ELECTROLYTES I N HYDROPHOBIC SYSTEMS
747
EFFECTS O F ELECTROLYTES I K HYDROPHOBIC SYSTERZS. I11
STUDIES WITH MIXTURES OF ELECTROLYTES: IONANTAGONISM FRED HAZEL Department of Chemzstry and Chemical Engzneerang, Unwersaty of Pennsylvanza, Phaladelphza, Pennsylvania Receaved October SO, 1940 INTRODUCTION
The flocculation value of a mixture of two electrolytes for an arsenic trisulfide sol can be computed additively, if the precipitating power of each of the electrolytes is of the same order of magnitude (4), while, if the electrolytes differ markedly in their precipitating power, as do potassium chloride and barium chloride, the flocculation value of the mixture is greater than the additive value (7). The phenomenon of ion antagonism has been observed for a number of electrolyte pairs with several colloidal systems, e.g., OdBn’s sulfur sol (l),copper ferrocyanide sol (6), mercuric sulfide sol (3) and selenium sol (3). The explanation offered by Freundlich and Scholz (l),namely, that hydration is a critical factor in determining ion antagonism, has been evaluated by Weiser (7), who has pointed out that other factors also must be considered: (1) the antagonistic effect of each precipitating ion on the adsorption of the other, and ( 2 ) the stabilizing influence of the ions having the same charge as the sol. Kruyt and van der Killigen (3) correlated flocculation data with electrophoretic measurements and reached the conclusion that, whenever a salt raises the charge (mobility) of the particles, its addition also will raise the flocculation value of a second salt. The present study is concerned with the electrophoretic and the coagulating behaviors of the salt pairs potassium chloride-barium chloride and potassium chloride-chromium chloride with a manganese dioxide sol and an arsenic trisulfide sol. The investigation is the third of a series of reports which have dealt with anomalies of monovalent ions as applied to the above colloidal systems. The same stock samples of sols were used throughout the investigations and the experimental methods were the same. RESULTS
Flocculation data for the two sols, each a t three different concentrations, are given in tables 1 and 2 in terms of millimoles of electrolyte per liter. The experiments were conducted by adding 2 cc. of sol to 3 cc. of electrolyte mixture. The amounts of barium chloride and of chromium chloride required to coagulate the colloidal systems were determined when each of these electrolytes was present with 25, 50, and 75 per cent of the flocculating concentration of potassium chloride.
748
FRED HAZEL
Ir'o ion antagonism was observed with either of the electrolyte mixture8 during flocculation of the manganese dioxide sols or with the potassium chloride-chromium chloride mixture in the coagulation of the arsenic trisulfide sols. The flocculation values of the mixtures were less than the additive value in a majority of these cases. Thus, the flocculation value of potassium chloride for the 100 per cent manganese dioxide sol was 6.0 millimoles per liter, while the flocculation value of barium chloride was 0.14 millimole per liter. In a mixture containing these two electrolytes it was found that when 1.5 millimoles per liter of potassium chloride was TABLE 1 Manganese dioxide sol 1M) PER CENT SOL; 0.6 5. OF M N 0 2 PER LITER
TABLE 2 Arsenic trisulfide sol 26 PER CENT BOL
PER LITER ~
0 12 24 36
BaCb
CrCli
0.24
0.012
present (corresponding to 25 per cent of the flocculation value of this salt), only 0.072 millimole per liter of barium chloride (51 per cent of the flocculation value) was required for coagulation. Since for the mixture to have an additive value, 75 per cent of the flocculation value, or 0.105 millimole per liter, of barium chloride should be required to precipitate the colloid in the presence of 25 per cent of the flocculation value of potassium chloride, the departure from additivity may be computed to be 24 per cent. As further illustrations: a mixture containing 50 per cent of the flocculation value of potassium chloride and 34 per cent of the flocculation value of barium chloride coagulated the 100 per cent manganese dioxide sol, while 75 per cent of the flocculation value of potassium chloride and 20 per cent
ELECTROLYTES IN HYDROPHOBIC SYSTEMS
749
of the flocculation vJue of barium chloride were sufficient to coagulate the same system. In the illustrations, the departures from the additive values are 16 per cent and 5 per cent, respectively. These percentages are recorded in table 3 and are given the negative sign to indicate that less than TABLE 3
I PERCENT
o
p
1
I
YANOANESE DIOXIDE SOL
1
1 ~ ~p ecent r ~ sol
I/
ARSENIC TRISULFIDE SOL
~ p e r~e s n t
sol 1 BaCh i CrCli I! BaCh 1 CrClr
PER CENT FLOCCULATION WITH
VALUE OF
Kl IN MIXTURES
BaCI,
FIG.1. Effect of mixtures of potassium chloride and barium chloride on the mobility of manganese dioxide sole. Curve I, 100 per cent sol; curve 11, 25 per cent sol; curve 111, 6.25 per cent sol.
