JOHNE. GORDON AND ROBERTL. THORNE
4390
Salt Effects on the Activity Coefficient of Naphthalene in Mixed Aqueous Electrolyte Solutions. I.
Mixtures of Two Salts'
by John E. Gordon and Robert L. Thorne Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 09643 (Received May 19, 1967)
From solubility measurements on naphthalene in aqueous solutions at 25", the Setschenow constants for sodium sulfate, calcium chloride, magnesium chloride, sodium chloride, potassium chloride, potassium bromide, cesium bromide, sodium butanesulfonate, tetramethylammonium bromide, and tetra-n-butylammonium bromide were determined. The solubility of naphthalene in pure water was determined to be 2.620 f 0.01; X lom4mole 1.-1 at 25.00'. Salting out of naphthalene was found to be an additive function of the effects of the individual salts for 1:1 mixtures of sodium chloride with sodium sulfate, magnesium chloride, calcium chloride, potassium bromide, and cesium bromide. All mixtures containing one of t.he organic salts listed above were nonadditive. The deviations, in each case, were in the direction of a decrease in the activity coefficient (increase in solubility). The magnitude of the excess solubility increased with increasing total salt concentration. The systems tetra-n-butylammonium bromide-sodium sulfate and tetramethylammonium bromide-sodium chloride were measured at constant total concentration (0.7 and 2.5 AI) and variable composition. Maximum excess naphthalene solubilities amounting to 0.30 and 0.091 log S unit were observed at mole fraction of organic salt equal t o 0.41-0.42 and 0.34-0.37, respectively. The electrolyte interactions giving rise to these observations were considered.
The past several years have seen considerable improvement in our understanding of the effects of salts on the activity coeficients of dissolved nonelectrolytes. A relatively adequate theory of nonelectrolyte salting2 and its further substantiation and elaborationa+ have furnished a semiquantitative physical account of the majority of these effects as salt-induced medium effects. The latter arise (in water) from the effect of the electrolyte upon the internal pressure of the solvent, and thus on the energy required to create a hole for the nonelectrolyte molecule. This (McDevit-Long) theory is applicable for nonpolar nonelectrolytes. It predicts the relative salt effects of inorganic salts well and accommodates in out. The theory's prediction of the salting-in effeCtS of organic salts is as good as for the inorganic salts in the five case, where the necessary electrostriction data that for the salts are availab1e' One View then the salting of nonpolar nonelectrolytes (and a large proportion of that of polar nonelectrolytes) by organic The Journal of Phymkal Chemistry
salts is accounted for by the salt-induced medium effectea In 50% aqueous dioxane, on the other hand, the strong salting in of naphthalene by organic salts contradicts the theory, although the latter again predicts the pattern displayed by the inorganic salts very weL6 Similarly, the specific salt effects of inorganic salts on acidic and basic nonelectrolytes in water have been qualitatively accounted for with some success by considering the effect of the electrolyte on the acid-base (hydrogen bonding) properties of water. However, the highly specific behavior of the large organic salts included in that study goes beyond the capabilities of this (1) Contribution No. 1930 from the Woods Hole Oceanographic Institution, Woods Hole, Mass. Supported by National Science Foundation Grant GP-5110(2) w. F. McDevit and F. A* Long, J. Am. Chem. S0c.r 74s 1773 (1952). (3) N. C. Den0 and C. H. Spink, J. Phys. Chem., 67, 1347 (1963). (4) M. A. Paul, J . Am. Chem. soc., 74,5274 (1952). (5) E. Grunwald and A. F. Butler, aid., 82,5647 (1960).
SALTEFFECTS ON THE ACTIVITY COEFFICIENT OF NAPHTHALENE
explanation.6 All of these large organic ion-organic nonelectrolyte salt effects have consequently been considered to be due to specific short-range ion-molecule interactions distinct from or superposed on salt-induced medium effect~.~>'J Very little is known about salt effects in mixed electrolyte solutions. The RScDevit-Long theory requires additive ionic contributions to the salting parameter, K , in the Setschenow equation (1),9but it applies to the log (f/f")
log ( S " / S ) = KC,
(1) prediction of K = d log f/dC, as the salt molarity, C,, approaches zero. We are more interested in the behavior of eq 1 in intermediate and concentrated electrolyte solutions. Setschenow constants have been semiquantitatively split into additive anion and cation cont8ributions.2~10~11~12b Introduction of more than two ionic species produces the obvious complications of a greater number of interaction possibilities. However, it offers more flexibility in testing additivity. Existing data on salt mixtures appear to be limited to two reports. LindeI3 reported linear dependence of diethyl ether solubility on mole fraction for the systems sodium sulfate-sulfuric acid, sodium sulfate-sodium hydroxide, and sodium chloride-sodium acetate at constant total concentration (0.5 N ) . Larsson12&computed activity coefficients for benzoic acid in sodium chloride and potassium chloride solutions from measurements on sodium benzoate and on sodium benzoatcrNaC1 and -KC1 mixtures by assuming additivity of the effects of individual salts. Sodium benzoate was present in lorn concentration. Good agreement with two independent methods was obtained. This paper investigates additivity of salt effects in various two-salt systems. Naphthalene was chosen as the nonelectrolyte in the present work because it is nonpolarg and is well suited to accurate spectrophotometric analysis. The salt systems chosen include several basic types : inorganic-inorganic, inorganic-organic, organic-organic, and 1 : 1-1 : 1, as well as both sorts of 1:1-2: 1 charge types. The results provide tentative rules for nonelectrolyte salting behavior in mixed electrolyte solutions and some new information on the underlying interactions. In addition, Setschenow constants for ten pure salts and a new determination of the solubility of naphthalene in water are reported. =
