Chemical reactions were involved in the preparation of several famous ointments in the past, e.g., the Strong Mercuric Nitrate Ointment of 1959 B.P.C.
(a) Ointment containing free
iodine
Iodine is only slightly soluble
in most fats and oils. Iodine is readily
soluble in a concentrated potassium iodide solution due to the formation of
molecular complexes KI⋅I2, KI⋅2I2, KI⋅3I2 etc.
These solutions may be
incorporated in absorption-type ointment bases. For example, Strong Iodine
Ointment (British Veterinary Pharmacopoeia) treats ringworm in
cattle. It contains free iodine. At one time, this type of ointment was used as a counter-irritant in treating human rheumatic diseases but they were not popular because they stained the skin a deep red colour. Due to improper storage,
the water dries up and the iodine crystals irritate the skin, hence glycerol is
sometimes added to dissolve the iodine-potassium iodide complex instead of
water.
Example:
Strong Iodine Ointment. Iodine – 4 g
Woolfat - 4 g
Yellow soft paraffin – 76 g
Potassium iodide – 4 g
Water – 12 g
Procedure
(i) KI is dissolved in water. I2
is dissolved in it.
(ii) Wool fat and soft yellow paraffin are melted together over a water bath. Melted mass is cooled to about
40°C.
(iii) The I2 solution is added to the melted mass in small quantities at a time, continuously stirring, until a uniform mass is obtained.
(iv) It is cooled to room
temperature and packed. Use: Ringworm in
cattle.
(b) Ointment containing
combined iodine
Fixed oils and many vegetable and
animal fats absorb iodine which combines with the double bonds of the unsaturated constituents,
e.g.
CH3⋅(CH2)2⋅CH
= CH⋅(CH2)7⋅COOH
+ I2CH3⋅(CH2)2⋅CHI
CHI⋅(CH2)7⋅COOH
Oleic acid Di-iodostearic
acid
Example:
Non-staining
Iodine Ointment B.P.C.
Iodine – 5 g
Arachis oil – 15 ml
Yellow soft paraffin - q.s. to
100 g
Method
(a) Iodine is finely powdered in
a glass mortar and the required amount is added to the oil in a glass-stoppered
conical flask and stirred well.
(b) The oil is heated at 50°C in
a water bath and stirred continually. Heating is continued until the brown
colour is changed to greenish-black; this may take several hours.
(c) From 0.1 g of the preparation
the amount of iodine is determined by the B.P.C. method and the amount of soft
paraffin base is calculated to give the product the required strength.
(d) Soft paraffin is warmed to
40°C. The iodized oil is added and mixed well. No more heat is applied because
this causes the deposition of a resinous substance.
(e) The preparation is packed in
a warm, wide-mouthed, amber colour, glass bottle. It is allowed to cool without
further stirring.