the additive amount of the second electrolyte was required to flocculate the sol. From the above results it is apparent that the presence of potassium chloride increases the coagulating power of barium chloride for colloidal manganese dioxide. Similarly, the data in the tables show that the effectiveness of chromium chloride as a coagulator for both colloidal manganese
750
FRED HAZEL
PER CENT FLOCCULATION VALUE OF KCI IN MWTURES WITH CrCI3
FIG.2. Effect of mixtures of potassium chloride and chromium chloride on the mobility of manganese dioxide sols. Curve I, 100 per cent sol; curve 11, 25 per cent sol; curve 111, 6.25 per cent sol.
>
!-
i 6
e
FLOCCULATION VALUE OF KCI, PER CENT
FIG.3. Effect of mixtures of salts on the mobility of the 50 per cent arsenic trisulfide sol.
ELECTROLYTES IK HYDROPHOBIC SYSTEMS
i5l
dioxide and colloidal arsenic trisulfide is increased by the presence of this salt.' In only two of the thirty-six systems investigated were strictly additive values obtained: 75 per cent of the flocculation valuc of potassium chloride and 25 per cent of the flocculation value of chromium chloride coagulated the 100 per cent arsenic trisulfide sol, while 50 per cent of the flocculation value of potassium chloride and 50 per cent of the flocculation value of chromium chloride coagulated the 25 per cent arsenic trisulfide sol. On the other hand, the marked ion antagonism exhibited by mixtures of potassium chloride and barium chloride in the flocculation of arsenic trisulfide was verified. In the present work the greatest departure from additivity (80 per cent) was encountered with the 25 per cent arsenic trisulfide sol, which gave with barium chloride a flocculation value of 0.32 millimole per liter: 130 per cent of the flocculation value of barium chloride (0.42 millimole per liter) was required for coagulation when 50 per cent of the flocculation value of potassium chloride was present in the mixture. A phase of the present investigation consisted of the nicaiurenient of the critical mobilities of the colloidal particles a t the flocculating conceiitrations of the electrolyte mixtures. These measurements n-cre limited to compositions that contained 25, 50, and 75 per cent of the flocculating concentration of potassium chloride, thus corresponding to the compositions employed in the coagulation studies. The results with the threc manganese dioxide sols of different concentration are plotted in figures 1 and 2. Similar results with the 50 per cent arsenic trisulfide sol are shcn-n in figure 3. The graphs clearly show that potassium chloride incrcases the critical mobilities against barium chloride and chromium chloride for both of these colloidal systems. DISCUSSION
I n a previous communication (2) an explanation was offered to account for t'he high critical mobilities toward certain poorly adsorbable monovalent ions. It was concluded that compression of the double layer at high electrolyte concentrations was of primary importance in the coagulation of sols by ions of this valence type. On the other hand, reduction of the charge of the particles by adsorption was considered to be the controlling factor in coagulation by polyvalent ions. It will be shown that the results of t'he present investigation can be explained from the same standpoint. When an electrolyte mixture containing ions of both the above ralence types, such as are to be found in a mixture of potassium chloride and either barium chloride or chromium chloride, is added t o a sol, the effect Xannow (5) recently has shown t h a t while potassium chloride decreases the precipitating power of barium chloride for an arsenic trisulfide sol, the prescncc of this uni-univalent electrolyte sensitizes the sol toward lanthanum chloride.
