Calcium thiosulfate has the molecular formula of
CaS2O3 and the molecular weight of 152.2082 g/mol. It
can be prepared by the reaction of sodium thiosulfate
upon the chloride:
CaCl2=Na2S2O3 ? CaS2O3+2NaCl
Since calcium thiosulfate is soluble in water
(50.2 g/100 ml at 20°C), the solution must be evaporated
at a low temperature to crystallize the product
(salt-out temperature ～15°F (-9.4°C). A hexahydrate,
CaS2O3·6H2O, is the result. Its CAS number is 10124-
43-3 while that of the anhydrate is 10124-41-1. Its density
is 1.87 g/cm3 and it loses four waters of hydration at
45°C and the other two at about 85°C. Further heating
to 112°C causes it to decompose, forming S, SO2 and
calcium sulfate:
2CaS2O3+heat ? 2CaSO4+S+SO2
Thiosulfuric acid could also be used to prepare this
salt at an acidic pH. The expected result is the hexahydrate.
However, there is no report of an investigation
concerning the product(s) formed as a function of pH
of precipitation. A high-purity calcium thiosulfate can
be prepared by a metathesis reaction of ammonium thiosulfate
and calcium hydroxide or calcium oxide.
Aqueous solutions of high-purity calcium thiosulfate,
in concentrations up to about 30% by weight, are
prepared by removing substantially all of the co-product
ammonia under conditions that minimize the decomposition
of the thiosulfate ion. Heating the reaction mixture
to a temperature of about the boiling point at atmospheric
pressure is used to prepare aqueous solution
of about 10% by weight calcium thiosulfate, and lower
reaction temperatures are employed to prepare more
concentrated aqueous solutions of calcium thiosulfate
up to about 30% by weight, which is the saturation
point.
Calcium thiosulfate is supplied as a solution commercially.
The clear solution has little odor, and the pH is in
the range of 6.5–7.5. It is used in dechlorination, ozone
and hydrogen peroxide quenching, concrete and cement
formulations, photographic processes, coatings, stabilizers,
and reactant for precious and nonprecious metals.
It is also used as a “liquid lime” in agriculture.
Calcium thiosulfate is a reducing agent. It is routinely
used as a titrant to determine concentrations of oxidants
such as hypochlorite in bleach and dissolved oxygen in
water. It instantly dechlorinates water, and is used to
stop a bleaching action in the paper making industry.
