ADSORPTION OF IONS AND THE PHYSICAL CHARACTER OF PRECIPITATES. I1
FERRIC OXIDEAND BENTONITE PRECIPITATES~ G. E. CUNNINGHAM, H. E. GABLER, AND W. 5.PEACHIN Department of Chemistry, Clarkson College of Technology, Potsdam, New York
Received June 1 1 , 1936
Among the various factors which influents the physical nature of a precipitate, the specific effect of adsorbed ions is often one of the most important. The importance of choosing the proper medium for the formation of precipitates in analytical work is too familiar to require discussion here. The experiments here recorded were suggested by previous observations made by Cunningham in collaboration with Weiser (5) on sulfur precipitates formed by salting Odh’s sulfur sol. Sting1 and Morawski (4) and O d h (3) had recorded various salts as giving precipitates ranging macroscopically from gelatinous through flocculent, slimy, and he-grained to plastic in character, depending upon the precipitating ion used. The work of Weiser and Cunningham showed that, in the last analysis, these precipitates were either gelatinous or non-gelatinous, depending upon the degree of hydration of the ions adsorbed. In the case of the formation of plastic sulfur, the neutralieing ions did not carry with them sufficient protective water to prevent complete coalescence of the particles. I n the case of a gelatinous precipitate, the neutralizing ions not only carried sufficient water to form a protective coating about the micelles and prevent contact of sulfur to sulfur, but the protective water at the same time usually acted as an adhesive, loosely binding the precipitated particles together. Ions of intermediate hydration gave intermediate types of precipitates. Weiser and Cunningham made motion pictures2 through the ultramicroscope which showed the outflow of dense, adsorbed water into ordinary supernatant water as a slightly adsorbed, highly hydrated ion was displaced by a highly adsorbed, slightly hydrated ion -added to the supernatant liquid. At the same time the sulfur micelles usually coalesced, Presented at the Thirteenth Colloid Symposium, held at St. Louis, Missouri, June 11-13, 1936. a Presented at the Sixth Colloid Symposium, held at Toronto, Canada, June, 1928.
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forming a particle of plastic sulfur very much smaller than the original clump. In an extreme case, the shrinkage was to less than 0.5 per cent of the original volume. Some views taken from the motion pictures are reproduced from the original article (5) in figure 1. In view of the results obtained with sulfur, it seemed reasonable t o expect that precipitates of substances not exhibiting the coalescing tendency of amorphous sulfur would be more or less gelatinous, depending upon the degree of hydration of adsorbed ions. Since the bulk of the sulfur clumps varied so greatly with the amount of adsorbed water, it seemed feasible to compare various precipitates on a basis of their bulk.
FIG.1. Ultramicroscopic views of the shrinkage of sulfur clumps on displacing a slightly adsorbed, highly hydrated ion hy a highly adsorbed, slightly hydrated ion. A, Li+ displaced by K+; B, Li+ displaced by Cs+; C, Li+ displaced by Ba++.
The effects of varying both positive and negative adsorbed ions on both a positive colloid, ferric oxide, and a negative colloid, bentonite, were studied. EXPERIMENTAL PROCEDURE
For the purpose of forming the precipitates under uniform conditions, the Weiser mixing device (6) was used in all cases. This device consists of a smaller test tube sealed in the bottom of a larger one. One of the two liquids to be mixed is placed in each compartment, the larger tube is then stoppered, and the apparatus is shaken quickly and vigorously.
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After forming the precipitates in the above manner, they, together with their supernatant liquids, were poured into 5 x 4 in. test tubes, centrifuged for five minutes, and allowed to stand. After at least two days, the depths of the precipitates were measured with a millimeter scale. The actual Ferric Oxide Precrpitotes
- Method
I
=Method 2 40 I
cr B i I‘
I I*
,
FIG.2. Comparison of the bulks of ferric oxide precipitates obtained by varying the precipitating ion
I
FIG.3. Comparison of the bulks of ferric oxide precipitates obtained by varying the stabilizing ion
volumes were not recorded, but the capacity of the test tubes of the size used is about 1 cc. per 6-mm. depth. The data are recorded in the charts, figures 2 to 5. The individual procedures are described more fully in the following paragraphs.
