A STUDY OF T H E PREPARATIOX XXD CERTAI?; PROPERTIES O F COLLOIDAL HYDROLS BERYLLIUM OXIDE SOLS* BY WILLARD H. M A D S O S AND FRAXCIS C. KRACSKOPF
Introduction One of the most interesting compounds of beryllium is the hydroxide. I t is easily prepared by precipitation from solutions by various alkaline reagents. J . 31. van Bemmelen’ distinguished two forms of beryllium hydroxide. The granular alpha-hydroxide prepared by precipitation from alkaline solution of the oxide, and the gelatinous beta-hydroxide precipitated by ammonia from a solution of beryllium sulfate. After considerable research on the two forms, van Bemmelen concluded that the alpha-hydroxide was a definite compound, Be(OH)?, but that the beta-hydroxide was of indefinite composition and behaved like gelatinous aluminum and ferric hydrox:de. “It may therefore be regarded as colloidal beryllium oxide for there is little to show that the water is other than adsorbed or mechanically held”’ The various hydrated beryllium hydroxides which A. Atterberg? claims to have prepared may be regarded as different stages in the desiccation of the gelatinous or beta-hydroxide. The properties of beryllium hydroxide vary greatly with the mode of preparation. One equivalent of concentrated solutions of normal salts can dissolve from one to five equivalents of the hydroxide depending on the salt used. The solubility of the hydroxide and its sensitiveness to reagents are greatly diminished if it has been dried by warming, or boiled for a long time in water. When the hydroxide is first precipitat.ed by strong alkalies, it is very voluminous, easily adsorbs carbon dioxide and is easily soluble in potassium hydroxide, potassium carbonate, ammonium carbonate or in acids. On standing it slowly changes to the more stable form, which is granular and much less active. These same differences of properties of the two forms of the hydroxide were also observed by Haber and van 0 0 r d t . ~ ” I t is a well-known fact that beryllium hydroxide must be washed with water containing an electrolyte to prevent the loss of colloidal beryllium hydroxide through the filter paper. Concerning colloidal beryllium hydroxide Weiser‘ states, “While colloidal solutions of hydrous beryllium oxide have not * Contribution from the Laboratory of General Chemistry, University of Wisconsin. * An abstract from a thesis presented by Willard H. Madson, in partial fulfillment of the requirements for the degree of Doctor of Philosophy, University of Wisconsin. J. prakt. Chern., 66, 2 2 7 (1882); Z. anorg. Chern., 18, 126 (1898). hlellor: ‘‘ A Comprehensive Treatise on Inorganic and Theoretical Chemistry,” Val. (1923). 3 Atterberg: Kongl. Svenska Vet. Akad. Hand., 12, I (1873). 3a Haber and van Oordt: 2. anorg. Chem., 38, 377-398 (1904). Weiser: “The Hydrous Oxides,” p. 163 (1926).
v
3238
WILLARD H . MADSOS AND FRASCIS C. KRACSKOPF
been described in detail Bohm and Xiclassens prepared a clear concentrated sol by peptizing a freshly made gelatinous oxide with a small amount of 0.0 j 31 HCl. Since the gelatinous oxide runs through the filter paper when an attempt is made to wash out adsorbed salts there is little doubt that a pure sol could be prepared by thorough washing of the hydrous gel using the centrifuge or supercentrifuge.” It was therefore the purpose of this investigation to prepare pure hydrous beryllium oxide sols and to study certa n of their properties. Preparation of Colloidal Hydrous Beryllium Oxide Attempts were made to prepare colloidal beryllium oxide by many of the well-known methods for the preparation of hydrous colloidal oxides, such as peptizing the precipitated hydroxide (beta) by various reagents, hydrolysis of salts under different conditions, etc. But in every case, however the conditions were varied, the solutions were perfectly clear (after standing a short time) and gave no colloidal tests, such as the Tyndall effect, cataphoresis or flocculation by electrolytes. Sols prepared by the method described by the authors6 (developed during this investigation) contained some colloidal hydrous aluminum oxide. Therefore the beryllium salt was purified by conversion of the commercial carbonate (C.P) to the basic acetate and recrystallization several times from hot glacial acetic acid solutions. The basic acetate was converted t o the chloride (for use in the preparation of sols) by boiling a small amount of the salt with a large excess of approximately six normal hydrochloric acid until a syrupy mass was formed. I t mas also found that in the preparation of the sols, a substitution of six inch porcelain casseroles for the pyrex beakers, greatly increased the concentration of the sols produced by the above method. T’arious methods were tried in an attempt to produce more concentrated sols. Syrupy beryllium chloride was slowly heated to a white paste and then poured into boiling water. X very concentrated sol was thus produced but after equilibrium of settling had been established, the sol was no more concentrated than sols prepared by the casserole method. Sols were made by substituting various concentrations of sodium hydroxide solutions for the boiling water. Dilute solutions of the alkali acted like electrolytes, slowly coagulating the sols, while the more concentrated solutions dissolved the hydrous oxide presumably forming a soluble beryllate. Dilute solutions of HC1 were substituted for the water but this did not increase the stability or concentration of the sols, although stable sols were produced. The more concentrated solutions of the acid dissolved the hydrous oxide. I t appeared that the heating of the salt and the formation of the “basic chloride” before the addition of the water was most important in determining the character of the sols. A study was made to determine the temperature that would give the best results. Samples of syrupy beryllium chloride were 5
Bohm and Xiclassen: Z. anorg. Chem., 132, 5 (1924). Madson and Krauskopf: ,I. Chem. Educ., 6, 334 (1929).
