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Introduction
Sodium ferrocyanide (Na₄[Fe(CN)₆]·10H₂O) is an inorganic complex salt used as an anticaking agent in table salt and road salt. Despite containing cyanide groups, the iron-cyanide complex is extremely stable and non-toxic; the cyanide is not released in the human body or in the environment under normal conditions. It is approved as a food additive (E535) in the EU and as a GRAS substance in the USA.
Sodium ferrocyanide is added to salt at very low concentrations (typically 10-30 mg/kg) to prevent clumping by modifying crystal growth. It is also used in wine and citric acid production to remove copper and iron ions (fining agent).
CAS Number
14434-22-1 (decahydrate). Anhydrous: 13601-19-9.
Synonyms
Sodium hexacyanoferrate(II), Yellow prussiate of soda (YPS), Sodium ferrocyanide decahydrate, E535.
Topic Headings
1. Mechanism of anticaking action in table salt (paragraph form)
Sodium chloride crystals naturally have a tendency to absorb moisture from the air, especially under humid conditions. The absorbed water dissolves a small amount of salt on the surface, and when the water evaporates, the salt recrystallizes, forming bridges between adjacent crystals. This results in hard lumps. Sodium ferrocyanide is added to salt at very low levels (typically 13-20 mg/kg of salt). The ferrocyanide ions ([Fe(CN)₆]⁴⁻) adsorb onto the growing faces of sodium chloride crystals, particularly on the {111} crystal faces. This adsorption poisons the crystal growth sites, preventing the formation of crystal bridges. The salt crystals remain discrete and free-flowing. The effect persists even under high humidity because the ferrocyanide remains on the crystal surfaces. No other anticaking agent works as effectively at such low concentrations. Sodium ferrocyanide also improves the whiteness of salt and prevents discoloration from trace metals.
2. Physical and chemical properties (list form)
• Appearance — Pale yellow to yellow crystalline powder or granules (decahydrate).
• Odor — Odorless.
• Molecular weight — 484.06 g/mol (decahydrate); 303.91 g/mol (anhydrous).
• Melting point — Decomposes above 435°C (loses water of hydration at 82°C).
• Solubility in water — 17.5 g/100 mL at 20°C (soluble).
• Solubility in other solvents — Insoluble in ethanol.
• Stability — Very stable; does not release cyanide under normal conditions; only decomposes in strong acids or UV light.
• pH (1% solution) — Approximately 9-10 (alkaline).
• Density — 1.46 g/cm³ (decahydrate).
• Color — Yellow due to the iron-cyanide complex.
3. Food applications and typical usage levels (list form)
• Table salt (sodium chloride) — 10-30 mg/kg (10-30 ppm) as an anticaking agent; maximum permitted 20-25 mg/kg in most countries.
• Sea salt and specialty salts — Same levels; ensures free-flowing properties.
• Salt for butter and cheese making — Used to prevent caking in cheese salt.
• Wine and juice fining (removal of copper and iron) — 0.1-0.5 g/L of wine; causes precipitation of copper ferrocyanide and iron ferrocyanide.
• Citric acid production — Used in purification steps to remove heavy metals.
• Industrial salt (non-food) — For de-icing and water softening, though not required.
4. Safety, regulatory status, and misconceptions (paragraph form)
Sodium ferrocyanide is permitted as E535 in the EU with maximum levels: 20 mg/kg (as ferrocyanide ion) in table salt. In the USA, it is GRAS under 21 CFR 184.1296 (as an anticaking agent in salt). JECFA has established an ADI of 0-0.025 mg/kg body weight expressed as ferrocyanide ion, but this is a safety limit far above typical intake. The safety concern about cyanide is common but unfounded. The ferrocyanide ion is a coordination complex in which the cyanide groups are tightly bound to an iron(II) center. The complex has very low toxicity (LD50 in rats >5000 mg/kg). It does not dissociate in the human gastrointestinal tract because stomach acid is not strong enough (requires concentrated mineral acids with heating). Even if some dissociation occurred, the resulting free cyanide would be immediately complexed by dietary iron and metabolized by the liver (rhodanese pathway). The salt contains about 30% cyanide by weight, but none is bioavailable. JECFA concluded that the ADI is not limiting because actual intake is far below the limit. Consumers concerned about "cyanide in salt" should know that many foods naturally contain cyanogenic glycosides (e.g., almonds, lima beans, cassava) which release more cyanide than ferrocyanide ever could. The use of sodium ferrocyanide in salt is considered safe by all major food safety authorities.
