RICHARD G. YALMAN AND JAMES F. CORWIN
1432
Vol. 61
TABLE I phase. The calculation of this ratio a t the boundTHEMOLERATIOFe10a:Nio.r?rZno,r:!8FezOc AS A FUNCTION ary line between the spinel.field and the spinelOF MOLE FRACTION OF FezO, IN THE INITIAL MATERIAL Fez03 field can be done as follows. In the twoTemp.,OC.
700 800 900 1000 1050 1100 1150 1200 1250 1300 1350 1400 Stoichiometric limit
0.119
.004 .015 ,035 .060
.OX5
Initial mole fraction BenOz 0.227 0.484 0.660
.007 ,012 .028 ,050 .OX3 .143 .159 .175
.I80 .187 .190
.09
.I94 .198
0.75
phase area the over-all composition of the system can be given as
+
aNizZnl- zFez0d.bFea04.cFep0, dFezOa ,054 .079 ,142 .237 .412 ,509 .542 .570 .585 ,625
.051 .090 .149 .247 .431 .688 1.07 1.27 1.15 1.20 1.33
and Merwinlo in the system Mg0-Fe0-Fez03. The composition of the spinel phase may be represented as (neglecting cation and anion vacancies)
(2)
The ratio b/(b+c+d) can be calculated from a knowledge of the initial composition and the ferrous iron content. I n the one-phase area d = 0, and the ratio becomes b/(b+c). As the temperature is increased further c approaches zero and the ratio approaches unity. The value of b/(b+c+d) has been plotted in Fig. 3 for three compositions. The temperature at which the second phase disappears has been marked. It can be seen that the value of b/(b+c) at this temperature is 0.7 for all compositions. Darken and Gurrgll found that magnetite prepared in air contdned excess oxygen. At -the boundary between the magnetite hematite phase and the magnetite field, which occurs at 1392" (in air), the composition of the magnetite phase in terms of the components Fe304and Fez03was found to be 0.7 mole fraction Fe304. I n the notation of equation 1, this corresponds to
+
aNi,Znl-sFezO1~bFe~Or~cFenOa (1) The mole ratio b/a represents the solubility of Fe304 in the Ni-Zn ferrospinel solid solution. BebFeaO4.cFe2Os (3) yond the solubility boundary a second phase, identified as a-Fe203 (hematite) by X-ray diffrac- These authors found that the mole fraction of tion, separates. Throughout the two-phase re- FeaOd = b/(b+c) = 0.7. Since equation 3 is idengion, this solubility @/a) would be expected to re- tical to equation 1,where a = 0, it appears that this main constant a t a given temperature. This is ratio is independent of the value of a of equation 1 verified within experimental error from the data throughout the composition range. over the two-phase region presented in Table I. The authors acknowledge the constructive adThe value of b/(b+c) can be interpreted as a meas- vice of Dr. D. M. Grimes relative to the above reure of the amount of excess oxygen in the spinel search. (10) H. 6. Roberts and H. E. Merwin, Am. J . Science, 21, 146 (1931).
(11) L. S. Darken and R. W. Gurry, J . A m . Chem. Boc., 68, 798
(1946).
HYDROTHERMAL REACTIONS UNDER SUPERCRITICAL CONDITIONS. 111. THE EFFFCT OF pH ON THE CRYSTALLIZATION OF SILICON DIOXIDE BYRICHARD G. YALMAN AND JAMES F. CORWIN Contribution from the Department of Chemistry, Antioch College, Yellow Springs, Ohio Received July 16, 1967
The hydrothermal reactions of silica glass have been studied under isothermal conditions at 400' and 5000 p.s.i. The res1llts indicate that no reaction occurs with sulfuric, phosphoric and hydrofluoric acids, in potassium hydrogen sulfate solutions Or in pure water. In weak sodium hydroxide solutions silica glass devitrifies to form cristobalite, and in stronger sodium hydroxide solutions quartz is formed. The mechanisms of these reactions involves the trihydrogen and dihydrogen silicate ions. Similar results are obtained with increasing pH in buffered phosphate, sulfate and fluoride solutions. The formation of quartz and cristobalite in these solutions is due to the hydrolysis of the anions present. There is no evidence that hydrothermal reactions of silica glass a t 400" in fluoride containing solutions are due to the formation of the fluosilicate ion.
