CHEMICAL DISINFECTANTS.
A chemical disinfectant should fulfil the following conditions: 1. It must be an efficient germicide. 2. Its germicidal power should not be destroyed by the fæcal or other polluting matter, with which the bacteria of infection are associated. 3. For many purposes, it must not be destructive to or liable to stain the skin, or fabrics, or other articles to which it is applied. 4. It should preferably not be a virulent poison; and should be moderately cheap. The search for a completely non-poisonous disinfectant is a chimera.
There are three great classes of chemical disinfectants.
1. Oxidising agents, as the halogens (chlorine, etc.) and permanganates.
2. Deoxidising agents, as sulphurous acid (SO₂) and formic aldehyde (CH₂O).
3. Other disinfectants, which act by coagulating protoplasm or otherwise, as carbolic acid, corrosive sublimate.
The number of disinfectants is legion. Only the chief ones can be mentioned and their chief properties described. It is a good rule to eschew the use of all disinfectants of which the exact composition is not given; and all disinfectants which are described by “fancy” names, which are not descriptive of their composition.
A. Oxidising Agents.
Chlorine has been most commonly used as chloride of lime (CaCl₂, Ca(ClO)₂). This is somewhat unstable in composition. A solution of sodium hypochlorite containing 10 per cent. of available chloride is preferable. Chloride of lime for sprinkling on decomposing matter should contain at least 10 to 15 per cent. of available chlorine. Sulphuretted hydrogen and other offensive gases are decomposed by it. Thus
SH₂ + Cl₂ = S + 2HCl. Its chief action is as an oxidising agent.
Thus H₂O + Cl₂ = 2HCl + O.
A large excess must be used in disinfecting, otherwise the chlorine may simply oxidise fœcal or other organic matter, and not effectually destroy contagia.
Methods of Use—(a) As a gas, by the action of hydrochloric acid on strong chloride of lime. The molecular density of chlorine is 35·5, of formic aldehyde 15, of sulphurous acid 32; and the rate of diffusion of gases being inversely to the square-root of their densities, clearly chlorine does not compare favourably as a gas with formic aldehyde. The bleaching effect of chlorine on coloured articles of apparel is a disadvantage.
(b) As a liquid: used thus chlorine is very efficient, applied either as a spray or brushed on walls and other surfaces. Delépine found that a solution of one part of chloride of lime in 100 parts of water applied to wall paper impregnated with tubercular matter, disinfected it in a few hours, or in a few minutes if the layer of infected matter was not thick.
Bromine, Iodine, and Euchlorine (a mixture of chlorine and Cl₂O₄) are efficient disinfectants.
Iodine Trichloride (ICl₃) was found by Behring to share with corrosive sublimate (HgCl₂) carbolic acid and cresol mixed with acids (see page [327]), the halogens (Cl, Br, and I), and chloride of lime a superiority over other disinfectants in their power of killing anthrax spores in a short time.
Permanganates have been largely used as disinfectants, but their value is small. Impure sodium manganate (Na₂MnO₄) with much common salt (NaCl) containing some permanganate is known as “Condy’s Green Fluid.” “Condy’s Red Fluid” consists of permanganate and sulphate of soda. To be of any use it must be employed in 5 per cent. solution. It stains fabrics brown, and it exhausts its feeble disinfecting power in first oxidising decomposing organic matter.
B. Deoxidising Agents.
Sulphurous Acid (SO₂) acts chiefly as a reducing agent on organic matter. It is used chiefly as a gaseous disinfectant, and for this purpose is generated (a) by burning 1 lb. of sulphur for every 1,000 cubic feet of space in the room (which will equal 1·12 per cent. of SO₂). The windows and chimney of the room are first closed; the sulphur is placed in a saucepan supported over a bucket of water, and its ignition is aided by a small quantity of methylated spirit. The door of the room is then sealed, and the room left until the next morning. (b) Carbon disulphide may be burned in a benzoline lamp. (c) SO₂ liquefied under pressure is supplied in cylinders available for convenient use. The experimental results of the action of SO₂ on various bacteria are somewhat discrepant. It probably is fairly efficient for some diseases, but not in tuberculosis.
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.
Mercuric Chloride (HgCl₂, corrosive sublimate) was found by Koch to destroy anthrax bacilli in a dilution of 1 in 20,000. Others have obtained less favourable results, but it is certainly a powerful germicide. The germicidal effect is greatly diminished by contact with organic matter, an insoluble albuminate of mercury being produced. For this reason HgCl₂ is not the best disinfectant for fæces unless mixed with acid, as in the following solution: HgCl₂ ½ oz., HCl 1 oz., aniline blue 5 grains to three gallons of water. This gives a solution of 1 in 960. The colouring is added to avoid accidental poisoning. HgCl₂ is not a good disinfectant for linen. Stains are apt to be fixed by it, and if linen soaked in it is subsequently washed with soap, without first carefully washing out the HgCl₂, it is darkened in colour. It attacks metals, and must not therefore be placed in metal receptacles.
Chloride of Zinc in a solution containing 25 grains to the fluid drachm is known as “Sir William Burnett’s solution.” It is a good deodorant, but an inefficient disinfectant.
Chinosol (C₉H₆NKSO₄) belongs to the quinoline group. It is an almost inodorous powder, very soluble in water, noncorrosive, and does not stain. A solution of 1 in 1200 forms an efficient germicide.