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of diamond dust. Further, the authors Irish to thank Dr. Ying Fu of this laboriltory for valuable suggestions relating to this work. REFEREKCES (1) ADAM:T h e Physics and Chemistry o j Surfaces, 3rd edition, pp. 17-105. Oxford University Press, London (1941). (2) AYON,Smith, A X D THORXHILL: Ind. Eng. Chem., Anal. E d . 16,257 (1943). (3) BRUNAUER: T h e Adsorption ojGases and V a p o r s . Vol. I . Physical Bdsorption, pp, 151-5. Princeton University Press, Princeton (1943). (4) BRUNAUER, EMMETT, AXD TELLER: J. Am. Chem. SOC.60, 309 (1938). (5) DEITZAND GLEYSTEEY: J. Research S a t l . Bur. Standards 29, 191 (1942). (6) DODGEA Y D DAVIS:J. Am. Chem. SOC.49, 610 (1927). (7) EMMETT: Advances i n Colloid Science, edited by E . 0. Kraemer, pp. 1-36. Interscience Publishers, Inc., Xew York (1942). (8) F c , HANSEN,ASD BARTELL: J . Phys. & Colloid Chem. 62, 374 (1948). (9) HARKINS AND J U R A J. : Am. Chem. SOC.66, 1362 (1944). (10) HARKINSA N D JCRA:J . Am. Chem. SOC.66, 1366 (1944). (11) JOYIER, WEINBERGER, ASD MONTGOXERY: J. Am. Chem. SOC.67, 2182 (1945). (12) LIVIHGSTON: J . Chem. Phys. 16, 617 (1947). (13) hIEYER AND RONGE:Angew. Chem. 62, 637-8 (1939). (14) RIES, NORDSTRAND, ASD KREGER: J. Am. Chem. SOC.69, 39 (1947). (15) SCOTT:“The Calibration of Thermocouples a t Low Temperatures” in Temperature, Its Measurement and Control in Science and I n d u s t r y , American Institute of Physics, pp. 21C-11. Reinhold Publishing Corporation, New York (1941). (16) SMITHAND BELL:S a t u r e 162, 109 (1948).
COLLOIDAL DIHYDROXY DIHYDROGES PHOSPHATES OF ALUMINURl AND IROK WITH CRYSTALLISE CHARACTER ESTABLISHED BY ELECTROS A S D X-RAY DIFFR.ICTIOS’, ? C . V . COLE
AXD
11. L. JACKSOS
Department o j S o i l s , Cnioersity of W i s c o n s i n , M a d i s o n , W i s c o n s i n Received August 28, 1949
Soluble phosphate fertilizers, when applied to acid soils high in minerals of advanced weathering stages (6), become fixed in much less soluble forms ivhich are only slowly available to plants. The nature and properties of the aluminum and iron phosphates formed have been the object of considerable investigation in attempts to develop reliable tests of phosphate fertility and fixing capacity of soils and to form the basis of more efficient agronomic practices. The demands for a better understanding of the processes of phosphate fixation have been 1 Presented a t the Twenty-third National Colloid Symposium, which was held under the auspices of the Division of Colloid Chemistry of the American Chemical Society at hlinneapolis, Minnesota, June 6-8, 1949. 2 This work was supported in part by the Graduate Research Committee through a grant from the Wisconsin Alumni Research Foundation.
