The synthetic process of bisulfite (CAS NO. ) contains 2 steps. The synthetic route is as follows:

In a 2 l round-bottomed flask, attached to an efficient reflux condenser and set in a hot water bath, are placed 222 g (1.72 moles) of selenious acid, 270 ml of paraldehyde, 540 ml of dioxane, and 40 ml of 50% acetic acid, and the mixture is refluxed for 6 hours. The solution is then decanted from the inorganic material, which is washed with two 150 ml portions of water. The combined solutions are steam-distilled through a still head (p. 65) until the paraldehyde and dioxane have been removed; this requires about 3.5 hours. The mixture is decanted from a little selenium, and to the solution, without filtration, is added a slight excess of 25% lead acetate solution. The lead selenite is removed by filtration, and the filtrate is saturated with hydrogen sulfide in a hood. Then 20 g of Norit is added; the whole is warmed to 40°C in a hood and filtered with suction. The water-clear solution is concentrated on a hot water bath under reduced pressure to about 150 ml in the usual apparatus.

This concentrate is added to a previously prepared and filtered solution of sodium bisulfite in 40% ethanol contained in a 4-l. beaker provided with a mechanical stirrer. The mixture is stirred for 3 hours, and the addition product then is filtered with suction on an 18-cm. Büchner funnel and washed, first with two 150 ml portions of ethanol and then with 150 ml of ether. The yield of air-dried product is 350–360 g (72–74%, based on the selenious acid used). Glyoxal bisulfite is pure enough for most purposes. It can be recrystallized by dissolving it in water and adding enough alcohol to make a 40% solution. The recovery is 90–92%.