420
LEOBREWER, T. R. SINONSON AND L. K. J. TONG
Vol. 65
A VAPOR PHASE EQUILIBRATOR FOR ACTIVITY COEFFICIENT DETERMINATIONS BY LEOBREWER, THOMAS R. SIMONSON AND LEEKARLJ. TONG The Department of Chemistry, University of California, Berkeley 4,Califmtia Received August 9, 1960
A vapor phase distributor haa been designed that allows rapid and accurate determinations of activity coefficienta of volatile solutes. It has been used to determine the activity coefficient of mesityl oxide in a variety of aqueous salt solutions, but it can be applied to many types of solutions. A deviation from Henry’s Law of 20% has been found for saturated mesityl oxide solutions. The distribution of iodine between water and carbon tetrachloride has been studied. It has been confirmed that the variation of distribution ratio with iodine concentration is entirely consistent with expected regular solution behavior and is not due to new species such aa I or 1,. +
Introduction Activity coefficients of organic solutes in aqueous salt solutions are commonly determined by the equilibration of the solute between the aqueous salt solutions and an organic phase, such as carbon tetrachloride or the pure organic solute (e.g., solubility determinations), or sometimes through direct vapor pressure determinations. The first two methods involve errors due to emulsification of the organic phases and are complicated by deviations from Henry’s Law in the organic phase or in the aqueous phase. The third method has been difficult to apply accurately. Friedman‘ also has pointed out that complete thermodynamic data cannot be obtained from solubility measurements since one cannot determine the effect of variation of solute concentration upon the solute activity coefficient. Jones and Kaplan2 have described an equilibrator for the distribution of volatile substances by way of the vapor phase, which avoids the difEcu1ties of the other methods. Their method is applicable to solutes of relatively low vapor pressure. The Jones and Kaplan equilibrator is inconvenient in that the entire apparatus must be kept in motion. The equilibrator used in the present work is of a different design in that no movement of the main part of the distribution apparatus is required. Experimental Apparatus.-The principle of the apparatus is illustrated in Fig. 1. Bulbs 1, 4,5 and 6 contain mercury. Bulbs 2 and 3 contain the solutions between which a volatile substance is to be distributed. If pressure is applied on bulb 6, that pressure is transmitted through the mercury into bulb 4 and causes compression of the gas in bulb 4. This compressed gas travels by two paths. It pushes down on the liquid in bulb 2 until the liquid has risen in the arm between bulb 1 and 2 to a height equal to the height of liquid in bulb 3. Any further compression is relieved by bubbling through bulb 3. In actual operation pressure is applied a t bulb 6 while suction is applied at bulb 5 and then this is reversed by applying pressure at bulb 5 and suction a t bulb 6. The result of this alternating suction and pressure is to cause gas to bubble from 4 through 3 to 1 and then to bubble from 1 through 2 and back to 4 again. The aource of the alternating pressure and suction is the oscillation of two bulbs of mercury which are connected with one another and are oscillated by a pulley system. In the actual apparatus, there are two bulbs in series in place of bulb 3. The salt solution is placed in bulb 2 and water in bulbs 3a and 3b. Mesityl oxide is added to bulb 3a and the alternating pressure and suction started. The gas in the system bubbles through bulbs 3b, 3a and 2 and (1) H. L. Friedman. J . Phya. Chsm.. 69, 161 (1955). (2) G. Jones and B. B. Kaplan, J. A n . Chem. Soc., 60, 1G00, 1855 (1928).
