Often referred to in literature under the proprietary name, Formalin.

Properties : Solution of formaldehyd is an aqueous solution containing not less than 37 per cent, of formaldehyd, CH20. It is a clear, colorless liquid, having a pungent odor and caustic taste. Solution of formaldehyd is miscible in all proportions with water and alcohol. On standing it sometimes loses its transparency, its cloudiness being due to the separation of paraformal-dehyd, a polymerization product of formaldehyd. Paraformaldehyd is also frequently formed on evaporation of the solution. Paraformaldehyd is a solid which is largely changed again into formaldehyd on heating.

Incompatibilities: Solution of formaldehyd is incompatible with oxidizing agents and with alkalies. With ammonia it forms hexamethylenamin.

Action and Uses : Formaldehyd is a powerful germicide, especially valuable in the form of gas because of its penetrating power, but it is active only in the presence of an abundance of moisture. The solution is germicidal in the strength of from 1 to 2 per cent, (percentages refer to amounts of absolute formaldehyd, HCOH), but it may require from twenty to thirty minutes for it to act. In a strength of 1 : 5,000 it restrains the growth of many organisms, and in many cases a strength of 1 : 20,000 or 1 : 30,000 is sufficient to prevent the multiplication of bacteria. It is useful as a preservative of urine, although its reducing properties interfere with copper tests for sugar, and it is likely after the urine has stood some time to cause a precipitation of albumin, if present.

It hardens tissues and is used in histology for this purpose. It has a similar hardening effect on the living skin; it is very irritating and if repeatedly or continuously applied produces reddening, inflammation and necrosis. It is applied to the skin to restrain unilateral and excessive sweating. From 1 to 10 per cent, solutions in alcohol are appropriate for this purpose. It is sometimes used for the disinfection of the hands, in connection with a solution of soap.

The use of formaldehyd for the preservation of food has been quite commonly condemned on account of the disturbance of digestion which often follows its ingestion.

The principal application of formaldehyd is in room disinfection. For this purpose the vapor must be generated in a tightly closed room, containing plenty of moisture. Several methods have been described for generating the vapor. the most convenient being by the use of potassium permanganate which, when added to the solution, by decomposing a part of the formaldehyd, generates sufficient heat to vapor ize the remainder. For an ordinary-sized room 2 pounds of potassium permanganate are placed in a vessel of at least 25 quarts' capacity and a mixture of 1 quart of formaldehyd solution and 1 quart of water poured on it. Intense heat is generated by the reaction of the two chemicals, and by this heat the formaldehyd is vaporized. The heat is so great as sometimes to cause fire, against which due precautions should be taken. When the mixture has been made the operator should leave the room instantly. After the disinfection is complete the irritating fumes can be neutralized by ammonia.