Phase formation in the system CaO-SeO{sub 2}-H{sub 2}O

Journal Article · Wed Oct 01 04:00:00 EDT 1997 · Journal of the American Ceramic Society

OSTI ID:549927

Dumm, J Q; Brown, P W ^[1]

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1. Pennsylvania State Univ., University Park, PA (United States). Dept. of Materials Science and Engineering

The chemistry of the system CaO-SeO{sub 2}-H{sub 2}O has been investigated at 23 and 80 C. The existence of four major phases in this system--CaSeO{sub 3}.H{sub 2}, Ca(HSeO{sub 3}){sub 2}.H{sub 2}O, Ca{sub 2}(SeO{sub 3})(Se{sub 2}O{sub 5}), and Ca{sub 2} (HSeO{sub 3}){sub 2}(SE{sub 2}O{sub 5})--has been confirmed, and the equilibria among them determined. At 23 C, CaSeO{sub 3}.H{sub 2}O and Ca(HSeO{sub 3}){sub 2}.H{sub 2}O are the stable calcium selenites. CaSeO{sub 3}.H{sub 2}O exists over the pH range 12.5--3.7, making it the most important phase for natural systems as well as most technological applications. Ca(HSeO{sub 3}){sub 2}.H{sub 2}O is the more acidic phase; it exists over the pH range 3.7--0.4. Incongruent dissolution of Ca(HSeO{sub 3}){sub 2}.H{sub 2}O forms CaSeO{sub 3}.H{sub 2}O. These results indicate Ca{sub 2}(HSeO{sub 3}){sub 2}(Se{sub 2}O{sub 5}) is not a stable phase at 23 C. At 80 C, CaSeO{sub 3}.H{sub 2}O, Ca{sub 2}(SeO{sub 3})(Se{sub 2}O{sub 5}), and CA{sub 2} (HSeO{sub 3}){sub 2}(Se{sub 2}O{sub 5}) are the stable calcium selenite phases. Ca(HSeO{sub 3}){sub 2}.H{sub 2}O has not been observed. Again, CaSeO{sub 3}H{sub 2}O exists over the broadest pH range, 11.0--3.9. Ca{sub 2}(SeO{sub 3})(Se{sub 2}O{sub 5}) and Ca{sub 2}(HSeO{sub 3}){sub 2}(Se{sub 2}O{sub 5}) dissolve incongruently. The formation of CA{sub 2}(SeO{sub 3})(Se{sub 2}O{sub 5}) and CA{sub 2}(HSeO{sub 3}){sub 2}(Se{sub 2}O{sub 5}) at 80 C demonstrates the increasing stability of the Se{sub 2}O{sub 5}{sup 2{minus}} ion and decreasing stability of the HSeO{sub 3}{sup {minus}} ion with increasing temperature. The threshold of Se toxicity has been established as 4 mg/(kg of animal weight), whereas the minimum intake of Se to avoid deficiency has been set at 0.04 mg/(kg of animal weight). Because of this relatively narrow gap between Se deficiency and toxicity, the delivery of Se to the biosphere from sources other than soil is a major concern.

• OSTI ID: 549927
• Journal Information: Journal of the American Ceramic Society, Journal Name: Journal of the American Ceramic Society Journal Issue: 10 Vol. 80; ISSN 0002-7820; ISSN JACTAW
• Country of Publication: United States
• Language: English

Related Subjects

36 MATERIALS SCIENCE
54 ENVIRONMENTAL SCIENCES
CALCIUM OXIDES
DISSOLUTION
EQUILIBRIUM
HYDRATES
LAND POLLUTION
PH VALUE
PHASE STUDIES
SELENIUM OXIDES
TEMPERATURE RANGE 0273-0400 K