COLLOIDAL HYDROUS BERYLLIUM OXIDE SOLS
3239
heated (by means of an oil bath) as follows: ( I ) a t 167~-1;0~C. for five hours; ( 2 ) at I ~ Z ~ - I ~for O seventeen ~C. hours, and (3) a t 118~-128~C. for seventytwo hours and after heating added to boiling water. Analyses of the sols thus produced (after they had been dialyzed for 4 days) showed no weighable beryllium oxide. Therefore the chloride must be quickly decomposed in order to produce the “oxychloride” apparently necessary for the preparation of a stable sol. Twenty-four liters of colloidal hydrous beryllium oxide were prepared, February I S , 1930, using the following method: Pure basic beryllium acetate was converted to a syrupy chloride as previously described. Fifteen ml. portions of this syrup (containing about j g. of BeC12) were heated in six inch casseroles, using a small flame of an adjustable burner. Vhen the material was nearly dry, that is when only a few syrupy bubbles were forming, the flame was turned up to full height and when the redness of the flame could be seen through the bottom of the casserole, one liter of boiling water was added. The sol thus formed was boiled for about a minute and then allowed t o cool in the air in three-liter flasks. About thirty liters were prepared in this manner. After twenty-four hours the colloidal “solutions” were siphoned into a z;r-liter pyrex Florence flask, leaving the beryllium oxide that had been heated too hot to form in colloidal suspension on the bottom of each of the three-liter flasks. The sol was designated as sol No. 2 4 L. The increased temperature just before the addition of the water produced colloidal systems which were from two to three times as concentrated as those prepared without increasing the temperature. Four months later this sol had a pH value of 5.61 measured by means of a glass electrode. Sol S o . 24 L was analyzed a t various times by the following method: one hundred ml. portions of the sol were evaporated in a platinum crucible by means of an electric hot plate and afterward heated to constant weight by means of a Meeker burner, and weighed as beryllium oxide. Table I gives the results of the analyses.
TABLE I Date of analysis mg. B e 0 per 1.
Analyses of Sol No.
24,
3-1-’30 I93
0
;-I
0-’3
186
L 7-16-’30 I8j
2-4-‘31
178
Three sols were prepared by evaporating the beryllium chloride syrup nearly to dryness in a quartz evaporating dish, then turning the flame up and quickly heating the dish and its contents to redness and immediately plunging it into a liter of boiling water. Very concentrated sols were thus produced but equilibrium of settling has not been established and therefore it is impossible to tell whether or not the sols will be more concentrated than the others.
WILLARD H. MADSON AND FRANCIS C. KFLAUSKOPF
3240
Purification of the Sols The sols were dialyzed in the dialyzer described by Sorum.’ The collodion bags were prepared from Mallinckrodt’s U.S.P. collodion. The perfectly clean 2 5 0 ml. Erlenmeyer flasks were partly filled with collodion. After a small amount had been poured out the rest was slowly allowed to flow out while the flask was completely rotated twice. The time of emptying was approximately fifteen seconds. The flask was then allowed to drain for thirty seconds, and then a moderate current of air passed into the flask for fortyfive seconds. Afterward the flask was immediately filled with distilled water. One minute later the collodion bag was removed, the neck inserted and the collodion allowed to dry on the neck thus holding the bag. Before using, the bags were thoroughly washed with distilled water. They were never used more than three times, usually only once or twice. About two liters of distilled water were allowed to flow through each of the twelve batteries of t,he dialyzer each hour. With the exception of sol S o . 17d (IZ), all sols were dialyzed at room temperature a few hours before they were heated to about 80°C. Usually the sols were partly cooled before being removed. Small amounts of distilled water were added at various times to replace water lost by evaporation. A11 sols were kept in well-stq:pred pyrex flasks. Utmost care was always exercised to prevent any contamination of the sols. Table I1 summarizes the purification of the sols used in this investigation. The stock sol No. 24 L F‘X t b source of all sols mentioned in the table. TABLE I1 Summary of the Purification of the Sols Sol No.
Total S o . hrs. dial.
Hrs. dial.
I 08
87
hot
mg. B e 0 per 1. after dial.
Temp
70-80 70-80
7 5-80 7 j-80 7 5-80 7 5-80 7 5-80 7 j-80 7 j-80
IO1
99
63 57
I43 168
130
53
162
51
12
I2
86
21
19 44
77
68 88
65
48 i 2
97
72
64
C.
In general dialyzed sols gave the following tests for chlorides when tested with acidified silver nitrate: dialyzed less than 50 hours, distinct test for chlorides; dialyzed 50-100 hours, faint test for chlorides; dialyzed more than I O O hours, no test for chlorides. 801s S o . 18d, No. I jd(Iz), and S o . 17d(72) showed absolutely no precipitation during eight months. The other dialyzed sols had formed a very slight precipitate after they had stood for two weeks or Sorurn: J. Am. Chern. SOC., 50, 1263 (1928).