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intensified by the need for phosphate fertilization of such soils in this country and in tropical areas being developed for food production or being considered for development. Full interpretation of the results of recent experiments on phosphate fertilizer uptake using radioactive phosphorus has been severely hampered by a lack of knowledge of the forms in which phosphate is present in the soil system and the properties of the various phosphate compounds in soils. Because of the extreme complexity of t'he soil system, many different concepts of the chemical mechanism of phosphate fixation have been advanced. A number of investigations (1, 5 , 7, Y,9, 11) support the view that precipitation of colloidal iron and aluminum hydroxy phosphates is the cause of fixation by highly weathered acid soils. Smenson: Cole, and Sieling (10) determined the pH values of series of solutions of iron and aluminum chloride, each containing a known excess of hydrochloric acid and a known amount of phosphate, after addition of increments of standard sodium hydroxide solution and digestion of t,he suspensions for 1 hr. a t 100°C. The inflection points of the resulting t,it,rat,ioncurves corresponded to the points of maximum precipitation of phosphate, pH 3-3.4 for the iron phosphate and pH 3.8-4.0 for the aluminum phosphate (10). It was found that the sum of the milliequivalents of hydroxyl ion added t,o reach the isoelectric point (after the neutralization of excess free hydrochloric acid) and the milliequivalents of phosphate chemically combined as HzPO; exactly equaled the milliequivalents of iron or aluminum in the original solutions, indicating that the precipitates were dihydroxy dihydrogen phosphates. The isoelectric point of the aluminum compound as determined by the inflection in the titration curve and the pH value of maximum phosphate precipitation was confirmed by observation of the migration of the precipitat'es in a microcataphoresis cell. They concluded that, at the isoelectric pH values, aluminum and iron dihydroxy dihydrogen phosphates, Al(OH),H,PO, and Fe(OH)zH2P04,were precipitated rather than the normal phosphates. The coordinated water given in their formulas (10) does not occur in the crystals and is therefore omitted as cited here. Precipitates formed under ihese conditions were further examined in the present study. Ensminger (3) has recently reported a study of the compounds formed as the result of the precipitation of aluminum from solution by addition of ammonium phosphate and phosphating aluminum oxide, kaolinite, and soil colloids. He concluded that the valence of the phosphate ion in the precipitate increased from one to tw-o or three on increasing the pH value of the phosphating solution. He reported x-ray diffraction patterns of crystalline preparations of aluminum phosphate which were described as being different from AlP04. The objectives of the present study were (a) t o determine whether the species of aluminum phosphate formed from solution are the same as those formed upon phosphating aluminum-containing minerals and ( b ) to determine whether these species formed by prolonged digestion at steam hotplate temperatures are the same as the colloidal aluminum and iron phosphates which precipitate a t room temperature, such as would occur under soil conditions. The crystalline character of the phosphates prepared from solution was investigated by means of x-ray
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diffraction, electron diffraction, and the electron microscope. The stoichiometric relations were studied by chemical and thermal analyses. IXSTRUMENTAL TECHNIQUES
X-ray diffraction patterns were obtained by means of the General Electric X-ray Diffraction Apparatus, unfiltered iron radiation being used for the film type (XRD-1) and nickel-filtered copper radiation being used for the General Electric Geiger counter type (XRD-3). Electron microscope photographs of the precipitates were made with the RCA electron microscope (EMU-I), equipped with a high-intensity “biased” electron gun. Electron microscope slides were prepared by placing a drop of a barely opalescent suspension of the colloid on a 200-mesh screen previously coyered with a thin film of Formvar, and then drying it in a desiccator. The electron diffraction patterns were made with the electron diffraction adapter of the same instrument. The electron beam was accelerate: with a fixed potential of 50 kv. corresponding to a wave length of about 0.05 .I.Transmission patterns were obtained by supporting the sample on a ZOO-mesh wire screen covered with a thin film of Formvar. Slides were prepared by drying down a sufficient amount of an aqueous suspension of the precipitate on the Formvar so that the film of precipitate was almost opaque to light. PREPARATION O F THE PHOSPHATE PRECIPITATES
The iron and aluminum phosphates studied were prepared by adding, \vith stirling, approximately 0.1 S sodium hydroxide to slightly acid solutions of aluminum or iron chloride containing a 2: 1 excess of phosphate (as KH2POa or XaH,P04) until the desired pH value was obtained. These phosphates nere prepared a t pH values ranging from 2 to 8 to observe the effects of pH on the type of compound formed. After the addition of sodium hydroxide, the sample of suspension was placed on a steam-pressure hotplate, where it reached a temperature of 90°C. After digestion on the hotplate for varying periods of time the precipitate was filtered on a Buchner funnel, redispersed, and washed three times in hot 1 per cent sodium chloride solution to remove the sorbed or saloid-bound phosphate. The precipitate was then washed with hot distilled xater until the filtrate was chloride-free. Portions of the still \vet, precipitate n-ere redispersed for preparation of electron microscope and electron diffraction slides and the bulk of the sample was air-dried, lightly crushed to pass a 160-mesh sieve, and stored for x-ray diffraction analysis, chemical analysis, and thermal weight-loss studies. CRYST.4L S P E C I E S O F PRECIPITATED A L I X I X U M . W D IRON PHOSPHhTES
Freshly precipitated aluminum and iron phosphates v w e found to be g-amorphous,3 producing no diffraction patterns with iron x-radiation (1.93 A. wave length). These precipitates were found to consist of extremely minute crystals 3 The term x-amorphous is employed to refer t o a material which does not give a ponder x-ray diffraction pattern. An x-amorphous material may or may not be crystalline enough t o give electron diffraction patterns, and may give a broad diffuse x-ray diffraction band typical of amorphous materials.
