INFLUENCE OF CONCENTRATION AND AGE ON SOME COLLOIDAL PROPERTIES OF FERRIC CHLORIDE SOLUTIONS HERBERT L. ELLISON AND F R E D HAZEL Laboratory of General Chemistry, University of Wisconsin, Madison, Wisconsin
Received November 1, 1Q34 INTRODUCTION
The effect of dialysis and aging on particle size and constitution of colloidal ferric oxide sols prepared by a condensation method has been investigated by Nichols, Kraemer, and Bailey (4), who found that during aging the particle size distribution shifts slightly toward larger sizes, the formation of secondary aggregates being more pronounced in concentrated sols. The age of the stock solution of ferric chloride was observed to affect the particle size of the resulting ferric oxide. The particles gradually decreased in size with increasing age of the stock solution, and it was suggested tentatively that this was due to the formation of more and more nuclei therein. Condensation methods for the preparation of colloidal suspensions depend upon creation of a supersaturation of the condensable phase, together with some starting points or nuclei to serve as centers for growth. The number of particles produced and the degree of dispersity depends upon the number of these starting points or centers. Further, the distribution of size of the condensed particles depends upon the ratte of migration of material to the centers, taking into account the amount of dissolved material and the number of condensing centers. Nuclei are often produced by the condensation process itself, but in some cases artificial centers can be introduced. The rate at which the nuclei appear affects the distribution of the size of the particles, since those introduced or formed first are supplied with more material for building particles than are those formed later, and will consequently become larger. If it were possible to introduce all of the centers at the beginning, before the condensation process sets in, and to avoid spontaneous production of centers, one might expect to get much more uniformly sized particles. In fact, such principles have been used in Zsigmondy’s nuclear method of preparing gold sols, with the result that fairly uniformly sized particles were obtained. The object of the present investigation was to determine the effect of age and concentration of ferric chloride solutions on the pH values, the mobility, and the number of colloidal particles. The selection of ferric 829
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HERBERT L. ELLISON AND FRED HAZEL
chloride systems for this work seemed desirable for two reasons: first, their property of spontaneous production of ultramicroscopically visible particles on standing at room temperature; and second, their frequent use in preparation of colloidal ferric oxide by high temperature hydrolysis.
AGE
I
TABLE 1 Measurements on ferric chloride solutions NO. OF PARTICLE8 P E R CC. 10-
x
1
MOBILITY p/8EC./V./CM.
1
PH
M/1000 ferric chloride 0 hours 20 hours 2 days 7 days 12 days 21 days
0.4 0.6 0.7 38. 86. 107.
3.6 3.1 3 .O 2.9 2.7 2.5
3.02 2.70 2.66 2.62 2.49 2.56
0 hours 20 hours 2 days 7 days 14 days 21 days 49 days
4.7 168. 655. 849 1090. 1155. 1336.
4.2 3.7 3.6 3.6 3.0 2.8
2.37 2.10 2.08 2.00 1.99 1.98 1.96
M/10 ferric chloride* 0 hours 20 hours 2 days 7 days 14 dayst 21 days
6.0 56. 59. 69. 93. 405.
3.4 2.8
1.so 1.61 1.65 1.64 1.63 1.58
* Mobility measurements for this concentration could not be accurately made because of a drift in the liquid due t o electrolysis of the ferric chloride solution. t Precipitation occurred. EXPERIMENTAL
Ferric chloride solutions of three concentrations, M/1000, M/100, and M/10, were prepared, and immediately after preparation measurements of the number of particles (19, electrophoretic velocity (2), and pH (3) were made by methods previously described. Similar measurements were made at definite time intervals thereafter up to twenty-one days, the solutions being allowed to stand undisturbed at room temperature. The
COLLOIDAL PROPERTIES OF FERRIC CHLORIDE SOLUTIONS
83 1
results of these measurements for the above concentrations are given in table 1. Table 2 shows the results of measurements on ferric chloride solutions and the resulting high-temperature hydrolysis products. Solutions of M/1000 and M/100 ferric chloride were prepared as before and similar measurements made. Then definite quantities of these solutions (50 cc. of M/100 and 100 cc. of M/1000), were dropped slowly into 950 cc. and 900 cc. respectively of boiling water, following the usual method of preparing ferric oxide sols. Number of particles, migration velocity, and pH were then determined for these hydrolysis products.