Introduction As a result of systematic investigations on the isothermal growth of quartz from sodium chloride solutions at 400", Swinnerton, Owen and Corwin2 (1) This research was supported in part b y the United States Air Force through the Air Force Office of Scientific Research of the Air Research and Development Command, under contract No. A F 18(600)1490. Additional support was received from the U. 8. Army Signal Corps (Contract No. DA 36-039 SC-64605) through its Signal Corps Engineering Laboratories a t Fort Monmouth, New Jeraey. Reproduction in whole or in part is permitted for any purpose of the United States Government.
found that optimum conditions were obtained with bombs containing a 50% charge of M/40 sodium chloride raised to a pH of 10 by the addition of sodium hydroxide. These authors, as well as o t h e r ~ , ~ ~ ~ have emphasized the importance of the hydroxide ion in the dissolution of silica and the growth of (2) A. C. Swinnerton, G. E. Owen and J. F. Corwin, Ddsc. Faraday Soc., 5,172 (1949). (3) R. Nacken, Report on Researah Contraot for Synthesis of Oscillator Crystals, U. S. Army Signal Corps Interrogation, 1946. (4) G. Van Praagh, Disc. Faraday SOD.,5, 338 (1949); Research (London), 1, 458 (1948).
a
EFFECT OF p H
Oct., 1957
ON
CRYSTALLIZATION OF SILICON DIOXIDE
1433
quartz. I n a series of experiments a t 365" using run) and 125 ml. of the desired solution. The autoclave was heated to 400 & 2" for 48 hours. Unless otherwise sodium bicarbonate and sodium carbonate solu- then designated, only successful runs, i.e., having less than 5% tions, Franke6 observed the formation of cristobal- leakage in 48 hours a t 400°, are included in the results. ite as well as quartz and concluded that the forma- In a number of experiments a quartz seed plate was sustion of these substances depended upon the pH of pended by means of a silver wire from the top of the autoclave. the nutrient solution. Reparation of Solutions.-All solutions were prepared On the other hand Wooster and Wooster6 as well from carbon dioxide free water using A. C. S. Reagent Grade as Corwin, et aZ.,7.8 have grown quartz from potas- Chemicals. Unless otherwise specified the total fluoride, sium hydrogen fluoride and sodium fluoride solu- sulfate and phosphate concentrations in the corresponding tions, respectively, and the latter authors have sug- solutions were M/40. This was done by preparing stock solutions of M / 4 0 hydrogen fluoride, M/40 potassium gested that the mechanism of the silica transfer in fluoride, etc., and mixing the desired volumes of these resolutions containing fluoride involves the fluosilicate agents. All fluoride solutions were handled in polyethylene ion, SiF6'. Although both cristobalite and quartz ware. Analysis.-Optical, X-ray and infrared analyses were were obtained in other alkali and ammonium halide made with a Bausch and Lomb Petrographic Microscope, solutions (including ammonium fluoride),9 this work a General Electric Counsel Recording X-ray apparatus, was not sufficiently extensive to determine whether and a Perkins-Elmer Model 21 infrared spectrophotometer, the formation of cristobalite was due to the p H of respectively. The p H of the various solutions was deterthe nutrient solution or to a "specific ion" effect. mined by means of a Beckman model G p H meter and silica was performed by the method of Kenyon and However, more recent experiments on the devitrifi- analysis Bewick" with the aid of a Beckman model DU spectrophocation of silica in solutions containing alkaline tometer. Total fluoride was determined by the method of earth hydroxides and halides indicate that in this Shell and Craig.Iz Preparation of the Autoclaves .