STUDIES I N COPRECIPITATION. VI
INTERNAL STRUCTURAL CHANGESON AGINGOF FRESHLY PREPARED PRECIPITATES I. M. KOLTHOFF
AND
E. B. SANDELL
School of Chemastry, University of Minnesota, Minneapolis, Minnesota Received June 6, 1032
Studies made in recent years show that the lattice of a real crystal is far from ideal. Crystals seem to be built of submicroscopic or amicroscopic blocks, the so-called Smekal (1) blocks, which fit together like bricks in a wall, giving rise to free spaces in the packing (Lockerstellen). In addition, ordinary crystals very often even contain microscopically visible cavities. A real crystal therefore has a very discontinuous structure and possesses a large internal surface. D. Balarew (2) has strongly advocated the theory that contaminating substances are adsorbed at the internal surface of a crystal, and goes so far as to attribute all coprecipitations to this cause. I n previous studies dealing with the corprecipitation of water and of foreign ions with calcium oxalate, it was found that contaminants appeared to diffuse out of the crystalline precipitates if the latter were allowed to age under the mother liquor before filtration. This phenomenon cannot always be attributed to a recrystallization, for the particle size is often too large for this to occur to any extent. Therefore, under these conditions, it may be assumed that the contaminant actually diffuses out through microscopically invisible cracks and capillaries in the crystals. I n other words, the fresh crystals are very imperfect immediately after precipitation, but on standing there is a tendency for the cracks and capillaries to become filled with lattice material, this process resulting in the pushing out of foreign substances. If the perfection of the crystals would take place in such a way that the capillaries were entirely filled with lattice ions, it might be expected that all the foreign material would be extruded. From the experimental evidence presented in this paper it would appear that the perfection does not take place in such an idealized manner; in most cases dams are formed in the capillaries, and these prevent further effusion of adsorbed material from the inside to the outside of the crystals. Such crystals may have a large “isolated interior surface,” which no longer is in open communication with the outside. Moreover, a large part of the 723
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I. M. KOLTHOFF AND E. B. SANDELL
internal surface becomes isolated during the growth of the crystalline precipitate. In the first part of this paper experiments will be described in which potassium iodate was added to a freshly formed suspension of calcium oxalate after the precipitation had taken place. After various times of shaking, the iodate in the supernatant liquid was removed by addition of iodide and dilute acid (insufficient to dissolve the precipitate but strong enough to allow a quantitative reduction of the iodate) and then thiosulfate; after the last trace of iodine had reacted, strong hydrochloric acid was added to dissolve the calcium oxalate, and the iodine liberated from the iodate in the precipitate quantitatively determined with dilute thiosulfate solution. In addition, experiments were made in which the effusion of iodate coprecipitated with calcium oxalate was investigated after various treatments. The experimental procedure in the determination of the iodate content of the precipitate was similar to that described above. I. EXPERIMENTS WITH CALCIUM OXALATE
A . Decrease of amount of coprecipitated iodate in calcium oxalate o n standing Twenty-four cubic centimeters of 0.25 N calcium chloride mixed with 10 cc. of 0.050 M potassium iodate was treated at room temperature with 20 cc. of 0.25 N ammonium oxalate. The precipitate was allowed to stand in the liquid for the period of time indicated in table 1; 40 cc. of 0.lON hydrochloric acid and potassium iodide was then added to the mixture. The iodine thus liberated was titrated first with 0.1 N thiosulfate and then carefully with 0.005 N until all the iodine was just removed (starch as indicator). Concentrated hydrochloric acid was next added to dissolve all the precipitate and to liberate the iodate held internally. The amount of thiosulfate required to titrate the iodine liberated by the internal iodate is given in table 1. The precipitate obtained under the conditions just described recrystallizes and grows to larger size on standing, as microscopic observations show,'so that the decrease in contamination is not due primarily to effusion of impurity from the crystals. These results are recorded for the purpose of comparison with the next series of experiments, which were made under conditions such that there could have been but little recrystallization. The precipitates described in table 2 were obtained by Hahn's procedure at room temperature in the presence of 10 cc. of 0.050 M potassium iodate. The time of precipitation was 5 minutes. Twenty cubic centimeters of 0.25 N ammonium oxalate and 21 cc. of 0.25 N calcium chloride were used, the latter being kept 1 cc. in excess during the addition. The volume a t the end of the precipitation was 100 cc. It is evident that more than 50 per cent of the iodate has escaped from
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STUDIES IN COPRECIPITATION. VI
Hahn’s precipitate after twenty-four hours standing. It is improbable that this purification can be attributed to recrystallization, for the crystal TABLE 1 Dependence of amount of iodate in calcium oxalate precipitated cold o n time of standing ajter precipitation TIME OF STANDINQ
IODATE I N PRECIPITATE (AS cc. OF 0.005 N THIOSULFATE)
iz 1 I
TZ iz I i$ 1
2 4
IODATE I N PRECIPITATE (AS cc. OF 0.005 N THIOSULFATE)
hours
hours
0 0
TIME OF STANDINQ
14.2 13.0 11.8 11.9 11.6 11.5 10.5 10.95
4 19 19 70 73 120 48*
10.3 8.4 8.65 8.7 8.5 7.8 2.7
* Liquid diluted to 225 cc. after precipitation. The high dilution decreases iodate concentration in solution and promotes recrystallization. TABLE 2 Decrease of iodate content in precipitates (prepared according to F . L,. H a h n ) on standing NO.