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which did produce electron diffraction patterns. Larger crystals were produced when the colloidal suspensions were digested a t temperatures of TO-100°C. The larger crystals formed in this way gave x-ray diffraction patterns with increasing sharpness of lines as the crystal size increased. The nature and relationships of the different crystal sizes are similar to those of oxide and hydroxide crystals as reported by ITeiser and hlilligan (12).
Electron microscopic obsercations Crystals of different sizes were observed under the electron microscope in precipitates which had been digested for increasing periods of time. Electron micrographs of the freshl3. precipitated phosphates of aluminum and iron (figures 1 and 4 , respectively) showed minute crystals. I n the next stages of growth, after digestion at 90°C. from 8 hr. t o 2 days depending on the method of initial precipitation, the precipitates consisted of minute \Tell-defined single hexagonal crystals (figure 2) or aggregates. These aggregates of fine crystals frequently formed hexagonal networks. Precipitates which had been digested for longer periods of time contained ivell-defined hexagonal crystals, approximately 1-5 microns in diameter and 0.5-1 micron in thickness. These crystals were opaque t o the electron beam (figures 3, 5, and 6). Rupturing and curling of the supporting film occurred x i t h an intense electron beam, and the crystals were thus rotated in the microscopic field and seen t o consist of thin hexagonal plates. The hexagonal aggregates of the finer crystals of precipitates in earlier stages of growth provide evidence that the mechanism of crystal growth in these precipitates is not only the usual process of the dissolution of the finer crystals and the deposition on single larger crystals. The mechanism of crystal growth includes also a process in which the smaller crystals aggregate to form the framexork of a much larger crystal which is gradually filled in by crystallographically oriented finer crystals, the whole being cemented together by the usual solution and deposition process. The occurrence of this phenomenon is probably indicative of the extreme insolubility of the crystal species involved. The growth of crystals to appreciable size by digestion a t hotplate temperature is extremely slow, requiring days or weeks to convert the finest crystals (figure 1) to the 1-5 micron range. The crystal size found in fresh precipitates was determined by the usually dominant factor-namely, rate of precipitation-as determined by the temperature, the concentrations of the solutions, and rate a t vhich they were mixed. The formation of relatively large crystals through control of the processes of precipitation is more effective than the growth of extremely small crystals to larger sizes, a fact indicative of the extreme insolubility of the crystal species involved. It was also observed that under the most intense electron beam (biased gun), crystals of both the iron and the aluminum phosphates soften and fuse into droplets. The fusion has also heen observed in the finer material, ivhich under a moderately intense beam softens and takes on regular hexagonal shape before fusing into droplets. Hexagonal aggregates of smaller crystals have been observed to fuse into single crystals under this heating effect of the electron beam. During
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C. 1'. C O L E . I S D M. L. JICKSOS
FIGS.1, 2, and 3 . Aluminum dihydroxy dihydrogen phosphates: 1, freshly precipitated; 2, after moderate digestion; 3 . after extensive digestion. FIGS. 1,5,a n l 6 . Ferric dihydroxydihydrogenphosphntes;4, freshly precipitated; 5 and 6, after extensive digestion. FIGS.7 a n J 8. Crystals of precipitated aluminum hydroxide FIGS.9 and 10. Crystals of precipitated ferric hydroxy oxide
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the other observations, the electron beam intensity was kept far belox that necessary t o cause fusion of the crystals. Hexagonal crystals very similar to those of the aluminum and iron phosphates have been observed in precipitates of both the aluminum and iron hydroxy oxides (figures 7, 8, 9, and 10). These crystals, in contrast t o those of the phosphates, did not fuse under the most intense electron beam. The presence or absence of these oxides in prepared precipitates may also be determined readily by both electron and x-ray diffraction techniques.
