Phenol Coefficient  is a measure of the bactericidal activity of a chemical compound in co relation to phenol. To calculate phenol coefficient, the concentration of phenol at which the compound kills the test organism in 10 minutes, but not in 5 minutes, is divided by the concentration of the test compound that kills the organism under the same conditions (or, probably more common, dividing the dilution factor at which the tested substance shows activity by the dilution factor at which phenol shows comparable activity). The phenol coefficient may be determined in the presence of a standard amount of added organic matter or in its absence.

A related way to express the bactericidal activity of an agent (at a given concentration) is by employing the formula k = N/C·T where N is the number of surviving cells, C is the concentration of agent applied and T is the time for which the agent is applied, so k is inversely proportional to dose (C·T is collectively called dose).

One way to compare disinfectants is to compare how well they do against a known disinfectant and rate them accordingly using the Phenol coefficient. The disinfectant to be tested is compared with phenol on a standard microbe (usually Salmonella typhi or Staphylococcus aureus). Disinfectants that are more effective than phenol have a coefficient > 1. Those that are less effective have a coefficient < 1.

Note: The phenol coefficient is defined as the killing power of a disinfectant against a test organism,as compared to that of phenol under identical conditions. The conditions must be specified as;

  1. Type of microorganism
  2. Age of culture
  3. Temperature of environment 
  4. Time of action of disinfectant 
  5. Presence of organic matter
  6. pH of the medium 
  7. Amount of organic matter
  8. Incubation Time
  9. Availability of Nutrient materials
  10. Proportion of disinfectant to the culture. 


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