SODIUM HYDROSULFITE

Sodium hydrosulfite is a whitish to light yellow crystalline solid having a sulfur dioxide-like odor. 
Sodium hydrosulfite is a powerful reducing agent. 
Sodium hydrosulfite has therefore been suggested to be used as an additive in kraft pulping to improve the yield. 

CAS Number: 7775-14-6
EC Number: 231-890-0
IUPAC Name: Disodium dithionite
Chemical Formula: Na2S2O4

Other names: 7775-14-6, Sodium hydrosulphite, Dithionous acid, disodium salt, Sodium sulfoxylate, Disodium dithionite, Sodium hypodisulfite, Vatrolite, 2K5B8F6ES1, CHEBI:66870, Blankit, Burmol, Hydros, sodiumdithionite, Blankit IN, Hydrosulfite R Conc, V-Brite B, Disodium hydrosulfite, Caswell No. 774, CCRIS 1428, HSDB 746, Sodium hydrosulfite (Na2S2O4), Sodium dithionite (Na2(S2O4)), EINECS 231-890-0, UN1384, EPA Pesticide Chemical Code 078202, UNII-2K5B8F6ES1, sodium dithionit, sodiumhydrosulfite, Sodiumhydrosulphite, sodium hydro sulfite, MFCD00011640, sodium sodium hydrosulfite, Dithionous aciddisodiuMsalt, Na2S2O4, EC 231-890-0, Sodium dithionite (Na2S2O4), SODIUM DITHIONITE [II], SODIUM DITHIONITE [MI], CHEMBL3410462, DTXSID9029697, Na2 (S2 O4), SODIUM DITHIONITE [MART.], SODIUM HYDROSULFITE [HSDB], SODIUM HYDROSULFITE [INCI], Dithionous acid, sodium salt (1:2), AKOS015904498, Sodium dithionite or sodium hydrosulfite, BP-13393, FT-0695294, S0562, Q414560, Sodium dithionite or sodium hydrosulfite [UN1384] [Spontaneously combustible], Sodium dithionite [Wiki], 14844-07-6 [RN], 231-890-0 [EINECS], 2K5B8F6ES1, 7775-14-6 [RN], Dinatriumdithionit [German] [ACD/IUPAC Name], Disodium dithionite [ACD/IUPAC Name], Dithionite [ACD/IUPAC Name], Dithionite de disodium [French] [ACD/IUPAC Name], Dithionous acid, disodium salt, MFCD00011640 [MDL number], Sodium hydrosulfite, Sodium hydrosulphite, Sodium hypodisulfite, 1340-77-8 [RN], Blankit, Burmol, Disodium hydrosulfite, disodium sulfinatosulfinate, Dithionous aciddisodiumsalt, EINECS 231-890-0, SODIUM DITHIONATE, Sodium dithionite (Na2S2O4), Sodium Hydrosulfite, 85per cent, Sodium hydrosulfite, Sodium hypodisulfite, Sodium hyposulfite, Sodium sulfoxylate, sodiumdithionite, UNII:2K5B8F6ES1, UNII-2K5B8F6ES1, Vatrolite, V-Brite B

Sodium hydrosulfite is a white crystalline powder with a sulfurous odor. 
Although Sodium hydrosulfite is stable in dry air, Sodium hydrosulfite decomposes in hot water and in acid solutions.

Sodium hydrosulfite is a whitish to light yellow crystalline solid having a sulfur dioxide-like odor. 
Sodium hydrosulfite spontaneously heats on contact with air and moisture. 

This heat may be sufficient to ignite surrounding combustible materials. 
Under prolonged exposure to fire or intense heat containers of Sodium hydrosulfite may violently rupture. 
Sodium hydrosulfite is used in dyeing and to bleach paper pulp.

Sodium hydrosulfite is a whitish to light yellow crystalline solid having a sulfur dioxide-like odor. 
Sodium hydrosulfite spontaneously heats on contact with air and moisture. 