-In our earlier experiseries the formation of silicates and cristobalite is ments which were primarily concerned with the optimuni due to a "specific ion" effect.10 conditions for the isothermal growth of quartz a very tightly Because of the many differences in experimental adhering coat of quartz formed on the exposed surface of the conditions in these studies it seemed worthwhile to autoclaves. This material was most easily removed by reinvestigate the hydrothermal devitrification of heating a 50% sodium hydroxide solution in the closed autoat 100' for several hours.' When new bombs were silica under as closely controlled conditions as pos- claves used, previous results were not always duplicated, and the sible. I n order to determine the effect of hydrox- differences included both the degree of devitrification and ide and fluoride ions on the formation of quartz the kind of crystalline material formed (Table I). Because the treatment with sodium hydroxide probably and cristobalite two series of experiments were absorbed hydroxide ions on the walls of the automade using similar concentrations of potassium hy- leaves claves we believe that the data obtained with the new droxide and potassium fluoride. A series of experi- bombs is more reliable. Consequently, in all of the work ments over a wide p H range, but with varying reported below we have attempted to reproduce the surface amounts of hydrofluoric acid, potassium fluoride of a new bomb by polishing the autoclaves after each run to a mirror-like finish. The autoclaves are further "condiand potassium hydroxide, but at constant fluoride tioned" by heating them to 400" for several hours with a concentration (0.025 M ) , also were performed. The 50% filling of the solution to be used in the next experiment. latter p H study was then compared with two similar sets of experiments using mixtures of sulfuric TABLEI acid, potassium sulfate and potassium hydroxide EFFECTOF BOMBSURFACE ON HYDROTHERMAL REACTIONS and phosphoric acid, sodium dihydrogen phosphate, 10 g. source, 400", 50% filling, 350 atm., 48 hr. disodium hydrogen phosphate and trisodium phosOldbombs New or polished bombs cleaned with NaOH phate. Due to difficulties described in the ExperiNutrient % % mental section, the data obtained may not be exDevit. so1n.a Devit. Productb Productb actly reproduced in other laboratories. However, it 3-4 Xonotlite' Ca(0H)z 100 Cristobalite NaF 100 Quartz G6 Cristobaliteis our conviction that the systematic trends ob(major) Quartz tained here can be obtained elsewhere, particularly NazS01 100 Quartz G3 Cristobaliteas they are in agreement with the existing literature. (major) Quartz CaSOc 100 Cristobalite 3 XonotliteC Experimental Equipment and Procedure.-The apparatus and control equipment and the conditions for maximum growth of quartz on a seed plate were those described in previous publications.2J Thus, in all of the experiments described herein 2 5 0 4 . stainless steel autoclaves were loaded with 10 g. of clear vitreous silica in the form of a solid rod (to ensure approximately the same surface area of silica in each (5) I. Franke, Bu21. SOC. franc. mineral cn'atallograph, '74, 207 (1951). The exact text reads: e t constate, qu'il y a, pour de tres faibles concentration (0.004 par ex). formation de cristobalite et pour des concentration plus forte formation de quartz. Ces resultats mettent en evidence que la forme metastsble (cristobalite) depend du pH de la solution." (6) N. Wooster and W. A. Wooster, Nature, 167, 297 (1946). (7) J. F. Corwin and A. C. Swinnerton, J . A m . Chem. Soc., '73, 3598
". . .
(1951). ( 8 ) J. F. Corwin, R. G. Yalman, G. E. Owen and A. C. Swinnerton. ibid., 76, 1581 (1953). (9) J. F. Corwin, A. H. Herzog, 0.E. Owen, R. G . Yalman and A. C. Swinnerton, ibid., 76, 3933 (1953). (10) J. F. Corwin, R. G. Yslman, J. W. Edwards and G.E. Owen, THIEJOURNAL,61,939 (1957).
Cristobalite
Each 250-ml. autoclave contains 125 ml. of a 0.025 iW solution or its equivalent and approximately 10 g. of silica glass rod. Product determined by optical analysis and representative samples confirmed by X-ray analysis. Major product listed first. Identified by chemical and Xray examination. a
Results In every experiment the devitrified silica glass is found as soluble silica, amorphous silica and crystalline silica. The amount of the amorphous silica formed was small and was probably due to the precipitation of soluble silica on cooling. The estimated amount of soluble silica (including the amorphous silica) was 1-2000 p.p.m. which is of the order of magnitude of that found in more careful solubility (11) 0. A. Kenyon and H. A. Bewick, Anal. Chem.. 26, 145 (1953). (12) H. R. Shell and R. L. Craig, ibid.. 26, 996 (1954).