1 2
3
4
-
PROCEDURE
cc.
OF
0.0050
N THIOSULFATE REQUIRED
Precipitate dissolved immediately 42.9 after precipitation Precipitate allowed to stand in solution from which precipitated, after 40 cc. of 0.10 N hydrochloric acid had been added and iodine removed, for 18 hours. Precipitate then dissolved and titrated with thiosulfate after removing free iodine. When titrated required. . . . . . . . . . . . . . . . . . . , . . , . , . , 24.6 Precipitate washed to remove io- Supernatant liquid contained iodate date and allowed to stand in 100 equivalent to 19 cc. of thiosulfate cc. of solution containing 4 cc. of and the precipitate 23.8 cc. 0.25 N calcium chloride for 25 hours As in 3; stood 48 hours Supernatant liquid then required 21.1 cc. of thiosulfate and the precipitate required 18.5 cc.
size of the precipitate is larger than that of one formed by the ordinary method in the cold and which has stood for a week in solution. It should
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I. M. KOLTHOFF AND E. B. SANDELL
also be remembered that Hahn’s precipitate formed a t room temperature contains very little higher hydrate, so that the purification cannot be ascribed to the transformation of the higher hydrates into the monohydrate. Precipitates formed in hot solution generally show only a slight loss of iodate or other contaminant on standing a t room temperature. The data will be omitted here.
B. Digusion of iodate into and internal adsorption by pre-formed calcium oxalate The results to be described furnish good evidence for the existence of internal surface in open communication with the exterior of the crystals of calcium oxalate. For the purpose of comparison, the results obtained with the precipitate formed in cold solution are presented; these results are neither distinctive nor conclusive, but lend greater weight to the conclusions drawn from the behavior of Hahn’s precipitate (table 5). The precipitates were obtained by adding 20 cc. of ammonium oxalate to 24 cc. of calcium chloride, both 0.25 N , a t room temperature. The time of addition was 1 minute. After the specified time of standing, 10 cc. of 0.050 M potassium iodate was added and the mixture shaken as indicated in table 3. To determine the amount of iodate which had gone into the interior of the crystals it was only necessary to add dilute hydrochloric acid (40 cc. of 0.1 N was used) and remove the liberated iodine with thiosulfate, then to add concentrated hydrochloric acid to dissolve all the precipitate and finally to titrate the iodine liberated by the iodate in the particles. The reproducibility of these experiments is not particularly good. The anomalous results are explained by the existence of two opposing effects: diffusion into the crystal versus recrystallization and transformation of hydrates. The next series of precipitates (table 4) was obtained from hot solution. Ten cubic centimeters of 0.25 N ammonium oxalate was added rapidly dropwise to a mixture of 50 cc. of water and 12 cc. of 0.25 N calcium chloride heated to 100°C. Twelve cubic centimeters more of calcium solution was then added to the main solution and again precipitated with 10 cc. of oxalate. This particular procedure was used to obtain crystals that would not be too small. The average crystal diameter was 0.5 to 1 micron. No recrystallization was apparent after a day, but it is not possible to say conclusively that none had taken place. The liquid containing the precipitate was cooled immediately after precipitation by cold water. Ten cubic centimeters of 0.050 M potassium iodate was added 5 minutes after the end of the precipitation. The amount of iodate that had permeated the crystals was determined in exactly the same manner as described above. The variation in amount of post-occlusion with time is very regular. The effect of the aging of the crystals is well marked.
