Calcium Thionates

Description

Calcium dithionate has the molecular formula of CaS2O4 and the molecular weight of grams per mole. The “thionite” and “thionate” salts of calcium include:
Calcium dithionite=CaS2O4
Calcium dithionate=CaS2O6
Calcium tetrathionate=CaS4O6
Calcium dithionite, CaS2O4, may be prepared by the reaction of calcium chloride with sodium dithonite:
CaCl2(solid)+ (NH4)2S2O4(solid)+ heat
? CaS2O4(solid)+ 2NH3(gas)+ HCl(gas)
Alternatively, it may be prepared by the solid-state reaction of SO2 gas upon the sulfite:
Ca(HSO3)2(solid)+ SO2(gas)+ heat
? CaS2O4(solid)+H2O
This reaction could probably be carried out in solution as well. However, the reaction would be dependent upon the pH of the solution and a pH of about 4.5 would produce the hydrosulfite:
CaCl2+ 2(NH4)2SO3+ SO2
? CaS2O4+ 2NH4Cl+H2O
The description of the physical and chemical properties of this salt are completely absent in the scientific and technological literature. Calciumdithionite is mentioned in a few patents as a reducing agent. The older reference books like Sidgewick or Remy (1933 or 1950) do not list such a compound as being known. It has no CAS number and has no commercial value. However, it is offered for sale either as “dithionite” or “hydrosulfite”.
There is some confusion in the commercial literature concerning calcium dithionite, CaS2O4. Most manufacturers list the dithionite as synonymous with “hydrosulfite”, Ca(H2SO3)2. Its CAS number is 15512-36-4. However, the two are completely different compounds.
Calcium hydrosulfite is supplied as a colorless or slightly yellow colored liquid with a strong sulfur odor. It is a strong reducing agent. It is corrosive to skin and eyes and may be toxic by ingestion or inhalation. It is spontaneously combustible upon exposure to air or moisture. In contrast, calcium dithionite is stable in air and reacts slowly with moisture, releasing SO2 gas. But, it dissolves in water, forming calcium hydrosulfite:
CaS2O4+ 2H2O0Ca(HSO3)2+ 2H⁺ ? Ca(OH)2+ 2SO2
Both compounds react with oxidizing agents to generate heat and products that may be flammable, combustible, or otherwise reactive. These reactions may be violent. Sulfites generate gaseous sulfur dioxide in contact with oxidizing acids and nonoxidizing acids.