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Ferric oxide precipitates Method 1 . Each precipitate was formed separately by placing 1 cc. of N ferric chloride in the inner compartment of the mixing device and 1 cc. of N s o d i m hydroxide plus 15 cc. of a normal solution of the salt supply-
II 'Cd"
Y'
FIG.4. Comparison of the
bulks of bentonite precipitates obtained by varying the precipitating ion
:i /
f
20
c/- Sodiw, AU;
FIG.5. Comparison of the bulks of
bentonite precipitates obtained by varying the stabilizing ion
ing the ion t o be studied in the outer compartment. The calculated mass of the ferric oxide was in each case 0.036 g. The results are indicated in figures 2 and 3. Method 2. A ferric oxide sol was prepared by adding 100 cc. of N sodium hydroxide to 900 cc. of N ferric chloride, while stirring. This pro-
+
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cedure resulted in a ferric oxide sol in 0.5 N ferric chloride. The sol was dialyzed for ten days, until the dialyzate, on evaporation to one-tenth its volume, showed no test for chloride. To form the precipitates, 2-cc. portions of this sol were placed in the inner compartment of the mixer and 15cc. portions of N salt solutions were placed in the outer compartment. By evaporation of 25cc. portions of the sol, it was found that the mass of solid contained in 2 cc. of sol was * 0.045 g. The results are indicated in figures 2 and 3. All the values plotted in figures 2 and 3, for ferric oxide precipitates, were measured after the precipitates had been standing six months. Bentonite precipitates
A bentonitg sol was prepared by adding 4 parts of powdered bentonite to 100 parts of boiling water, with stirring, and allowing to stand at least two days before using. To form the precipitates, 10 cc. of sol and 10 cc. of 2 N salt were mixed in the mixing device. Each precipitate contained 0.4 g. of bentonite. The results are indicated in figures 4 and 5. All bentonite precipitates stood at least two days before being measured. DISCUSSION OF RESULTS
There is no good agreement among various workers or methods as to the actual degree of hydration of ions. In fact, there is some disagreement as to their order of hydration. Recent investigators (1, 2 ) have claimed that the degree of hydration is a linear function of the electrostatic charge on the ion. I n general, the degree of hydration decreases as the atomic weight increases, for the ions of elements in a given family in the periodic classification. As far as possible, the ions in the charts (figures 2 to 5) have been grouped according to the periodic classification. Runs plotted on vertical depthlines tied together by horizontal tie-lines were made simultaneously. Results not tied together by horizontal lines in the charts are not to be compared with one another for the reason that, since they were made at different times, the sols were in different stages of ageing. With ferric oxide, the bulks of the precipitates obtained with halogencontaining ions by the first method (metathetical formation of ferric oxide in the presknce of various salts) were in the order C1- > Br- > I-and By the second method (salting.out the dialyzed ClO, > BrO; > IO;. sol), the order was C1- > Br-, I-, and ClO, > BrO; > IO,. I n each case the order is as predicted. The bulks of the ferric oxide precipitates obtained by the first method with the divalent ions studied were in the order CrOY-, CrzO;- > W0;> SOX-. By the second method, the order was CrOY-, Cr20T- >SO;WO;-, the order of SO;and WOY- being reversed. No information THE JOWNAL OF PEYSICAL CHEMISTRY, VOL.
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was available to the authors as to the findings of other workers with regard to the hydration of these ions. The bulks of the ferric oxide precipitates obtained with various cations by the first method were in the order Ba++ > Sr++, Ca++ and Mg++ > Cd++ > Be++. By the second method, the order was: K+, Na+ > Li+ > N H t ; Ba++ > Sr++, Ca++; Cd++ > Mg++ > Be++. With the exception of the case Mg++ > Cd++ by the first method, the positions of the ions in these series are all the reverse of those expected on a basis of their positions in the periodic classification. However, since ferric oxide is a positive sol, these are stabilizing ions, and the heavier ions are much more highly adsorbed than the lighter ones. It is believed that the heavier stabilizing ions are so highly adsorbed as to bring about the adsorption of a sufficiently larger quantity of negative precipitating ions to account for the increased bulk of the precipitate. No attempt has been made to prove this hypothesis but, in the absence of evidence to the contrary, it seems reasonable. With bentonite, a negative sol, the bulks of the precipitates obtained with various cations were in the order: Li+ > Na+ > K+; Mg++ > Ca++ > Zn++ > Be++ > Cd++; Ca++ > Sr++ > Ba++; and ?*In++> Fe++ > Co++. With the exception of Be++, these ions all fall in the order predicted. The bentonite precipitates obtained in the presence of various stabilizing ions ran in the order NO; > NO, > C1- and C1- > Br- > I- > SCN-. As far as predictable from the periodic classification, the positions of these ions are in agreement with the theory. I n practically all the above cases, the order of the ions is in agreement with the familiar Hofmeister series for their effect on the swelling of gelatin. That is, those ions which, in these experiments, gave precipitates of least bulk are the ones which make gelatin the least gelatinous, or most fluid, and vice versa. The indication is that in the case of the thinner gelatin gel there is less bound water and more lubricating water than in the thicker gel. The results of these experiments could not be duplicated with arsenious sulfide sol, for the reason that the arsenious sulfide itself is not gelatinous enough to give precipitates of sufficient bulk to show the differences in bulk. This does not mean that the adsorbed ions have no effect on the character of the micelles, however. It is hoped that the results of these experiments may be found of practical use in the control of the nature of precipitates and the consistency of pastes. SUMMARY
1. A study has been made of the effect of adsorbed ions on the bulks of ferric oxide and bentonite precipitates.
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2. The bulk of the precipitate is symbatic with the degree of hydration of the adsorbed ions. REFERENCES (1) BRINTZINQER AND RATANARAT: Z. anorg. allgem. Chem. 222, 113 (1935). (2) BRINTZINQER, RATANARAT, AND OSSWALD: Z. anorg. allgem. Chem. 223, 101 (1935). (3) O D ~ NDer : kolloide Schwefel, pp. 134,157. Akademische Buohhandlung, Upsala (1912). (4) STINGLAND MORAWSKI: J. prakt. Chem. [2] 20, 76 (1879). (5) WEISERAND CUNNINGHAM: Colloid Symposium Monograph 6, 319-41 (1928); J. Phys. Chem. 83, 301-16 (1929). (6) WEISERAND MIDDLETON: J. Phys.Chem. 24,48 (1920).