1434
RICHARD G. YALMAN AND JAMES F. CORWIN
experiments.la*l4 The crystalline silica was distributed in three areas: on the seed plate (when present), on the surfaces of the autoclave, and in a sheath or envelope surrounding the unreacted source material, ie., in situ. The characteristics of the latter material have been described in detail elsewhere.4--7 The amount and kind of crystalline deposit depends upon the composition of the nutrient solution. I n all of the experiments reported here only a-cristobalite and a-quartz were detected. Both of these were identified by optical and X-ray analyses. A number of white, cherty specimens having an index of refraction in the range 1.464-1.470 were obtained in the more dilute sodium hydroxide solutions and in the buffer solutions a t low pH. X-Ray examination showed that this material consisted of quartz, as well as cristobalite. No loss in weight was observed when the sample was heated at 1000° for up to 17 hours. After heating, the index of refraction increased slightly to 1.474; while X-ray analysis showed that some of the quartz originally present was converted to cristobalite. Infrared patterns obtained on the samples before and after heating corresponded to the X-ray results, and there was no indication of the presence of water or hydroxyl groups. For comparison “triple baked” silica gel was heated to 400” for several hours. Upon further heating at 1000° for several hours this material lost water and the product had indices of refraction of 1.48-9 and 1.54-5. Although the major portion of the crystalline product obtained from the phosphate runs at a pH of 6.2 consisted of well defined crystals of cristobalite and quartz, some material having a mean index of refraction of 1.52-1.53 was present. This material is similar t o that observed by K e a P and may consist of this form of silica.16 Hydroxide Series.-The results of the sodium hydroxide experiments and the observations made using distilled water are given in Table 11. Although little devitrification occurs in water, the amount of devitrification in the sodium hydroxide solutions (column 5 ) is nearly independent of the concentration. No crystallization occurs in pure water, while only cristobalite is formed in 0.001 Jl NaOH. In the concentration range of 0.002-0.005 M NaOH no quartz is formed on the autoclave walls, but due to nucleation and a high concentration gradient some quartz is transferred to the seed plate and formed in situ, respectively. Suddenly at 0.006 M NaOH all of the devitrified silica is converted to quartz. The increase in the rate of formation of quartz with increasing hydroxide ion concentration is also indicated by the change in distribution of quartz in the autoclave. Thus, in 0.005 M NaOH over 90% of the quartz was found on the autoclave walls. This amount decreased to 84y0 in 0.006 M NaOH and t o 74y0 in 0.007 M NaOH, and finally, in 9.025 M NaOH the conversion of silica to quartz was so rapid that all of the quartz was found in situ and no silica was transferred through the bulk of the solu(13) G. C. Kennedy, Econ. Gsol., S9, 25 (1944). (14) G. W. Morey and J. M. Hesselgesser, ibid,, 46, 821 (1951). (16) P. P. Keat, Sczence, 120,328 (1954). (16) P. P. Iceat, private communication.
Vol. 61
TABLE I1 ISOTHERMAL DEVITRIFICATION OF SILICAGLASSIN SODIUM HYDROXIDE SOLN. 10 P. source, 400”, 50% filling, 350 atm., 48 hr.
NaOH moles/l.
Seed Plate
In situ
Autoclave surfaces
Total
A. Distribution, Grams SiOp 0.0000 negl. negl. negl. negl. .OOl 5.43 negl. 0.68 4.75 .002 5.61 0.06 .79 4.76 .004 6.88 .09 .29 6.50 .005 6.95 .10 .45 G.4 .006 6.45 .85 5.43 .17 7.13 .007 .18 1.70 5.25 IO.00 .025 0.0 10.00 0.00 B. Quartz Present, per cent.b 0. ooooc 0.0 0.0 0.0 0.0 .001 0.0 .o 0.0 0.0 ,002 5 25 .o 80 5 ,004 50 .o 80 5 .005 50 .o 90 100 .006 100 100 100 100 100 100 .007 100 100 100 100 .025 100 a No great accuracy is to be associated with these figures due to difficulties in removing material from autoclave walls, etc. No great accuracy is to be associated with these figures due to difficulties in sampling, etc.; each of these figures represents the average of a t least two “successful” runs. The only products were a-cristobalite and a-quartz. Cristobalite was identified in one experiment; its presence was questioned in two experiments; and no crystalline silica was found in two experiments.