X-ray diffraction data The aluminum phosphate preparations were classified, by comparison of their x-ray diffraction patterns (table l), into three types, termed for convenience A, B, and C. The preparations of type A were formed by precipitation at a pH value of 3.8 (the isoelectric pH) and digestion of the colloidal suspensions at 90°C. The x-ray diffraction data (table 1) of preparation A showed it to consist mostly of the crystal species ~ a r i s c i t e ,an ~ aluminum dihydroxy dihydrogen phosphate mineral, Ak1(OH)&P04.T'ariscite is given the formula A1PO4.2H20 by Winchell (13), a formula which is equivalent to .41(OH),H2P04, the latter being more appropriate terminology according to its chemical properties and dehydration data considered below. Preparation A also contained a small amount of the crystal species ~ t e r r e t t i t e . ~ The aluminum phosphates of type B were obtained by precipitation at a pH value of 3.3 and digestion of the colloidal suspensions at 90°C. These precipitates consisted mainly of the crystal species sterrettite,' an aluminum dihydroxy monohydrogen dihydrogen phosphate, [LkI(OH),],HP04H2P04.This precipitate also contained a small amount of the crystal species variscite. I t is significant, that sterrettite was also prepared by digesting a suspension of aged aluminum hydroxide, rll(OH)r, in a phosphate solution at 90°C. X-ray diffraction patterns of the Geiger counter type obtained from the aluminum preparation B are shown in figure 11. Curve a represents the x-ray crystalline precipitate prepared by extensive digestion at 90°C. Curve b represents the x-amorphous precipitate prepared by digestion for only a short time. The correspondence of position and intensity of the peaks in the x-amorphous preparation (curve b ) to the peaks of the more highly crystalline preparation (curve a) suggests that the two precipitates might consist of the same crystal speciesnamely, sterrettite. The arrows in figure 11 indicate the position ofelectrondiffraction maxima of the x-amorphous preparation. These maxima correspond to those of variscite and sterrettite (table 1). Preparation C was formed Tvhen a suspension of colloidal aluminum phosphate was taken to dryness on the hotplate at approximately 105'C. The crystal species of this preparation has not as yet been identified. The species does not correspond to the aluminum hydrosy phosphate minerals wavellite or vashegyite. This RIineral specimen obtained f r o m Ward's Satural Sciences Establishment, Rochester, Xew York. American Society for Testing Materials X-ray Diffraction Card File KO.I1 338.
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C. V. COLE A N D M . L. JACKSON
135
COLLOIDAL PHOSPHATES O F BLUMIXUM 4 K D IROX
3
N Y
N
c!
L
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C. V. COLE A N D M . L. JACKSON
species is considered unlikely to form in soils because of the high temperature involved in its preparation. The iron phosphate preparations were differentiated, by comparison of their x-ray diffraction patterns, into two types termed for convenience P and Q. The preparations of type P were formed by precipitation at pH values ranging from 2 to 5 and digestion a t 90°C. The crystal species formed was found to be strengite,6 a ferric dihydroxy dihydrogen phosphate, Fe(OH)2H2POa.Strengite is given the formula FeP04.2H20by Winchell (13),but the dihydroxy dihydrogeri terminology is more correct according to the chemical properties and dehydration
-
0
I-
=o LL LL
c3
50! 20 0
IO
20
30
DIFFRACTION
40
50
ANGLE
60
-
70
8C
2 0
FIG.11. Geiger counter x-ray spectrometer patterns of the precipitated aluminum dihydroxy (mono, di)-hydrogen dihydrogen phosphate, sterrettite, [A1(OH)213HP04H2P0~. Curve a , x-ray crystalline pattern after extensive digestion of the precipitate; curve b , amorphous band, after slight digestion of the precipitate.
data considered below. The two minerals variscite and strengite are end members of a dihydroxy dihydrogen phosphate series, (A1,Fej (OHj2H2P04, of which barrandite' is an intermediate member (13, p. 138). The aluminum and iron apparently may replace each other in all proportions. The x-ray diffraction film patterns of variscite and strengite (figure 12) illustrate the similarity of the two crystal species and the clear definition of the x-ray diffraction lines of these phosphate preparations. Further study of the longer spacings in the strengite lattices (table 1) is needed to relate the presence or absence of lower order (hkl) spacings to crystal habit and conditions of preparation. American Society for Testing Materials X-ray Diffraction Card File No. I1 515.
