US Pharm. 2009;34(1):40-41.
Method of Preparation: Calculate the quantity of each ingredient for the amount to be prepared. Accurately weigh or measure each ingredient. Disperse the Carbopol 940 into the ethyl alcohol, stirring rapidly, until mixed and uniform. Add the trolamine dropwise to a pH in the range of about 6.5 and a suitable viscosity. Add the coal tar solution; mix until uniform. Package and label.
Use: Coal tar alcohol gel has been used to treat psoriasis and other coal tar-responsive dermatoses.
Packaging: Package in tight, light-resistant containers.
Labeling: For external use only. Keep tightly closed. Keep out of the reach of children. Store away from flame and heat. Use only as directed.
Stability: A beyond-use date of 30 days can be used for this preparation.1
Quality Control: Quality-control assessment can include theoretical weight compared with actual weight, pH, specific gravity (SG), active drug assay, color, clarity, texture-surface, texture-spatula spread, appearance, feel, rheologic properties, and physical observations.2
Discussion: Coal tar topical solution contains 20% coal tar and 5% polysorbate 80 in alcohol USP; it contains between 81.0% and 86.0% ethanol. It should be preserved in tight containers. Coal tar is a local irritant that is used to treat chronic skin diseases. It generally decreases the rate of epidermal synthesis of deoxyribonucleic acid and suppresses hyperplasia. It should not be applied to the eyes or to raw, weeping, or blistered surfaces. Temporary skin discoloration may occur while it is being used. Coal tar (Pix Carbonis) is a byproduct of the destructive distillation of bituminous coal. It is a nearly black, viscous liquid; it is heavier than water, has a characteristic, naphthalene-like odor, and produces a sharp, burning sensation on the tongue. It is slightly soluble in water and partially soluble in acetone, alcohol, ether, methanol, and hexane. It is more soluble in benzene and almost completely soluble in nitrobenzene. When placed in water, coal tar imparts its characteristic odor and taste and produces a faintly alkaline reaction.1,3
Carbomers are synthetic, high-molecular-weight polymers. They are composed of acrylic acid cross-linked with either allyl sucrose or allyl ethers of pentaerythritol, and they occur as white-colored, fluffy, acidic, hygroscopic powders with a slight characteristic odor. The molecular weight of carbomer 940 is approximately 4 ´ 106. The pH of a 0.5% to 1.0% dispersion is in the range of 2.5 to 3.5. Carbomers are soluble in water; after neutralization, they are soluble in 95% ethanol and glycerin. When carbomers are dispersed in water, an acidic colloidal solution of low viscosity forms that will thicken when an alkaline material, such as trolamine, is added. To ease the initial dispersion process, the carbomer should be sprinkled on rapidly agitated water, with care taken to minimize the formation of lumps. Numerous neutralizing agents can be used to thicken the gel, including amino acids, borax, potassium hydroxide, sodium bicarbonate, sodium hydroxide (0.4 g sodium hydroxide will neutralize about 1 g carbomer), polar organic amines such as trolamine, and lauryl and stearyl amines. Incorporation of air bubbles into the gel should be kept to a minimum. Maximum viscosity generally can be obtained in a pH range of 6 to 11.4
Trolamine (TEA, triethanolamine) is an alkalizing and emulsifying agent. It occurs as a variable mixture of alkanolamines and is a clear, colorless to pale yellow-colored, viscous liquid with a slight odor of ammonia. Its SG is 1.120 g/mL to 1.128 g/mL, and it has a pH of 10.5 in a 0.1 N aqueous solution. Trolamine is highly hygroscopic, melts at 20°C to 21°C, and boils at 335°C. It is miscible with water, 95% ethanol, methanol, and acetone. When exposed to air and light, it may turn brown; at temperatures less than 15°C, it may stratify (this can be corrected by warming and mixing). Store trolamine in a cool place in airtight containers protected from light.5
Alcohol (C2H5OH, MW 46.07, ethyl alcohol, ethanol, grain alcohol) is a clear, colorless, mobile, volatile liquid with a slight, characteristic odor and a burning taste. Alcohol USP refers to 95% ethanol; dehydrated alcohol refers to 99.5% alcohol. Its SG is between 0.812 and 0.816, and its boiling point is 78.15°C. It is miscible with chloroform, glycerin, and water.6
REFERENCES
1. US Pharmacopeial Convention, Inc. USP Pharmacists' Pharmaco peia. 2nd ed. Rockville, MD: US Pharmacopeial Convention, Inc; 2008:135-136,775-779.
2. Allen LV Jr. Standard operating procedure for performing physical quality assessment of ointments/creams/gels. IJPC. 1998;2:308-309.
3. Harvey SC. Topical drugs. In: Gennaro AR, ed. Remington's Pharmaceutical Sciences. 16th ed. Easton, PA: Mack Publishing Co; 1980:724.
4. Koleng JJ, McGinity JW. Carbomer. In: Rowe RC, Sheskey PJ, Owen SC, eds. Handbook of Pharmaceutical Excipients. 5th ed. Washington, DC: American Pharmaceutical Association; 2006:111-115.
5. Goskonda SR, Lee JC. Trieth anolamine. In: Rowe RC, Sheskey PJ, Owen SC, eds. Handbook of Pharmaceutical Excipients. 5th ed. Washington, DC: American Pharmaceutical Association; 2006:794-795.
6. Owen SC. Alcohol. In: Rowe RC, Sheskey PJ, Owen SC, eds. Handbook of Pharmaceutical Excipients. 5th ed. Washington, DC: American Pharmaceutical Association; 2006:18-20.