COLLOIDAL HYDROUS BERYLLIUM OXIDE SOLS
3241
more. This slight precipitate may have been caused by slight contamination during the removal of samples for the determination of flocculation values. Some of the undialyzed sols have shown no precipitation in more than two years. Flocculation Values I. Theoretical. Hydrous beryllium oxide sols belong to the class of sols usually known as lyophobic or suspensoid colloidal systems. The stability of the sols is due to the three factors: ( I ) the Brownian movement, ( 2 ) attraction between the dispersed particle and the dispersion medium, and (3) the electric charge of the particle. The last factor is the most important in this case. I t is generally conceded that the charge of a colloidal particle is due to the diffuse Helmholtz electric double layer. If this charge is destroyed the stability of the sol is decreased and the sol, if a suspensoid, coagulates. I t was first thought that it was necessary to completely neutralize the charge of a suspensoid in order to cause coagulation. This complete neutralization was called the iso-electric point. Later investigations of Powiss have indicated that a lowering of the potential of the interface to a certain characteristic value, which he calls the “critical potential,” is sufficient to produce flocculation. Flocculation depends upon the probability of collision and upon the probability of adhesion. Murrayg outlines four conditions for comparable work in the coagulation of colloids by electrolytes: ( I ) uniform size of colloidal particles, ( 2 ) uniform concentration of the colloids, (3) quick and uniform mixings of colloid and electrolyte, and (4) uniform treatment after mixing. Thus flocculation values vary with the method used in their determination. However, the values give a relative indication of the stability of the various sols. 11. Experimental determination of flocculation oalues. a-Standard solutions. All solutions were carefully prepared, using calibrated weights and calibrated volumetric flasks. The solutions were kept in pyrex flasks. The preparation of the solutions is given in some detail because the flocculation values vary with the impurity of the solution. Concentrations are expressed in mols per liter of solution. Potassium Chloride: molar. Mallinckrodt’s reagent quality dried at I IoOC. Potassium Acetate, .so3 molar; Potassium Mono-chlor Acetate, ,404 molar; Potassium Di-chlor Acetate, ,404molar; Potassium Trichlor Acetate, .404 molar. The acetates were prepared by using a definite amount of standard KOH solution and neutralizing it with the respective acids using phenolphthalein indicator. (The potassium hydroxide solution was prepared from a saturated solution of C.P. KOH and standardized with a standard sulfuric acid solution.) Powis: 2. physik. Chem., 89, 186 (1915). 9 M ~ r r a yChem. : News, 123, 277 ( 1 9 2 1 ) . 8
3242
WILLARD H. MADSOS AND FRANCIS C. KRAUSKOPF
Potassium Chromate, molar/zo. The C.P. salt was recrystallized from water solution and dried at I Z O O C . Magnesium Sulfate, molar/zs. Mallinckrodt’s reagent quality MgSOa.7Hz0,undried. Potassium Sulfate, rnolar/s j. The C.P. salt was recrystallized several times from water and dried at I IoOC. Sodium Sulfate, molar/z j. Mallinckrodt’s (dried) reagent quality, dried at I IoOC. Tertiary Potassium Phosphate, (KSPO4),molar/n j : Mallinckrodt’s C.P. salt dried a t IIO’C. Primary Potassium Phosphate (KH2P04)molar/n j : hfallinckrodt’s C.P. salt was recrystallized several times from wrater solution and dried at I IoOC. Secondary Potassium Phosphate (K2HP04.3Hs0), molar/z 5 : Large transparent colorless crystals of Mallinckrodt’s reagent quality potassium phosphate, dibasic; not dried. Tertiary Potassium Arsenate, (K&04), molar/2 5 : Mallinckrodt’s potassium arsenate was recrystallized three times from water and dried a t
Ioj°C. Potassium Ferricyanide, (KsFe(CK)6)molar/z 5 : Mallinckrodt’s reagent quality salt was not dried for previous experiments showed that drying did not change the flocculation power. Solutions were made using carbon dioxide free water and were always prepared just before using for their flocculation power changed slightly with age. Potassium Ferrocyanide, (K,Fe(CS),.3H20), molar/jo: Mallinckrodt’s reagent quality potassium ferrocyanide was used. The salt was not dried for previous experiments showed that more uniform results were obtained without drying. The solutions were made with carbon dioxide free water and kept in “blackened” pyrex flasks. The flocculation power of ferrocyanide solutions kept in this manner remained practically constant for two weeks. b-Method
and apparatus used in the determination of flocculation values.
Test tubes: the five-inch pyrex test tubes were so thoroughly cleaned that the solutions would not stick to the sides of the tube. Utmost care had to be taken to have them absolutely clean or varying results were obtained. The test tubes were stoppered (when used) with carefully cleaned rubber stoppers, All pipets and the micro burets were calibrated before using. The preparation of the solutions has been previously described. Procedure-The concentration of electrolyte, such that one to two ml. of the electrolyte added to two ml. of sol in a total volume of j ml. would cause complete flocculation within 2 4 hours, was first determined. Then this concentration of electrolyte was placed in a calibrated micro buret which was graduated to 1/20 ml. Xater was placed in the other micro buret which was graduated to 1/10 ml. Then various concentrations of the electrolyte were placed in test tubes by adding from 0.1 to 3.0 ml. of the electrolyte and mak-
COLLOIDAL HPDROCS BERYLLIUM OXIDE SOLS
3243
ing up the total volume added to 3.0 ml. by adding the required amount of water from the other buret. Usually a rough run was first made by varying the electrolyte in each tube from 0.2 to 0.5 ml. To each of these solutions was added a ml. of the sol and the tubes stoppered. After the sol was added each tube was shaken five times. The sol was always added from a calibrated two ml. pipet and allowed to flow down the side of the test tube, never allowed to drop directly into the solution. The solutions were allowed to stand for twenty-four hours and the approximate flocculation value determined in this manner. After the approximate flocculation value was determined, solutions were made by varying the concentration of the electrolyte in the successive tabes by 0.1ml. Csually duplicates of each of eight concentrations were used in each run. The lowest concentration of electrolyte which caused complete flocculation was usually easily determined. Often the sol in one tube of a series would be partly flocculated and in the next completely flocculated as was the case with all the others in which the electrolyte was more concentrated. Occasionally there would be two tubes of partial flocculation but never more than two if the concentrations were varied by only 0.I ml. of added electrolyte. It was practically impossible to determine what concentration produced complete flocculation if the data were taken by artificial light. Therefore all observations were made by daylight. In this work, flocculation value has been defined as the number of millimoles of electrolyte per liter necessary to completely precipitate the sol within 24 hours. The concentration is computed for the total volume of sol plus added electrolyte. o-Experimental
results of the determination of flocculation values.
