limit Kii < 0.2 was adhered to. Finally 6 = A;,, - (& + Jc) was calculated for each concentration. Solutions containing 60, 70, and 80% dioxane showed ion association and were treated using eq. 2. Again the method outlined in Fuoss and Accascina was followed. Here the graphical method of finding A,was not as clearcut and necessitated several trials until a straight line y us. x was obtained. J, the intercept, was used to calculate ii, the new value was introduced in the expression for f and the procedure repeated. KA is obtained as the slope of they tls. x line. The whole procedure was tested for consistency by substituting S, E, J, KA, f 2, and y back into eq. 2 and obtaining the difference from A exp. A, decreases from in aqueous solution to in 80% dioxane-water. The values of ii increase from 2.74 in water to 7.60 in 80% dioxane. These results are shown in Fig. 1. The viscosity of dioxane-water mixtures passes 1I 90-20% no 60-40% 50-60% ;:/b 40-70% 80% ; 4 2 O C DIELECTRIC CONSTANT FIG. 1. &, ii us. dielectric constant. through a sharp maximum at 60% dioxane. This seems to have no marked effect on A,, because the A, us. D plot shows no change in curvature in this range. The ii values begin to increase steeply at around 60% dioxane, suggesting that the solvation sheath may be predominantly dioxane. Indeed, the change in A, with dioxane content can only be explained in terms of solvation. A plot A,q, us. dioxane content shows a marked decrease beyond 40% dioxane, even though the viscosity increases by 17% in the range % dioxane. Ion association as determined by KA does not become marked until a dioxane content of approximately 60% has been attained. The data of Table A may be summarized in Table 2. The ii and K, are very similar to those of Lind and Fuoss for potassium chloride. The Concentrated Region The conductance-viscosity data in the concentrated region were fitted to the Wishaw- Stokes equation (10) Generally, the ii obtained from the Fuoss-

5 KARTZMARK: ON SOLUTIONS OF SODIUM NITRATE 2849 TABLE 2. Summary of Table A in the Depository of Unpublished Data Wt.% tl A0 Dioxane D (p (mho) J fi KA Onsager equation gave the best results. Where the dilute region showed marked association, i.e., in dioxane > 40%, the Wishaw-Stokes equation failed completely. Taking a 3% error as maximum, the data for aqueous solutions fitted the equation up to c = 1.95 M; the range was very much reduced in solutions containing - dioxane (c 0.15 M). The failure of the equation in aqueous solution at higher concentrations may be attributed to inadequacy of the viscosity correction but is probably due also to association. The viscosity factor is more pronounced in 20 and 40% dioxane. Table B gives calculations of A, using Gellings' equation where a, /?, B, d have the same significance as in Debye-Hiickel theory and z is an empirical constant. For 6, Gellings used the effective radii of the ions in the solution as calculated by Bottcher (1 1 ) from refractive index and density of electrolytic solutions. Z is evaluated from the conductance data by the method of least squares. No correction is made for the viscosity of the solution. The data of Gellings for sodium nitrate in water plot on the same A 0s. fi curve but do not extend to saturation nor to the very dilute region. The present paper extends Gellings' data to c = 7.79 M in aqueous solution. With d = 2.74 (the value obtained from the dilute region) and z = , the equation fits the data over the entire concentration range, within about 5%. The 20 and 40% dioxane solutions are equally well handled but, at higher dioxane concentrations, no constancy in z was found for reasonable d values. Admittedly, while Gellings' equation fits the experimental data over a much wider range than Wishaw- ' WT. '10 DIOXANE FIG. 2. q:., us. wt. % dioxane. Stokes', it contains an empirical constant, which has no obvious theoretical basis and it ignores the viscosity correction entirely. Viscosities and Associated Properties The relative viscosities (solvent = 1 )of sodium nitrate solutions increase with concentration (to 2.98 in saturated aqueous solution) and also with increase in dioxane content in the mixed solvents. When the viscosities relative to water (qi,,) are plotted against % dioxane, for 1, 2, 3, and 4 M solutions of sodium nitrate, Fig. 2

6 2850 CANADIAN JOURNAL OF CHEMISTRY. VOL is obtained. The ratios of q:el/q,el increase with dioxane content and this indicates that the same concentration of salt increases the viscosity of solvent to a greater extent, the greater the dioxane content. The viscosity of dioxane-water solutions (0.0 M curve) passes through a maximum at 60% and falls off rapidly. The limited solubility of sodium nitrate does not allow investigation beyond the solvent maximum; from 1 to 4 M there is no suggestion of a maximum. In 80% dioxane-water two liquid layers are produced shortly after M sodium nitrate. The apparent molar volumes 4, (Table B) increase with concentration of sodium nitrate and also with dioxane content to an apparent maximum between 40 and 60% dioxane. This was also found to be true of sodium chlorate (12), which approached a maximum at 64.5% dioxane. The difference 4, (ClO; - NO;) remains roughly constant between 1 and 6 M, in water, and in 40% dioxane, the values being around 1.3 ml/mol greater in the latter. A plot ofaq us. (Fig. 3) shows a minimum around 4 M in aqueous and 20% dioxane solutions; the minimum is not observable in solutions of higher dioxane content, which have limited solubility of sodium nitrate. Figure 4, a plot of A/A, us. (c/d)'i2, shows an interesting regularity in solutions in 0, 20, and 40% dioxane. All data points fall on the same curve, to within about 5%, a type of reduced conductance. In the very dilute region, solutions with dioxane content less than 60% show the well known "anabatic phoreogram" typical of complete dissociation, i.e., the limiting slope lies below the A us. Jc plot. For 60 and 80% dioxane, the limiting slope lies above the A us. fi plot, "catabatic phoreogram", a sign of association. 1. A. N. CAMPBELL, E. M. KARTZMARK, and B. G. OLIVER. Can. J. Chem. 44,925 (1966). 2. H. C. JONES. Carn. Inst. Wash. Pub. No. 170, 11 (1912). 3. P. J. GELLINGS. Rec. Trav. Chim. 75,209 (1956). 4. K. HESS and H. FRAHM. Ber. 71,2627 (1938). 5. J. E. LIND and R. M. Fuoss. J. Am. Chem. Soc. 65, 999 (1960). 6. P. R. FREY and E. C. GILBERT. J. Am. Chem. Soc. 9, 1344 (1937). 7. H. S. HARNED and B. B. OWEN. The physical chemistry of electrolytic solutions. 3rd ed. Rheinholdt Publishers, New York p M. J. CANNON and M. R. FENSKE. Ind. Eng. Chem. Anal. Edit. 10, 299 (1938). 9. R. M. Fuoss and F. ACCASCINA. Electrolytic conductance. Interscience Publ. New York B. F. WISHAW and R. H. STOKES. J. Am.Chem. Soc. 76,2065 (1954). 11. C. J. F. BOTTCHER. Rec. Trav. Chim. 65, 14 (1946). 12. B. G. OLIVER and A. N. CAMPBELL. Can. J. Chem. 47, 4207 (1969).