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STUDIES IN COPRECIPITATION. VI
TABLE 3
D i f f u s i o n of iodate into and adsorption by calcium oxalate precipitated at room temperature NO.
PIME OF STANDING BEFORE ADDITION OF IODATE
TIME OF SEAKING AFTER ADDITION OF IODATE
0 minute 0 minute 0 minute 0 minute 0 minute 0 minute 0 minute 0 minute 0 minute 0 minute 0 minute 5 minutes 10 minutes 15 minutes 30 minutes 60 minutes 80 minutes 200 minutes 24 hours 26 hours 48 hours 2 days 3 days
$ minute 1 minute 2 minutes 5 minutes 10 minutes 15 minutes 60 minutes 180 minutes 20 hours 48 hours 68 hours 15 minutes 15 minutes 15 minutes 15 minutes 15 minutes 100 minutes 24 hours 29 hours 3 days 25 hours 5 days 5 days
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23
IODATE I N PRECIPITATE (CC. OF
,0050 N THIOSULFATE)
0.15 0.3; 0.45 0.55 0.7 1.0; 1 . 4 1.05; 1 . 1 1.85 1.7 4.7; 5 . 4 5.5; 7 . 1 8.5 0.55 0.35 0.25 0.20 0.15 0.25 5.3 3.0 0.3 0.5 0.6; 0 . 2 0.3
TABLE 4
Diffusion of iodate into calcium oxalate precipitated hot TIME OF STANDING BEFORE ADDITION OF IODATE
5 minutes
5 minutes 5 minutes 5 minutes 5 minutes 5 minutes 5 minutes 5 minutes 1 hour 1 hour 2 hours 20 hours 3 days
TIME OF SHAKING A m E R ADDITION OF IODATE
5 minutes 10 minutes 20 minutes 30 minutes 60 minutes 120 minutes 180 minutes 20 hours (not shaken) 20 hours (not shaken) 2 hours 20 hours (not shaken) 2 hours 2 hours
T E E JOURNAL OF PEYSICAL CEEMISTRY, VOL. XXXVII, NO.
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IODATE I N PRECIPITATE (CC. OF 0.0050 N THIOSULFATE)
0.4 0.55 0.6 1.05 1.1 1.1 1.15 1.85 1.0 0.9 0.85 0.4; 0 . 5
0.3
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I. M. KOLTHOFF AND E. B. SANDELL
Finally we have the behavior of Hahn’s precipitate. Twenty cubic centimeters of oxalate and 21 cc. of calcium were added simultaneously to 50 cc. of water a t room temperature, the calcium being kept 1 cc. in excess; time, 5 minutes. At the end of the precipitation, 3 cc. more of calcium chloride was added and immediately thereafter 10 cc. of 0.050 M potassium iodate; the mixture was then shaken (table 5). The internal iodate was determined as before. These results are decisive. The particle size of Hahn’s precipitate is large enough to preclude the possibility of any appreciable recrystallization on standing at room temperature. The diffusion of iodate into calcium oxalate is rapid a t first and then falls off. More than 25 per cent of the TABLE 5 Diffusion o j iodate i n t o Hahn’s precipitate jormed cold TIME OF STANDING BEFORE ADDITION OF IODATE
TIME OF SHAKING AFTER ADDITION OF IODATE
0 minute 0 minute 0 minute 0 minute 0 minute 0 minute 0 minute 20 minutes 40 minutes 70 minutes 20 minutes 40 minutes 1 hour 2 hours 4 hours 20 hours 44 hours
2 minutes 5 minutes 10 minutes 30 minutes 60 minutes 2 hours 21 hours 10 minutes 10 minutes 10 minutes 2 hours 2 hours 2 hours 2 hours 2 hours 2 hours 2 hours
IODATE I N PRECIPITATE (CC. OF THIOSULFATE)
---0.0050 N
1.2
1.5 2.1 2.4
3.15 4.05 3.7
1.6 1.0 0.9 2.7 2.0 1.75 1.6 1.0 0.4 0.25
total amount of iodate has entered the crystal 2 minutes after precipitation and 50 per cent after 10 minutes. The aging of the crystals has a very marked effect on the amount of iodate taken up. The permeability of the crystal has decreased to 50 per cent of the original after an hour or two. These effects quite definitely indicate that calcium oxalate crystals have a discontinuous structure and that they tend to perfect themselves on standing in solution. DISCUSSION OF THE RESULTS
1. Table 1 shows that calcium oxalate freshly precipitated a t room temperature from fairly concentrated solutions loses fairly much occluded
STUDIES IN COPRECIPITATION. V I
729
iodate on aging; e.g., immediately after precipitation the amount of iodate corresponds to 14.2 cc. of 0.005 N thiosulfate, but after a few days equilibrium is reached, and the figure falls to 7.8. The case here is rather complicated, since the primary precipitate consists of the higher hydrates of calcium oxalate and these undergo transformation into the monohydrate on standing, accompanied by an entire structural change and recrystallization. Table 3 shows the pronounced porous character of calcium oxalate formed under the above conditions. The amount of iodate diffusing into, and being adsorbed at, the internal surface continually increases with the time of shaking and finally, after about two days, reaches a value almost identical with that found in calcium oxalate containing coprecipitated iodate, shaken for two days (see table 1). From this behavior it is not justifiable to infer, however, that the processes of the effusion and diffusion are reversible, because other factors, such as transformation of higher hydrates with recrystallization, play a predominating r81e here. The figures in table 3 indicate that the changes take place within a short time after precipitation. 