X-RAY STUDIES ON THE HYDROUS OXIDES. VI1 FERRIC OXIDE HARRY B. WEISER
AND
W. 0. MILLIGAN
Department of Chemistry, The Rice Instilute, Houston, Texas Received J u n e 14, 1934 THE BROWN GEL
The interaction of solutions of ferric salts and soluble bases gives a highly gelatinous precipitate of hydrous ferric oxide, frequently misnamed ferric hydroxide. The composition and other properties of this brown gel depend upon the conditions of formation and the age of the sample. Heating under water ( 1 , 4 , 5 , 8 , 12) or dilute salt solutions (2) for a long time, or aging a t room temperature (14) for a much longer time, gives an almost anhydrous, more or less brick-red product. X-ray analysis by Bohm (2) showed that boiling in the presence of water, potassium chloride, or ammonium chloride solutions gives a-Fe2O3,whereas aging a t 150°C. in the presence of 2 M potassium hydroxide gives a-Fe203.HzO. Katsuria and Watanabe (9) obtained the a-FeZ03x-ray pattern from a gel heated to 150°C. in an autoclave. Nichols, Kraemer, and Bailey (11) obtained the a-Fe2Oa pattern for the coagula from aged sols. Although all the evidence points to the aged gel being a-FezOs, it is generally agreed that the x-ray diffraction pattern of the freshly precipitated oxide shows no lines or bands (2,3,6,7,15). The results of a systematic study of the transformation from the brown gel to a-Fez03are given in the following section.
Aging under water and ferric chloride solutions The brown gel was precipitated a t about 20°C. by the addition of 300 cc. of 15 M ammonium hydroxide to 1 liter of 0.2 M ferric chloride solutiog. The precipitate was washed with dilute ammonia by the use of the centrifuge until free of chloride, and finally with water until only a trace of ammonia remained. Samples of the washed brown gel were aged as indicated in table 1. X-ray analysis of the products shows that, although no lines or bands can be detected in the original gel, the lattice of a-Fez03appears gradually upon aging in the cold, and more rapidly a t 100°C. I n figure 1 are given densitometer curves (10) of the x-ray negatives for the products aged a t 100°C. These show clearly the progress of the transformation from the brown gel to a-Fe20a. 25
26
HARRY B. WEISER AND W. 0. MILLIGAN
Aging in the presence of electrolytes Since the results in table 1 show that the presence of ferric chloride delays the transformation from the brown gel to a-FeZ03,a systematic study of the effect of added electrolytes was undertaken. The brown gel was precipitated a t 32”C., as described above, and all washings were carried out by decantation without using the centrifuge. The washing was facilitated by this procedure, since the matting of the precipitate was avoided. Samples were aged in the presence of ammonium hydroxide, ammonium chloride, ferric chloride, and hydrochloric acid. Ten additional samples were aged under pure water as a control experiment. The results are TABLE 1 Aging of Pe~Os.xHzOin the presence of water and ferric chloride solutions AGING
MEDIUM
FEMPERATURE
‘IME OF AGING
RESULTS OBTAINED B Y X-RAY ANALYBIS
COLOR
Water
Room temperature
0 8 days 3 months 5 months 6 months 1 year 2 years
No lines or bands visible No lines or bands visible No lines or bands visible One or two weak broad bands Weak a-FezOs pattern Good a-Fe203pattern Good cu-FezOs pattern
Brown Brown Brown Brown Brown Brick-red Brick-red
Water
100°C.
0 1 hour 2-16 hourr 19 hours
No lines or bands visible a-FezOa pattern, broad bands a-Fez03 pattern, broad lines a-Fe2O~pattern, sharp lines
Brown Brown Bric k-red Bric k-red
Ferric 100°C. c hloridr solutior
No lines or bands visible a-FezOa pattern, few broad bands 6 hours a-Fe20s pattern, few broad bands 24-36 hourr a-Fez03pattern, broad lines 0 1 hour
Brown Brown Brown Some brick-red
summarized in table 2. It is apparent that added electrolytes retard or prevent the transformation from the brown gel to a-FezOs. The higher temperature of precipitation of 32’C., as compared with 20°C. in the preceding experiments, allowed the transformation to proceed more rapidly. This conclusion is further substantiated by the observation that a separate sample, precipitated a t 100°C. and cooled rapidly to room temperature, gave a good pattern of a-FezOa without further aging. THIESSEN AND KOPPEN’S “HYDRATES”
Thiessen and Koppen (17) have recentIy reported the preparation of a series of ferric oxide hydrates by the slow hydrolysis of ferric ethylate.