This heat may be sufficient to ignite surrounding combustible materials. 
Under prolonged exposure to fire or intense heat containers of this material may violently rupture. 
Sodium hydrosulfite is used in dyeing and to bleach paper pulp.

Sodium hydrosulfite is an inorganic sodium salt that is the disodium salt of dithionous acid. 
Sodium hydrosulfite has a role as a reducing agent and a bleaching agent. 

Sodium hydrosulfite contains a dithionite(2-).
Sodium hydrosulfite is unstable under physiological conditions, Sodium hydrosulfite degradation rate increases with increasing acidity.

After contact with moisture, Sodium hydrosulfite is oxidized to hydrogen sulfide (hso3-), sulfite (so32-), and hydrogen sulfate (hso4-).
Sodium hydrosulfite can release sulfur dioxide under strongly acidic conditions.

Under anaerobic conditions (as in the lower gastrointestinal tract), hydrogen sulfide (hso3-) and thiosulfate (s2o32-) can occur. 
Hydrogen sulfide (hso3-) can be absorbed after ingestion.
Sodium hydrosulfite is metabolized efficiently and most of Sodium hydrosulfite is rapidly excreted as sulfate in the urine.

Sodium hydrosulfite is widely used in industry due to Sodium hydrosulfite reducing properties and ability to react with oxygen.
Sodium hydrosulfite is used as a reducing bleaching agent to make yellow discoloration of cellulose-based products in the textile industry, in the pulp and paper industry as a reducing bleach, as an oxygen scavenger in boilers, for preservation and water treatment to remove iron stains on cultural artifacts.

Iron flash control on white fabrics in bleaching environments.
Sodium hydrosulfite is also used in photographic film, clay, wine, leather goods, food and beverages, polymers, cleaners, gas cleaning, environmental remediation, metal recovery and chemical processing.

Sodium hydrosulfite (Na2S2O4) is a powerful reducing agent. 
Sodium hydrosulfite has therefore been suggested to be used as an additive in kraft pulping to improve the yield. 

However, Sodium hydrosulfite easily decomposes and Sodium hydrosulfite is thus important to determine the effect of different conditions. 
The stability of Sodium hydrosulfite was found to decrease with increasing heating temperature, concentration of Sodium hydrosulfite, heating time and pH. 

Sodium hydrosulfite was found to be relatively stable at moderate alkaline pH: 11.5 and 12.5, while a rapid decrease in stability with time was noted at higher heating temperatures and concentrations of Sodium hydrosulfite.
Based on this study on the thermal stability of Sodium hydrosulfite, the following conditions are suggested as the most promising, when adding Sodium hydrosulfite to the kraft cooking as an additive; pH 12.5, with 0.4 M concentration of the solution, at a heating temperature of 100 °C.

Uses of Sodium hydrosulfite:
Sodium hydrosulfite is used to dye fibers and textiles, to strip dyes from fabrics, and to bleach sugar, soap, oils and wood pulp.
Sodium hydrosulfite is used as a chemical reagent and an oxygen scavenger in the production of synthetic rubber.

All uses of Sodium hydrosulfite are based on Sodium hydrosulfite reducing properties. 
In the textile industry, Sodium hydrosulfite is primarily used as reducing agent for vat dyes and sulfur containing dyes, and for the removal of pigments on textiles. 
Sodium hydrosulfite is also used as a bleaching agent in reductive bleaching processes, for instance, in the bleaching of mechanical paper pulp, and the bleaching of cotton and wool, as well as sugar.

Industry Uses:
Bleaching agent
Bleaching agents
Cleaning agent
Dye
Not Known or Reasonably Ascertainable
Other (specify)
Oxidizing/reducing agents
Reducing agent
Solvents (which become part of product formulation or mixture)

Consumer Uses:
Not Known or Reasonably Ascertainable
Reducing agent

Usage Areas of Sodium hydrosulfite:
In the textile sector, Sodium hydrosulfite provides a reductive reaction with the dye remaining on the fiber after dyeing, allowing excess dye to be removed from the fiber.
Sodium hydrosulfite is used in the paper industry for bleaching pulp.

