SILICA IN
July, 1957
THE
PRESENCE OF ALKALINEEARTH METALOXIDES
939
HYDROTHERMAL REACTIONS UNDER SUPERCRITICAL CONDITIONS' BYJAMES F. CORWIN,R. G. YALMAN,J. W. EDWARDS AND G. E. OWEN Contribution from the Department of Chemist?y and the Department of Physics, Antioch College, Yellow Springs, Ohio Received February 67,1967
The hydrothermal reactions between dilute water solutions of alkaline earth metal oxides and silica glass were studied under carefully controlled experimental conditions a t temperatures and pressures well above those associated with the critical conditions for water and water solutions. Under these conditions calcium and strontium oxides reacted rather rapidly with the silica and the reaction products, calcium and strontium silicates, were sufficiently soluble in water so that the silica glass continued to devitrify. The oxides of barium and magnesium reacted much less ra idly and beryllium oxide almost not a t all. Barium and magnesium oxides form coatings on the silica surface that inhibit further reaction. The beryllium oxide does not react with silica under these conditions.
Introduction Over a period of years, we have investigated the mechanism of the formation of a-quartz from silica glass in the presence of alkali metal ions.2-6 The same techniques have now been applied to the reactions of the alkaline earth metal oxides with silica. Very little information concerning these reactions is available in the literature except for calcium oxide which has been very thoroughly investigated. The present work describes the effect of dilute solutions of these oxides on the devitrification of clear silica glass rod. The conditions of 400°, 340 atmospheres pressure, which were developed by filling the reaction vessel to one-half its volume, were arrived a t during the early work2 and were maintained in this work for comparison. These conditions made sure that the reactions were taking place in the supercritical region. Experimental The apparatus and control equipment have been described in a previous publication.* The reaction vessels were 250-ml. stainless steel autoclaves. Constant conditions were used in all experimental runs, Le., isothermal 400 f 2", 340 atmospheres pressure, 50% liquid filling. The autoclaves were polished with a wire brush after each run, and then conditioned by heating to 400' while containing the solution of the oxide to be used in the subsequent experiment. After conditioning, a weighed piece or pieces of clear silica glass were placed in the autoclaves, 125 ml. of the solution to be'used was added, the autoclave was sealed and heated to 400 After 48 hours the autoclave was allowed to cool to room temperature. When the reactant was insoluble in water, a weight sufficient to make an 0.025 N solution was added to 125 ml. of water in the reaction vessel. After cooling the solution was decanted and centrifuged to remove suspended material. After centrifugation the liquid showed no Tyndall effect to an arc light. The unreacted glass was cleaned by scraping, dried and reweighed to determine the extent of reaction. The devitrified material from the solution, from the rod, and the remainder which was scraped from the sides and bottom of the reaction vessel was dried at 110' and retained for X-ray analysis and for chemical analysis when sufficient material was available. Silica.-Pure, transparent silica glass was obtained from the Thermal American Fused Quartz Company, and was used directly in the experiments.
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(1) The results and interpretations presented here are derived from work supported on contract between Antioch College and the U. S. Army Signal Corps through its Signal Corps Engineering Laboratories a t Ft. Monmouth, New Jersey. (2) A. C. Swinnerton, G. E. Owen and J. F. Corwin, Disc. Faraday Soc., 6, 172 (1949). (3) J. F. Corwin and A. C. Swinnerton, J . A m . Chem. Soc., 73,3598 (1951). (4) J. F. Corwin, R. G. Yalman, G. E. Owen and A. C. Swinnerton, ibid., 7 6 , 1581 (1953). (5) J. F. Corwin, A. H. Herzog, G. E. Owen, R. Yalman and A. C. Swinnerton, ibid., 75, 3933 (1953). (6) R. G.Yalman, J. F. Corwin, G. E. Owen and Neil Fetter, ibid.. 17, 4779 (1955).
Chemicals.-C.P. A. C. S. grade chemicals were used and treated according to analytical directions for making u the solutions. The distilled water had been freed of C& by boiling.
Results and Discussion The results and conditions under which they were obtained are listed in Table I. TABLE I"vb Oxide concn. 0.025 N , 20 g. silica glass, 400', 125 mi. vol., 340 atm., 48 hr. Wt. loss silica, g.
Oxide
pHi
pHj
Be0
4.5
4.5