AGE
I
TABLE 2 Measurements o n .ferric chloride solutions NO. OF PARTICLES P E R CC.
x
Original
0 hours 20 hours 2 days 7 days 14 days 21 days
0.04 0.06 0.07 2.2 3.4 10.1
I
Product
0.4 0.9 1.o 18.8 37.7 117.0
MOBILITY
PH
r/sEC./V./CM.
Original
1
3 .O 2.9 2.8 3 .O 2.8 2.5
I
Product
Origins1
1.9 2.3 1.8 2.2 2.4
3.02 2.70 2.68 2.63 2.58 2.56
3.40 3.46 3.47 3.41 3.34 3.30
3.2 3 .O 2.9 2.8 2.7 2.7
2.36 2.12 2.09 2.00 1.99 1.98
2.84 2.77 2.71 2.60 2.56 2.50
Product
100 solution
0 hours 20 hours 2 days 7 days 14 days 21 days
0.47 3.5 67.3 88.1 107.3 118.4
11.8 85. 1151. 1640. 1992. 2302.
4.1 3.6 3.1 3.1 3.0 2.9 DISCUSSION
As was previously mentioned the rate of growth of colloidal particles is determined by the number of nuclei present and the degree of supersaturation of the system. In the system under consideration there are no nuclei present a t the starting point, hence they must be produced by growth from the ferric chloride molecules in solution. The rate of nucleus formation is itself dependent upon the degree of supersaturation, so that ferric chloride solutions of different concentrations should exhibit initial growth stages of different lengths. When the particles have reached the minimum size necessary for nuclear action, i.e., when the initial growth
832
HERBERT L. ELLISON AND FRED HAZEL
stage is over, the particle growth is governed by the concentration of dissolved matter. If this concentration is sufficiently great a sudden appearance of particles will be evident. If a solution contains C gram-equivalents of ferric chloride per liter and the degree of hydrolysis is denoted by x , the following expression may be written' C F ~ ( O H )= ~
CH+
XC
= 3xC (assuming complete dissociation)
cF&t =
(Y
(1
- s)c
where CY denotes the degree of dissociation of ferric chloride. Therefore,
The amount of ferric hydroxide formed is then directly proportional to the concentration of hydrogen ions. If the amount of ferric hydroxide in colloidal form is assumed to be directly proportional to the amount of ferric hydroxide in true solution, a linear relation should exist between the hydrogen-ion concentration and the number of particles observed under the ultramicroscope.2 In figure 1 these quantities are plotted for M/10, M/100, and M/1000 solutions. Apparently the linear relationship is valid for certain stages of the process. It will be observed, however, that the initial points are in each case off the curve. Speculation regarding the reason for this behavior must be reserved until more experimental data can be obtained. A consideration of the relationships between the age of the solution and the number of particles, graphically represented in figure 2, should lead to a more intimate understanding of the mechanism of particle formation. The abrupt changes in the courses of the curves indicate that the process may be considered as a summation of steps, rather than one step which 1
Making the simplifying assumption that no basic chlorides are formed.
* The observed number of particles may not exactly represent the amount of colloidal material, since obviously a number of amicrons may be present. The reaction is of course very complicated, notably because basic chlorides of varying composition are formed. The basicity of the latter possibly changes with aging of the solution, and this is accompanied by changes in the hydrogen-ion concentration. The rate of the reaction is opposed by increased hydrogen-ion concentration, and is favored by adsorption (which removes hydrogen ions from solution) and factors which tend to increase the adsorbing surface, such as partic!e formation. It is apparent from the data, however, t h a t a consideration of t h e simple hydrolytic equilibrium given above cuts across these secondary phenomena and established a relation between the rate of particle formation and the hydrogen-ion concentration.