Sodium hydrosulfite is used in the food industry to bleach sherbet and maltose.
Sulfonation in water treatment, bleaching of minerals by removing iron ions, production of chemicals

Industrial Processes with risk of exposure:
Pulp and Paper Processing
Textiles (Printing, Dyeing, or Finishing)
Toxic Gas from Spilling Chemical in Water

Applications of Sodium hydrosulfite:

Industry:
Being water-soluble, Sodium hydrosulfite is used as a reducing agent in some industrial dyeing processes. 
In the case of sulfur dyes and vat dyes, an otherwise water-insoluble dye can be reduced into a water-soluble alkali metal salt (e.g. indigo dye).

Sodium hydrosulfite can also be used for water treatment, aquarium water conditioners, gas purification, cleaning, and stripping. 
Sodium hydrosulfite has also been applied as a sulfonating agent. 

In addition to the textile industry, this compound is used in industries concerned with leather, foods, polymers, photography, and many others, often as a decolourising agent. 
Sodium hydrosulfite is even used domestically as a decoloring agent for white laundry, when Sodium hydrosulfite has been accidentally stained by way of a dyed item slipping into the high temperature washing cycle. 
Sodium hydrosulfite is usually available in 5 gram sachets termed hydrosulfite after the antiquated name of the salt.

Sodium hydrosulfite is the an active ingredient in "Iron Out Rust Stain Remover", a commercial rust product.

Laboratory:
Sodium hydrosulfite is often used in physiology experiments as a means of lowering solutions' redox potential (Eo' -0.66 V vs SHE at pH 7).
Potassium ferricyanide is usually used as an oxidizing chemical in such experiments (Eo' ~ .436 V at pH 7). 

In addition, Sodium hydrosulfite is often used in soil chemistry experiments to determine the amount of iron that is not incorporated in primary silicate minerals. 
Hence, iron extracted by Sodium hydrosulfite is also referred to as "free iron." 
The strong affinity of the dithionite ion for bi- and trivalent metal cations (M2+, M3+) allows Sodium hydrosulfite to enhance the solubility of iron, and therefore dithionite is a useful chelating agent.

Aqueous solutions of Sodium hydrosulfite were once used to produce 'Fieser's solution' for the removal of oxygen from a gas stream.
Pyrithione can be prepared in a two-step synthesis from 2-bromopyridine by oxidation to the N-oxide with a suitable peracid followed by substitution using Sodium hydrosulfite to introduce the thiol functional group.

Photography:
Sodium hydrosulfite is used in Kodak fogging developer, FD-70. 
This is used in the second step in processing black and white positive images, for making slides. 
Sodium hydrosulfite is part of the Kodak Direct Positive Film Developing Outfit.

Preparation of Sodium hydrosulfite:
Sodium hydrosulfite is produced industrially by reduction of sulfur dioxide. 
Approximately 300,000 tons were produced in 1990.

The route using zinc powder is a two-step process:
2 SO2 + Zn → ZnS2O4
ZnS2O4 + 2 NaOH → Na2S2O4 + Zn(OH)2

The sodium borohydride method obeys the following stoichiometry:
NaBH4 + 8 NaOH + 8 SO2 → 4 Na2S2O4 + NaBO2 + 6 H2O

Each equivalent of H− reduces two equivalents of sulfur dioxide. 
Formate has also been used as the reductant.

Structure of Sodium hydrosulfite:
The structure has been examined by Raman spectroscopy and single-crystal X-ray diffraction. 
The dithionite dianion has C2 symmetry, with almost eclipsed with a 16° O-S-S-O torsional angle. 
In the dihydrated form (Na2S2O4·2H2O), the dithionite anion has gauche 56° O-S-S-O torsional angle.

