422
FRED HAZEL
SILICIC ACID AS A PROTECTIVE COLLOID FOR MANGANESE DIOXIDE SOLS FRED HAZEL Department of Chemistry and Chemical Engineering, University of Pennsylvania, Philadelphia, Pennaylvania Received August 16. 1939
Kruyt (10) reproduces data showing that silicic acid has no protective action when measured by the criterion of the gold number (17). Some evidence of the protective action of silica has been reported, however. Thus, clay suspensions are stabilized by addition of silica sols (14). Soil colloids of high silica: sesquioxide ratios, e.g., Sharkey soil colloid, are more stable toward electrolytes than those of low ratios, e.g., Norfolk soil colloid (12). It has been shown, also, that silica sols (prepared by acidifying sodium silicate with tartaric acid) have a protective influence on colloidal silver (2) and colloidal gold (3). In the present study an investigation was made of the protective action of colloidal silicic acid for manganese dioxide sols. PREPARATION O F SOLS
The silica sol was prepared by adding three volumes of a 1.6 per cent sodium silicate solution slowly, with stirring, to one volume of 2 normal hydrochloric acid. After aging for 48 hr. the mixture was dialyzed for 10 days in collodion membranes. The dialyzate had a silica concentration of 14.7 g. per liter and a pH of 4.8. Colloidal manganese dioxide was prepared by a slight modification of a method recommended by Cuy (1). Three-fourths molar ammonium hydroxide was added dropwise a t the rate of one drop every 10 sec. to M/100 potassium permanganate a t 90°C. The sol was purified by dialyzing for 10 days with “Visking casing.” The purified sol had a manganese dioxide concentration of 0.66 g. per liter and a pH of 6.0. FLOCCULATION DATA
The protective action of colloidal silicic acid on manganese dioxide sols is illustrated by the data in table 1. Flocculation values, obtained by a method described previously (4), are given for the unprotected sol and for sols containing different amounts of silicic acid. The concentration of manganese dioxide, 0.013 g. per liter, was the same in all systems. Two series of experiments were conducted in which the concentration of silica was kept constant a t 2.9 g. per liter and the manganese dioxide concentration increased to 0.066 g. and 0.33 g. per liter, respectively. No
423
SILICIC ACID AS A PROTECTIVE COLLOID
coagulation was observed in either system a t the end of 24 hr. in the presence of 60 millimoles of potassium chloride or of barium chloride.
MnOt.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . MnOl-SiOl: 0.12 g. Si02 per liter.. . . . . . . . . . . . . . . . . . . . 0.59 g. Si02 per l i t e r . . . . . . . . . . . . . . . . . . . . . 2 . 9 g. Si02 per liter.. . . . . . . . . . . . . . . . . . . . .
5.6
0.044
0.002
60*
60* 60t
0.012
60 t 60t
60 t
* No coagulation a t the end of 24 hr. t Weak gels were formed in these systems over periods ranging from 48 hr.
to
9 days.
-3
3
i
\
H
1
\
-z
1 z
c
I
0
-2
0 MILLIMOLES
2
4
POTASSIUM CHLORIDE PER LITER
0
003
MlLLlMOCES B A R N CHLORICE PER
006 LITER
FIQ 2
FIG.1 MOBILITY STUDIES
Mobility measurements were made by an ultramicroscopic method. A velocity-depth curve was constructed and the value of the true mobility obtained from the area under the curve (16). The following data are on manganese dioxide sols containing 0.013 g. of the dispersed substance per liter and on mixtures of manganese dioxide
424
FRED HAZEL
with colloidal silicic acid. The concentrations of manganese dioxide and silica in the mixtures were 0.013 g. and 0.59 g. per liter, respectively.
-3
-2
3’ \ -I
z
6
5
4
MILLIMOLES T W U M NITRATE PCR LITER
3
2
1
PH
FIQ.3
FIQ.4
-2
3
\ 5
0
$ I->
3
s
+2
t4
0
0.02
004
0.08
MLLIMOLES THORIUM NITRATE PER LITER
FIQ.5
The effects of potassium chloride, barium chloride, and thorium nitrate on particle mobility are shown in figures 1, 2, and 3. The mobility curves for the protected and unprotected systems demonstrate that the buffering effect of silicic acid is much greater with thorium nitrate than with the other two electrolytes.
SILICIC ACID AS A PROTECTIVE COLLOID
,
425
Mobility data for the systems comprising the mixture and for the mixture itself are given in figure 4 in relation to hydrogen-ion activities. The acidities were adjusted with nitric acid. The mixture and the manganese dioxide coagulated a t pH 3. The curve for colloidal silicic acid was obtained by a re-plot of the data from a previous study (5). Neither silicic acid (5, 9), manganese dioxide (6), nor the mixture' was recharged a t the acidities investigated. The mobility behavior of colloidal silicic acid with thorium nitrate is recorded in figure 5. The particles coagulated when their mobility was reduced to about 0.7 p per second per volt per centimeter. Higher concentrations of thorium ions recharged the silica and yielded a stable sol.* DISCUSSION
The critical potentials of potassium chloride, barium chloride, and thorium nitrate for the manganese dioxide sol used in preparing the mixtures were, in terms of mobilities, 2.4, 0.4, and 2.0 p per second per volt per centimeter. The first and last of the electrolytes lowered the mobility of the particles in the mixture below their critical potentials for manganese dioxide without the precipitation of this substance occurring. The critical mobilities of potassium chloride and barium chloride for silica sols of the same order of concentration as employed here have been determined to be less than 0.4 p per second per volt per centimeter ( 5 ) . The corresponding value for thorium nitrate is 0.7 per second per volt per centimeter. Since the critical potentials of the electrolytes are lower for silica and for mixtures of silica and manganese dioxide than for pure manganese dioxide, it seems plausible to assume that the stability of the particles in the mixtures is due to a silica covering established by adsorption. For purposes of studying the state (5,8) of the particles in the mixtures, a criterion was sought which would enable a rigid distinction to be made between the electrokinetic behaviors of colloidal silicic acid and of colloidal manganese dioxide comprising the mixtures. Sharp contrasts of kind were not found, however. Some of the parallel behaviors are summarized here. Thus, both systems were recharged by thorium ions but iMattson (13) haa shown that soil colloids having a high si1ica:sesquioxide ratio are not recharged in acid solution. 2 It has been pointed out t h a t silicic acid sols are difficult to recharge ( 5 ) . Many substances containing silica have been recharged, however. Among these is glass, for which data are available with thorium ions. White, Monaghan, and Urban (15) determined the isoelectric concentration to be of the order of 10-lM. In the present investigation an isoelectric concentration of 1.2 X 1 V M was found for a silicic acid sol containing 0.59 g. of silica per liter, while for a so1 containing 0.12 g. per liter the concentration of thorium ions corresponding t o zero mobility was slightly less than 4 x 10-'M.
