Sodium Sulfite and Sodium Hydrosulfite

Sodium Sulfite
Sodium sulfite is a compound that is very easily oxidized. For this reason, it is employed in many cases where a gentle reducing agent is desired, e.g. to bleach wool and silk, as an antichlor after the bleaching of yarns, textile and paper, as a preservative for foodstuffs, and to prevent raw sugar solution from colouring upon evaporation. It is very widely used in the preparation of photographic developer to prevent from oxidation from hydroquinone and other agents. It has a small application in the field of medicine as an antiseptic and as an antizymotic for internal use. About 60% of the total merchant market is in the paper Industries. While merchant capacity is about 145,000 t/year, the paper mills have twice capacity to captive use. The demand for boiler feed water treatment is about 15%. It is use to remove oxygen from water and thus help to prevent corrosion and scale formation. These were four major production in 1891, one of which provided 50% percent of the product.

The most important commercial procedure for preparing this compound involves passing sulphur dioxide into a solution of soda ash until the product has an acid reaction. At this point the solution consists chiefly of sodium bisulphite. This may be converted into sodium sulphite by adding more soda ash to the solution and boiling until all the carbon dioxide has evolved. After the solution has settled, it concentrated, and crystals of Na2SO3. 7 H2O settled out upon cooling.

Sodium Hydrosulphite
One of the most important chemicals in the dyeing and printing industries is sodium hydrosulphite. It is a very powerful reducing agent and has a specific action on many dyes, particularly vat dyes, reducing them into soluble form. This reducing agent in employed for stripping certain dyes from fabrics and for bleaching straws and soaps. Formerly, one of the main difficulties with this compound was a lack of stability, however, this has been corrected, and it may be shipped and stored. Dye house worker call it simply "hydro".

There are two methods of manufacture, which require zinc dust. In the first process the zinc dust is allowed to reduce sodium bisulphite at room temperature.

2 NaHSO3 + H2SO3 + Zn ─→  ZnSO3 + Na2S2O4 + 2 H2O

The production of the reaction are treated with milk of lime to neutralize any free acid, and this reduces the solubility of the ZnSO3, which is filtered off. Sodium chloride is added to salt out the Na2S2O4 . 2H2O, which is dehydrated with alcohol and dried. The crystal are stable only in a dry state. The second process consists of treating an aqueous suspension of zinc dust in formaldehyde with sulfur dioxide at 80oC. A double decomposition reaction is then carried out with soda ash to form the sodium ash. A pure product is obtained by evaporation under vacuum. Another method is to reduce sodium sulfite with sodium amalgam (formed in the production of chlorine using a mercury cell at a controlled pH in the range of 5 to 7).

Sodium hydrosulfite can also produced from sodium formate formed as a by-product in the manufacture of pentacrythritol. The sodium formate in an alkaline alcoholic solution is reacted with sulfur dioxide.

HCOONa + 2SO2 + NaOH  →  Na2S2O4 + H2O + CO2

Rising coat of zinc metal and adverse pollution regulation with shift manufacture away from the zinc process. Over the long term sodium hydrosulfite will be manufactured by the amalgam or formate process.


Anonymous said…
3rd sentence second para should be;This may be converted into sodium sulphite-----
Kesington Blog said…
Thanks again for your correction, you right.
Hmehta said…
I didnt understand manufacturing process of sodium hydrosulphite with sodium formate. can you explain in detail.
Margi Asih said…
As on the above description that sodium format is by product on producing Pentacrythritol Manufacturing, so sodium sulfit will have low price. The detail of this production you can see on this blog also.

The sodium format still in liquid form dilute in sodium hydroxide, while SO2 is in gas state, so this reaction is using scruber that can contacting two substance state. In certain condition both solution can be reacted to form sodium sulfite

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