Tables 111, IV, V, and VI, give the flocculation values of sixteen different electrolytes, using different sols. A11 sols were not used in each determination but comparisons were made which showed that the values of all sols were in agreement. At first the technique required to obtain reproducible results was somewhat difficult, but after this mas developed the flocculation values were easily checked. No results were omitted which were obtained after such satisfactory technique had been developed for each electrolyte. Satisfactory results of the flocculation value of potassium chloride were very difficult to obtain. The technique was quickly developed for all the other electrolytes. Flocculation values of some electrolytes, such as potassium dichromate, could not be determined because the comparatively deep color of the solution made it impossible to visibly tell when flocculation was complete. The flocculation value of but one univalent inorganic anion (Cl-) was determined because of the large value and therefore comparatively inaccurate results. Other univalent anions, such as nitrate, bromide, and iodide, were tried insofar as to be sure that their flocculation values were also comparatively large.
3244
WILLARD H. MADSON AND FRANCIS C. KRAUSKOPF
TABLE I11 Effect of Change of Valence of Inorganic Anions on Flocculation T'alue of Electrolytes Flocculation value
Electrolyte
mg.Be0 per 1.
dialyzed
Potassium chloride
52
168
240
65
72
220
86
I2
220
52
I 68
0.22
HOW8
240
220
Potassium chromate
220
0.20 0.20
53
I43
0.24 0.24
0.24
63
108
57
101
0.24 0.20 0.24
0.24
65
72
0.22 0.22
72
48
77
24
86
I2
0.24 0.22 0.22 0.22 0.26
0.26
185
0
I.20 1,20
Tertiary potassium arsenate
1.20 52
168
0.048 o ! 048 0,044
53
I43
64
97
0.044 0.040 0.042
Potassium ferricyanide
0.042 0.042
52
168
0.026 0.024 0.024
0.024 0.026 0.026
COLLOIDAL HYDROUS BERYLLIUM OXIDE SOLS
3245
TABLE I11 (continued) Sol No.
Electrolyte
I 8d
mg. Be0 per 1. 64
Hours dialyzed
97
Flocculation
value
0.024 0.024
Potassium ferrocyanide
I 6d
52
168
0.008 0.010
0.008
0,008 0.010 0.010
0.009 0.011 1 5d
53
I43
0,008
18d
64
97
0.010
0.010 0.010 0.012 0.012 0,013 0.013 0.013
TABLE IV Flocculation Values of the Potassium Acetates Electrolyte
Potassium monochloracetate
Potassium dichloracetate
Hours
mg.BeO per 1.
dialyzed
value
18d
64
97
145 145 137 145 145
I 8d
64
97
Sol
KO.
Flocculation
81
SI
Potassium trichloracetate
I 8d
Potassium acetate
I 8d
64
97
73 73 73
73 64
97
67 61 57 57 57 57
3246
WILLARD
n. MADBON AND
FRANCIS
c. KRAUSKOPF
TABLE V
Electrolyte
Magnesium sulfate
Flocculation Values of Various Sulfates mg. Be0 Hours Sol No. I 6d
per 1 52
dialyzed
168
Flocculation value 0.22 0.22
1
5d
53
I43
12d
57
IO1
0.22 0.22 0.22 0.22
Ammonium sulfate
I 6d
52
I 68
1
5d
53
I43
0.22
12d
57
IO1
0.20
0.22 0.22
0.22
0.20
Potassium sulfate
16d
52
168
5d
53
'43
12d
57
IO1
0.20 0.20
1
0.20 0.20 0.22 0.22 0.20
0.20
72
0.22 0.20
48
0.22 0.22
24
0.22 0.22
I2
0.22 0.22
0
0.26 0.26
Sodium sulfate
I 6d
52
168
1 5d
53
I43
0.20
I2 d
57
IO1
0.20
0.20
0.20
0.20
0.20 0.20 0.20
COLLOIDAL HYDROUS BERYLLIUM OXIDE SOLS
3247
TABLE VI Flocculation Values of the Potassium Ortho Phosphates Electrolyte
Tertiary potassium phosphate Primary potassium phosphate
Secondary potassium phosphate
mg.BeO per 1.
Hours
Flocculation
Sol No.
dialyzed
value
I 8d
64
97
0.048 0.048
I 6d
52
168
53
I43
57
IO1
0.044 0.044 0,044 0,044 0,044 0.048 0.048 0.048
64
97
65
72
72
48
77
24
86
I2
185
0
0.044
0.044 0.048 0,044 0.048 0.048 0.048 0.048 0.048 0.048 0.84 0.84 0.80
52
168
I43
0.032 0.032 0,034 0.034 0.032 0.032
0.032 0.032 97
72
0.035
0,035 0.036 0.036 0.034 0.034
WILLARD H. MADSON AND FRANCIS C. XRAUSKOPF
3248
There are several points to be given consideration concerning these flocculation values. First, all results for each electrolyte agree very closely with each other with the possible exception of those for potassium ferrocyanide. With the ferrocyanide one must take into account the fact that the solutions used were extremely dilute, and also that they change in the presence of light. Although this fluctuation was practically eliminated by using freshly prepared solutions and keeping them in the dark, it was impossible to get as good results with such unstable solutions. However the results of ferrocyanide ion show the order of magnitude of the flocculation value and considering all the variables they are in very good agreement. Second, the flocculation values are reproducible over a long period of time. This is especially shown with secondary potassium phosphate where more than two months elapsed between the first and last determinations. Also the last determinations were made with entirely new standard phosphate solutions. All values agreed with those previously determined irrespective of the time interval, though no determinations were made over a time interval of more than seventy-one days. Third, the flocculation values are of the same order of magnitude as those for ferric oxide." The values for ferric oxide are uniformly lower than those for beryllium oxide, especially for potassium chloride. The sols of ferric oxide are more concentrated than the beryllium oxide sols; this may account, in part for the variation. Table VI1 is a condensed summary of the flocculation values of hydrous beryllium oxide sols as given in the four preceding tables. This summary contains data for dialyzed sols only.
