Sodium Hydrosulfite

Properties and reactions

Hydrolysis

Sodium dithionite 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 dithionite cannot be stored for a long period of time.

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

Redox reactions

Sodium dithionite is a reducing agent. At pH=7, the potential is -0.66 V vs NHE. Redox occurs with formation of sulfite:

S₂O₄^2- + 2 H₂O → 2 HSO₃⁻ + 2 e⁻ + 2 H⁺

Sodium dithionite 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 dithionite 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.

Applications

Industry

This compound is a water-soluble salt, and can be used as a reducing agent in aqueous solutions. It is used as such in some industrial dyeing processes, primarily those involving sulfur dyes and vat dyes, where an otherwise water-insoluble dye can be reduced into a water-soluble alkali metal salt (e.g. indigo dye). The reduction properties of sodium dithionite also eliminate excess dye, residual oxide, and unintended pigments, thereby improving overall colour quality.

Sodium dithionite can also be used for water treatment, gas purification, cleaning, and stripping. It can also be used in industrial processes as a sulfonating agent or a sodium ion source. In addition to the textile industry, this compound is used in industries concerned with leather, foods, polymers, photography, and many others. Its wide use is attributable to its low toxicity LD₅₀ at 5 g/kg, and hence its wide range of applications. It is also used as decolourising agent in organic reactions.

Biological sciences

Sodium dithionite is often used in physiology experiments as a means of lowering solutions’ redox potential (E^o’ -0.66 V vs SHE at pH 7). Potassium ferricyanide is usually used as an oxidizing chemical in such experiments (E^o’ ~ .436 V at pH 7). In addition, sodium dithionite 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 dithionite is also referred to as “free iron.” The strong affinity of the dithionite ion for bi- and trivalent metal cations (M²⁺, M³⁺) allows it to enhance the solubility of iron, and therefore dithionite is a useful chelating agent.

Geosciences

Sodium dithionite has been used in chemical enhanced oil recovery to stabilize polyacrylamide polymers against radical degradation in the presence of iron. It has also been used in environmental applications to propagate a low E_h front in the subsurface in order to reduce components such as chromium.

Photography

It 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. It is part of the Kodak Direct Positive Film Developing Outfit.

Home

The main use of sodium dithionite at home is as a decoloring agent for white laundry, when it has been accidentally stained by way of a dyed item slipping into the high temperature washing cycle. It is usually available in 5 gram sachets termed hydrosulfite after the antiquated name of the salt.

Laboratory

Aqueous solutions of sodium dithionite 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 dithionite to introduce the thiol functional group.