JOURNAL O F CHEMICAL EDUCATION
SPONTANEOUS COMBUSTION WILLIAM F. EHRET New York University, New York
Tam REACTION between white phosphorus and the oxygen of the air lends itself readily to a demonstration of spontaneous ignition. The subsequent combustion of the phosphorus is rapid and highly exothermal. P, 50n -t P401af 740,OM) calories
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In order to simulate as nearly as possible the spontaneous combustions that plague us ordinarily the phosphorus is given a large specific surface by spreading it very thimly over the strands of steel wool (No. 0), glass wool (medium), or cotton in little balls of these materials. When the slow initial reaction has raised the temperature to the kindling point, and the phosphorus on the hall inflames, it may easily he extinguished by grasping the ball with a pair of tongs and dropping it into water. The balls are about 1 in. in diameter and are made by rolling the original fibers between the palms of the hands. Phosphorus is applied to the balls by dipping them, with the aid of tongs, into a solution of the element in carbon disulfide. The concentration of the solution is not critical; we have found that a 1-in. in. diameter) when length of a stick of phosphorus s%'( dissolved in 100 ml. of carbon disulfide gives a suitable coating. The ball is squeezed tightly by the tongs while it is in the solution and while draining just above
it. This prevents excessive "drag-out" of solution. If solution of the same concentration is spread on three different substrates, cotton, glass wool, and steel wool, the time required for ignition is influenced in part. by the physical properties of these substances, viz., their heat capacity, thermal conductivity, and ability to radiate heat. In general, the order of ignition, from first to last is cotton, glass, iron. The actual time seldom exceeds 30 minutes. Many factors other than those just mentioned have an influence upon the time required for the phosphorus to reach its kindling temperature (ca. 45'). Some of these are: 1. Concentration of phosphorus in the carbon disulfide solution. 2. Tightness and size of the ball. This determines the amount of solution trapped in the ball. Almost all of the carbon disulfide must evaporate before the phosphorus can come into effective contact with the air. 3. Size of strands (in the ball) and the way they pack. The influence is like that in (2). 4. Temperature of the surroundings. This governs the rate of evaporation of the carbon disulfide and the rate of oxidation of the phosphorus.
MAY. 1948 5. Drafts. These influence the rates mentioned in (4). 6. Humidity. While water may he a catalyst for the reaction between phosphorus and oxygen, high humidity has a retarding effect. The phosphorus pentoxide which slowly forms on the surface of the hall is extremely hygroscopic and on moist days will attract enough water 'to form a blanket of phosphoric acid which hinders the influx of air. 7. The age of the solution. Phosphorus reacts slowly with oxygen and moisture in the container. A yellow substance is slowly precipitated. 8. Unknown catalysts upon the iron, glass, or cotton.
In view of the many factors involved, and the difficulty of their control, it is perhaps not surprising to find that the ignition times are not reproducible, nor is the order of inflammation always that given ahove. In this respect the experiment simulates
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closely the more usual type of spontaneous combustion wherein kindling conditions develop at most unexpected times. In performing this experiment caution should he exercised along these general lines: 1. White phosphorus must he kept and cut under water, and exposed to the air only momentarily during transfer from one vessel to another. Immediate immersion in water is called for shonld any phosphorus catch fire accidentally and become attached to the skin or clothes. 2. Once a ball has been immersed in the phosphoms solution and then exposed to air it is a fire hazard. It will eventuallv inflame! If the ex~erimenterd m not wish to wait for it to ignite spontaneously, a match or other flame will of course set it off. Blowing nu the ball also causes it to inflame immediately. Another way to temporarily inactivate the ball is to immerse it in water. If the hall later becomes dry its incendiary properties reappear. 3. The experiment should he performed on a fireproof surface.
