STUDIES ON AGING OF FRESH PRECIPITATES. XVIII
THE MIXED-CRYSTAL FORMATION BETWEENBARIUM SULFATEAND POTASSIUM PERMANGANATE I. M. KOLTHOFF AND G . E. NOPONEN School of Chemistry, Institute of Technology, University of Minnesota, Minneapolis, Minnesota
Received October 10, 1937
According to Grimm et al. (4,5, 6, 10, 11) mixed-crystal formation between two chemical individuals may occur when the following conditions are satisfied: (1) the chemical building types of the two compounds must be the same; (6)the lattice types must be the same; (3) the lattice constants must be of the same order of magnitude. Similarity of chemical building types refers to pairs of compounds with the same number of cations and anions in which the corresponding ions are made up of the same number of atoms. The above conditions include not only the classical ones of mixed-crystal formation but also the formation of Grimm’s “neuartige Mischkristalle.” Examples of systems forming this new kind of mixed crystals are barium sulfate-potassium permanganate; barium sulfate-potassium fluoborate; calcium carbonate-sodium nitrate; lead sulfide-sodium bromide, etc. Grimm and his coworkers (4,6) studied especially the new kind of mixed-crystal formation between barium sulfate and potassium permanganate. They coprecipitated potassium permanganate with barium sulfate and studied the properties of the red precipitate. Briefly, they found that the crystals were homogeneously colored; that the permanganate in the crystals was inert towards various reagents; that the rate of solution in concentrated sulfuric acid increased with the potassium permanganate content of the crystals; that the amount of potassium permanganate in the crystals was approximately proportional to the concentration of potassium permanganate in the precipitating solutions; that it was possible to obtain precipitates containing as much as 80 mole per cent of potassium permanganate; that the x-ray pictures made by the Debye method showed a regular displacement of lines (6, ll),such as would be produced if the potassium permanganate were dissolved in the barium sulfate lattice ; finally, that equivalent amounts of potassium and permanganate are coprecipitated. The latter fact is strongly indicative of mixed-crystal formation; the results of the x-ray investigation of 237
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I. M. KOLTHOFF AND G . E. NOPONEN
Wagner (6, ll),however, give conclusive evidence of the formation of solid solutions of potassium permanganate in barium sulfate.1 Various authors (I 2, 3) have tried to invalidate Grimm’s conclusions. Balarew (l),for example, criticizes Grimm’s work and attributes the coprecipitation of potassium permanganate to an internal adsorption during the growth of barium sulfate. Balarew’s claim that the coloring of barium sulfate by potassium permanganate is not completely homogeneous does not disprove mixed-crystal formation. Even in cases in which mixed crystals of the classical type are formed, a heterogeneous distribution often occurs (compare, e.g., Kolthoff and Noponen (8, 9) for the systems barium sulfate-lead sulfate and barium sulfate-barium chromate). It depends upon the manner of preparation of the mixed crystals whether they are homogeneous or not. Neither Wagner etal. nor his opponents have considered the conditions under which homogeneous mixed crystals are obtained, and therefore one cannot arrive at any exact statement of the rules determining the distribution of permanganate between solution and solid. Benrath and Schackmann (2), criticizing Grinm’r conclusions, showed that fairly perfect barium sulfate, on being shaken for five weeks with potassium permanganate solutions, incorporated hardly any potassium permanganate. Such a result was to be expected, as hardly any recrystallization occurs upon shaking the fairly perfect barium sulfate. Interesting studies on the coprecipitation of permanganate with barium sulfate have been made by Karaoglanov (7), the results of which are