X-RAY STUDIES O N THE HYDROUS OXIDES. VI1
27
From breaks in dehydration isotherms they deduce the formation of Fe,Oa.. nHeO where n = 4, 7/2, 3, 5 / 2 , 2,3/2, 1, 1/2, and probably 5 and 9/2.
FIQ. 1. DENSITOMETER CURVES FOR Fe20a.zFI.0 AUED UNDER WATER AT loO°C. Time of aging in hours: (1) 0; (2) 1; (3) 2; (4) 4; ( 5 ) 19
The existence of such a series of oxide hydrates is so unusual that it seemed advisable to confirm or disprove Thiessen and Koppen’s experiments.
28
HARRY B. WEISER AND W. 0. MILLIGAN
a * z
8I ”2 V
g
h d
n
3 a
% x
X-RAY STUDIES ON T H E HYDROUS OXIDES. VI1
29
Preparation of ferric ethylate Samples of ferric ethylate were prepared by the interaction of solutions of anhydrous ferric chloride and sodium ethylate in absolute alcohol. In the first experiments the alcohol was dehydrated with metallic calcium, as was done by Thiessen and Koppen. At all times the apparatus and solutions were protected from the action of moisture in the air by the use of large drying towers containing calcium chloride and phosphorus pentoxide. A solution 15 millimolar with respect to ferric oxide in absolute alcohol was prepared from the ferric ethylate. Moisture from the air was allowed to diffuse into the solution, causing a slow hydrolysis which was complete a t the end of three or four weeks, the exact time depending upon the atmospheric conditions prevailing during the hydrolysis period. Several samples prepared in this way were separated from the supernatant liquid and washed thoroughly, using the centrifuge, with alcohol alone or first with alcohol and then with water. The samples washed with alcohol alone contained considerable adsorbed alcohol, which was largely removed by washing with water. The following analytical results are typical: Sample washed with alcohol alone: Fez03 = 68.03 per cent; HzO = 24.53 per cent; CZH~OH= 7.54 per cent. Sample washed with alcohol and with water: Fe2O3 = 64.14 per cent; H2O = 34.85 per cent; C2H60H = 1.01 per cent. The amount of adsorbed alcohol was determined by a combustion method. The samples actually used in dehydration experiments were always washed with water to remove excess alcohol. Since only 1 per cent or less of alcohol remained in the samples washed in this way, its presence may be neglected in the dehydration experiments described in the next section. In a second series of experiments an improved method of drying the alcohol was adopted (16). “Absolute alcohol” was further dried with excess metallic calcium and distilled directly into a carefully dried flask containing metallic sodium and ethyl oxalate. Traces of water in the alcohol react with the sodium ethylate to give alcohol and sodium hydroxide. The reaction is shifted in the desired direction by.allowing the sodium hydroxide to hydrolyze the ethyl oxalate, giving alcohol and sodium oxalate. The resulting highly anhydrous alcohol was distilled directly into the reaction vessels containing anhydrous ferric chloride and metallic sodium. The solutions of ferric chloride and sodium ethylate were mixed, giving ferric ethylate. The entire apparatus and solutions were protected from the action of moisture in the air by calcium chloride and phosphorus pentoxide towers. The slow hydrolysis of this sample of ferric ethylate was carried out in warmer weather than the first; hence the resulting ferric oxide gel contained less adsorbed water than the first preparation.