2. Table 2 shows the slow effusion of coprecipitated iodate from calcium oxalate prepared a t room temperature according to F. L. Hahn’s procedure. Although no recrystallization takes place here on aging, the iodate content has decreased to about half of its original value after two days. The reverse process, namely, the diffusion into, and adsorption of iodate by, the internal surface is demonstrated by the figures in table 5. The aging process takes place very rapidly. Comparison of the amount of iodate adsorbed a t the internal surface after a day (table 5 , 3.7 cc.) with the amount of coprecipitated iodate in the crystals after the same period (table 2, 24.6 cc.) indicates that on aging only a small part of the capillaries is filled by lattice ions, and that the “isolated internal surface” is fairly large. Actually most of the iodate coprecipitated is buried within the crystal and has no chance to escape. 3. Table 3 shows that calcium oxalate precipitated at 100OC. and allowed to stand a t room temperature takes up only a very small amount of iodate. In agreement herewith it was found in the previous paper that calcium oxalate prepared under the above conditions loses hardly any coprecipitated iodate, if allowed to age a t room temperature. Still the amount of coprecipitated iodate is fairly large (compare table 1 in previous paper (3)), thus showing that the “isolated internal surface” of calcium oxalate precipitated at 100OC. is fairly large and that the iodate is buried within the crystal. The internal surface in open communication with the outside is distinctly smaller than the same of a precipitate prepared at room temperature according t o Hahn, even though the latter has a larger crystal size. (Compare tables 4 and 5 . ) 4. Summarizing, it appears that: (a) Coprecipitated ions quite gener-
730
I . M. KOLTHODF .4ND E. B. SANDELL
ally are adsorbed at the internal surface. (b) A distinction is to be made between (1) internal surface in open communication with the exterior of the crystal, and (2) isolated internal surface. A freshly prepared precipitate very rapidly undergoes internal structural changes that result in an increase of the isolated internal surface and prevent any further effusion of coprecipitated material from within, or diffusion into, and adsorption of ions at its internal surface. The most effective purification of a precipitate is to be expected if the crystals by a slow dissolution and deposition process undergo an entire recrystallization. Calcium oxalate precipitated a t 100°C. and aged a t room temperature loses hardly any coprecipitated iodate, whereas an effective purification is achieved if the digestion is carried out a t higher temperature for a longer time. Under the latter conditions a real recrystallization takes place. 11. PHENOMENA WITH POTASSIUM PERCHLORATE-PERMANGANATE
To confirm the interpretation of the phenomena described with calcium oxalate and potassium iodate, experiments of a similar nature were made with potassium perchlorate and potassium permanganate. The object of these experiments was to determine whether permanganate could diffuse into crystals of potassium perchlorate and be adsorbed internally. If fractures and discontinuities exist in a real crystal of potassium perchlorate, such a crystal placed in a saturated solution of potassium perchlorate containing dissolved potassium permanganate should become stained by the latter. Those two salts are strictly isomorphous and if the effect expected actually exists it should be plainly shown by this pair. No detailed description of the experiments and the results will be given here-only the salient facts. It was found that crystals of C.P. potassium perchlorate immersed in a saturated solution of the same salt in 0.1 N potassium permanganate for a short time-an hour was sufficient- were colored distinctly pink after removal from this solution and after washing with a saturated solution of potassium perchlorate to remove adhering permanganate. Recrystallized potassium perchlorate obtained by evaporating a solution of the salt a t room temperature also took up permanganate, but