in harmony with the concept of the formation of solid solutions. It is peculiar that the authors criticizing Grimm’s conclusions ignore the conclusive evidence of mixed-crystal formation obtained by him and Wagner from the x-ray diagrams. I n the present study the authors have made a preliminary study of the illcorporation of potassium permanganate in recrystallizing barium sulfate with the ultimate purpose of learning the rulcs governing the distribution of potassium permanganate between the solid and liquid phases. I n previous work by the authors (8, 9) it was shown that homogeneous mixed crystals of barium sulfate and lead sulfate and of the former with barium chromate could be obtained upon adding lead nitrate or sodium chromate, respectively, t o a fresh suspension of barium sulfate and shaking for considerable periods of time until distribution equilibrium was attained. The fresh, imperfect barium sulfate is subject t o repeated recrystallizations upon aging in the mother liquor, thus incorporating the mixed-crystal1 After this paper was submitted for publication, P. R. Averell and G. II. Wslden, Jr., (J. Am Chem. Soc. 69,906 (1937)) reported conclusive evidence, based upon x-ray investigations, that hydronium permanganate (HsOMn04)forms a solid solution in barium Rulfate
MIXED-CRYSTAL FORMATION
239
forming component in the recrystallisate, and eventually yielding homogeneous crystals. I n the present study an attempt was made to reach distribution equilibrium from two sides : ( I ) by shaking freshly precipitated barium sulfate with solution containing potassium permanganate until the concentration of the latter in the solid became constant, (2) by shaking barium sulfate containing coprecipitated potassium permanganate with the supernatant liquid until the permanganate concentration in the solid became constant. EXPERIMENTAL
A 0.5 molar solution of potassium permanganate was prepared and filtered through an asbestos filter after one day of standing. The general procedure was to precipitate barium sulfate quickly at room temperature from 0.3 M solutions of barium nitrate and sodium sulfate, using a slight excess of barium. Immediately after precipitation, various volumes of 0.5 M potassium permanganate were added and the suspensions shaken at room temperature (8, 9). After various periods of time the mixtures were centrifuged and the mother liquor was poured off. The precipitate was washed four or five times with 50-ml. portions of C.P. acetone which did not contain impurities reacting with permanganate. It was found that water could not be used for washing the precipitate, as permanganate was extracted continuously. With acetone, on the other hand, the fourth or fifth washing was always colorless. The washed precipitate was dried at 130°C. for bwenty-four hours. Weighed amounts of the dried precipitate (from 0.3 to 0.8 g. depending upon the permanganate concentration) were treated with 10 ml. of concentrated sulfuric arid in a glass-stoppered iodine flask. After the crystals had completely dissolved, 40 ml. of water and 0.25 g. of potassium periodate were added, and the contents of the flask were heated for ten to fifteen minutes. The solution was then cooled and made up to a volume of 100 ml. The permanganate content of the solution was determined colorimetrically by comparing with suitable standards in a colorimeter, taking the average of ten readings. T h e rate of entrance of permanganate into freshly precipitated barium sulfate
To 26 ml. of 0.3 M barium nitrate were added 25 ml. of 0.30 M sodium sulfate and 25 mi. of 0.5 M potassium permanganate. The mixtures were shaken for various periods of time and further treated as described above. The amounts of permanganate found in the precipitate after various periods of time are given in table 1. The occurrence of a maximum in the amount of permanganate in the precipitate after one day of shaking is comparable to that of lead in the precipitate upon shaking fresh barium sulfate with lead nitrate (8).
