Phenol, USP, remains the standard to which the activity of most germicidal substances is compared. The phenol coeffi cient is defined as the ratio of a dilution of a given test disinfectant to the dilution of phenol that is required to kill (to the same extent) a strain of Salmonella typhi under carefully controlled time and temperature conditions. As an example, if the dilution of a test disinfectant is 10-fold greater than the dilution of phenol, the phenol coefficient is 10. Obviously, the phenol coefficient of phenol itself is 1. The phenol coefficient test has many drawbacks. Phenols and other germicides do not kill microorganisms uniformly, so variations in the phenol coefficient will occur. Moreover, the conditions used to conduct the test are difficult to reproduce exactly, so high variability between different measurements and laboratories is expected. Hence, the phenol coefficient may be unreliable.
Several phenols are actually more bactericidal than phenol itself. Substitution with alkyl, aryl, and halogen (especially in the para position) groups increases bactericidal activity. Straight-chain alkyl groups enhance bactericidal activity more than branched groups. Alkylated phenols and resorcinols are less toxic than the parent compounds while retaining bactericidal properties. Phenols denature bacterial proteins at low concentrations, whereas lysis of bacterial cell membranes occurs at higher concentrations.
Phenol (carbolic acid) is a colorless to pale-pink crystalline material with a characteristic "medicinal odor." It is soluble to the extent of 1 part to 15 parts water, very soluble in alcohol, and soluble in methanol and salol (phenyl salicylate).
Phenol exhibits germicidal activity (general protoplasmic poison), is caustic to skin, exerts local anesthetic effects, and must be diluted to avoid tissue destruction and dermatitis.
Sir Joseph Lister introduced phenol as a surgical antiseptic in 1867, and it is still used occasionally as an antipruritic in phenolated calamine lotion (0.1%-1% concentrations). A 4% solution of phenol in glycerin has been used to cauterize small wounds. Phenol is almost obsolete as an antiseptic and disinfectant.
Liquified phenol is simply phenol containing 10% water. The liquid form is convenient for adding phenol to various pharmaceutical preparations because it can be measured and transferred easily. The water content, however, precludes its use in fixed oils or liquid petrolatum, because the solution is not miscible with lipophilic ointment bases.
p-Chlorophenol p-Chlorophenol is used in combination with camphor in liquid petrolatum as an external antiseptic and anti-irritant. The compound has a phenol coefficient of about 4.
p-Chloro-m-xylenol p-Chloro-m-xylenol (PC-MX; Metasep) is a nonirritating antiseptic agent with broad-spectrum antibacterial and antifungal properties. It is marketed in a 2% concentration as a shampoo. It has also been used topically for the treatment of tinea (ringworm) infections such as athlete's foot (tinea pedis) and jock itch (tinea cruris).
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