around again until analyses indicate that equilibrium has been reached when the mesityl oxide concentration in bulbs 3a and 3b has become the same. Bulbs 1, 2, 3a, 3b, 4, 5 and 6 are arranged compactly on a wooden latform which is immersed in a thermostat. The two osci&ting bulbs are outside of the thermostat and the alternating pressure and suction is transmitted by gas pressure through rubber tubing to bulbs 5 and 6 and through them into the a paratus. Bulbs 2, 3a and 3b have ground glass stoppers wLch may be removed to take samples. The rate of distribution depends upon the rate of bubbling which can be regulated by the height through which the bulbs oscillate and by the rate of oscillation. A variable speed d.c. motor with a gear reducer was used to operate the pulley. Equilibrium could be approached from either side by putting mesityl oxide initially in any one of bulbs 2, 3a or 3b. Within the analytical uncertainty of O.l%, equilibrium is attained in less than 12 hours. The matenals used as solutes in this study, mesityl oxide and iodine, have lower vapor pressures than water. Thus it might seem surprising that one could expect to reach equilibrium with respect to the solute without transporting large amounts of solvent and thus diluting the salt solutions that are being compared with water. However, the success of this method does not depend upon the actual value of the partial pressure of the material being transported but depends upon the Henry’s law constant, the ratio of partial pressure to concentration, or upon a corresponding quantity, K , which is defined as the ratio of concentration in the liquid phase to concentration in the gaseous phase using moles per liter for both phases. If one sets up the differential equations for the transport of material from one bulb to the next due to saturation of the gas bubbling through each bulb, one fmds that the concentration of solute varies in an oscillatory manner in its approach to its equilibrium value. Thus the concentration in the second bulb, CZ, is given by c2 =
e-3V12R
[f
(2CZO
-
c10
- d)cos ( 4 3 V / 2 K ) +
where the cO values are initial concentrations in the three bulbs, K has been defined above, and V is the ratio of the volume of gas bubbled through each bulb to the volume of solution in the bulb. This equation is given for equal volumes of solution in each bulb and equal values of K for each solution. A similar but more complicated expremion results when K is different for each solution. The equation takes a simpler form when one solution is saturated and thus maintains a constant concentration. This occurs for the solute when excess solute is present and for the solvent when no salt or other non-volatile solute is present in one of the bulbs and all volatile solutes are a t low concentrations. Thus if c? is now the concentration of solvent in the first bulb, which contains no salt, and cz is the concentration of solvent in the second bulb
V is the same as above. The c and K values apply to the solvent. Examination of these equations shows that a solute like mesityl oxide will approach equilibrium more rapidly than will water. The value of K is 4 X lo4 for
March, 1961
T'APOR P H A S E &JUILIBRA4TIONFOR
pure water, 620 for mesityl oxide in dilute aqueous solution, and 81 for iodine in dilute aqueous solution. A value of V of 1000 or a volume of gas 1000 times larger than the volume of solution in a bulb is sufficient to bring the mesityl oxide concentrations to closer than 99.9% of the equilibrium values. The same value of V will transnort onlv 2.7 grams of water pel: liter of solution from p;re wate; to a 2 M NaCl solution. The above equations assume saturation of each gas bubble by the volatile constituents of the solution. One can derive similar equations based on reasonable rates of saturation of the bubbles and reach similar conclusions. The observed rates for mesityl oxide solutions indicate rapid rates of saturation, and thus e uilibrium is attained within the analytical accuracy of 0.1 before sufficient water can distil to have any significant effect. This source of error is largest for concentrated salt solutions. Distillation of water dilutes the Eialt solution by less than 1%. Such a dilution of a 5 M ITaC1 solution would decrease the activity coefficient of mesityl oxide by less than 0.2%. The final salt concentration was used and any error in the equilibrium value of the activity coefficient due to water distillation is less than the analytical error. There waa no error from spray as no test for halide ion could be detected in the water bulbs when halide salt solutions were used. This method appears to be quite generally applicable to distribution of solutes between immiscible phases such as CCL and water or between miscible hases such as alcohol and water or water and aqueous sa& solutions as long aa the Henry's law constant for the solute is large enough to permit approach to equilibrium within analytical uncertainty for the solute before appreciable amounts of solvent have been transported. Materials.-Mesityl oxide was chosen to test the apparatus aa its activity coefficient was desired for interpretation of kinetic studies. The mesityl oxide was prepared by distillation from purified Eastman Kodak diacetone containing 0.01% iodine.3 The water layer was separated and the mesityl oxide was washed once with half-saturated sodium chloride solution containing 0.1 M NaOH and then successively with half-saturated sodium chloride solutions with a ten-fold reduction in sodium hydroxide each time until the salt solution contained only lo-' M NaOH. Calcium chloride hydrate and calcium hydroxide were added to the mesityl oxide and the saturated calcium chloride solution withdrawn. I3rying was completed with anhydrous calcium chloride. 