ANOMALOUS DIFFUSION OF QUINONE I N SALT SOLUTIONS HERBERT FREUNDLICH A N D DEODATA KRUGER’ School of Chemistry, Institute of Technology, University of Minnesota, Minneapolis, Minnesota Received September 16, 1938
In previous investigations (1, 2) anomalies of diffusion were observed that were at first not readily explained. Quinone diffused in pure water according to Fick’s law with a well-defined diffusion coefficient, agreeing with the value that could be derived from the molecular weight, but if the quinone were allowed to diffuse in a uniformly distributed salt solution, for instance, in a solution of potassium sulfate, diffusion proceeded so irregularly that no constant could be determined. The well-known method of Oeholm and a similar one were used. In both, the solution of quinone w&s covered with a solution of the same composition but free of quinone, and after a certain time the column of liquid was carefully divided into layers which were analyzed. The characteristic anomaly in the salt solutions consisted in the appearance of too large amounts of quinone in the top layers of the column. It seemed as if a convection current had started and had transported some of the quinone solution upward. The density gradient a t the outset was correct, Le., the liquid above was distinctly less dense than the quinone solution below. G. S. Hartley (3) suggested that this behavior might be due to the following mechanism: A uniformly distributed solute could show a so-called “diffusion r6trogradeJ” i.e., it might become unequally distributed if a second solute were allowed to diffuse in the liquid, the unequal distribution occurring mainly where the concentration gradient was steep near the juncture of the two solutions. This may result from a general type of Donnan effect. Owing to a mutual change in solubility, the potassium sulfate in the quinone solution develops other forces than in pure water, and these fields of force produce an unequal distribution of the potassium sulfate. This unequal distribution of the potassium sulfate, occurring during the process of diffusion, may cause an inadmissible density gradient, although the latter started out correctly. Thus, a thin layer of the liquid a t the junction of the two solutions becomes less dense than the solution above. This irregular density gradient may produce convection currents, which disturb the normal process of diffusion. The correctness of this 1
Berlin-Wilmersdorf, Germany. 981
982
HERBERT FREUNDLICH AND DEODATA K R ~ ~ Q E R
explanation was proved by reducing the concentration of the pure potassium sulfate solution above to a sufficiently marked degree so that it was less dense than the solution of potassium sulfate and quinone below. The anomalies then disappeared. It was not proved by analysis that the uniformly distributed solute actually changed its concentration near the borderline of the two solutions, owing to the diffusion of the second solute. Under the conditions of the experiments mentioned this would have been difEcult to do, because it would have meant the determination of a small change in the concentration of a concentrated solution of a salt such as potassium sulfate. Since the effect is due to a mutual change in the solubility of the solutes, it should be possible to interchange them. If the quinone is uniformly distributed at the outset, and potassium sulfate is diffusing, owing to a strong concentration gradient, a change in the distribution of the quinone should be observed near the junction of the two solutions. Quinone can be determined very exactly by an iodometric method (7), hence it was likely that a change in its distribution might be determined. This direct proof appeared to be desirable, since experiments on “diffusion r6trograde” have been performed infrequently (3, 6). The following experiments were done under conditions similar to those described previously (2) (cf. Tables I11 to IX). I n the first of the previous papers it was found that potassium chloride, in contrast to potassium sulfate, showed a less marked anomaly of diffusion. I n the first experiments the concentration of quinone was rather small, 27.8 millimoles per liter. I n parallel experiments potassium chloride and potassium sulfate, both about 1 normal in a solution of quinone (27.8 millimoles per liter), diffused into an aqueous solution of quinone, also containing 27.8 millimoles per liter. Diffusion was allowed to go on only for a short time, as long as no salt had permeated to the top of the diffusion cylinder; hence the concentration gradient on the borderline of the two solutions always remained steep. The results are given in table 1. There was no change in the distribution of the quinone when potassium chloride was diffusing, but a very distinct change was observed with potassium sulfate. The quinone had a tendency to pass out of the potassium sulfate solution and to accumulate in the aqueous solution ; hence there was a decrease in the quinone concentration just below the liquid junction and an increase above. At a somewhat higher concentration of quinone-55.6 millimoles per liter-a change of concentration was also shown in the presence of a concentration gradient of potassium chloride, as was to be expected. The concentration of the salt was 1 normal. Two experiments gave similar results; the results of one experiment are shown in figure 1. Here the distribution of the diffusing salt is also represented, and the decrease
983
DIFFUSION OF QUINONE I N SALT SOLUTIONS
of the quinone concentration below the junction and the increase above are seen clearly. The abscissae are the layers of the column of diffusion. The ordinates on the left-hand side refer to the continuous lines representing the concentration of potassium chloride, in per cent of the original concentration; the ordinates on the right-hand side refer to the dotted TABLE 1 Diffusion of potassium chloride and potassium sulfate in a 0.0678 normal solution of quinone* T = 11.5 hr.; t = 19.1"C.