only slightly, so that the crystals were only faintly tinted. If the recrystallized perchlorate was ground in a mortar to give fragments 10 to 100 microns in diameter and these placed in permanganate as before, then it was found that so much potassium permanganate penetrated the crystals that they were colored a vivid red, The permanganate was actually in the interior of the crystals and not a t or near the surface, as was proved by taking the colored crystals and shaking them with a volume of water insufficient to dissolve them all. The fragments remaining, though greatly reduced in size, were still colored. Even more drastic treatment failed to remove the color; the red crystals were shaken with a dilute solution of
STUDIES IN COPRECIPITATION. VI
731
hydrochloric acid and hydrogen peroxide; after a week in this mixture the perchlorate was still strongly colored. Microscopic examination of the colored crystals in reflected light under low magnifications showed that they were homogeneously colored, although some fragments were more strongly colored than others. It is impossible that the coloration of the crystals can be due to permanganate seeping into microscopically visible cavities or cracks. Beyond a manner of doubt, the permanganate has diffused into microscopically invisible breaks, faults, fractures, discontinuities, or capillaries in the lattice of the perchlorate and has been adsorbed a t the internal surface. The phenomenon is not simply one of diffusion alone, for then any imperfect crystal would be colored by permanganate. This was found definitely not to be true. Thus, ground sodium chloride kept in permanganate for a long time remained perfectly uncolored except for an isolated particle or two which had been colored by permanganate that had penetrated into some microscopic crack or cavity. The coloration of a crystal appears to be dependent upon adsorption or incorporation of the colored substance in the lattice. Thus in the case of potassium perchloratepermanganate, the permanganate, being isomorphous with the firstnamed salt, will tend to perfect the lattice of this crystal if given the opportunity to do so, i.e., a real crystal of potassium perchlorate immersed in permanganate will be colored by the latter as a consequence of the building up of the lattice. If this be the correct explanation, an imperfect crystal of potassium perchlorate allowed to stand in its own saturated solution should no longer be able to take up permanganake. This was verified. Ground crystals of perchlorate which became strongly colored if first placed in potassium permanganate would no longer take up this substance l o more than a very slight extenl if first immersed in a saturated solution of potassium perchlorate. The behavior is very striking. The explanation is obvious. When the imperfect crystals were kept in contact with saturated perchlorate, potassium and perchlorate diffused into the discontinuities and filled them by building up the lattice. Permanganate was therefore unable to color the crystals when they were later brought into contact with it. It is evident that crystals of perchlorate which have formed by slow evaporation and which have stood for some time in the saturated solution should take up but little permanganate as already described. Crushing the crystals of the salt should expose the openings of more internal surfaces. Therefore the ground crystals should be colored much more strongly, as was actually found. It is also possible that the stress of grinding causes more faults to appear in the lattice, The first well authenticated instance of artificial coloration of a crystal appears to have been reported by Vater (4),who found that an artificial crystal of calcite immersed in Thoulet’s liquid (a solution of mercuric iodide
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I. M. KOLTHOFF AND E. B. SANDELL
in potassium iodide) became colored yellowish-brown; the coloration was hastened by boiling the crystals with the solution. Cleavage fragments of Iceland spar became similarly colored. The coloring was entirely homogeneous; the crystals were pleochroic. The last observation is especially worth noting. If the crystals were immersed in water they became colorless after a time. The coloration of Vater’s crystals was probably due to iodine. Retgers ( 5 ) emphasized the extreme importance of these results. Strangely enough these observations of the permeability of crystals to solutions do not appear to have aroused much attention. The number of instances of artificial coloration of crystals has remained small.1 Instances of permeability of crystals to gases are also known, for example, oxidation and change of color of olivine kept in the atmosphere. These facts all speak for a mosaic structure of real crystals. 111. PHENOMENA WITH BARIUM SULFATE-POTASSIUM PERMANGANATE
