Chemical Properties
Iron oxides occur as yellow, red, black, or brown powder. The color
depends on the particle size and shape, and crystal structure.
Production Methods
Three main manufacturing processes are currently applied for iron
oxide pigments:
(a) Solid-state reactions (red, black and brown): calcination of
black or yellow iron oxides to red iron oxide; thermal
decomposition of ferrous sulfate.
(b) Precipitation process (red, orange, yellow and black):
treatment of ferrous sulfate solutions with alkali and
oxidation. The Penniman–Zoph process uses ferrous sulfate,
alkali, iron powder and air or oxygen.
(c) Laux process or aniline process (red, yellow, and black):
reduction of nitrobenzene to aniline with iron.
Pharmaceutical Applications
Iron oxides are widely used in cosmetics, foods, and pharmaceutical
applications as colorants and UV absorbers. As inorganic
colorants they are becoming of increasing importance as a result of
the limitations affecting some synthetic organic dyestuffs. However,
iron oxides also have restrictions in some countries on the quantities
that may be consumed, and technically their use is restricted because
of their limited color range and their abrasiveness.
Safety
Iron oxides are widely used in cosmetics, foods, and oral and topical
pharmaceutical applications. They are generally regarded as
nontoxic and nonirritant excipients. The use of iron oxide colorants
is limited in some countries, such as the USA, to a maximum
ingestion of 5 mg of elemental iron per day.
LD50 (mouse, IP): 5.4 g/kg
LD50 (rat, IP): 5.5 g/kg
storage
Iron oxides should be stored in well-closed containers in a cool, dry
place.
Incompatibilities
Iron oxides have been reported to make hard gelatin capsules brittle
at higher temperatures when the residual moisture is 11–12%. This
factor affects the use of iron oxides for coloring hard gelatin
capsules, and will limit the amount that can be incorporated into the
gelatin material.
Regulatory Status
Accepted for use as a food additive in Europe. Included in
nonparenteral medicines licensed in many countries including
Japan, UK, and USA.