30
HARRY B. WEISER AND W. 0 . MlLLIGAN
Dehydration zsotherms The samples of hydrous ferric oxide prepared as described above were dehydrated isothermally in an apparatus shown in figure 2, which needs no further description. The general method of procedure consists in pumping off a definite amount of water and measuring the vapor pressure of the resulting product a t constant temperature by means of a manometer filled with “vacuum pump oil” of known density. In the case of the sample made from the alcohol dried with metallic calcium only, the weighed sample was sealed into the weighed tube, A, and changes in composition
\
Sample
Manometer
1
Scale 0
10
-
cm 50
Fro. 2. APPARATUS FOR ISOTHERMAL DEHYDRATION
were followed by closing the stopcock and reweighing the tube with contents. Typical isotherms are given in figure 3. An improvement was made in the apparatus used in the isothermal dehydration of the samples made from the drier alcohol. Tube B was substituted for tube A, and changes in composition were followed by collecting the water vapor in a. drying tube filled with magnesium perchlorate trihydrate. Otherwise the apparatus was unchanged. Typical isotherms for this sample are given in figure 4. The equilibrium pressure at a given temperature and composition is established in half an hour or less. The pressure was read only after it had remained constant for half an hour. It is obvious that the isotherms contain no breaks which would suggest the existence of hydrates in the gel. We have no explanation to offer for
X-RAY STUDIES ON T H E HYDROUS OXIDES. VI1
31
the series of breaks a t just the right place in the isotherms of Thiessen and Koppen. Since these observers made no statement to the contrary, it is probable that they neglected to remove the alcohol from t,heir samples before dehydration. This might account for one point of inflection in the
Pressure
- mrn H3
F ~ Q3. . DEHYDRATION ISOTHERM FOR FezOa.zHzO
curve, but it would come a t a point corresponding to a hydrate only by accident. X-ray analysis
The freshly precipitated samples, either unwashed, washed with alcohol, or washed with both alcohol and water, gave x-ray patterns showing no THE JOURNAL OF PHYSICAL CHEMISTRY, VOL.
39, NO. 1
32
HARRY B. WEISER AND W. 0. MILLIGAN
lines or bands. A sample washed with water and heated to about 185°C. gave a weak pattern corresponding to a-Fez03. The broad bands indicated eirtremely small particle size. HYDROLYSlS O F FERRIC SALTS
A . Ferric sulj'ate The system Fez03-S03-Hz0 has been examined in detail by Posnjak and Merwin (13, IS), using phase rule methods.' These investigators have proven that a-Fez03.Hz0may be prepared by the hydrolysis of ferric sulfate at low concentrations and at temperatures below 130°C. More concentrated solutions give a basic salt of composition 3Fez03.4s03.9HzO. The following experiments are in agreement with these observations. Solutions of ferric sulfate were prepared as indicated in table 3. The solutions were heated to the boiling point on a hot plate, using reflux condensers to prevent loss of water vapor. The more dilute solutions gave a-FezOlc. TABLE 3 Hydrolysis of ferric sulfate solutions CONCENTRATION
I
Ow PRECIP1TATE
SOLID PHASE IDENTIFIED BY X-RAY ANALYSIS
M
1.00 0.50 0.25
0.10 0.05 0.01
Yellow Yellow Yellow Yellow Yellow Yellow
Mostly basic sulfate Mostly basic sulfate Mostly basic sulfate Mostly basic sulfate Mostly basic sulfate Mostly cu-Fe20s.Hz0
HzO,while the yellow precipitate obtained in the more concentrated solutions gave the same x-radiogram as the basic salt reported by Posnjak and Merwin.
B. Ferric nitrate
A solution containing 25 g. of ferric nitrate hexahydrate per liter was heated slowly to 100°C. on a hot plate; a yellow precipitate was obtained which gave the x-ray pattern for a-Fez03.Hz0. A 0.1 N solution was allowed to stand a t room temperature for about seven months. This was likewise identified as a-FezOs HzO. More concentrated solutions were not examined. Simon and Schmidt (1 5 ) dialyzed a ferric nitrate solution and examined the precipitate from the resulting sol by x-ray diffraction methods. The pattern was reported as being different from a-Fe203;it was probably that for the a-monohydrate.
1
X-RAY STUDIES ON T H E HYDROUS OXIDES. VI1
33
C. Ferric bromide and oxalate Hydrolysis of ferric bromide and ferric oxalate gives a yellow precipitate which was proven by x-ray analysis to be cr-Fe2O3.H20.
D. Ferric chloride Slow hydrolysis of ferric chloride solutions, either dilute or concentrated, always gives a third polymorph of ferric oxide monohydrate, which has been designated elsewhere (20) as P-Fe203.H20.
