Formaldehyde or Formic Aldehyde (CH₂O) is produced by the slow and incomplete oxidation of methyl alcohol (CH₃OH) under access of air. A saturated solution in water containing 40 per cent. of the formaldehyde gas is known as formalin. The simple evaporation or heating of formalin is liable to produce the polymeric paraform which is solid and inert. To prevent the formation of paraform when formalin is evaporated, Trillat adds to it a solution of calcium chloride (CaCl₂), the mixture being known as formochloral. It is stated that when the air of a room is charged with less than one per cent. of the vapour, rapid and complete disinfection of surfaces occurs, and that it possesses a certain amount of penetrating power into loose fabrics. No damage is done to textile fabrics; and disinfection by this means possesses the advantage over disinfection by sulphurous acid or chlorine that the room can be entered without serious discomfort soon after the disinfection is carried out. In solution formalin is undoubtedly a powerful disinfectant, and in the gaseous condition it is at least equal in value to SO₂, probably better. Formaldehyde is used as a disinfectant.
(a) By evaporating a 60 per cent. solution of CH₂O in methyl alcohol (trade name holzine) over pieces of glowing coke placed under an asbestos plate (Opperman-Rosenberg apparatus). (b) By subliming tabloids of paraform by the heat of a lamp. A methylated spirit lamp is employed, and the moisture from the combustion in this causes the transformation of a considerable proportion of paraform into CH₂O vapour. It is doubtful if the quantity of the latter evolved is sufficient for efficiency. (c) In Trillat’s apparatus formalin (i.e. the 40 per cent. solution in water of CH₂O) with CaCl₂ solution is heated in an autoclave worked at a pressure of 40 lbs., provided with a pressure gauge and thermometer. In all these methods the room must be carefully sealed, as the tendency for the disinfectant to escape is greater than with SO₂ or Cl. (d) The best method is to spray a solution of formalin 4 oz. to one gallon of water on all the surfaces of the room (see page [333]). This is equal to a strength of 1 in 40 of formalin, or 1 in 100 of formic aldehyde.
C. Other Disinfectants.
Tar Acids.—When coal tar is treated by acids and alkalies in succession, it becomes separated into (1) hydrocarbons, (2) phenols or tar acids, carbolic, cresylic, etc., (3) aniline and other basic substances. The hydrocarbons are known in commerce as “neutral tar oils.” They are brown and syrupy, turning milky with water, and feebly disinfectant. The two most important “tar acids” are phenol or carbolic acid (C₆H₅OH) and methyl-phenol, also called cresol or cresylic acid (C₆H₄(CH₃)OH). The higher members of this same group yield milky emulsions with water, and are less poisonous than phenol. Various mixtures of them are used as disinfectants, and sold as creolin, Jeye’s and Lawes’ fluids. Izal belongs to the same series.
Carbolic Acid (phenol) did not kill anthrax spores until a 3 per cent. strength of its solution was used for 7 days (Koch), but sporeless anthrax bacilli were destroyed in a few minutes by a 1 to 2 per cent. solution. The disinfecting power of carbolic acid is greatly increased by adding mineral acids. Carbolic acid and lysol are superior to creolin for disinfecting stools. A 5 per cent. solution of carbolic acid destroys tubercle bacilli in sputum in 24 hours. Carbolic acid powders are in common use. In my opinion quicklime is more valuable.
Cresol is obtained from “crude carbolic acid” by fractional distillation at a temperature between 185° and 205° C. A one-half per cent. solution has equal disinfecting power to a 2 or 3 or sometimes a 5 per cent. of phenol (carbolic acid).
Creolin consists of cresol emulsified in a solution of hard soap. Behring classifies the comparative germicidal power of phenol, cresol, and creolin on bacteria in broth as 1, 4, and 10 respectively. When albumen is present, creolin loses a part of its disinfectant power.
Lysol contains 50 per cent. of cresol, dissolved by means of neutral potash soap. It is completely soluble in water and does not turn milky as creolin does when water is added. It is more effective than creolin, and still more than HgCl₂ in albuminous liquids.
Soap has, owing to its alkalinity, disinfectant as well as cleansing action. A temperature of 55°-75° C. greatly aids its action. Antiseptic soaps possess no special value as germicides, but carbolic soap is a useful insecticide.
Lime in a one-tenth per cent. solution destroys typhoid and cholera microbes.