' American Society for Testing Materials X-ray Diffraction Card File No. I1 276
4 5
5 4
I
TT--P-+rr-T+-*
A
12
3
-b /
B
. :x L
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C. V. COLE AND M. L. JACKSON
pattern (XRD 3146, table 1) at 4.84, 2.43, and 1.217 i., identified as that of sterrettite. But the powder diffraction pattern shows a large number of other (hkl) spacings, a fact which suggests that the electron diffraction specimen was highly oriented with respect to one (hkl) index of the crystals. The electron micrographs (figures 1 to 6) show the crystals to be flat and hexagonal, and the flat crystals t'o be mainly oriented with their flat surface normal to the electron beam. It is likely therefore that the diffraction pattern represents the (0001) indices of the hexagonal crystals. I t is interesting to note the hexagonal crystal habit observed as contrasted to the orthorhombic crystallographic system listed in the literature (table 2). A further explanation of the limited number of electron diffraction lines can be formulated on the basis of the platy crystal habit observed in the electron microscope. One dimension may exceed the critical size range for an electron diffraction pattern by the transmission method. If the long axis is opaque to the electron beam, no spacings may correspond to this axis, since the entire electron beam may be absorbed or deflected. However, if the crystals are thin enough so that the electron beam easily passes through, the diffraction spacings may represent the indices along the short (c) axis. When the sample for electron diffraction mas digested to increase crystal size and the specimen on the formvar film was stirred up to give random instead of oriented crystals, various other electron diffraction spacings were obtained (table 1).Under these circumstances, reflection patterns could be produced by the random crystals without the necessity of penetration through the thick crystals. A crystallographic analysis of the diffraction spacings in terms of (hkl) indices is needed. COMPOSITION AND MOLAR RATIOS O F T H E PRECIPITATED PHOSPHATES AND T H E CORRESPONDING MINERAL PHOSPHATES
The chemical composition of the precipitated phosphates compares closely with that of the corresponding mineral phosphates (table 2). The chemical formulas presented in the table correspond more closely to the properties of the compounds than do the formulas given in earlier literature, for example, AIPOl. 2H20 (13). In this regard, it is significant that the ignition losses occur gradually over a temperature range considerably above 105°C. in much the same may that wat,er is lost from gibbsite, Al(0H)S. The ignition loss represents in both cases OH- of the lattice rather than molecular H20. The isomorphous series variscite-barrandite-strengite, (A1,Fe)(OH),H2POc (table 2), is a dihydroxy dihydrogen phosphate series in which aluminum and iron are isomorphously substituted. The ignition losses at 105-800°C. by the precipitated phosphates variscite and strengite show good agreement with the theoretical values (table 2). The x-ray diffraction patterns of variscite (Al) and strengite (Fe) are very similar (table l ) , with the strengite pattern having slightly greater spacings due to the larger size of the ferric ion as compared with the aluminum ion. The barrandite spacings' are intermediate between those of variscite and strengite. For sterrettite, the ratio of aluminum to phosphate and the chemical char-
COLLOIDAL PHOSPHATES OF ALUMINUM AND IRON
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V. COLE AND M. L. JACKBON
acteristics of the aluminum phosphate precipitates support the formula [ A ~ ( O H ) Z ] ~ H P O ~ H(table Z P O ~2). Only consideration of the crystal structure of this species will finally establish the formula. The ignition losses of the sterrettite precipitates did not show good agreement with the theoretical values according to the above formula for the mineral sterrettite (table 2). Because of the small particle size of the precipitated sterrettite, lattice OH- ions may be lost a t temperatures lower than 105’C. Similar loss of water derived from lattice OH- ions is observed in precipitated gibbsite and various hydroxy oxides. The formulas of other mineral phosphates-wavellite, dufrenite, vashegyite, and palmerite-are also listed in table 2 for comparison. The process of precipitation of aluminum and iron dihydroxy phosphates is pictured as the polymerization of Werner-type complexes Al(0H)(H20)rHzPO: and Fe(OH)(HzO)4HzPO:from solution. The tendency for ions of aluminum and iron to form complexes with various anions such as phosphate is well known (4, p. 820). During precipitation of the phosphate crystals, water is visualized as being replaced from the complex ion by shared crystal lattice OH- ions forming, for example, the sterrettite or variscite lattices, depending upon the conditions of precipitation. In the present investigation, rapid precipitation and digestion a t pH values of 5 to 6 favored the formation of sterrettite containing both HZPO; and HP0;ions, in 1:1 ratio. Slower precipitation and digestion a t lower pH values ( 2 . 