'The washing and drying operations were done quickly to minimize condensation, hydration and isomerization reactions. The presence of products of the condensation reactions considerably reduces the solubility of the resulting material. The d mesityl oxide was fractionally distilled at 50 mm. d i s c a z i g the low boiling fraction of low refractive index and the highest boiling fraction. The mesityl oxide then was slowly frozen and the lowest melting portion was discarded. The mesityl oxide was then fractionally dfstilled at 50 mm. again discarding the fraction of low boiling point and low refractive index and the very highest boiling fraction. The final material had a refractive index nD 1.443 a t 25'. Comparison with the data given by Strom, Monger and Finch' for mesityl oxide and iso-mesityl oxide indicates that our final roduct had less than a per cent. of iso-mesityl oxide. The geshly made mesityl oxide was used a t once to prepare 0.25 M aqueous solutions, as the pure mesityl oxide reacts with oxygen and undergoes other reactions on standing. The a ueous solutions did not form peroxides and were quite s t a b e although they decreased in strength due to volatility losses. The sodium perchlorate solution was prepared by neutralization of perchloric acid. The other salt solutions were prepared from reagent grade salts. The salt solutions were close enough to neutrality to ensure no appreciable hydration of mesityl oxide to diacetone alcohol. Analyses.-The mesityl oxide was determined by bromination as described by Pressman, Brewer and Lucas.6 A t high concentrations of mesityl oxide, the separation of a mesityl oxide dibrornide phase removed some of the mesityl oxide from the aqueous phase and low bromination results
ACTIVITY COEFFICIENT DETERIMIXATIOKS
421
2
(3) H. Hibbert, J . Am. Cham. Soo.. 57, 1748 (1915). F. Stross, J. M. Monger and H. de V. Finch, {bid., 69, 1627 (1947). (5) D. Pressman,L. Brewer and H. J. Lucas, ibid.. 64, 1117 (1942). (4)
K A Fig. 1.-Illustration
of principle of distribution apparatus.
were obtained unless a considerable excess of bromine was used or unless the mixture was shaken. The salt solutions were analyzed by evaporating to dryness below the boiling point of the saturated solution and weighing the residue. The analysis was accurate even for salts such as sodium acetate. Testing of the residue showed that only 10-4 mole of acetic acid escaped per liter of 2.5 M sodium acetate solution. As the salt solutions were handled volumetrically, they were kept in an air thermostat inasmuch as some concentrated salt solutions expand as much aa 0.1% per degree. The halide solutions were checked by the Volhard titration. Equilibration Procedure.-The procedure for adding salt and mesityl oxide to the bulbs has been described above. The time for e uilibration could be considerabl reduced from the norma 10-12 hours by adding mesityroxide to all bulbs, but with different concentrations in bulbs 3a and 3b. The test for equilibration was equality within the experimental reproducibility of 0.1 yo of mesityl oxide concentrations in bulbs 3a and 3b. The mesit 1 oxide was d e termined in the three bulbs using the speciarpipet described by Eberz and Lucase to prevent loss of mesityl oxide by vaporization. The solution was forced up into the special pi et by nitrogen which was bubbled through mesityl oxide sorution of about the same concentration as that in the equilibrator. This procedure ensured that the progress of the equilibrator toward equilibrium was not affected by the sampling. Nitrogen was used as the equilibrator gas although mesityl oxide does not form peroxides in the aqueous solution. Check runs with air and oxygen yielded distribution ratios differing not more than 0.2% from the ratios obtained with nitrogen. As the apparatus could be run continuously, two to three runs could be done every 24 hours. The ratio of the mesityl oxide molality in the water to that in the salt solution yields directly the activity coefficient of mesityl oxide in the salt solution when the mesityl oxide molality is low enough to allow assumption of Henry's law. Appreciable deviations from Henry's law were found even below 0.1 M. As corrections can be applied for this deviation, it was possible to use molalities as high as 0.1 M without loss in accuracy. The apparatus had to be modified for the few test distributions of iodine between water and carbon tetrachloride as the mercury in bulbs reacted with the iodine. The mercury was replaced with saturated sodium nitrate solution for iodine runs. Following each run, a known volume of salt solution was pipetted into a weighed bottle, weighed, and then evaporated to dryness to yield the weight of salt and weight of solvent (water plus mesityl oxide). The mesityl oxide concentration was known; thus the weight fractions of all components could be calculated. One can express molalities aa moles per kilogram of water or as moles er kilogram of solvent (mesityl oxide plus water). The Patter choice waa adopted. The ratio, c/m, of the concentration in moles per liter to the molality is obtained directly from the ratio of weight of solvent to pipet volume. The elm ratios for dilute salt solutions containing 0.1 M mesityl oxide are about 0.1% lower than for pure salt solutions. For some of the more concentrated salt solu(6) W. F. Eberr and H. J. Lucas, ibid., 116, 1230 (1934).