752
FRED HAZEL
of the potassium chloride is to compress the double layer. This effect decreases the stability of the particles and is responsible for the fact that the coagulating power of chromium chloride for manganese dioxide and arsenic trisulfide sols and of barium chloride for a manganese dioxide sol is increased in the presence of this salt. During coagulation by mixtures containing less than 100 per cent of the flocculation value of potassium chloride it is necessary that adsorption of barium ions (or chromium ions) occur. Potassium chloride raises the critical mobilities for polyvalent ions, however, as is apparent from figures 1, 2, and 3; from this it may be concluded that, a smaller amount of adsorption need occur than would otherwise be the case. Accordingly, it is to be expected that the effectiveness of barium and chromium ions as coagulators will be increased by the presence of potassium chloride, unless this salt markedly cuts down the adsorption of the polyvalent ions from the mixture. It may be observed from the results in table 1 that while potassium chloride increases the coagulating power of chromium chloride for an arsenic trisulfide sol, it decreases the coagulating power of barium chloride for the same system. The latter result may be ascribed to the antagonistic effect of potassium chloride on the adsorption of barium ions (7). From the above behavior of arsenic trisulfide with electrolyte mixtures it i s apparent that the conclusion of Kruyt, previously mentioned, is not generally valid. Thus a salt which raises the charge (mobility) of the particles will raise the flocculation value of a second salt only providing its presence is antagonistic to the adsorption of the ions of the second salt.2 The probability of ion antagonism occurring during the flocculation of sols with electrolyte mixtures is somewhat greater, however, providing one of the salts tends to raise the mobility of the particles. Thus, potassium chloride decreases the stability of arsenic trisulfide particles, owing to two complimentary effects :-compression of the double layer and elevation of the critical mobility. On the other hand, when potassium chloride is added to a manganese dioxide sol, the mobility of the particles is decreased; from this it may be concluded that appreciable adsorption of potassium ions has occurred. This factor, which was not present with the arsenic trisulfide sol, decreases the stability of the particles. At the same time the double layer is compressed, owing to the electrolyte concentration, and
* This paper is not concerned with the mechanism of coagulation by binary electrolyte mixtures containing a common coagulating ion and non-common stabilizing ions, as illustrated in a potassium chloride-potassium ferrocyanide mixture. Since cation antagonism is impossible in this mixture, the decreased precipitating power of potassium chloride in the presence of potassium ferrocyanide is t0.bassigned t o the stabilizing effect of the strongly adsorbed ferrocyanide ion (7). In keeping with this, i t has been found generally t h a t the flocculation values of potassium ferrocyanide are greater than those of potassium chloride for negative sols.
753
ELECTROLYTES IN HYDROPHOBIC SYSTEMS
because of this effect the critical mobility is relatively high. All of these factors tend to decrease the stability of the particles and minimize the amount of polyvalent ions that need be adsorbed to complete the coagulation. The data in tables 1, 2, and 3 are rearranged in table 4,where the values given represent the percentages of the theoretical additive amounts of barium chloride required to coagulate manganese dioxide and arsenic trisulfide sols of different concentrations in the presence of 25, 50, and 75 per cent of the flocculation value of potassium chloride. It may be observed from the data in the table that the precipitating power of barium chloride for manganese dioxide, in the presence of potassium chloride, increases as the sol is diluted. Thus, in the presence of 25 per cent of the flocculation value of potassium chloride, 68 per cent of the theoretical additive amount of barium chloride was required to precipitate the 100 per cent manganese dioxide sol, while only 33 per cent of the theoretical TABLE 4 PER CENT OF FLOCCULATION
OF
KCl
100 per cent
25
68 68 80
50
75
I
MANOANEII DIOXIDE SOL
Sol
26 per cent
Sol
52 44 44
8.26 per cent
ARSENIC TRISULFIDE SOL
sol
100 per cent Sol
M per cent Sol
33 34 32
170 210 224
180 236 236
p e r cent 801
200
260 300
754
FRED HAZEL
