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.