1
LAYrRt
o,75 om,; e =
mole per liter
AMOUNT OF QUINONE TITRATED$
-
33-25 25-20 2C-15 15-10 10-5 5-0 0-5
-
-
DIFTUEION OF POTABBIUM SULFATE
DIFFUSION OF POTABSIUM CHLORIDE
1
h
0.70 om.;c 0.84 equivalent wr liter
AMOUNT OF-QUINONE TITRATED
5.37
5.52
5.34 5.37 5.38 5.39 5.37
5.50 5.54 5.41 5.24
t
IO
-30 -20 -10 0 IO 20 30 FIG. 1. Concentration gradient of potassium chloride; quinone uniformly distributed. h = 0.80cm.; T = 11.5hr.; t = 25.6"C.;~(potassiumchloride) = 1 normal; c (quinone) = 0.9556 molar. , potassium chloride; ----------, quinone.
lines, which indicate the concentration of quinone given in cubic centimeters of a 0.02 normal solution of thiosulfate needed for titrating 2 cc. of the liquid in the diffusion column. The results with potassium sulfate were confirmed by four further experiments (two groups of two parallel experiments). They were done with special care, using the smaller concentration of quinone (27.8 milli-
984
HERBERT FREUNDLICH AND DEODATA KRUGER
moles per liter). The experiments of the first group will be discussed more fully. Pure quinone and potassium sulfate were used. The aqueous solution and the one containing salt had ex.actly the same concentration of quinone. Two cubic centimeters of the solutions needed 5.65 and 5.66 cc., respectively, of a 0.02 normal solution of thiosulfate for titration. Table 2 gives the complete results of one experiment in this group; the TABLE 2 Difusion o j potassium suljate i n a 0.0678 normal solution of quinone h = 0.70 cm.; T = 10 hr.; t = 213°C.; c = 1 equivalent per liter AMOUNT OF QUINONE TITBATED
LAYER
33-15
15-10 10-5 5-0 0-5 5-10 10-15 15-20 20-25 25-30
Actual determinations
Mean value
5.58; 5.58; 5.59 5.72; 5.69 5.77; 5.74 5.64; 5.61 5.48; 5.45 5.45; 5.40 5.54; 5.56 5.64; 5.61 5.64; 5.66
5.58 5.70 5.75 5.62 5.46 5.42 5.55 5.62 5.65
Constant f
30 20
*'
.
5.2
"z V
30 20- 105 0 510 20 30 FIG. 2. Concentration gradient of potassium sulfate; quinone uniformly distributed. h = 0.70 cm.; T = 10 hr.; t = 22.4"C.; c (potassium sulfate) = 0.827 normal; c (quinone) = 0.0278 molar.
second was in good agreement, and both confirmed the results given in table 1. In figure 2 the results of an experiment of the second group are given in a manner similar to figure 1. Again the distribution of the diffusing salt was determined, and the decrease in concentration of quinone below the junction and the increase above were very marked. The difference between the highest and lowest value of the quinone concentration observed
DIFFUSION O F QUINONE I N SALT SOLUTIONS
985
was 6.1 per cent of quinone (table 2). For the second experiment of this group with potassium sulfate it was found to be 5.7 per cent. In the experiments with potassium chloride the corresponding differences in quinone were 2.7 per cent (cf. figure 1) and 2.3 per cent, respectively. It is obvious that a similar change in distribution must occur if quinone diffuses in a solution of potassium sulfate, the salt being uniformly distributed a t the outset, and that this effect may cause the disturbing anomalies previously investigated. To a minor degree this also holds for the diffusion of potassium chloride. Some experiments with uniformly distributed succinic acid in a field of diffusion of sodium chloride and potassium chloride, respectively, were in good agreement with these results. According to Linderstr$m-Lang (5)
C
EO
0 IO 20 30 FIG.4 FIG. 3. Relative solubilities of quinone in salt solutions. The solubility of quinone in water is taken as 100. FIG.4. Concentration gradient of potassium nitrate; quinone uniformly distributed. h = 0 . 7 0 c m . ; ~= 11.5hr.;t = 21.5"C.;~(potassiumnitrate) = 1.5normal; c (quinone) = 0.0833 molar.