tization of a manganese dioxide sol consists of a compression of the double layer. This effect is proportional to the potassium chloride concentration and is responsible for the fact that, as the percentage of the flocculation value of potassium chloride in the mixture was increased from 25 per cent to 75 per cent, the percentage of the flocculation value of barium chloride decreased regularly. As a result, it may be stated that, for a manganese dioxide sol of a given concentration, the precipitating power of barium chloride, in a mixture with potassium chloride, is independent of the concentration of potassium chloride. Supporting data for the above conclusion are shown in table 4 for sols of three different concentrations. These data make it appear probable that the presence of potassium chloride has no effect on the adsorption of barium ions by manganese dioxide. The behavior of arsenic trisulfide was found to be quite different in the above respects: ( 1 ) For a sol of a given concentration, an increase in the potassium chloride concentration decreased the coagulating power of barium chloride. ( 2 ) In the presence of a given percentage of the flocculation value of potassium chloride, the coagulating power of barium chloride decreased as the sol was diluted. The first result appears to have been due to the antagonistic effect of potassium chloride on the adsorption of barium ions, which decreased the efficiency of the latter as a coagulator. Although it is not so apparent, the same antagonistic effect may have influenced the second result. Thus, even though the percentage of the flocculation value of potassium chloride remained constant as the sol was diluted, higher concentrations of potassium chloride (64 millimoles per liter for the 25 per cent sol) were required to coagulate the diluted sols than was required for the 100 per cent sol (48 millimoles per liter). A second factor also may have played a r6le in the coagulation of arsenic trisulfide sols of different concentrations with mixtures of potassium chloride and barium chloride: chloride ions were preferentially adsorbed when potassium chloride was added to this system, resulting in an increase in charge and mobility. This effect, which increases with dilution of the sol, may have contributed to cause an increase in the amount of barium chloride required to complete the coagulation. SUMMARY
1. Flocculation studies have been made on manganese dioxide sols and arsenic trisulfide sols, each a t three different sol strengths, with the electrolyte mixtures potassium chloride-barium chloride and potassium chloride-chromium chloride. 2. No ion antagonism was observed with the manganese dioxide sols and either of the electrolyte mixtures or with the arsenic trisulfide sols and the potassium chloride-chromium chloride mixture. In these cases the presence of potassium chloride sensitized the sols toward the second electrolyte.
SPECIFIC HEATS OF SODIUM CHLORIDE SOLUTIONS
755
3. The marked ion antagonism which occurs when an arsenic trisulfide sol is coagulated by mixtures of potassium chloride and barium chloride was verificd. 4. An electrophoretic study was made of the systems, and the mobilities of the particles at the coagulating concentrations of the electrolyte mixtures were determined. 5 . It was found that as the concentration of potassium chloride, in a coagulating mixture with barium chloride or with chromium chloride, was increased, the critical mobilities likewise increased, 6. The results were discussed and explanations for the various behaviors were proposed. REFEREXCES FREUNDLICH A X D SCHOLZ: Kolloid-Beihefte 16, 267 (1922). H A Z E LJ: Phys. Chem 46, 736 (1911) KRCYTA X D VAK DER M'ILLIGEX:Proc Roy. h a d . Amsterdam 29, 484 (1926). J. Chem. Soc 67, 67 (1895). L I X D E R A NPICTOX: D WASXOW:Kolloid-Beiheftc 60, 367 (1939). WEISER: J. Phys. Cheni. 30, 1531 (1926) (7) WEISER:Inorganzc CoUozd Chemzstry, Yol. 111. John Wiley and Sons, Inc., K e w York (1938). (8) WEISERAND MILLIGAX:J. Am. Chem. Soc. 62,1921 (1940). (1) (2) (3) (4) (5) (6)
THE SPECIFIC HEATS OF SOME AQUEOUS SOLUTIOSS OF SODIUM ASD POTASSIUM CHLORIDES AT SEVERAL TEMPERATURES. I11
RESULTS FOR DILUTE SODIUM CHLORIDE SOLUTIONS AT 25°C. C. B. HESS Department of Chemzstry, The C'nacerszty of Rochester, Rochester, XezL. Yorli' Recezved April 18, 2940
I n the adiabatic twin calorimeter described earlier (4), the calorimeter covers were in direct contact with the jacket water and thus presumably were a t jacket temperature. Since the temperature in the two calorimeters is seldom, if ever, the same, a temperature head might exist between the covers and the solutions in the calorimeters which might lead t o different condensation effects in the two calorimeters. If the respective covers could be kept at the temperatures of the solutions in the calorimeters, such condensation effects would probably be eliminated. I n this Present address: E. I. du Pont de Nemours and Company, Inc., R. & H. Chemicals Department, Xiagara Falls, New York.
