A disinfectant is a chemical liquid that destroys micro-organisms. Disinfectants are used to destroy hhful micro-organisms or to inhibit their activity. Disinfectants are either complete or incomplete. Complete disinfectants destroy spores as well as vegetative forms of organisms. Incomplete disinfectants destroy vegetative forms of organisms but do not injure spores. Some disinfectants are: (a) mercury compounds (mercuric chloride, phenylmercuric borate), (b) halogen and halogen compounds (chlorine, iodine, fluorine, bromine and calcium and sodium hypochlorites), (c) phenols, including cresol from coal tar and orthophenylphenol, (d) synthetic detergents (anionic, such as sodium alkyl benzene sulphonates and cationic, such as quaternary ammonium compounds), (e) alcohols with low molecular weights, except methanol, (f) natural products (pine oil), and (g) gases (sulphur dioxide, formaldehyde, ethylene oxide, etc.). Heat and electromagnetic waves are also used as disinfectants. A number of compounds like mercurous and mercuric chlorides, copper sulphate or carbonate and a mixture of zinc oxide and zinc hydroxide are employed as seed disinfectants. The effectiveness of disinfectants is rated by their phenol coefficient. Phenol is the standard and the corresponding disinfectant measuring system is called phenol coefficient. The disinfectant to be tested is compared with phenol on a standard microbe (usually Salmonella typhi or Staphilococcus). The disinfectants that are more effective than phenol have a coefficient more than 1. Those that are less effective have a coefficient less than 1.
