Preparation of calcium dithionate tetrahydrate
50g of highest quality (>95%) finely powdered manganese dioxide are placed in a 500ml suction flask with 250ml of water. The flask is fitted with an inlet tube leading almost to the bottom of the vessel. The mixture is cooled in an ice-water bath and a stream of sulfur dioxide is led in (3 bubbles/sec) for a period of about two hours, during which time the flask is frequently swirled and good cooling maintained. The black dioxide disappears gradually in the course of the reaction, leaving a light brown slurry. The crude manganese dithionate solution is heated to boiling and small portions of 50g of calcium hydroxide is added in small portions. The liquid with the precipitate must be strongly alkaline to litmus paper. An excess of slaked lime does no harm, as it is removed in the next step. The entire mixture is diluted to about one liter with boiling water and a rapid stream of carbon dioxide is passed into the boiling mixture for 15-30 minutes. After cooling to about 50°C the slurry is filtered, washed with 150ml of boiling water, and the combined filtrates are evaporated on the steam bath to a volume of about 30ml. Several crops of crystals are isolated and filtered without washing. The remaining calcium salt in the final mother liquor is precipitated with 50ml of alcohol. The material is dried in air. Total yield 85-90%.
Inorganic laboratory preparations, by G. G. Schlessinger, 61, 1962