Sodium Nitrite — Frequently Asked Questions

By passivation. Nitrite oxidises the steel surface to form a thin, adherent, self-repairing oxide film that stops further attack. That is different from a barrier coating or a sacrificial anode; the metal protects itself once the film is established. It is used in closed heating and cooling circuits and in concrete admixtures to protect reinforcement.

Because nitrite oxidises haemoglobin to methaemoglobin, which cannot carry oxygen, and the lethal dose is small. Nitrate is comparatively inert; nitrite is chemically reactive and acutely toxic by ingestion. That difference of one oxygen atom is why the two salts require quite different handling and why the names must not be confused.

It is the reaction in which nitrous acid, generated from sodium nitrite and acid, converts an aromatic amine into a diazonium salt. That salt then couples with another aromatic compound to form an azo dye. Essentially the entire azo dye industry, the largest class of synthetic dyes, depends on this reaction and therefore on sodium nitrite.

Acids, which liberate toxic nitrous fumes, and ammonium salts, which can form unstable ammonium nitrite. It should also be kept from combustible material, since it is an oxidiser, and from reducing agents. Storage should be dry, since it is hygroscopic, and clearly segregated and labelled to avoid confusion with sodium nitrate.