s
tion to the autoclave surfaces or to a seed plate. A similar variation in the distribution of quartz was found in the phosphate runs (see Fig. 1). Fluoride Series.-The results of the devitrification of silica in potassium fluoride solutions over the molar concentration range of 0.0049-0.040 are given in Table 111. I n these experiments both the per cent. devitrification and the distribution of the devitrified material are essentially the same as those observed in the low concentration NaOH experiments. However, the amount of quartz formed was considerably less, and no quartz was observed on the autoclave walls. TABLE I11 DEVITRIFICATION I N POTASSIUM FLUORIDE SOLUTIONS 10 g. source, 400°,50% filling, 350 atm., 48 hr. KF, moles/I.
% Devitrification”
?’ & Quartzb
0.0049 .0053 .0061 ,0077 .0085 .010 .012 .025
12 0.0 60 0.3 55 1 41 1 45 1.6 30 4 49 4 80 9.5 .040 82 8.5 I n each experiment 66-95% of the devitrified mateNo quartz was rial was deposited on the autoclave walls. formed on the autoclave walls.
The results of a number of “unsuccessful1’ runs,
Le., leakage in excess of 5-10%, are given in Table IV, together with the corresponding data for “successful” runs, When leakage occurs, there is an in-
c
EFFECTOF pH
Oct., 1957
ON
CRYSTALLIZATION OF SILICON DIOXIDE
crease in the devitrification of the silica glass and the amount of quartz formed. The results of fluoride analysis on these runs (Table IV) indicate that fluoride is lost during leakage, presumably as hydrofluoric acid. This was also indicated by the formation of copper(1) fluoride on the upper surface of the copper gaskets used to seal the autoclaves.
?k-5
1435 - 250
TABLE IV
EFFECT OF LEAKAGE ON FLUORIDE RUNS 10 g. source, 400°, initial vol. 125 ml. soln., 48 hr. KFI, moles/l.
%
Final vol., ml.
Devit.
yo Qtz.
% FremainIng
121 30
14 51
0.0 8.5
97 35
1.2 3.8 100" 100" 9.6 100"
99 85 51 28 97 33
0.0049 .0049
56 ,0061 123 75 100 .0061 100" 52 .0061 looa 23 .0061 66 .025 122 100 .025 26 a 100% devitrification t o quartz.
When hydrofluoric acid was present in the nutrient solution, the extent of devitrification due to leakage during the run decreased and no quartz was formed. The effect of hydrofluoric acid can also be seen from the results of the series of experiments in which the total fluoride concentration was maintained at M/40 over a wide p H range (Table V). No devitrification (and no quartz formation) occurred in hydrofluoric acid. The degree of devitrification increased markedly when potassium fluoride was present, but remained nearly constant over the pH range of 3-10 (see Fig. 2). At the same time the amount of crystalline silica deposited in situ gradually increases corresponding t o the observations made in the sodium hydroxide (Table 11) and sodiumlphosphate (Fig. 1) experiments.
PH.
Fig. 1.-Distribution of devitrified silica in M/40 phosphate at 400": A, % devitrification; 0, % devitriiied silica deposited on autoclave surfaces; 0, mg. growth on seed plate.
90
TABLE V DEVITRIFICATION OF SILICAGLASSIN BUFFERED FLUORIDE SOLUTIONS 10 g. source, 400°, 50% filling, 350 atm., 48 hr. Concn. HF 0.025 .020 .005
.0025
...
Moles/ 1. KF
...
0.005 .020 .0225 ,025 .025
.... KOH added.
pH 2.5 3.0 3.9 4.3 6.4 10.Oa
Devitrification, g. Quartz present. g. AutoAutoIn clave In clave situ sursitu surfaces faces
..
..