439 1
without purification. Sodium butanesulfonate (Distillation Products Industries) was recrystallized four times from 95% ethanol. Tetra-n-butylammonium bromide and tetramethylammonium bromide, from the same source, were recrystallized three times from ethyl acetate and from methanol or aqueous ethanol, respectively. The organic salts were dried for 24 hr at 50-60" and 0.005-0.001 mm. Sodium butanesulfonate was analyzed by passing weighed samples through a column of Biorad AG50-X8 200-400 mesh ion-exchange resin (H + state) in water and titrating the efluent with standard alkali: equiv wt calcd, 160.2; found, 159.9, 159.7. Tetramethylammonium bromide was titrated with silver nitrate: equiv wt calcd, 154.06; found, 154.24, 154.22, and 154.30. Naphthalene was either Distillation Products Industries White Label material, recrystallized four times from ethanol and sublimed at ca. 1 mm, or zone-refined material, 99.99+%, obtained from James Hinton, Valparaiso, Fla. Laboratory-distilled water was passed through a mixed-bed deionizing column, distilled from potassium permanganate, and redistilled from an all-glass still. Solubility Measurements. An excess of naphthalene and 1.5 ml of water, or of the appropriate salt solution, were sealed in 20-mm 0.d. Pyrex ampoules. These were rocked in a constant-temperature bath maintained at 24.91 f 0.03" until equilibrium had been attained (see Table I). Some ampoules were heated in a water bath at 90" with frequent shaking for 3 hr before placing in the constant-temperature bath for equilibration, in order to approach equilibrium from the direction of supersaturation. That these samples were indeed supersaturated was demonstrated by observing the optical density of one of the (filtered) samples at the time of immersion of the others in the 25" bath; it was 140% of the equilibrium value and falling. As Table I shows, equilibrium appears t o be reached within 7 hr ; however, subsequent samples were equilibrated for a t least 24 hr. Analysis of the saturated naphthalene solutions was accomplished after filtering through a modified Swinney filter adapter attached to a hypodermic
Experimental Section
(6) R. L. Bergen, Jr., and F. A. Long, J . P h y s . Chem., 60, 1131 (1956). (7) A. F. Butler and E. Grunwald, ibid., 67, 2330 (1963). (8) E. F. J. Duynstee and E. Grunwald, Tetrahedron, 21, 2401 (1965). (9) F. A. Long and W. F. hlcDevit, Chem. Rev., 51, 119 (1952).
Inc., Beverly, RIass.; they were dried at 125" and used
(13) E. Linde, Arkiv K e m i , 6, 20 (1917).
V o l u m e 71,N u m b e r I S
December 1967
JOHN E. GORDON AND ROBERT L. THORNE
4392
Table I : Solubility of Naphthalene in Water a t 24.91
Sample5
Equilibration Under or time, supersaturated hr initially
1 1 1 1 1 1 1
7 18 24 24 36 48
2 2 2 2
72 72 72 72
48
U U U U U U U U
U S S
&
0.03"
*
Solubility, mole/l. X 104
2.609 2.616 2.598 2.588 2.621 2.609 2.613 2.619 2.611 2.625 2.608 Mean 2.611 f 0.015
' 1, Distillation Products Industries White Label, recrystallized four t,imes from ethanol and sublimed; 2, James Hinton, zone-refined "99.99+ %." Average of determination a t three wavelengths. Precision measure is 95% confidence interval.
syringe. The filter unit employed two layers of Whatman No. 40 filter paper of 1.3-cm diameter compressed between Teflon gaskets by the metal Swinney fitting. Division of the filtrate into successive 3-ml aliquots and measurement of the optical density of each showed that the first was about 1.5% lower than subsequent aliquots, whose optical density was constant. This is presumed to represent the loss due to adsorption on and partition into the filter materials, an important potential source of error14which is minimized in the present procedure by the compact filtration assembly. After discarding the initial filtrate, the portion to be measured was introduced into the absorption cell directly from the syringe. The use of cells of lo-, 3-, 1-, 0.3-, and 0.1-mm path length made it possible to measure all solutions at an optical density near unity without the necessity of making dilutions. The entire procedure thus also minimizes evaporation losses. The optical density was measured a t 2832, 2760, and 2662 A, using a Clary Model 14 spectrophotometer, and corrected at each wavelength for the solvent blank, which was measured under identical conditions. The equilibrium naphthalene concentration was taken as the average of that calculated from measurement a t the three wavelengths. Extinction coefficients were measured (1) b,y directly dissolving weighed portions of naphthalene in water, using moderate ultrasonic treatment, and diluting to volume and (2) diluting a small weighed aliquot of a concentrated solution of naphthalene in methanol to volume in a large quantity of water so that the final concentration of methanol was