E. Ferric acetate Fifteen years ago one of us (19) found that boiling freshly prepared ferric acetate solutions gave a brick-red colloid or precipitate, whereas solutions aged for ten days or more before boiling gave a definitely yellow colloid or precipitate. At that time the available evidence pointed to the conclusion that this yellow product consisted of hydrous ferric oxide, differTABLE 4 Slow hydrolysis of ferric salts
I
BALT UBED
PRODUCT IDENTIFIED BY X-RAY ANALYSIS
Sulfate Nitrate Nitrate inoculated with a trace of 8FepO3.H20 Bromide Oxalate Chloride Acetate
ing from the red-brown and brick-red varieties only in particle size. These experiments have been repeated, and the yellow product has been identified as a-FezOs.HzO and the brick-red powder as a-FezOs. Rapid hydrolysis of ferric salts always yields a-Fe203,and slow hydrolysis, except in the case of the chloride, yields the a-monohydrate. Slow hydrolysis of the aged ferric acetate solutions in the cold gives nuclei of a-Fe203.Hz0,which determine the course of the reaction a t higher temperatures. Table 4 summarizes the results obtained by the slow hydrolysis of various ferric salts. SUMMARY
The brown gel formed by the interaction of solutions of ferric salts and bases in the cold gives an x-radiogram showing no lines or bands. Aging under water of the oxide freshly precipitated in the cold for a few weeks or
34
HARRY B. WEISER AND W. 0. MILLIGAN
months a t room temperature or a few hours near the boiling point brings about a transformation from the brown gel to brick-red a-Fez03. Excess electrolytes retard or prevent the transformation. X-ray examination of the products formed by systematic aging of the original gel indicates that the transformation consists in the growth of particles too finely divided to give a characteristic x-radiogram, to crystals of a-FezOa large enough to give sharp diffraction lines. Gels thrown down near 100°C. give a t once a weak x-radiogram of a-Fen03. Dehydration isotherms of the ferric oxide gel formed by the slow hydrolysis of ferric ethylate show no breaks indicative of the existence of definite hydrates. This is contrary to the results of Thiessen and Koppen. Rapid hydrolysis of ferric salts gives a brick-red precipitate identified by x-ray analysis as a-Fez03. Slow hydrolysis of ferric salts gives a yellow precipitate of hydrous a-Fe20s. H20, except in the case of ferric chloride where /3-Fe203.H 2 0 is formed; ferric sulfate gives either a-Fez03.H20 or a definite hasic sulfate, depending on the conditions. REFERENCES (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) (15) (16) (17) (18) (19) (20)
ATTFIELD:Chem. News 17, 303 (1868). BOHM: Z. anorg. allgem. Chem. 149,210 (1925); Kolloid-Z. 42,276 (1927). BOHMA N D NICLASSEK: 8. anorg. allgem. Chem. 1 3 2 , l (1923). BRUSHAND RODMAN: Chem. News 1 7 , 5 5 (1868). DAVIES:J. Chem. SOC.19, 69 (1866). HABER:Ber. 66, 1717 (1922). H ~ ~ T TAND I GGARSIDE:Z. anorg. allgem. Chem. 179, 49 (1928). JEANNEL: Compt. rend. 66, 199 (1868). KATSURIA AND WATANABE: Sci. Papers Inst. Phys. Chem. Research Tokyo 13, 89 (1930). MILLIGAN:Rev. Sci. Instruments 4,496 (1933). NICHOLS, KRAEMER, AND BAILEY:J. Phys. Chem. 36,326 (1932). P ~ ADE N ST. GILLES:Ann. chim. phys. [3l 46, 47 (1856). POSNJAK A N D MERWIN:J. Am. Chem. SOC.44,1965 (1922). SCHIFF:Chem. Zentr. 31, 768 (1860). SIMONAND SCHMIDT: Kolloid-Z., Zsigmondy Festschrift., p. 65 (1925). SMITH:J. Chem. SOC.1927, 1288. THIESSEN AND KOPPEN:Z. anorg. allgem. Chem. 188, 113 (1930). TUNELL AND POSNJAK: J. Phys. Chem. 36, 929 (1931). WEISER:J. Phys. Chem. 24, 277 (1920). WEISERA N D MILLIQAN:J. Am. Chem. SOC., in press.