0.2 1.74 2.12 2.4 3.30
6.5 6.52 6.33 5.65 3.15
0.0 .OG .22 .23 3.14
0.0
.O .O
.O .16
No quartz was formed in either pure hydrofluoric acid or in the solution buffered a t p H 3. At higher pH's there was a gradual build up of quartz in the sheath surrounding the source material, but the amount was small until the p H of the nutrient solution was raised by the addition of potassium hydroxide. No quartz deposited on the autoclave walls until the pH was raised to 10. Sulfate Series.-The results of the runs in buffered sulfate solutions are recorded in Table VI.
Fig. 2.-Devitrification of silica glass in M/40 buffered solutions a t 400": 0, hydroxide; o, phosphate; A, fluoride; m, sulfate.
No devitrification was observed in any solutions containing sulfuric acid. Some devitrification occurred in potassium hydrogen sulfate solutions and the degree of devitrification increased with increasing potassium sulfate concentration in much the same way as it occurred in the buffered fluoride system (see Fig. 2). No quartz formed in situ until the concentration of potassium sulfate was twenty times that of potassium hydrogen sulfate. The amount of quartz in situ did not exceed 25% of the total deposit, and when a seed plate was present the deposit on the seed plate was only 50% quartz in M/40 potassium sulfate and 80% quartz at a p H of 10. No quartz was found on the autoclave walls. Phosphate Series.-The results of a very large number of experiments using nutrient solutions containing phosphate are summarized in Table VII.
RICHARD G. YALMAN A,ND JAMES F. CORWIN
1436
TABLEVI DEVITRIFICATION OF SILICA I N SULFATE SOLUTIONS 10 g. source, 400", 50% filling, 360 atm., 48 hr. PH
Devitrifioation, g. Autoclave surfaces
I n situ
Quartz present, g. Autoolave surfaces
In situ
1.68" 0.0 0.0 .. .. 1.75 .o .o .. *. 1.85 .o .o .. .. 2. l o b .o .G1 .. 0.0 2.15 .o 1.31 .o 2.50 .20 1.82 0.0 .o 2.65 .40 4.55 .o :0 3.00 .4G 4.71 .o .o 4 . 05c .62 6.08 .07 .o 5.81d .63 6.30 .08 .o 10.0" .49 6.10 .13 .o M/40 HtSO,. b M/40 KHSO,. C 0.00025 d l KHSO4 plus 0.2475 M KzS04. M/40 KzS04. * M/40 KzSOa, KOH added.
..
I n all of these runs the total phosphate concentration was maintained a t 0.025 M , but unlike the previous experiments the source material consisted of 20 g. of silica glass tubing. TABLEVI1 DEVITRIFICATION OF SILICA I N PHOSPHATE SYSTEM 20 g. source, 400°, 50% filling, 350 atm., 48 hr.
Vol. 61
drochloric acid is a weak acid, sodium hydroxide is a weak base, and that the normally strong electrolyte, potassium chloride, is a weak electrolyte which hydrolyses t o form alkaline solutions (see Table VIII). The hydrolysis of sodium chloride in superheated steam to form hydrochloric acid has also been observed.'* The ionization constants of phosphoric acid, l9 hydrofluoric acid20 and the hydrogen sulfate ion1g decrease with increasing temperature and the salts of these acids should hydrolyze more a t 400" than solutions of potassium chloride. The hydrolysis of the sulfate ion a t 400" has been demonstrated by means of oxygen exchange experimentsZ1; while the hydrolysis of fluoride ion a t high temperatures has been reported on several occasions22and was observed in the experiments reported here. TABLEVI1118 PROPERTIES OF ELECTROLYTES I N THE CRITICAL REGION T = 400°, 50% filling, 350 atm. Compound
K
HzO HCl KOH KCl
7 x 10-18 1.1 x 10-4 1.7 X 3 x lo-'