3 4 ) , on the other hand, favored the formation of variscite containing phosphate as the H2PO; ion only. Swenson et al. reported (10) that precipitated aluminum phosphate contains a higher aluminum to phosphate ratio than 1:1 when freshly precipitated, but that this ratio approaches 1: 1 as the precipitate is digested. Increases of the phosphate concentration in solution also favored the approach to this ratio. This equilibrium type of reaction is interpreted, in view of the present identification of the crystal species formed, to represent the conversion of the sterrettite component of the precipitate, with an aluminum to phosphate ratio of 3:2 (table 2), to variscite with an aluminum to phosphate ratio of 1:l. In the work of Swenson el al. (lo), the mole ratio of phosphate to aluminum or iron did not exceed 1:1 in the precipitate, even in the presence of a ninefold excess of phosphate in solution, when sorption of saloid-bound phosphate was prevented by the presence of a high concentration of sodium chloride. Digestion of the precipitates in solutions of high phosphate concentration is readily seen to favor formation of the crystal species with the maximum phosphate content (minimum phosphate valence HzPO;)-namely, variscite. The fact that the precipitates prepared a t room temperature are crystalline (to electron diffraction) and are the same species as the precipitates made x-ray crystalline by digestion at hotplate temperatures is important new information relative to the interpretation of phosphate precipitation (“fixation”) in soils. Dean and Rubins (2, p. 378) had visualized phosphate fixation as replacement by HzPO; of OH- from the crystal lattices of soil minerals. Sieling (9) had concluded that the mechanism of phosphate precipitation by ground or alkali-activated kaolinite involved reaction with “free (hydroxy) alumina” rather than with the
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kaolinite lattice. Low and Black (8) similarly had concluded that phosphate precipitation by kaolinite was through reaction with alumina released from the kaolinite lattice, and they showed a simultaneous release of silica from the kaolinite lattice as alumina was lost. Ensminger (3) had established that x-ray crystalline aluminum ammonium phosphates are produced by the reaction of ammonium phosphates with kaolinite, aluminum oxides, and soil colloids. Aluminum and iron hydroxy phosphates are so extremely insoluble that they do not at room temperature form crystals of large enough size to be x-ray crystalline. The identification by electron diffraction of the crystal species formed ut room temperature (cf. soil temperatures) constitutes the first direct evidence of the particular crystal species of aluminum and iron phosphates which probably precipitate during phosphate fixation under acid soil conditions: namely, members of the variscite-barrandite-strengite isomorphous series, (A1,Fe) dihydroxy dihydrogen phosphate, and sterrettite, aluminum dihydroxy (mono, di)-hydrogen phosphate. SUMMARY
Aluminum and iron phosphates precipitated from slightly acid solutions have been investigated to identify the crystalline species formed after treatment to increase the crystal size. The objective was also to determine whether these crystal species are the same as those formed immediately upon precipitation from solutions a t room temperature and thus probably formed under acid soil conditions. The results of the investigation may be summarized as follows: 1. Electron micrographs of precipitated phosphates of aluminum and iron which had been digested for several hours a t 90°C. showed the presence of minute, but well-defined, hexagonal crystals occurring singly and in aggregates. Precipitates which had been digested for several days contained well-defined hexagonal crystals approximately 1-5 microns in diameter and 0.5-1 micron in thickness. 2. Extended digestion a t 90°C. of the freshly precipitated dihydroxy dihydrogen phosphates which were x-amorphous (amorphous to x-rays) produced precipitates which gave sharp line x-ray diffraction patterns. Intermediate digestion periods resulted in precipitates which gave amorphous-type, broad-line x-ray diffraction patterns. 3. The crystal species variscite and sterrettite were identified in the aluminum phosphate precipitates by means of their diffraction spacings. Precipitate A, precipitated and digested at p E 3.8, consisted mostly of variscite, .41(OH)zHzPOI, with some sterrettite. Preparation B, digested at pH 5.5, consisted mostly of sterrettite, [Al(OH)z],HPO4H2PO4, with some variscite. 4. The iron phosphate precipitate was identified as the strengite crystal species, Fe(OH)zH,PO,, by means of its x-ray diffraction spacings. This preparation was found in precipitates formed at pH values ranging from 2 to 5 . 5 . Electron diffraction patterns showed that the dihydroxy dihydrogen phosphates freshly precipitated from solutions at room temperature or at 90°C. consisted of minute crystals which were of the same species as those made x-ray crystalline by digestion of the suspensions a t 90°C. for several hours or days.
