CALCIUM THIOSULFATE

Description

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.