A weak S-S bond is indicated by the S-S distance of 239 pm, which is elongated by ca. 30 pm relative to a typical S-S bond.
Because this bond is fragile, the dithionite anion dissociates in solution into the [SO2]− radicals, as has been confirmed by EPR spectroscopy. 
Sodium hydrosulfite is also observed that 35S undergoes rapid exchange between S2O42− and SO2 in neutral or acidic solution, consistent with the weak S-S bond in the anion.

Properties and Reactions of Sodium hydrosulfite:

Hydrolysis:
Sodium hydrosulfite is stable when dry, but aqueous solutions deteriorate due to the following reaction:
2 S2O42− + H2O → S2O32− + 2 HSO3−

This behavior is consistent with the instability of dithionous acid. 
Thus, solutions of Sodium hydrosulfite cannot be stored for a long period of time.

Anhydrous Sodium hydrosulfite decomposes to sodium sulfate and sulfur dioxide above 90 °C in the air. 
In absence of air, Sodium hydrosulfite decomposes quickly above 150 °C to sodium sulfite, sodium thiosulfate, sulfur dioxide and trace amount of sulfur.

Redox reactions of Sodium hydrosulfite:
Sodium hydrosulfite is a reducing agent. 
At pH 7, the potential is -0.66 V compared to the normal hydrogen electrode. 

Redox occurs with formation of bisulfite:
S2O42- + 2 H2O → 2 HSO3− + 2 e− + 2 H+

Sodium hydrosulfite reacts with oxygen:
Na2S2O4 + O2 + H2O → NaHSO4 + NaHSO3

These reactions exhibit complex pH-dependent equilibria involving bisulfite, thiosulfate, and sulfur dioxide.

With organic carbonyls:
In the presence of aldehydes, Sodium hydrosulfite reacts either to form α-hydroxy-sulfinates at room temperature or to reduce the aldehyde to the corresponding alcohol above a temperature of 85 °C.
Some ketones are also reduced under similar conditions.

Manufacturing Methods of Sodium hydrosulfite:

Zinc dust process:
An aqueous slurry of zinc dust is treated in a stirred reactor with cooling at ca. 40 °C with liquid or gaseous sulfur dioxide to give zinc dithionite. 
After completion of the reaction the solution is passed through a filter press to remove unreacted zinc dust and impurities from the zinc. 

The zinc is then precipitated from the zinc dithionite by adding sodium carbonate or sodium hydroxide in stirred vessels. 
The zinc carbonate or hydroxide is removed in filter presses. 

Anhydrous Sodium hydrosulfite is precipitated from the clarified Sodium hydrosulfite solution by concentration under vacuum and addition of sodium chloride at > 60 °C. 
Sodium hydrosulfite is filtered, washed with methanol, and dried at 90 - 100 °C. 

Besides the evaporation process the salting out process, which was more widely used previously, is still known. 
In this process the dithionite is obtained from the solution by the addition of sodium chloride and methanol.

Amalgam Process:
In the amalgam process, sodium hydrogensulfite is reduced to Sodium hydrosulfite in aqueous solution in a cooled, stirred vessel using the sodium amalgam of a chloralkali electrolysis cell. 
The sodium-free mercury is returned to the electrolysis cell where Sodium hydrosulfite is recharged with sodium. 

During reaction of the amalgam with the hydrogensulfite solution a pH of 5 - 6 must be maintained. 
Sodium hydrosulfite is obtained by precipitation with salts or methanol or both.

Formate Process: 
Sodium formate, dissolved in 80% aqueous methanol, is charged to a stirred vessel. 
At a pressure of 2 - 3 bar sulfur dioxide and sodium hydroxide are introduced into this solution such that a pH of 4 - 5 is maintained.

Sodium Borohydride Process Sodium borohydride is stable in strong aqueous alkali and can be used in this form for the production of Sodium hydrosulfite by adding SO2 and sodium hydroxide.