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Phenol Coefficient Definition

Disinfectants can be categorized on their ability to disinfect microbes. If the power of disinfectants is high it can kill pathogens, fungi, microbes and endospores. Disinfectants can also lead to sterilization, if used continuously.

Disinfectants with low power cannot destroy vegetative cells, and some virus possessing envelope, whereas intermediate level disinfectants are bit more efficient. The disinfection power can be determined from the disinfectant application.

Thus, to measure how efficient a chemical disinfectant is, there are various methods like surface time kill test, qualitative and quantitative suspension test, Kelsey Sykes test, Dilution test, American association of official analytical chemist are the tests. Apart from that, another test to know how effective is the disinfectant, phenol coefficient test can be determined.

Phenol coefficient can be obtained when degree of dilution of test disinfectant is divided by the degree of dilution of phenol. If the value obtained is greater than 1, it means its extremely strong and is better than phenol and if the value comes below 1 then, phenol is better than the disinfectant tested.

What is Phenol Coefficient?

The chemical formula of phenol is C6H6O, which is a carboxylic acid, with an aromatic ring. Phenol possess antimicrobial and antifungal properties, and is one of the most ancient disinfectant. Phenol at 0.1-1% concentration stops the bacterial growth and at higher concentration like 1-2% kills bacteria and fungus. 5% concentration of phenol can kill Anthrax, within a time period of 48 hours.

Although phenol is an antiseptic, it is quite toxic to skin, hence not used and if consumed orally can be fatal. As phenol is an ideal disinfectant, its used to compare with the other disinfectant to check their disinfecting power. For example, hydrogen peroxide, chlorine and others.

Phenol coefficient = dilution ratio of disinfectant ÷ dilution ratio of phenol

For example, when 1 part of phenol is diluted in 100 parts, kills microbes in 10 minutes and when another disinfectant’s 1 part is diluted in 500 parts, it will kill those microbes in the same time.

500/(100) = 5 is the phenol coefficient.

This, states that the disinfectant tested is stronger than phenol.

Phenol Coefficient Test

i. Rideal walker method for phenol coefficient determination

To find the disinfection ability of other disinfectants, in comparison to the phenol, Rideal walker developed a method. A liquid containing sodium chloride, peptone and distilled water is mixed, sterilized and allowed to cool down.

The pH of the medium is set between 7.3-7.5. The organism Salmonella typhi to be checked is subcultured. Various conc of phenol is made when 1gm of phenol is mixed with increasing conc of water like 95, 100 and so on. In the same way, the disinfectant to be tested, its concentration are also made.

These subcultures are put on plate at a time period of 2.5, 5, 7.5 and 10 minutes. Further through the empirical data, the results can be determined.

Phenol coefficient = dilution ratio of disinfectant ÷ dilution ratio of phenol

Rideal Walker has limitations like it does not check the amount of organic matter and it only determines the ability of phenol disinfectants.

ii. Chick Martin Test

This test is performed using yeast liquid or a 4% concentration of human feces which have been dried. Thus, the presence of organic matter is determined by this method and it takes around 30 minutes. To determine the disinfectant efficiency, S. aureus and S. typhi bacterial cultures are used.

Phenol Coefficient Limitation

These tests have been customized this way to determine the disinfecting power of disinfectants. However only phenolic type of disinfectants can be checked through this method. Although germicides like iodine, formalin, picric acid, chlorine, hydrogen peroxide differ from phenol but can be compared.

Comparing pure/ diluted form of phenol with hydrophobic compounds results in error, due to the wrong use of phenol coefficient. Phenol coefficient is not used on antiseptics, as they do not kill Bacillus typhosus, which is used in phenol test.

Different bacteria is killed by different antiseptic at varying power. For instance, tincture iodine is 760 times stronger in disinfection than 5% phenol, although tincture iodine is not germicidal 760 times than phenol practically.

Factors Affecting Phenol Coefficient Test

There are to be precise 4 factors, which could result in error in test results. The factors are interfering substances, surface activity, temperature and pH. By increasing disinfectant property elevates. Between pH 6-8, best growth is obtained.

Thus, the ideal pH is 7.5. Substances like salts can be interfering substances which will impact the disinfectant activity. The disinfecting power gets elevated when in small amounts, surface active compounds are present.


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