30
IO
FIG.3
the solubility of succinic acid is reduced more strongly by sodium chloride than by potassium chloride. We found a distinct shift of concentration of the succinic acid with sodium chloride and a smaller one with potassium chloride, both in the same direction as in the preceding experiments with quinone, Le., an increase in concentration on the water side and a decrease on the salt side of the borderline. The original concentration of the succinic acid waa 254 millimoles per liter and that of the salt solutions was 2 moles per liter; the temperatures were 20.9' and 2O.l0C., respectively. As was mentioned above, we must assume the forces affecting the solubilities of the solutes to be the cause of this phenomenon. If this be so, it may be expected that substances which increase the solubility of quinone should produce a shift of concentration of the quinone in the
986
HERBERT FREUNDLICH AND DEODATA KRUQER
opposite direction, Le., an increase on the salt side and a decrease on the water side of the borderline. Kruyt and Conmar Robinson (4) have investigated the salting-out and salting-in effects of a number of salts upon quinone. Some of their results are represented in figure 3; the abscissae are the salt concentrations, c, in moles per liter, and the ordinates are the solubilities of quinone, L, on a relative scale, the solubility in pure water being taken as 100. Obviously we might expect particular strong effects in the opposite direction for potassium thiocyanate and potassium iodide, but both have a TABLE 3 Diffusion of potassium nitrate i n a 0.0839 normal solution of quinone h = 0.70 cm.; T = 11.5 hr.; t = 21.5"C.;e = 1.5moles per liter. The solution in the diffusion column contained 0.01 mole of hydrochloric acid per liter
Mean value
POTILIBIVY ~ ~ E A T E CONCENTBATION I N P E B CENT OF OR101N I L CONCENTBITION
8.30;8.31
8.305
1 .o
8.30;8.32 8.30;8.27;8.27;8.29 8.30;8.31 8.36;8.37 8.44;8.44 8.41;8.46 8.42;8.43 8.37;8.38
8.31 8.28 8.305 8.365 8.44 8.435 8.425 8.375
1.9 2.5 7.1 18.7 38.5 62.8 79.8 94.0 95.9 98.3
8.39;8.35
8.37
AMOUNT OF QUINONE TITRATED
-
LAYEB
Actual determimtiod
'
30.6-25 25-20 20-15 15-10 10-5 5-0 0-5 5-10 10-15 15-20 20-25 25-30
Amount of quinone titrated in original aqueous solution, 8.35,8.34; mean value, 8.345 Amount of quinone titrated in original potassium nitrate solution, 8.38,8.40; mean value, 8.39
* Cubic centimeters of a 0.01 normal solution of thiosulfate needed for 1 cc. of quinone solution. certain tendency to react with quinone and can therefore not be used. Potassium nitrate seemed to be the most hopeful substance, since it caused a considerable increase in solubility of the quinone without tending to react in any marked degree. The first experiment gave a small effect in the direction that had been expected, but it was not quite satisfactory. Titration values of the same layer did not agree as well as desired. A second experiment was done with special care. The potassium nitrate solution was 1.5 normal, and both the salt and the aqueous solution were 0.01 normal as to hydrochloric acid, quinone being more stable in weakly
DIFFUSION OF QUINONE IN SALT SOLUTIONS
987
acid solutions (5). The quinone concentration was somewhat higher than in the previous experiments,-namely, 83.3 millimoles per liter. The samples were taken with carefully standardized 1-cc. pipets having a particularly good outflow. This experiment showed beyond doubt (cj. table 3 and figure 4) that again the uniformity of distribution of the quinone was disturbed by the diffusing salt, but now, as had been expected, in the opposite direction, as an increase in the concentration of quinone was found on the salt side of the borderline and a decrease on the water side. Figure 4 corresponds to figure 1 and figure 2. The quinone concentration is given in cubic centimeters of a 0.01 normal thiosulfate solution needed to titrate 1 cc. of the liquid in the diffusion column. The total quinone content was found to be unaltered after diffusion had taken place, indicating that there had been no chemical reaction. Probably no simple correlation exists between