Chloride of Lime.—Chloride of lime, bleaching powder, which consists of calcium hypochlorite, the active agent, and chloride and some unchanged quicklime is one of the most useful disinfectants. It is employed to sterilize water for drinking purposes on a large scale and to disinfect sewage plant effluents. A 5 per cent. solution is the proper strength for ordinary disinfection. Only a supply which is fresh or has been kept in air-tight containers should be used, as it rapidly loses strength on exposure to the air. The active agent is nascent oxygen liberated from the decomposition of the hypochlorite.
Sodium Hypochlorite.—Sodium hypochlorite prepared by the electrolysis of common salt has been used to some extent.
Bichloride of Mercury.—Bichloride of mercury, mercuric chloride, corrosive sublimate (HgCl2), is the strongest of all disinfectants under proper conditions. It is also extremely poisonous to men and animals and great care is necessary in its use. It is precipitated by albuminous substances and attacks metallic objects, hence should not be used in the presence of these classes of substances.
It is used in a strength of one part HgCl2 to 1000 of water for general disinfection. Ammonium chloride or sodium chloride, common salt, in quantities equal to the bichloride, or citric acid in one-half of the amount should be added in making large quantities of solution or for use with albuminous fluids to prevent precipitation of the mercury ([Fig. 109]).
None of the other metallic salts are of value as practical disinfectants aside from their use in surgical practice. In this latter class come boric acid, silver nitrate, potassium permanganate. The strong mineral acids and alkalies are, of course, destructive to bacteria, but their corrosive effect excludes them from practical use, except that “lye washes” are of value in cleaning floors and rough wood-work, but even here better disinfection can be done more easily and safely.
Fig. 109.—Tanks for bichloride of mercury, government quarantine disinfecting plant.
ORGANIC COMPOUNDS.
Carbolic Acid or Phenol.—Carbolic acid or phenol (C6H5 OH) is one of the commonest agents in this class. It is used mostly in 5 per cent. solution as a disinfectant and in 0.5 per cent. solution as an antiseptic. For use in large quantities the crude is much cheaper and, according to some experimenters, even more active than the pure acid, owing to the cresols it contains. The crude acid is commonly mixed with an equal volume of commercial sulphuric acid and the mixture is added to enough water to make a 5 per cent. dilution, which is stronger than either of the ingredients alone in 5 per cent. solution.
Cresols.—The cresols (C6H4CH3OH, ortho, meta and para), coal-tar derivatives, as phenol, are apparently more powerful disinfectants. A great number of preparations containing them have been put on the market. Creolin is one which is very much used in veterinary practice and forms a milky fluid with water, while lysol forms a clear frothy liquid owing to the presence of soap. Both of these appear to be more active than carbolic acid and are less poisonous and more agreeable to use. They are used in 2 to 5 per cent. solution.