COLLOIDAL PROPERTIES OF FERRIC CHLORIDE SOLUTIONS
833
might be represented by a continuous function. It is well known that precipitation proceeds slowly until the nuclei have reached a size sufficient to function as condensation centers. I n this connection Nordenson ( 5 ) , during conductivity studies on the formation of gold sols, observed a similar stepwise character in the course of particle formation. He concluded that after the original precipitation the gold molecules slowly condense to colloidal gold particles until a great number of particles have reached the minimum size of nuclear action. After the nuclear limit is reached the reduction process proceeds rapidly, depositing the rest of the gold on the gold nuclei formed during the growth period. Nordenson demonstrated that the nuclear growth period could be arrested a t will by the addition of colloidal gold particles, which then functioned as condensation nuclei causing a rapid particle growth. If it be assumed that a similar series of steps holds for the growth of colloidal particles from ferric chloride solu-
a
tions, then the different stages for M/100 solution (figure 2) may be identified thus: (1) A to B-a slow nuclear growth stage; (2) B to C-a rapid condensation stage with large increase in the number of visible particles; (3) C to D-a slow growth stage similar to (1). Upon longer standing it might be expected that a fourth stage, similar to (2), would occur. Although no such stage was observed for the relatively short periods used in this investigation, a year-old sample of M/100 ferric chloride, originally clear, was found to have undergone marked precipitation, while still retaining a large number of colloidal particles in the supernatant liquid. It is a well-known fact that norm'al growth processes follow an exponential law. This behavior affords a means of differentiating more rigidly between successive stages in the curves of figure 2. If y denotes the number of particles and t denotes the time (age), the law of exponential growth is given by y = yoeCt
834
HERBERT L. ELLISON AND FRED HAZEL
where yorepresents the number of particles a t the beginning of the growth stage and c is a constant. Taking logarithms,
Y = ct In Yo
2.30310g y = ct
+ 2.30310g yo
so that plotting log y against t should yield a straight line. This operation is carried out in figure 3. It will be observed that the linearity is quite good 12.5 I
P" ffif
m
DAYS
FIG.4
FIG.3
FIG.5 in all cases. The stepwise character of the process is clearly shown by the abrupt discontinuities in the curves. The fact that the number of particles in the M/1000 solution is smaller than the number in either of the other two solutions at any stage appears reasonable, as does the fact that the stepwise character of the curve (figure 2) is less pronounced. However, the position of the curve for the M/10 solution is anomalous. The graphical representation of the relation between age and pH of the
COLLOIDAL PROPERTIES OF FERRIC CHLORIDE SOLUTIONB
835
solutions is shown in figure 4. The relative positions of these curves for the different concentrations of solutions are quite normal, and the similarity of the curves is good. They are all typically logarithmic curves, the initial drop corresponding to a more rapid hydrolysis a t the beginning of the process. An examination of the tables reveals that with increasing age of the ferric chloride solutions a gradual decrease occurs in the migration velocity of the particles. At the start, when few particles are present, adsorption of positive ions (Fe+++and H+) is great, resulting in a high particle velocity. As more particles form, ferric ions are constantly used up, making less available for adsorption. At the same time the concentration of the solution gradually becomes less. With more particles and less available ions the charge on each particle is lowered, with a proportional drop in the migration velocity. The results in table 2 show that the number of particlesin the hydrolysis product of a ferric chloride solution is governed by the number of particles present in the original stock solution. This is shown graphically by the linear curve in figure 5. SUMMARY
1. An investigation of the effect of age and concentration on some of the colloidal properties of ferric chloride solutions has been made. Ultramicroscopic methods were used to follow changes in particle number and particle velocity, while changes in pH were measured with a glass electrode. 2. Spontaneous formation of nuclei was found to take place in all concentrations of ferric chloride studied. The number of these nuclei present a t the start was affected by the original concentration of the solution. 3. As the solutions of ferric chloride aged the number of colloidal particles increased in a stepwise manner. 4. The hydrogen-ion concentration of the solutions increased as hydrolysis proceeded. This increase when plotted against the increase in particle number gave an approximately linear relation. 5. Aging of the ferric chloride solutions resulted in a lowering of the electrophoretic velocity. 6. The number of particles in the ferric chloride solutions influenced the number of particles in the resulting high-temperature hydrolysis product. REFERENCES (1) (2) (3) (4) (5)
AYRES:Dissertation, Wisconsin, 1930. HAZELAND AYRES:J. Phys. Chem. 36,2930 (1931). AND SORUM: J. Am. Chem. SOC.63,49 (1931). HAZEL NICHOLS, KRAEMER, AND BAILEY: J. Phys. Chem. 36, 326, 505 (1932). NORDENSON: Dissertation, Upsala, 1914.