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The fact that the precipitates prepared at room temperature are crystalline (to electron diffraction) and are the same species as are formed by digestion a t hotplate temperatures is important new information relative to the interpretation of phosphate precipitation (“fixation”) in soils. This relationship constitutes the first direct evidence of the particular crystal species of aluminum and iron phosphates which are probably precipitated under acid soil conditions: namely, members of the variscite-barrandite-strengite isomorphous series, (A1,Fe) dihydroxy dihydrogen phosphates, and sterrettite, aluminum dihydroxy (mono, di)-hydrogen phosphate. REFERENCES
(1) COLEMAN, R.: Soil Sci. SOC.Am. Proc. (1944)9, 71 (1945). (2) DEAN,L.A., AND RUBINS,E. J.: Soil Sci. 89, 377 (1947). (3) ENSMINQER, L.E.: Soil Sci. SOC.Am. Proc. (1948) 13, in press (1949). (4) GRAHAM, R . P., AND THOMAS, A. w.: J. Am. Chem. Soc. 69, 816 (1947). (6) HECK,A. F.: Soil Sci. 37, 343 (1934). (6) JACKSON, M.L.,et al.: J. Phys. & Colloid Chem. 62, 1237 (1948). (7) KELLEY,J. P., AND MIDQLEY,A. R . : Soil Sci. 66, 167 (1943). (8) Low, P. F., AND BLACK,C. A.: Soil Sci. SOC. Am. Proc. (1947)12, 180 (1948). (9) SIELINO,D.H.:Soil Sci. SOC.Am. Proc. (1946) 11, 161 (1947). (10) SWENSON, R . M., COLE,c. V., AND SIELING, D. H.: Soil Sci. 67, 3 (1949). (11) TRUOG, E.: J. Am. SOC.Agron. 22,874 (1930). (12) WEISER,H.B.,AND MILLIOAN,W. 0.: J. Phys. Chem. 44, 1081 (1940). (13) WINCHELL,A. N.:Elements of Optical Mineralogy. I I . Descriptions of Minerals, 2nd Edition. John Wiley and Sons, Inc., New York (1927).
BEHAVIOR OF T H E ALUMINA-WATER SYSTEM’ W. E . HAUTH, JR.’
Iowa Engineering Experiment Station and Ceramic Engineering Department, Iowa State College, Ames, Iowa Received August 28, 10.69 INTRODUCTION
This work was initiated for the purpose of investigating and improving the slip casting method for the formation of articles from non-plastic, pure refractory oxides, aluminum oxide being the specific material used. In conjunction with this, a theoretical basis was offered for this process. The theory is based upon the principles of colloid chemistry, since the system studied exhibits many of the phenomena so familiar to the colloid chemist. It should be noted, however,
* Presented a t the Twenty-third National Colloid Symposium, which was held under the auspices of the Division of Colloid Chemistry of the American Chemical Society at Minneapolis, Minnesota, June 6-8, 1949. 2 This paper is based on material submitted by W. E. Hauth, Jr., as a thesis in partial fulfillment of the requirements for the degree of Doctor of Science at the Massachusetts Institute of Technology.