426
FRED HAZEL
neither gave an irregular series.8 Likewise, neither of these acidic oxides was recharged by hydrogen ions at a concentration corresponding to pH 1.5.4
The fact that the particles in the mixtures were homogeneous with respect to migration velocity supports the conclusion that the silica and manganese dioxide were mutually adsorbed. It was found that addition of increments of colloidal silicic acid to the manganese dioxide sol progressively lowered the mobility of the particles until, at the composition corresponding to a silica content of 0.59 g. per liter, the particles had the same mobility as a pure silica sol of the same concentration. This result is comparable to that obtained on adding gelatin to a manganese dioxide sol (7), a case in which the idea of mutual adsorption is accepted. TABLE 2 Sign o j charge on the particles
millimoled p a l i t a
0.0120 0.0128 0.0132 0.0136
0.0140
Negative. Negative. and positive. Negative. and positive. Positive Positive
The velocity values were of varied magnitude in these cases.
The rule of homogeneous particle mobility was violated in certain of the systems containing electrolytes. Thus, in mixtures with potassium chloride in concentrations greater than 2 millimoles per liter, more than one mean mobility value was observed. A more pronounced anomaly was encountered with thorium nitrate over a very narrow range of concentrations in mixtures near the isoelectric point.6 Migration of particles to both the anode and the cathode was observed. The results are shown in table 2. SUWRY
1. Silica sols increase the stability of colloidal manganese dioxide toward potassium chloride, barium chloride, and thorium nitrate. 2. The protective action has been explained on the basis of mutual adsorption. a Concentrations to 60 millimoles per liter were used in an effort to coagulate the positive systems. 4 Losenbeck (11) has reported the recharging of silica at hydrogen-ion concentrations between 10-3 and lo-*. See, however, references 5 and 9. 6 Precipitation occurred in these mixtures within 24 hr.
FREEZING POINTS IN MIXTURES OF STRONG ELECTROLYTES
427
3. No protective action was observed with nitric acid as the added electrolyte. 4. Colloidal silicic acid is flocculated a t low concentrations of thorium ions. The sol is recharged at concentrations of the order of to M. 5. Neither silica sols nor manganese dioxide sols yield an irregular series with thorium ions. REFERENCES
(1) CUY: J. Phys. Chem. %, 415 (1921). (2) DUMANSKII AND SEERSHNEY: J. Russ. Phys. Chem. SOC.80, 1593 (1928). AND PUCAKOVSKII: J. Russ. Phys. Chem. SOC.62, 469 (1930). (3) DUMANSKII J. Am. Chem. SOC.63,49 (1931). (4) HAZELAND SORUM: (5) HAZEL:J. Phys. Chem. 43,409 (1938). (6) HAZEL AND MCQUEEN:J. Phys. Chem. 37, 553 (1933). (7) HAZELAND KING:J. Phys. Chem. 39, 515 (1935). (8) KOMAGATA: Electroohem. Lab., Tokyo, Bull. 348 (1933). (9) KRESTINSKAYA AND NATANSON: Acta Physicochim. U.R.S.S. 7, 915 (1937). (10) KRUYT:Colloids. John Wiley and Sons, Inc., New York (1929). (11) LOSENBECK: Kolloid-Beihefte 16,27 (1922). Soil Sci. 28, 373 (1929). (12) MATTSON: (13) MATTSON:J. Am. SOC.Agron. 18, 458 (1926). (14) SMITH:J. Am. Chem. SOC.43, 460 (1920). AND URBAN:J. Phys. Chem. 39, 611 (1935). (15) WHITE,MONAGHAN, (16) WILLEYAND HAZEL:J. Phys. Chem. 41, 699 (1937). Z. anal. Chem. 40,697 (1901). (17) ZSIQMONDY:
FREEZING POINTS I N MIXTURES OF STRONG ELECTROLYTES' MERLE RANDALL
AND
BRUCE LONGTIN*
Department of Chemistry, University of California, Berkeley, California Received August 16, 1059
Randall and Vietti (17; cf. also 9, 10, 16) have proposed a plot of (l/z+z-) log -y* against the square root of the ionic strength3 as a means 1 Clerical assistance of the Works Progress Administration 0. P. No. 665-08-3-144 is gratefully acknowledged. * Shell Fellow in Chemistry. * The notation in general is that of Lewis and Randall (6). The mean activity coefficient, y*, is defined as 1,
In yi =
Y+
where y+ is the activity coefficient and
In y+ Y+
+
Y-
In y-
(1)
is the number of the positive ions per