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I. M. KOLTHOFF AND G. E. NOPQNEN
T h e rate of exit of coprecipitated potassium permanganate f r o m barium sulfate
To a mixture of 26 ml. of 0.3 M barium nitrate and 25 'ml. of 0.5 M potassium permanganate was added 25 ml. of 0.3 M sodium sulfate. The suspensions were shaken for various periods of time and further treated as described above. The amounts of permanganate found in the precipitate are reported in table 2. Apparently, the fresh precipitate obtained in the presence of permanganate contains much more permanganate than corresponds to equilibrium conditions. Grimni et al. (4,5 , 6 ) filtered their precipitates within half an hour after the preparation. Therefore their results do not correspond to a state of equilibrium, and no exact relation can be inferred from them regarding the distribution of the permanganate between the solid and liquid phases. Comparing the data in tables 1 and 2 shows that the same state of distribution of permanganate is approached TABLE 1 Rate of entrance of permanganate into barium sulfate Time of shaking 1\Iilligrams of KIInOa per 1 g of
Bas04
1 hour
' 22 hours
6 days
1
96
1
113
1
' 15 days
'
8 1
8 3
TABLE 2
Rate of exit of coprecipitated permanganate Time of shaking. , . . . . . , . . 10 minutes AIilligrsms of KhlnOd per 1 g. of BaS04~ 19.3 , ,
, , , ,
,
,
~
~
1 hour 17.3
1
25 hours
11.5
Ii
7 dags 9.2
from both sides after sufficient periods of time of shaking, indicating that distribution equilibrium can be attained from both sides. T h e amount of potassium permanganate in the mixed crystals of the concentrataon of the former in solution
LIS
a function
The experiments described in table 1 were duplicated, except that differm t volumes of 0.5 M potassium permanganate were added after the precipitation. The suspensions were shaken for one month, centrifuged, nashed with acetone, etc. The results of table 3 tend to show that there is no linear proportionality between the amounts of permanganate in the solid and liquid phases. However, they are not quite conclusive, since it will be shonn below that the amount of permanganate taken up by the iolid decreases upon addition of sodium nitrate to the solution. I n the above experiments the supernatant liquid contained sodium nitrate, the concentration of which decreased with increasing volumes of permanganate added. Moreover, it may be expected that the relative effect of sodiuni
24 1
MIXED-CRYSTAL FORMATION
nitrate upon the amount of permanganate incorporated in the solid will decrease with increasing concentration of potassium permanganate in the liquid. This effect of sodium nitrate may explain why the amount of incorporated permanganate increases more than linearly with the potassium permanganate concentration in the solution (table 3). The experiments will be repeated under conditions in which the effect of sodium nitrate is eliminated.
Effect of other balts upon the amount of potassium permanganate incorporated i n barium sulfate The experiments of table 1 were repeated, except that in addition weighed amounts of some electrolytes were added immediately after the precipitation to give mother liquors of various compositions. The susTABLE 3
Relation between concentration of permanganate in solid and i n solution Milliliters of 0.5 M KMnOa added. . . . . . . . . . Milligrams of KhhOo per 1 g. of BaSO4. . , ,
I
50 31.9
1
h055
1
5 1.2
TABLE 4
Effect of salts upon distribution of permanganate
1
+
Composition of mother liquor* 0.2 M 1.6 M 1.3ilfKNO3f 1.3MNaCl NaKOs NaNOal 0.21M S a x 0 3 0.2A4 NaNOa Milligrams of KMn04per 1 g. of BaS04. . . . . . . . . . . . . . . . . . . . . . 8.3 3.9 12.5 j 6.5
I
I
pensions were shaken for one month in order t o attain distribution equilibrium of the permanganate. It is seen (table 4) that the amount of permanganate incorporated in the barium sulfate decreases upon addition of sodium nitrate and sodium chloride, the effect of the former being greater than that of the latter. On the other hand, the distribution into the solid phase is favored by the addition of potassium nitrate. DISCUSSIOX
It has been shown that the rate of establishment of the distribution equilibrium of potassium permanganate between barium sulfate and solution is small. The same equilibrium distribution is reached upon shaking when potassium permanganate is added to the fresh suspension of barium sulfate as when it is coprecipitated, indicating the formation of a solid solution. In the case of mixed-crystal formation in the classical sense (Bas04 + PbS04; BaS04 BaCr04) we have shown (8, 9) that large
+
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I. M. KOLTHOFF AND G. E. NOPONEN