the shifts in concentration of the quinone and succinic acid, respectively, and the absqlute values in the change of their solubilities caused by the salts. The change in solubility would hardly allow us to predict unambiguously the amount of transported molecules of quinone and water caused by thediffusion gradient of the salt. It is believed that an inadmissible density gradient and hence disturbing convection currents may occur more probably with potassium nitrate as a diffusing salt, where there is, passing from below to above, a maximumminimum distribution of the organic solute than in the other cases, where the distribution is a minimum-maximum one, because just above the junction, in a zone containing practically nothing but the organic solute, the concentration of the latter will decrease in the case of a maximumminimum distribution. This zone, therefore, would become less dense than the layers above containing the same solute in higher concentration. An inadmissible density gradient will not be so easily set up when the organic solute is accumulated above the liquid junction. It appears that there is a difference in this direction in these experiments. The distribution of the salt was determined gravimetrically. In the experiments with potassium sulfate, as shown in figure 2, the top layers contained no salt, as should be expected for normal diffusion in such a shortJime. With potassium nitrate, however, an appreciable amount of salt was found in the upper layers. It amounted to about 1 per cent in the top layer and increased to 1.9 per cent in the second layer from the top. This indicated an anomalous diffusion, Le., one disturbed by convection currents. The occurrence of convection currents in the case of potassium nitrate would tend to level the changes'in concentration of the organic solute and thus make them less distinct. This was probably the reason why it was found much more difficult to discover a case of maximum-minimum distribution than distributions in the opposite direction.
988
HERBERT FREUNDLICH AND DEODATA K R ~ G E R SUMMARY
1. I t was shown by direct analysis that if salts (potassium chloride,
potassium sulfate, sodium chloride) diffuse in an aqueous solution of a second solute (quinone] succinic acid), the latter being uniformly distributed a t the outset, the second substance changes its distribution in the region where the concentration gradient of the salt is steep. We are dealing with the phenomenon of “diffusion r6trograde.l’ 2. If the solubility of the second solute is decreased by the salt, as is the case, for instance, with quinone and potassium sulfate, then we find a decrease of quinone concentration on the salt side and an increase on the water side of the borderline. A shift of concentration in this sense was observed with quinone in solutions of potassium chloride and potassium sulfate, and with succinic acid in solutions of sodium chloride and potassium chloride. The stronger reduction of the solubility of quinone by potassium sulfate, as compared with that by potassium chloride] corresponds to a greater shift in concentration caused by potassium sulfate; accordingly there was also a greater shift of concentration caused by sodium chloride] as compared t o potassium chloride] in the case of succinic acid, the solubility of which is reduced more strongly by sodium chloride. 3. If the solubility of the second solute is increased by the salt, as is the case with quinone and potassium nitrate, the shift in concentration has the opposite sense: there is an increase in quinone concentration on the salt side and a decrease on the water side of the borderline. REFERENCES
(1) FREUNDLICH, H.,AND KROGER,D.:Z. Elektrochem. 36, 305 (1930). (2) FREUNDLICH, H.,AND K R ~ ~ G E D.: R ,Trans. Faraday SOC.31,906 (1935). (3) HARTLEY, G. S.: Trans. Faraday SOC.27, 10 (1931). (4) KRUYT,H. R., AND ROBINSON, CONMAR: Koninkl. Akad. Wetenschappen Amsterdam 29, 1244 (1926). (5) LINDERSTR~M-LANQ, K . : Compt. rend. trav. lab. Carlsberg 15, No. 4 (1924). (6) THOVERT, J.: Ann. phys. [SI2, 369 (1914). (7) VALEUR, A.: Compt. rend. 129, 552 (1899).
