Iron oxides

• USP–NFFerric Oxide

CI 77491 (red), CI 77492 (yellow), CI 77499 (black); ferric oxide; ferrosoferric oxide; iron oxide red, yellow and black; pigment red 101; pigment yellow 42; pigment black 11; Sicopharm; Sicovit; E172

Iron(III) oxide (red); hydrated iron(III) oxide, iron oxide hydroxide (yellow); iron(II,III) oxide (black) · [—]

Fe2O3 (red); FeO(OH)·xH2O (yellow); Fe3O4 (black) · 159.69 (Fe2O3); 88.85 (FeOOH); 231.54 (Fe3O4)

Colouring agent

Typical concentrations: — · Dosage forms: Solid

Used as colourants and opacifiers in tablet film coatings, sugar coatings, hard and soft capsule shells and topical products, usually at low levels within the coating formulation. Blends of red, yellow and black oxides give a range of brown, orange and beige shades. They also provide some protection of light-sensitive drugs and are widely used in cosmetics.

Fine, odourless powders: red (haematite type), yellow (goethite type, hydrated) and black (magnetite type). Pharmaceutical grades are synthetic, which allows close control of colour, particle size and heavy-metal impurities.

Solubility

practically insoluble in water and organic solvents; soluble in hot concentrated mineral acids

Density

high, about 4–5 g/cm³ depending on type

Thermal behaviour

yellow oxide loses water above about 200 °C and turns red; black oxide oxidises to red on strong heating in air

Magnetism

black oxide is strongly magnetic

Stable to light and to the conditions of normal storage and processing. Store in a well-closed container in a cool, dry place.

Generally inert, but traces of soluble iron can catalyse the oxidation of sensitive drugs and may form coloured complexes with phenolic compounds. Strong acids dissolve them.

Produced synthetically, for example by precipitation from iron(II) salt solutions followed by oxidation (yellow and black), by calcination of yellow or black oxide (red), or by the reduction of nitrobenzene with iron (Laux process).

Considered non-toxic and non-irritant at the levels used as colourants; absorption of iron from them is very limited. Inhalation of the dust over long periods should be avoided.

Observe normal precautions for the quantity handled. Use a dust mask and eye protection; the powders stain skin and surfaces.

Accepted as a food colour in Europe (E172), re-evaluated by EFSA. In the USA synthetic iron oxides are permitted as colour additives in drugs, with ingested drugs limited to 5 mg of elemental iron per daily dose. Included in the FDA Inactive Ingredient Database (oral capsules and tablets; topical preparations) and in non-parenteral medicines licensed in Europe.

Titanium dioxide; aluminium lakes; calcium carbonate (as opacifier).

With the restriction of titanium dioxide as a food additive in the EU, iron oxides are increasingly used in combination with other opacifiers in film coatings. Colour of the final coat depends on particle size and dispersion, so pigment suspensions should be milled or homogenised and the colour checked against a standard.