All of our results can now be related in terms of the relative acidity or basicity of the nutrient solution at 400". No crystallization occurs in solutions PH of sulfuric acid, phosphoric acid, hydrofluoric acid, 2.00' potassium hydrogen sulfate or water itself. Cristo2.15 .o .o .o 2.90 .o .o .o balite is formed in weak sodium hydroxide solu3.64 .O .o .0 tions and the conversion of all of the silica to quartz 4.20 .o .o .o does not occur until the concentration of sodium 4.65) .01 .o .o hydroxide is 0.006 M . I n more basic solutions the 5.5 * 08 .o .o 6.0 .21 .28 .20 conversion of silica glass into quartz occurs so rap6.2 .24 .90 .44 idly that no silica is transported through the bulk of 6.5' 100% Quartz the solution to the seed plate or the autoclave sur7.04d 100% Quartz faces (Table I1 and Fig. 1). These results indicate 100% Quartz 8.0 8.5 100% Quartz that hydroxide ion is required for the crystalliza9.2' 100% Quartz tion of cristobalite as well as quartz, i.e., that the 10.0 100% Quartz mechanism of crystallization of cristobalite involves a M/40 phosphoric acid. M / 4 0 sodium dihydrogen the trihydrogensilicate ion, HaSi04-; while quartz phosphate. Only quartz found at this and higher pH's. Seed plate was clear with well developed faces. 100% is formed from the dihydrogensilicate ion, H2Si04=. devitrification. e M/40 disodium hydrogen phosphate. It also has been suggestedg that the repulsion of on the latter ion contributes to the spiral charges Neither cristobalite nor quartz formed in pure of a-quartz. phosphoric acid solutions. I n pure sodium dihy- formation The formation of cristobalite and quartz in the drogen phosphate solutions quartz was deposited salt solutions is due to the hydroxide ion concentraalong with cristobalite on the seed plate and the tion formed by the hydrolysis of the various anions amount of quartz in this deposit rapidly increased present and follows the same trend as that observed to 100% with the addition of small amounts of di- with increasing concentrations of hydroxsodium hydrogen phosphate. Simultaneously, the ide: no crystallization in solutions ofsodium the pure acids; character of the deposit changed from a milky, flaky material to clear quartz. The amount of ma- cristobalite formation in solutions containing mixsalt and quartz formation terial transferred to the seed plate and to the walls tures of the acid and its of the autoclave was nearly constant through the in solutions containing a high salt to acid ratio. By (18) 0. Fiichs-Frankfurt, Z . Elektrochem. Angew. phyaik. Chem., 47, pH range of 6.0-8.5. At still higher pH's the growth (1940). on the seed plate decreased rapidly and at a pH of 101(19) H. S. Harned and B. B. Owen, "The Physical Chemistry of 10.0 almost no devitrified material was found on the Electrolyte Solutions," Reinbold Publ. Corp., New York, N. Y.,1943, p. 580. walls of the autocIave (see Fig. 1). (20) H. H. Broene and T. DeVries, J . A m . Chem. Soc., 69, 1644 Discussion (1947). (21) R. G . Yalman, J. F. Corwin, G. E. Owen and N. Fetter, ibid.. By means of conductivity experiments a t 400" 4779 (1955). and 350 atmospheres Franckl7 has found that hy- 77,(22) E. Baur, Z . phyaik. Chem., 48, 497 (1904); W. Eitel, R. A. Distribution, g. Autoclave Seed In surplate situ faces negl. negl. negl. negl. negl. 0.3 negl. negl. .5 0.04 negl. 2.0 .05 negl. 4.0 .08 7.2 .50 .12 1.0 10.5 .21 4.0 9.7 .24 5.0 11.0 .25 5.1 10.5 .25 10.8 8.6 8.0 .25 11.5 .26 7.7 11.8 .07 9.3 10.3 .02 19.2 .4
Quartz present, g. Auto clave Seed In surplate situ faces 0.0 0.0 0.0
(17) E. U. Franck, 2. physik. Chcm., Neus Seris, 8 , 192 (1956).
Hatch and M. V. Denny, J . A m . Ceram. Soc.. 86, (10)October (1953).