LEOBREWER, T. R. SIMONSON AND L. K. J. TONG
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Vol. 65
TABLE I c/m RATIOSFOR AQUEOUS MESITYLOXIDIO SOLUTIONS AT 25” r.--
m
0.25
0.5
NaAc KAc NaCl KC1 NaBr KBr NaI NaN03
0.987 ,983 ,993
0.977 ,973 .988 .984 .885 ... .978 ,983
.D90
... ...
...
...
-0.1 1
,If MO2.5
...
0.916
0.897
0. 964
.952
0.940
.947
.931
.954
.939
0.947 ,978
r
2
1.*5
-0.02
M R10-
4.5
5.5
0.907
0.887
,969
.961 .962 .968
tions, the difference is considerably greater. For example, for 1.5 M NaC1 a t 25’ containing 0.1 M MO, elm is 0.6% .smaller than for the pure salt sol&ion. The distribution ratio of mesityl oxide was determined volumetrically. This ratio was converted to the molality basis in terms of moles of mesityl oxide per kilogram of solvent by multiplying by the ratio of c/m values for the two solutions. The c/m values for the salt solutions studied are given in Table I.
of mesityl oxide relative to the hypothetical solute standard state in pure water unless one corrects for the deviation from Henry’s law. The value of -0.3 which was found for k33 of equation 1 allows one to make this correction. If y* = (MO)w/(MO)s is the observed distribution ratio between water and a salt solution when there is a molality (M0)w in the water bulb and a molality Discussion (M0)s in the salt solution, the error in log y* due t o deviation from Henry’s law is -0.30. Deviation from Henry’s Law.-Pressman, Brewer and Lucas6 have determined the activity [(MO)w - (MO)s] = -0.30(MO)w (1 - l/y*). coefficient of mesityl oxide in sodium perchlorate Thus and sodium nitrate solutions by means of distrilog y = log y* - 0.3O(MO)w(l - l/-y*) (2) bution experiments between carbon tetrachloride gives the relationship between the distribution and the aqueous salt solutions. They have ascribed coefficient, y, obtained at low mesityl oxide molality the variation of the distribution ratio of mesityl and the coefficient, y*’ obtained at high mesityl oxide between water and carbon tetrachloride with oxide molality for a given salt solution. the mesityl oxide concentration to deviation from corrections are illustrated in Table I1 where Henry’s law in the carbon tetrachloride. This theThese results of three experiments a t approximately deviation becomes substantial for mesityl oxide con- the same salt molality but with different mesityl centrations in the CC1, greater than 0.2 mole/liter. oxide molalities are presented. The quantities Thus, the aqueous concentration must be kept k~ and k*32 were obtained by dividing the respective below 0.0076 mole/liter to avoid corrections for log y* and log y values by the sodium chloride deviations from Henry’s law in the CCld phase. molality. Because sodium chloride molalities were By use of the vapor phase distribution method one not, quite the same, one must compare k values need not apply any corrections for deviations than y values to reduce the observations to from Henry’s law until one has attained much rather the same sodium chloride molality. The ratio of the fugacity higher concentrations. of mesityl oxide to its aqueous molality is 7% TABLE I1 smaller in a 0.1 M solution and 20% smaller in a EFFECTOF HIQH MESITYLOXIDE MOLALITYUPON DISsaturated solution of mesityl oxide (0.304 M or TRIBUTION BETWEEN WATERAND SALTSOLUTIONS 0.302 mole/l. at 25’) than in very dilute solut#ions. 0.30(MO)wy kat The activity coefficient of mesityl oxide (com- mNnc1 y* k*at (MO)W (1 - l / ~ * ) log y ponent 3) relative to the hypothetical solute 1.467 1.873 0.186 0.133 0.0186 0.2539 1.795 0.173 1.941 ,190 .162 .0236 ,2648 1.840 1745 standard state in water (component 1) can be 1.517 1.619 2.060 ,194 ,209 ,0323 ,2816 1 912 .174 expressed as a function of both salt (component 2) and mesitzyloxide molality A series of experiments of the type illustrated in 0 Table I1 indicated that log y = - 0 . 