The Phenol coefficient, is now largely of historical interest, although the principles upon which it is based are still used. It is a measure of the

bactericidal A bactericide or bacteriocide, sometimes abbreviated Bcidal, is a substance which kills bacteria. Bactericides are disinfectants, antiseptics, or antibiotics. However, material surfaces can also have bactericidal properties based solely on their ...

activity of a chemical compound in relation to

phenol Phenol (also called carbolic acid) is an aromatic organic compound with the molecular formula . It is a white crystalline solid that is volatile. The molecule consists of a phenyl group () bonded to a hydroxy group (). Mildly acidic, it req ...

. When listed numerically, the figure expressing the disinfecting power of a substance by relating it to the disinfecting power of phenol may be a function of the

standardized test A standardized test is a test that is administered and scored in a consistent, or "standard", manner. Standardized tests are designed in such a way that the questions and interpretations are consistent and are administered and scored in a predete ...

performed. For example, the Rideal–Walker method, introduced in 1903 gives a

Rideal–Walker coefficient The Rideal–Walker coefficient, now only of historical interest, is a figure expressing the disinfecting power of any disinfectant. It is the ratio of the dilution of the disinfectant that kills a microorganism to the dilution of phenol that kill ...

and the

U.S. Department of Agriculture The United States Department of Agriculture (USDA) is the federal executive department responsible for developing and executing federal laws related to farming, forestry, rural economic development, and food. It aims to meet the needs of com ...

method gives a U.S. Department of Agriculture coefficient. 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 Rideal–Walker test was widely used, but the test conditions chosen were unrealistic, and impossibly high values for the coefficient were claimed by disinfectant manufacturers. Distinguished bacteriologist Sir Ashley Miles, reviewing the subject, described the test as "...at best a grossly over-simplified answer to a difficult problem and, at worst little short of bacteriological prostitution". Modifications were made by Dame

Harriette Chick Dame Harriette Chick DBE (6 January 1875 – 9 July 1977) was a British microbiologist, protein scientist and nutritionist. She is best remembered for demonstrating the roles of sunlight and cod liver oil in preventing rickets. Biography ...

and

Sir Charles James Martin Sir Charles James Martin (9 January 1866 – 15 February 1955) was a British scientist who did seminal work on a very wide range of topics including snake toxins, control of body temperature, plague and the way it was spread, dysentery, typhoid ...

in 1908. They used more realistic conditions, including 3% sterile faeces to mimic the conditions in which many disinfectants were used. The Chick–Martin test was then widely used until replaced by more suitable tests not reliant on phenol and reflecting the conditions in which modern disinfectants are used. Calculations by Harriette Chick showed that the killing of bacteria by disinfectants followed first order kinetics. So, the bactericidal activity of a particular disinfectant at a given concentration can be expressed as a constant (k) calculated 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 greater than 1; those that are less effective have a coefficient less than 1.

References

* {{cite book, title=Dorland's Illustrated Medical Dictionary , year=2004 , url=http://www.mercksource.com/pp/us/cns/cns_hl_dorlands.jspzQzpgzEzzSzppdocszSzuszSzcommonzSzdorlandszSzdorlandzSzdmd_c_44zPzhtm , at="Phenol c." entry , accessdate=2006-11-06 , url-status=dead , archiveurl=https://web.archive.org/web/20061102021742/http://www.mercksource.com/pp/us/cns/cns_hl_dorlands.jspzQzpgzEzzSzppdocszSzuszSzcommonzSzdorlandszSzdorlandzSzdmd_c_44zPzhtm , archivedate=November 2, 2006 Antiseptics