TACLE S'II Summary of the Flocculation Values of Colloidal Beryllium Oxide Electrolyte
KC1 CHzClCOOK CHClzCOOK CC13COOK CHICOOK KzCrO4 MgSO4 (xH4)zSO4 Na2S04 KHzPOa KsA~04 KzHP04.3HzO
K ,Fe (CW), KIFe (CN)e 10
Floc.
No. dif.
No. values
value
sols
averaged
227
3
I43 81 73 59 0.229 0.220 0.213
I
6 5
I
2
I
4 6 18 6 6 16 8
I
8 3 3
0.211
7
0.200
3
0.048 0.046 0.044 0.033 0.0248 0.0103
Hazel and Sorum: J. Am. Chem. SOC., 53, 49-54 (1931).
I
2
8 3
20
4 2
5
8 I4 8 17
COLLOIDAL HYDROUS BERYLLIUM OXIDE SOLS
3249
T h e Injtuence ojthe Length of T i m e of Dzalysis on Certain Properties of Hydrous Beryllium Oxide Sols a-Flocculation
values.
I t is a well known fact that the properties of lyophobic and lyophillic colloidal systems are very dependent upon the purity of the system. Therefore dialysis markedly changes the properties of the sols. Often many colloidal systems are completely destroyed by dialysis, sometimes in a very few hours. After a certain purity is obtained further dialysis does not change the properties to any noticeable degree. The time necessary t o obtain this degree of purity is dependent upon the type of sol used, upon the impurities and stabilizing materials present, and upon the method of dialysis. A study was made of the variation of flocculation values of certain electrolytes, for colloidal hydrous beryllium oxide sols, with the length of time of dialysis. The flocculation values of the four electrolytes, potassium chloride potassium chromate, potassium sulfate and primary potassium phosphate, were determined for the undialyzed sol KO.2 4 L and for other sols which had been dialyzed from twelve to one hundred sixty hours. The experimental results are given in Table VIII.
TABLE VI11 Flocculation Values for Beryllium Hydroxide Sols Variation with Time of Dialysis Hours dialyzed
Electrolytes KC1
0
> 600
I2
220
24
48 72
97 IO1
I08
I43 I 68
-
220
-
-
240
KICr04
K2S04
KH2POI
20
0.26
0.830
0.26
0.22
0.048
0.22
0 22
0.048
0.23
0.22
0 22
0.21
0.048 0.046 0.048 0.048
I
-
-
0.24
0.21
0.22
-
0.24
0.20
0.21
0.20
The data show that the flocculation values for the electrolytes decrease rapidly a t first, become practically constant after twelve hours of dialysis with the exception of those for potassium chromate and even these become constant after twenty-four hours. b-Concentration
of the sols.
The previous experiment led to a study of the change of concentration of the hydrous beryllium oxide sols with the time of dialysis. Table I X and Fig. I give the results of such a study.
WILLARD H. MADSON AND FRANCIS C. KRAUSKOPF
3250
-
0
40
0
0
0
50
U
8
+
I
100
150
I
Time in hours
FIO.I Variation of Concentration with the Length of Time of Dialysis
TABLE IX Variation of the Concentration of Beryllium Oxide with Time of Dialysis Hours dialyzed 0
I2
24
mg. Be0 per 1.
185 86 77
48
72
72 IO1
65 57 63
I 08
I43
5.3
168
52
The data show that there is a rapid decrease of beryllium content during the first few hours of dialysis. After fifty hours there is a rather constant decrease as shown by the straight line part of the graph (Fig. I). It is impossible t o duplicate exactly the pore size of the membranes by the method used in the preparation of the sacs. This may account for the slight variations in the decrease of beryllium content. The large decrease of Be0 content during the first hours of dialysis was expected. The concentrations of the various sols were determined by evaporating a definite amount of the sol in a platinum dish, heating to redness and constant weight and weighing as beryllium oxide. Thus any soluble beryllium salt in the undialyzed or partly dialyzed sols was determined as the oxide and
COLLOIDAL HYDROUS BERYLLIUM OXIDE SOLS
3251
not as its proper salt, and no account was taken of its solubility. Any soluble beryllium salt diffuses through the membrane very quickly. Thus the data show a large apparent decrease of colloidal beryllium hydroxide which is probably not the case but is largely a loss of soluble beryllium salts and a rather constant decrease of colloidal hydrous beryllium oxide. c-The hydrogen ion concentration of colloidal hydrous beryllium oxide systems. The colloidal systems of hydrous beryllium oxide as prepared were apparently the result of the hydrolysis of some basic salt or salts of beryllium. Therefore the undialyzed sols should contain some free acid and consequently
I
I so
100
Tlme of dabus
in
hours
Id0
PO0
FIG.2 The Influence of the Length of Time of Dialysis on the Hydrogen Ion Concentration of Colloidal Hydrous Beryllium Oxide Sols.
have a pH value less than seven. This value should increase with dialysis. A study was made of the influence of dialysis on the hydrogen ion concentration of the sols. The hydrogen ion concentration of five sols were determined by means of the glass electrode and apparatus described by Hazel and Sorum.ll Table X and Fig. 2 give the experimental data obtained.
