Description
Calcium selenite has the molecular formula of
CaSeO3 and the molecular weight of 167.0410 g/mol. It
can be prepared by the addition of sodium selenite to
a solution of magnesium chloride:
CaCl2(aq)+ Na2SeO3(aq) CaSeO3·H2O(solid)
It can also be prepared by treating a solution of
calcium chloride with selenous acid and adding sodium
carbonate to start precipitation. The monohydrate is
a colorless crystal with a density of 2.09 g/cm7.27 that is
insoluble in water. The Ksp is 1.2×10
-7.27. It is soluble
in dilute acids. A dihydrate has also been observed
whose molecular weight is 203.0669 g/mol. The monohydrate
has a molecular weight of 185.0569 g/mol and
is a monoclinic crystal with a=7.622, b =6.745 ,
c=7.922, b=108.46, V=386.4
3, Z=4 with
a density of 3.18 g/cm3. The polyhedron around the
calcium atoms is pentagonal bipyramidal and the selenite
group forms a trigonal pyramid with Se and O
atoms at the apices. These pyramids form layers parallel
to the bc plane which are connected by hydrogen bonds
involving crystal water. The selenite group acts as
a bidentate ligand.
The chemistry of the system CaO–SeO2–H2O has been
investigated at 23 and 80°C. The existence of four major
phases in this system:
CaSeO3·H2O
Ca(HSeO3)·2H2O
Ca2(SeO3)(Se2O5)
Ca2(HSeO3)2
These have been confirmed, and the equilibria among
them determined. At 23°C, CaSeO3·H2O and
Ca(HSeO3)·2H2O are the stable calcium selenites. CaSe-
O3·H2O 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(HSeO3)·2H2O is
the more acidic phase; it exists over the pH range 3.7–
0.4. Incongruent dissolution of Ca(HSeO3)·2H2O forms
CaSeO3·H2O. These results indicate Ca2(HSeO3)2(Se2O5)
is not a stable phase at 23°C. At 80°C, CaSeO3·H2O,
Ca2(SeO3)(Se2O5), and Ca2(HSeO3)2(Se2O5) are the stable
calcium selenite phases. Ca(HSeO3)·2H2O has not
been observed. Thus, CaSeO3·H2O exists over the
broadest pH range, 11.0–3.9. Ca2(SeO3)(Se2O5) and
Ca2(HSeO3)2(Se2O5) dissolve incongruently.
Uses
CALCIUM SELENITE is used as a feed additive to replace sodium selenite, and its function in feed is the same as that of sodium selenite, but with reduced toxicity.