D. Balarew (2) found that freshly precipitated barium sulfate allowed to stand in potassium permanganate solution was able to take the latter substance into the crystals and give a colored precipitate; he also noted the effect of aging and drying. Such treatment diminished markedly the power of barium sulfate to become colored. Experiments of a similar nature have been made in this laboratory; they can be carried out in a very simple way and offer a striking demonstration of the secondary structural changes taking place in a fresh precipitate on aging. Freshly prepared barium sulfate shaken immediately after precipitation with a permanganate solution assumes a slightly pink color after a few minutes. The intensity of the latter-an indication of the amount of permanganate in the ‘crystals-increases with time, and approaches a maximum after a day or two. Such a precipitate after washing is still red, and retains the color if shaken for a week or longer with a solution of hydrogen peroxide in strong hydrochloric acid. If the barium sulfate is allowed to stand after precipitation for some days and then treated with permanganate, hardly any of the latter compound is taken up, thus showing that the porous character has materially decreased or that most of the pores have been blocked by dams of barium sulfate.
* Toward the end of the nineteenth century the European jewelers were alarmed by reports that yellowish diamonds were being transformed into clear colorless ones by immersion in solutions of blue dyestuffs. Retgers ( 5 ) , who mentions this incident, regarded it as quite possible that the dye had actually diffused into the crystal in these cases and rendered the diamond colorless (blue and yellow being complementary colors). It appears improbable that a film of dye on the exterior could produce this effect. The alchemists were familiar with the decoloration of yellow diamonds by indigo solution (Ferrandus Imperatus, 1695). (See Janettaa: Bull. SOC. min. 14, 65 (1893).)
STUDIES I N COPRECIPITATION. V I
733
It may be mentioned in conclusion that ground crystals of potassium sulfate kept in potassium chromate and potassium manganate solutions become faintly yellow and bluish-green respectively. A more thorough study of internal adsorption by crystals and of a secondary structural change taking place in a fresh precipitate is planned for the future. SUMMARY
1. A fresh precipitate of calcium oxalate or barium sulfate prepared a t room temperature forms very inhomogeneous crystals, showing a porous structure. Diffusion of solution into the capillaries followed by adsorption of solution a t the internal surface takes place, if the adsorbent is added after the precipitation. Precipitates prepared a t 100°C. show a similar behavior, but one not as pronounced as when obtained a t room temperature from relatively concentrated solutions. 2. Freshly prepared precipitates on aging undergo rapid structural changes which result in an incomplete perfection of the crystals. Only %c small part of the capillaries is entirely filled by the lattice material, most of them being blocked by the latter, thus isolating the internal surface from the outside. The isolated internal surface is much larger in a precipitate obtained from hot than from cold solutions. 3. Coprecipitated foreign substances in crystalline precipitates are adsorbed on the internal surface. A purification occurs on aging by effusion of part of the contaminants through the capillaries; the impurities at the isolated internal surface, however, cannot be expelled. 4. A slow recrystallization of a precipitate via the solution yields relatively perfect and pure crystals. 5. Most crystalline salts may be expected to have a relatively large isolated internal surface. The latter can be partly exposed by grinding the crystals to smaller dimensions. REFERENCES
(1) SMEKAL, A.: Physik. Z.26,707 (1925);Z.angew. Chem. 42,489 (1929);Z.Elektrochem. 36, 567 (1928). (2) BALAREW, D.: Kolloidchem. Beihefte 30, 249 (1930); 32, 304 (1931); 33, 310 (1931). (3) KOLTHOFF, I. M., AND SANDELL, E . B.: J. Phys. Chem. 37,443 (1933). (4) VATER: Z.Krist. 24, 366 (1895). (5) RETGERS:Z.physik. Chem. 20, 534 (1896).