concentrations of electrolytes in solution, the ions of which do not participate in the exchange with ions in the lattice, do not affect the distribution of the isomorphous ion between the solid and the solution, if the added electrolyte does not change the ratio of the activities of the two isomorphous ions in the solution. Thus, it was found that the distribution of lead ion between barium sulfate and solution was not affected by 1 molar sodium nitrate and the distribution of chromate was not affected by 2 molar ammonium acetate. The system barium sulfate-potassium permanganate differs from the above classical systems in as much as indifferent electrolytes, such as sodium nitrate and sodium chloride, decrease the amount of permanganate taken up by the solid under equilibrium conditions. I n interpreting these results it should be considered that Walden and Cohen (12) found that barium nitrate can be incorporated into barium sulfate in the form of a solid solution. Although barium nitrate, and possibly also other salts, which are not expected to form solid solutions in barium sulfate according to the classical rules or those of Grimm, do not affect the distribution of an isomorphous ion, they apparently do affect the distribution in the case of the formation of the new kind of mixed crystals of the type described by Grimm. The decreasing effect of sodium nitrate upon the distribution of the potassium permanganate in the barium sulfate may be responsible for the fact that no linear relation has been found between the concentration of potassium permanganate in solution and in the solid under equilibrium conditions. If such a h e a r relation exists within a certain range of mole per cent of potassium permanganate in the solid, it might be represented by the following expression: (cK+* CMn0r)solution = K(CKMnO4 )solid
in which concentrations are written instead of activities. According to this expression addition of potassium salt to the solution should favor the distribution of permanganate in the solid phase. Actually, addition of potassium nitrate to the solution was shown to have the expected effect (table 4). Apparently the increasing effect of the potassium ions is greater than the decreasing effect of nitrate ions upon the distribution of the potassium permanganate in the solid phase. From this preliminary study it may be inferred that the factors determining the distribution are much more involved in the case of the formation of the “new kind of mixed crystals” than in the case of the distribution of an isomorphous ion between solid and solution. The investigations are being continued. SUMMARY
1. Upon shaking freshly precipitated barium sulfate with a potassium permanganate solution, distribution equilibrium is reached after fifteen
MIXED-CRYSTAL FORMATION
243
days to one month. The same equilibrium is reached when the permanganate is coprecipitated with the barium sulfate and the suspension shaken for a period of about fifteen days. The amount of coprecipitated permanganate decreases considerably during the time of shaking. 2. Sodium nitrate, and to a lesser degree sodium chloride, in the solution decreases the equilibrium concentration of potassium permanganate in the solid phase. 3. The presence of sodium nitrate in the mother liquor may account for the fact that the concentration of potassium permanganate in the barium sulfate under equilibrium conditions was found to increase more than linearly with the potassium permanganate concentration in the solution. 4. Potassium nitrate added to the liquid increases the distribution of potassium permanganate in the solid phase. This is accounted for by assuming that within certain limits the distribution of potassium permanganate is given by the expression (cK+* CMnOrholution = K(CKYnOd) solid
5. The factors affecting the distribution are more involved in the case of the formation of the “new kind of mixed crystals” than in the case of the distribution of an isomorphous ion between solid and solution.
(1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12)
REFERENCES BALAREW, D.: Kolloid-Beihefte 32,304 (1931). H.: Z. anorg. allgem. Chem. 218, 139 (1934). BENRATH, A., AND SCHACKMANN, V., AND NIKITIN,B.: Z. physik. Chem. A146, 137 (1929). CHLOPIN, DAHLMANN, H.: Thesis, Wiirzburg, 1930. GRIMM,H. G . : Z. physik. Chem. 98, 353 (1921); Z. Elektrochem. 28, 75 (1922); 30,467 (1924); Z. Krist. 67,574 (1922); Handbuch der Physik 24,581 (1927). GRIMM, H. G., AND WAGNER, G.: Z. physik. Chem. 132,131 (1928). Z . : Z. anorg. allgem. Chem. 222,249 (1935). KARAOGLANOV, I. M., AND NOPONEN, G. E.: J. Am. Chem.Soc. 60,197 (1938). KOLTHOFF, KOLTHOFF, I. M., AND NOPONEN, G. E.: J. Am. Chem. SOC.60,39 (1938). PETEHS,CL.: Thesis, Wiirzburg, 1930. WAGNER, G.: Z. physik. Chem. B2, 27 (1926). WALDENAND COHEN,U.:J. Am. Chem. SOC.67,2591 (1935).