3 0 ~ ~or~ the log YS = I c s ~ r n 2 k 3 m (1) second term of eq. 1appears to adequately represent was found to hold rather well. The term the variation of the mesityl oxide activity coefrepresents the deviation from Henry’s law. The ficient as a function of mesityl oxide molality for deviation from Henry’s law may be found readily both water and salt solutions of mesityl oxide. by use of the vapor phase distribution apparatus. It is of interest to compare the value of k33 = -0.30 If mesityl oxide is distributed between water and f 0.005 with the value of k3,=* = -0.07 f 0.01 a salt solution and the distribution repeated at obtained from the data of Pressman, Brewer several mesityl oxide molalities, it is found that the and Lucas5 for the effect of diacetone alcohol mesityl oxide molality in the water to that in the upon the activity coefficient of mesityl oxide in salt solution increases as the mesityl oxide molality aqueous solution. rises since the deviation from Henry’s law changes It will be noted that k* varied by 0.008 cormore rapidly in water than in the salt solution. responding to a 3% variation in y* and that k* This is due to the smaller mesityl oxide molality at the highest (MO), differs from k by 0.020 corin the salt solution than in the water. Thus the responding to a 77, error in y* due to the deviation ratio of the mesityl oxide molalities in the solution from Henry’s law. The average deviation from at equilibrium does not give the activity coefficient the mean value of k32 is only 0.0005 corresponding
+
March, 1961
VAPORPHASE EQUILIBRATION FOR ACTIVITY COEFFICIENT DETERMINATIONS 423 TABLE I11 SALTING-OUT COEFFICIENT k32 FOR MESITYL OXIDEAT 25.00'
m
NaAc
KAc
0.25 0.5
0.190 .191
0.176 0.179
1
1.5 2 2.5 4.5 5.5
,180
... .168 .164
NaCl
0.190 .180
,174 .173 .170 .1695 .1648
KC1
NaBr
KBr
NaNOs
NaI
NaClOi
0.158 .159
...
... ...
...
...
0.056
0.03
... ...
0.024
0.022
.149
0.117
0.102
.lo4 .lo5
( .OS)
.074 ( .082)
,025 .033
.1638
to 0.2% deviation in y. Thus, it is seen that the use of eq. 1 and a value of k33 = -0.3 allows an accurate correciion for deviation from Henry's law. The correction for (M0)w = 0.0015 M is 0.1% and for (M0)w = 0.015 M, the correction is 1%. As mesityl oxide solutions of the order of 0.015 M can be analyzed with an accuracy of O.l%, activity coefficients accurate to 0.1% can be obtained if the 1% Henry's law correction is applied. It is of interest to note that the linear dependence can be derived as a limiting equaof log 7 3 upon m,% tion from the regular solution equation' log y03 = ( B / R T ) z where ~ ~ y3 is the activity coefficient of mesityl oxide relative to the pure mesityl oxide standard state and $1 is the mole fraction of water. The derivation cbonsists of transforming mole fraction to molality, changing the standard states and neglecting higher powers of molality. One can make some generalizations about the deviations from Henry's law that might be expected for the two homogeneous portions of a liquid system with a miscibility gap. One finds that the regular solution equation predicts an increasing deviation from Henry's law at a given concentration of solute if mixtures of liquids of increasing differences of internal pressure are examined. On the other hand, the maximum concentration of solute increases as the internal pressure difference decreases. If one compares the deviations from Henry's law for various solutes in their saturated solutions, one finds that the deviations increase as the internal pressure differences decrease and reach a maximum when the miscibility gap has just closed. For a two liquid mixture just below the critical mixing temperature one would predict a deviation from Henry's law of about a factor of two in the saturated solutions. Salting-out of Mesityl Oxide.-In Table I11 the distribution data, a t 25" are presented in terms of the k32 values at rounded values of the salt molality. The correction for deviation from Henry's law has been applied. The NaNOa values in parentheses are from Pressman, Brewer and L U C ~ SThe .~ variation of the k32 values with molality is small. The maximum variation is 0.03 even up to 5 M . Interpolated values may be obtained readily within 0.002. A change in k32 of 0.002 for a 1 M salt solution corresponds to 0.5% change in 7 3 . The activity coefficients of mesityl oxide which are calculated from eq. 1 by use of the values of k 3 2 obtained by interpolation in Table I11 together with use of the value of k33 = -0.30 are activity (7) J. H. Hildebrand and R. L. Scott, "The Solubility of Nonelectrolytes," Third Edition, Reinhold Publ. Gorp., New York, N. Y..1950.