TABLE X The Influence of the Time of Dialysis on the Hydrogen Ion Concentration of Colloidal Hydrous Beryllium Oxide Systems Sol No.
L 17412) 17424) 18d I 6d
Hours dialyzed
24
l1
0
5.61
I2
97
6.46 6.86 6.98
68
7.00
24 I
PH
Hazel and Sorum: J. Am. Chem Soc., 53, 49 (1931).
3252
WILLARD H. MADSON AND FRANCIS C. KRAUSKOPF
T h e Lyotropic Series and Hydrous Beryllium Oxide. The behavior of colloidal systems with added electrolytes shows certain trends some of which are illustrated by well known rules, Le., Traube’s rule, the Schulze-Hardy rule, the Burton-Bishop rule etc. One of the most general of such phenomena is the lyotropic series. The series vary for different colloidal systems expecially if the sols are of different classes. However they usually follow a general order. The following lyotropic series were obtained from the flocculation values for hydrous beryllium oxide sols as given in Table VII. Anions (increasing flocculat’ion values) : ferrocyanide, ferricyanide, secondary phosphate, tertiary arsenate, primary phosphate, tertiary phosphate, sulfate, chromate, acetate, trichlor acetate, di-chlor acetate, mono-chlor acetate and chloride. Cations (increasing flocculation values) : Sodium, potassium, ammonium and magnesium. These series follow the general order of lyotropic series and agree especially well with the series for ferric oxide the only reversal being the sulfate and chromate.
Hydrous Beryllium Oxide Sols and the Irregular Series. Many colloidal systems, both lyophobic and lyophillic, exhibit the property of being either positively or negatively charged depending on the method of preparation or upon the dispersion medium and its impurities. Concerning the phenomenon of irregular series KruytI3 states “it will always occur when the potential-lowering effect of the cation is far in excess of the potentialraising effect of the anion. This may be due to a high valence of the ion or to a high degree of adsorbability. Polyvalent cations give, therefore, irregular series when they are combined with monovalent anions. But monovalent organic cations act in the same way.” Von Kohei Hakozaki,I4 has shown that positive colloidal iron oxide can be recharged negatively by the addition of potassium ferrocyanide. This suggested that the charge of hydrous beryllium oxide sols might be reversed by the addition of various amounts of ferrocyanide. Table XI gives the various concentrations of potassium ferrocyanide added t o sol ?To. Isd. In every case from molar 0.012 to molar o.ooooo8ferrocyanide, there was complete flocculation within twenty-four hours, while with the more dilute solutions there was no flocculation. Thus it was impossible to change the charge of the sol by the above method using the stated concentrations. If there had been a change of the charge there would have been a region of no flocculation somewhere above 0.000008 molar potassium ferrocyanide. 12 Weiser and Middleton: J. Phys. Chem., 24,641 (1920); Pauli and Wittenberger: Kolloid-Z., 50, 228 (1930). l a Kruyt: “Colloids,” p. 90,trans. by van Klooster (1930). 14 Von Kohei Hakozaki: Kolloid-Z., 39, 319 (1926).
COLLOIDAL HYDROUS BERYLLIUM OXIDE SOLS
3253
TABLE XI Coagulation of Sol No. I j d by Potassium Ferrocyanide (2 ml. of sol added to 3 ml. of electrolyte soln.) Conc. of K4Fe(CN)a 0.012 0.0104 0.0088 0.0072 o.ooj6 0.0040 0.0032 0.0024 0.00208 0 . O O I9 2
0.00176
0,00160 0.00144 0.00128 0.00I I2
Observations
*
*
* * * *
*
* * * * * * * *
* Complete flocculation in
Cone. of KZe(CNh 0.00096 0.00080 0.00064 0,00060 0 . 0 0 0j4
0.00048 0.00044
o .00040 0.00036 0.00032 0.00028 0.00024 0.00020 0.00016 0.00012
Observations
*
* *
* * *
* * * * * * * * *
Cone. of K4Fe(CNh 0.000080 0,000080 0.0000j6
0.000054 0.000048 0.000044
o.000040 0.000036 o.00003 2
0.000028 0.000024 0.000020
o . 0000 I 6 0.00001 2
o.000008
Observations
* * * * * * *
* * *
* * * *
hours. Concentrations, mols per liter of total volume. concentrations less than o.000008 molar did not cause flocculation in 24 hours. 24
Discussion The previous descriptions of the various sols show that their properties are, in general, independent of the preparation except for the heat treatment just before the addition of the water. Many details of preparation (other than those already described) were varied in order to find out if the properties of the sols were changed thereby. Some sols were cooled immediately after preparation, others were allowed to cool a t room temperature; some were dialyzed immediately after preparation, even before they had cooled, while others were dialyzed after several days, weeks, or months; still others were siphoned from various depths of an eight liter aspirator bottle of sol (after the sol had stood in the bottle for one month); but in no case were appreciable variations of the properties observed. Some of the sols were very milky white in appearance when they were first formed, afterward they gradually became less opaque because of the settling of the larger particles. Occasionally a stratification would form and the heavy layer settle slightly and then in about a week either rise again or be destroyed in some unknown manner. This phenomenon w m observed when the sols were kept perfectly still and special care taken not to destroy the stratification. In a few cases the stratification would form and the heavy layer slowly and continually settle until the entire sol had the bluish white appearance of the upper layer. This continual settling of the heavy layer was observed only when the sols were very milky in appearance and apparently rather concentrated. Usually there was no stratification of any kind.
