Sodium Dithionite: Bisulphite Reduced with Zinc Dust and Sulphur Dioxide (Thorpe 1913)

Sodium dithionite, Na2S2O4, called sodium hyposulphite or hydrosulphite in 1913, is a strong reducing agent used to dissolve vat dyes such as indigo. Thorpe's Dictionary of Applied Chemistry (1913) gives the method: shake a concentrated solution of sodium bisulphite with zinc dust while passing in sulphur dioxide, take the zinc out with milk of lime, salt the filtered liquid and cool it to crystallise the dihydrate, then dry it with boiling alcohol to the more stable anhydrous salt. The amounts are our arithmetic from his equation; the water, lime, salt, alcohol and glassware are ours. Source: Thorpe, Dictionary of Applied Chemistry vol. V (1913), SULPHUR, p. 306. Picture: that page.