coefficients taken with respect to the solute standard state of mesityl oxide in pure water with molality units used throughout. A solution designated as 0.1 M MO and 1 M NaCl will contain 0.1 mole of MO and 1 mole of NaCl per 1000 grams of solvent (990.2 g. of H2O and 9.8 g. of MO). The data of Table I11 can be converted to volumetric units by use of the c/m values of Table I. For NaCl below 2 M the k32 values a t 20" are 0.001 higher than those a t 25". The 1.003 ratio of yz0/y26,corresponding to an enthalpy of transfer of 100 cal./mole, is just slightly larger than the experimental uncertainty. Between 4.5 %nd 5 M , y2o/y25 = 1.02, which corresponds to AH = 690 f 200 cal./mole for MO(H20) = MO(5 M NaC1) with the same mesityl oxide molality in each solution. Pressman, Brewer and Lucas5 obtained one determination of YMO a t 30" in 1 M NaC104 and one in 1 M NaN03 which indicated a 3.5 to 4% reduction of y ~ upon o heating from 25 to 30", corresponding to a A H of transfer of around 1400 cal./mole. Comparison with the present determinations indicates that their 1 M NaN03 value at 25" is 2% high. As the change of y with temperature is close to the limit of their CC14 distribution method, there is some question whether salt solutions such as 1 M NaC104 and 1 M NaN03 with low salting-out coefficients really do display apDreciable temoerature coefficients in contrast to ihe low temperature coefficient observed here for 1 M NaC1. Combination of our results with those of Pressman. Brewer and Lucas5 vields the followine: order of decreasing salting-out coefficients: A C T CI-, Br-, Ko3-, I-, (3104- and Na+, K* and H+. This is in general agreement with order reported in the literatureSfor other substances. Comparison of the k values of mesityl oxide in salt solutions with the k values of the related diacetone alcohol as determined by Akerlofgshows that the k values of diacetone alcohol in NaCI, KC1, NaBr and KI are about 7570 of the corresponding k values of mesityl oxide. Haugen and Friedman'" have verified experimentally for nitromethane solutions the theoretical relationship between the coefficient k32 which characterizes the effect of an electrolyte 2 (8) Pressman, Brewer and Lucass present a aummary of literature references in therr footnotes B and 7. F. *4. Long and W. F. McDevit, Chem. Reua., 61, 119 (1952).have reviewed later work. (9) G. Akerlof, J. Am. Chem. 800.. 61,984 (1929). (10) G. R. Haugen and H. L. Friedman, J. P k y s . Chem., 60, 1303 (1956).