3254
WILLARD H. MADSON AND FRANCIS C. KRAUSKOPF
Several dialyzed sols were carefully frozen solid and allowed to melt a t room temperature. In every case complete coagulation was caused by a single freezing and melting of the dialyzed sol. In no case was coagulation caused by freezing undialyzed sols even though some were frozen and allowed to melt several times. I t was thus shown that dialysis decreased the stability of the sols. The decrease of the stability of the sols w-ith dialysis was also illustrated by boiling. Undialyzed sols were not coagulated by continuous boiling (refluxing) for as long as one week. Boiling dialyzed sols caused them to slowly coagulate, the speed of this coagulation being dependent on the length of time of boiling. Also dialyzed sols changed appearance upon being boiled; the color changed from bluish white, nearly transparent, to definitely opaque milky white. This color change was not perceptible when undialyzed sols were boiled. With many sols there was no settling when centrifuged at 1200 r.p.m. for an hour while a few sols were slightly coagulated by this treatment, These properties indicate that the sols were very stable and had the characteristics of lyophobic colloidal systems. Since the sols were prepared from beryllium chloride and water, it might seem that they were the result of simple hydrolysis of the salt with the formation of colloidal beryllium hydroxide and hydrochloric acid; and thus also the stability be attributed to an excess of beryllium chloride similar t o certain theoretical explanations of the stabiiity of colloidal hydrous aluminum and ferric oxide systems. The amount of chloride present in the sols decreased with continued dialysis from an easily detected amount after 1 2 hours of dialysis to no detectible amount (using acidified silver nitrate) after 140 hours. The flocculation values of electrolytes, and therefore the stability of the sols, remained constant during the same length of time of dialysis. Apparently, then, the sols are not the results of the simple hydrolysis mentioned above and their stability is not entirely due to the excess of beryllium chloride. The method of formation of the sols suggested that they may not be He(OH)2 but some l o w r hydrate. Therefore the following experiment was carried out. Sols No. 2 8 and S o . 29 were prepared frcm the stock sol 2 4 L by dialyzing for IOO hours. One month later no precipitate had formed jn either sol and at that time each sol Tyas coagulated by the addition of I O O ml. of hI/joo primary potassium phosphate. The supernatant liquid was decanted off and each of the precipitates separated from most of the mater by centrifuging. Each coagulum was then dried for one month, at room temperature, in sulfuric acid desiccators through which carbon dioxide free air was allowed to slowly pass. After one mcnth each residue had become constant as was indicated by repeated Iveighings. The amount of phosphate present in each coagulum was calculated frcm the loss of phosphate in the supernatant liquid frcm each sol. The experimental data in Table S I 1 show the loss of weight of hydrous beryllium oxide in each coagulum upon heating, (all weights given in grams).
COLLOIDAL HYDROUS BERYLLIUM OXIDE SOLS
3255
TABLE XI1 Analysis of Coagulum Sol. No.
28
0 . 0 2 7 23 wgt. of KHzPOa added jl JJ ” left in soln. 0.01593 9, ” adsorbed 0.01130 ” ’’ coagulum dried a t 0.1229 room temperature 0.0113 ” ’’ KH2POa in coagulum 0.1116 ” ” Be(0H)Z” ” 0.1021 ” coagulum after heating loss of wgt. by heating coagulum 0.0208 2J )J by heating ads. 0.0016* XH2PO4 ,, ,, by heating 0.0192 ‘LBe(OH)2” 17.20% experimental theoretical 41.87% * Due t o change from KH2P04 t o KPOa.
Sol No.
0.0IsjI
J )
J
J
29
0.02723
0.011 7 1
0.0866 0.0117
0.0749
0.0709 0.01j’i
J
*
0.0016
J
o 0141 18 83c-c 41.87%
The data indicate that hydrous beryllium oxide sols are not colloidal beryllium hydroxide but a hydrous oxide in which there is less than one molecule of water for two molecules of beryllium oxide. This small amount of hydration is also indicated by the fact that solutions of ethyl alcohol of concentrationsvaryingfrom 2jFCto 80% will not coagulate the sols within twentyfour hours, while even 90% ethyl alcohol will coagulate but a few. It appears that the sols are not the result of simple hydrolysis but a complicated combination of decomposition and then hydrolysis (or partial hydrolysis) of the resulting product or products. Thus the stability of the sols may be due to: (I) some minute amount of beryllium chloride or oxy-chloride, ( 2 ) some minute amount of basic beryllium acetate, (3) the character of hydrolysis, (4) the slight hydration of the oxide, and ( j ) the amphoteric nature of the oxide. The slight hydration of the oxide tends to decrease the stability of the sol rather than stabilize it. This also explains, in part, why it was impossible to make more concentrated sols, for it is more difficult to make anhydrous oxide sols than those which are highly hydrated. Since the method of preparation used to make the chloride does not eliminate all acetate, the acetate must be considered in the stability factors although it was present in so small amounts that it could not be detected by the ordinary methods. The properties of highly purified colloidal hydrous beryllium oxide sols are very similar to those of highly purified hydrous ferric oxide sols with one exception, that of the Burton-Bishop rule. An attempt was made to determine the influence of the variation of the concentration of the sols on the flocculation values. Since all beryllium oxide sols that were prepared were compara-
3256
WILLARD H. MADSON AND FRANCIS C. KRAUSKOPF
tively very dilute, no convincing experimental data were obtained, for the amount of change of flocculation value decreases with the dilution of the original sol. It is quite probable that the Burton-Bishop rule might hold for beryllium oxide sols if a stable sol of sufficient concentration could be prepared. In general the flocculation d u e s of the sixreen anions studied were of the order expected. The flocculation values of the various phosphates and acetates need explanation. Table VI gives the flocculation values of the primary, secondary, and tertiary potassium phosphates as 0.045,0.035 , and 0.048 respectively. According to the present theories of flocculation, coagulation is caused by the decrease of the charge of the colloidal particle to a certain minimum called the critical potential. The flocculation is due also primarily to the ions of a charge opposite to that carried by the colloidal particle. Thus the flocculation of colloidal hydrous beryllium oxide sols must be due primarily to the negative ions, for the sols are positively charged. Therefore some explanation must be given for the fact that primary and tertiary potassium phosphates have higher flocculation values than does secondary potassium phosphate. The ionization constants for phosphoric acid* have been given as 9.4 X IO-: 1.4x 10-7, and 2 . 7 X 1 0 - l ~for the first, second and third hydrogens respectively. Therefore the three phosphate solutions will effectively ionize as follows: KHzP04 KHzP04 7r=’c K+ IIzPo4HzPOaH+ HPO4-HPO4-- Ft H+ Pod--HOH @ H+ OH-
*
+
+ + +
KzHPOa
**+
K3P04 3Kf f PO&--HOH H+ OHPO4--H+ & HPOI-HPO4-H+ $ H2P04-
+
+
Each solution contains the same ions and the only variations are in the number of ions and their proportions. Experiments have shown that the pH values for phosphate solutions of the concentrations used, increase from 3-5 for the primary to about 8 for the secondary. Therefore the flocculation values can not be explained on any simple adsorption of hydroxyl or hydrogen ions, for if that were the case, there would be a regular change of the floccu* Britton: “Hydrogen Ions,” p. 13j (1929).
