Preparation of dithionite salts

Here's a nice little collection of proc's related to the manufacture of dithionites, used as reducing agents. It's always been my impression that these agents are undeservedly overlooked here at the Hive - well, who knows maybee one day their time will come (e.g., for selectively reducing a-oximino-aceto/propiophenones to cathinones - and who knows what else).

I know that Na2S2O4 is fairly easy to get OTC for many bees - OTOH, in some parts of the world (like mine) its use is strictly limited to chem labs.

1. Preparation of zinc dithionite - as submitted to Hyperlab by Chemister.

Into an Erlenmeier flask there's placed 5-10g Zn powder, 50 mls abs. EtOH; the flask is stoppered and thru a tube extending almost to the bottom, SO2, dried w/conc H2SO4, is bubbled into the rxn. The flask is also equipped w/an outlet tube, the other end of which is immersed into mercury. The rxn is over when all the Zn is gone. The crystalline precipitate is filtered and dried in an exicator (preferrably in vacuo) over H2SO4. As dry crystals, ZnS2O4 is relatively stable to air. Its aqueous solution is a very strong reducing agent.

2. Preparation of sodium dithionite from Na formate and SO2 (US Pat 3947559)

Note - in the 'classical' method the reaction is carried under superatmospheric pressures - albeit not very high, like 1,5-3 atm, yet very inconvenient for the kitchen bees. This one describes such a prep'n under atmospheric conditions - the main point of the patent is usage of methoxyethanol as solvt, but it appears that decent results are obtained using EtOH and esp. EtOH/Et(OH)2 system.

a) With EtOH as solvent.

A slurry of 160g (2.35 moles) sodium formate, 240g ethanol (74 DEGo.p. industrial methylated spirit) and 120g water was charged into the reaction vessel and heated up to 75 C with stirring under reflux in an inert atmosphere (N2). To the stirred slurry was added a solution of 60g (1.5 moles) sodium hydroxide in 70g water concurrently with a solution of 200g (3.13 moles) sulphur dioxide in 610g ethanol. The sulphur dioxide solution was added at a uniform rate over 1 hour and the alkali at such a rate that the first 30% was added 21/2 times as fast as the remaining 70%, the total time of addition being 1 hour. The temperature was kept at 75 C at all times. A sticky solid was formed initially but this quickly dispersed to give a normal-looking white solid. The mixture was stirred at 75 C for 2,5 hours after addition was complete and then filtered off under an inert atmosphere, washed with 200g methanol at a temperature above 60 C and finally dried. The white solid obtained weighed 211.9g and contained 70.7% sodium dithionite, Na2S2O4. The yield as Na2S2O4 was 58.6% calculated on the sulphur dioxide consumed.

b) With EtOH/ethyleneglycol as solvent.

A slurry of 160g (2.35 moles) sodium formate, 180g ethanol (74 DEGo.p. industrial methylated spirit) 60g ethylene glycol and 120g water was charged into the reaction vessel and heated up to 80 DEGC with stirring under reflux in an inert atmosphere (N2). To the stirred slurry was added a solution of 60g (1.5 moles) sodium hydroxide in 70g water concurrently with a solution of 200g (3.13 moles) sulphur dioxide in a mixture of 457g ethanol and 153g ethylene glycol. The sulphur dioxide solution was added at a uniform rate over 1 hour and the alkali at such a rate that the first 30% was added 21/2 times as fast as the remaining 70%, the total time of addition being 1 hour. When addition was complete the product was stirred at 80 DEGC for 21/2 hours, filtered off under an inert atmosphere, washed with 200g methanol at a temperature above 60 DEGC and then dried. The white solid obtained weighed 212.7g and contained 82.3% sodium dithionite, Na2 S2 O4. The yield as Na2 S2 O4 was 67.0% calculated on sulphur dioxide consumed.