424
LEOBREWER,T. R. SImxsox
AND
L. K. J. T o m
Vol. 65
upon y 3 and the coefficient k2$which characterizes the effect of component 3 upon y2, the mean activity coefficient of the electrolyte. The application of the cross differentiation equation"
are considerably higher than most determinations in the literature. Only the data of Linhart15 (after correction for hydrolysis) and the data of Davies and Gwynnele yield distribution ratios over 90 and some values in the literature range as low as 80. This would clearly seem to indicate that the aqueous phases did contain appreciable to eq. 1 yields 2(d log y2/b mJm, = k32 or log y2 = amounts of the CC4 phase in all determinations 1/2k32m3 for those 1-1 electrolyte solutions for which other than the two mentioned. The trend to k33 and k32 are essentially independent of m2. higher values of the distribution ratio with inFor those solutions which show appreciable varia- creasing iodine reported here and also that retion of h2with m2, it is necessary to express k32 ported by Davies and GwynnelBis in fair agreeas a power series in m2 before differentiating eq. 1. ment with the 4% increase predicted above on the We may now apply this result to predict the basis of a regular solution equation calculation. effect of mesityl oxide upon the activity coefficient of electrolytes. For a 2 M NaC1 solution, we calTABLE IV culate that the addition of 0.1 M mesityl oxide DISTRIBUTION OF 12 BETWEEN CCI, AND H20 AT 25" increases the mean activity coefficient of NaCl 11 in CCh (molesfl.) Distribution ratio by 2y0. Similar results may be obtained for the 0.013 89.8 other salt solutions. .045 90.6 Iodine Distributions.-To illustrate the use of the .OM 91.6 equilibrator for distribution between two im,065 93.1 miscible solvents, the distribution of iodine between .073 91.1 water and carbon tetrachloride was measured as ( .119) (91.0) a function of iodine concentration. This distribution has been studied directly by many observers. Most of them have noted an increase of the distriKeefer and Allen" and DeMainelBhave observed bution ratio or ratio of concentration in carbon the spectrum of 1 4 in CC4. The stability of the tetrachloride to concentration in water as the spectrally characterized I4 is much too small to iodine concentration was increased. Winther12 account for the trend of distribution ratio with attributed this trend to formation of I+ in dilute iodine concentration. Davies and Gwynnel* state solutions. Hildebrand and Scott? have pointed that Hildebrand and Scott7explain the variation of out that such an effect can be expected on the basis distribution ratio by an association of the iodine of expected deviation from Henry's law in the in the carbon tetrachloride. The use of the term carbon tetrachloride phase. Using the internal "association" is most unfortunate here and is pressures given by Hildebrand and Scott, one can certainly not in the spirit of Hildebrand's regular solution treatment, The regular solution treatcalculate that ment is based upon the unbalance of weak London In 7 = 2(59)2(5.6)2(0.01147)/ (97)(1.987)(298)(0.9885) = 0.0435 forces and it would not be profitable for most purposes to describe these weak interactions in terms where y is the activity coefficient of IZin a saturated of associated molecules. There also would not CC14solution, ZI,= 0.01147, at 25" relative to the appear to be any basis for Winther's contention hypothetical solute standard state of I 2 in CCla. that the trend of distribution ratios is due to I+ Thus, the regular solution equation would predict formation in the aqueous solutions. The regular a 4.4% increase in the distribution constant as the solution treatment is more than adequate to iodine concentration is increased from low values account for the entire observed effect. to saturation. Winther and others had observed The two examples of distribution of mesityl variations of 7 to 10%. A possible explanation oxide and of iodine illustrate the wide possibilities that one should consider is the possibility of varying for the use of the vapor phase equilibrator for emulsification of some of the CC4 phase in the distribution of solutes between miscible or immisaqueous phase. The vapor phase equilibrator cible phases. The apparatus is particularly valuprovides all of the same conditions of direct con- able for the high accuracy of the data and for its tact of the two phases in that both phases are satu- ability to yield more nearly complete thermodyrated with respect to water and CCl4, but it is im- namic data for multicomponent systems than can possible for any of the CCl, phase to appear in the be obtained easily by other methods. water bulb as long as there is no temperature (14) Values in the literature of the solubility of 11in CCh vary over 8 gradient. Thus one can check the emulsification range of 4%. The value chosen here is that of Jakowkin. whioh w a ~ theory. accepted by Hildebrand and Scott7 in their Table I on page 208 a6 the Table IV presents the results of such a check.'3 best value. The recent determination of the solubility of iodine in The values in parentheses are calculated from water by L. I. Katrin and E. Gebert, J . Am. Chem. SOC.,77, 6814 (1955) waa used to obtain the distribution ratio. The solubility of solubility measurements.l4 These distribution ratios iodine in both dry and wet CCL muet be accurately determined before an accurate comparison with the distribution data can be made. (11) H. A. C. McKay. Trana. Faraday SOC..49, 237 (1953). (12) C. Winther, 2. phustk. Chem., BS,299 (1929). (13) These determinations were made by Thomas L. Allen for an undergraduate special problem. The aqueous solutions were made 10-6
N in HrSOa to inhibit hydrolyeia.
(15) (16) (17) (18)
G. A . Linhart, J . Am. Cham. SOC.,40, 158 (1918). M. Dsvies and E. Gwynne, {bid., 74, 2748 (1952). R. M. Keefer and T. t.Allen, J . Cham. Phys., 26, 1059 (1856). P. A . D. DeMaine, Can. J . Phys., 36, 573 (1957).