COLLOIDAL HYDROUS BERYLLIUM OXIDE SOLS
3257
lation values, either increase or decrease, from the primary, through the secondary to the tertiary phosphates. The flocculation values show that this is not the case. Also it is well known that the flocculation power of ions increases enormously from the monovalent, to the bivalent, to the trivalent, etc. Therefore PO- - - ion should have a much greater flocculating power than either HPOa- - or H,PO,-, with the last having the least flocculating power of the three ions. Ionization constants show that, for phosphate solutions of the same molar concentrations, the tertiary phosphate solution has the greatest number of PO,- - - ions. Therefore the tertiary phosphate might be expected to have the greatest flocculation power. Again data show that this is not the case. Simple ionization and adsorption alone do not explain the differences of flocculation values of these phosphates. Beryllium oxide and hydroxide are thought to be amphoteric and thus their properties are changed as the pH of the solutions changes. Therefore the solubility of the hydroxide (or oxide) changes with the pH of the solution, and there must be a minimum solubility at some definite pH value, which may be called the “minimum solubility pH.” As the pH of the solutions changes from approximately 3-5 with the primary phosphates to about 9 with the tertiary, phosphate, this pH of minimum solubility of the hydroxide is passed. It is also reasonable to assume that the properties of the colloidal oxide or hydroxide also change with the amphoteric nature of the compounds, thus having a minimum stability of the sols a t or near the transition pH for the change from basic to acidic properties of the oxide. Then also it is easily seen why the secondary phosphate, which has an intermediate pH value to the other two phosphates, should have a higher flocculating power than either the primary or tertiary phosphates. The secondary phosphate solution has a pH value nearer the pH of minimum stability of the sol and the sol is more easily coagulated under those conditions, which gives the secondary phosphate lower flocculation value. The other two phosphate solutions have pH values farther away from this pH of minimum stability and therefore the sol is more stable and the salts have higher flocculation values. However more work should be done to definitely prove that this and other sols of amphoteric oxides have a decreased stability at some rather definite pH value of the system. Table VI1 shows that the flocculation power of the acetates decrease in the order, CHSCOO-, CC~SCOO-, CHClzC00-, and CH&lCOO-. The flocculation value of the potassium salts of the acetates are 59, 7 3 , 8 1 , and 143, respectively. Neither the order of the increasing degree of dissociation of the acids, the order of the increasing molar conductances of the salt, nor the order of the increasing ionic mobilities of the acetate ions, follows this order of increasing flocculation values. Also there is not sufficient variation in the conductance and in the ionic mobilities to account for the large variation of the flocculation values.
3258
WILLARD H. MADSOS AND FRANCIS C. KRAUSKOPF
Thus the flocculation values are dependent upon the three properties, ionization, adsorbability and the size of the ions. The acetate has the greatest flocculation power for it is highly adsorbed due to its organic nature. The other three acetates are all organic but their degree of ionization increases with increase of chlorine content. Thus of the three chloride substituted acetates the flocculation power decreases in the following order: tri-chloracetate, di-chloracetate and mono-chloracetate.
Summary A new method for the preparation of hydrous beryllium oxide sols has been developed, and a study made of the effect of changing the conditions of preparation. 2. h detailed study has been made of certain properties of highly purified colloidal hydrous beryllium oxide systems. 3. The flocculation values of sixteen different electrolytes for hydrous beryllium oxide have been determined by the classical method. 4. The nature of the coagulum of hydrous beryllium oxide sols has been determined. 5 . h study has been made of the influence of the length of time of dialysis of hydrous beryllium oxide sols on:a) flocculation values of various electrolytes b) the concentration of the sols c) the hydrogen ion concentration of the sols. 6 . Theoretical explanations have been given for:a) the variations of the flocculation values of the phosphates and of the acetates. b) the change of concentration of the sols with the length of time of dialysis. c) the change of flocculation values of electrolytes with the length of time of dialysis. d) the stability of hydrous beryllium oxide sols prepared by the met hod developed in this investigation. I.
Madison, Viseonsin June, 1931.