Oct., 1957
REACTION BETWEEN STRONTIUM HYDROXIDE AND SILICA
using the amount of cristobalite and quartz formed in each experiment (see Figs. 2 and 3) as a measure of the hydroxide ion concentration present a t 400°, the strengths of the various acids (at 400”) are found to decrease in the order sulfuric, phosphoric, hydrosulfate, hydrofluoric and dihydrogen phosphate. Thus only 0.61 g. of cristobalite was deposited-on the autoclave walls from M/40 potassium hydrogen sulfate solutions; while over 7.5 g. of cristobalite “was formed in sodium dihydrogen phosphate. No quartz was observed in either of these solutions, but traces of quartz were found in M/40 potassium sulfate with considerable quartz found in M / 4 0 potassium fluoride solutions. Finally, all of the silica was converted into quartz in M/40 disodium hydrogen phosphate solutions. In a previous paper on the growth of quartz in sodium fluoride solutions* we suggested that the mineralizing action of the fluoride ion was due to the formation of the fluosilicate ion by the reaction 2H20
+ Si02 + 6F- F? SiF6- + HOH
(1)
At that time a volumetric procedurezafor the determination of fluosilicate in the presence of fluoride based on the precipitation of potassium fluorosilicate in acid solution containing alcohol and saturated with potassium chloride was used. We have examined this procedure and have found that the fluosilicate ion is formed rapidly and nearly quantitatively when solujions containing sodium silicate and sodium fluoride are acidified The second step in this analysisz3is to reverse reaction 1 by digesting the precipitate of potassium fluosilicate. But this is essentially the conditions in the bombs, ie., any fluosilicate present in the bomb reactions would be decomposed a t the temperature of the run. Equilibrium studiesz4also have shown that the fluosilicate ion is most stable in acid solutions at low temperatures ; while preliminary studies in this Laboratory indicate that the rate of form(23) I. M. Kolthoff and V. A. Stenger, “Volumetric Analyses,” Interscience Publishers. Inc., New York, N. Y.,1947, p. 120. (24) J. G. Ryss, Zhur. Fir. Khim., 36, 654 (1951).
1437
PH.
Fig. 3.-Quartz (total) formed from silica glass in M/40 buffered solution at 400’: 0, hydroxide; 0, phosphate; A, fluoride; 0 , sulfate.
ation of the fluosilicate ion increases with increasing acidity. But these are just the conditions (M/40 HF) which prevent the devitrification of silica at 400’. The effect of the fluoride ion as a “mineralizing” agent therefore is not due to the formation of the fluosilicate ion, but like the anions of other weak acids, to its hydrolysis and the subsequent formation of hydroxide ion. Acknowledgments.-The authors wish to thank Dr. J. W. Edwards and the Monsanto Chemical Company, Dayton, Ohio, for making the X-ray analysis and Dr. Harry Knorr of the Kettering Foundation, Yellow Springs, Ohio, for the infrared analysis. We are indebted to Mr. Elwood Shaw of the Chemistry Department, Antioch College, and Dr. John White of the Geology Department, Antioch College, for their help with the optical measurements and for many valuable suggestions during the course of this work.
HYDROTHERMAL REACTIONS UNDER SUPERCRITICAL CONDITIONS. IV. THE REACTION BETWEEN STRONTIUM HYDROXIDE AND SILICA1 BY J. F. CORWIN, R. G. YALMAN, J. W. EDWARDS AND E. R. SHAW Contribution from the Department of Chemistry, Antioch College, Yellow Springs, Ohio Received J u l y 16, 1067
The hydrothermal reaction between dilute solutions of strontium hydroxide and silica glass was studied under carefully controlled experimental conditions at 400’, 340 atmospheres pressure, and in a time range from 0 to 192 hours. Through the devitrification of a controlled amount of silica glass rod, silica was added slowly to the solution during the reaction. Under these conditions strontium metasilicate was formed first, and on continued reaction the solid phase modified through a series of crystalline and amorphous materials to 8-cristobalite and finally to a-cristobalite. I n this respect the reaction closely resembles that of calcium hydroxide under the same conditions. (1) This research was supported in part by the United States Air Force through the Air Force Office of Scientific Research of the Air Research and Development Command, under contract No. A F 18(600)1490. Additional support was received from the U. S. Army Signal Corps (Contract No. DA 36-039 80-64605) through its Signal Corps Engineering Laboratories a t Fort Monmouth, New Jersey. Reproduction in whole or in part is permitted for any purpose of the United States Government. (2) Paper No. I, THISJOURNAL, 61, 939 (1957).
Introduction The investigation Of the reactio’ls Of the earth metal oxides with silica glass2 showed that calcium and strontium oxides have similar characteristics. The reaction was relatively rapid and showed signs of continuing devitrification of the glass even after equimolar quantities of silica had
