Element Phosphorus, P, Non Metal

The name phosphorus (light-bearer) was formerly used to designate all substances which possess the property of emitting light without at the same time having a correspondingly high temperature. The name phosphorescence, used in physics for the after luminescence shown by certain substances after a previous exposure to light, is a relic of that usage. At the present day, the name phosphorus is confined to one element, which also exhibits the above property of cold luminescence, although for a different reason.

Phosphorus was discovered about the year 1670 by an alchemist Brandt, who obtained it by the distillation of the residue left on the evaporation of human urine. He kept his method secret, but it was soon found out by Kunkel in Germany, and Boyle in England. Gahn and Scheele also soon found that the bones of the vertebrate animals were a much richer source of phosphorus, and at the present day it is still chiefly prepared from these.

The method of obtaining phosphorus depends on the fact that the oxygen compound of phosphorus, phosphoric acid, which is contained in the bones, is reduced by charcoal. The charcoal combines with the oxygen, and the phosphorus is set free and distils over.

In nature, phosphorus occurs only in the form of salts of the just- mentioned phosphoric acid. These compounds are very wide-spread, although they do not occur anywhere in large quantities. They are of great importance for organic life, since the " protoplasm " of the cells, the substance to which the actual vital activity is attached, always contains small amounts of phosphorus compounds. The nerve and brain substances, more especially, are comparatively rich in phosphorus, which is there present in the form of phosphoric acid derivatives.

Salts of phosphoric acid or the phosphates are also indispensable for the growth of plants. As the soil does not usually contain much of it, this substance is, for the purpose of high cultivation, added to the soil.

The artificial manures containing phosphoric acid play a very important role in agriculture.

Phosphorus does not appear to occur in the free state in nature. If it were produced by the reduction of phosphatic minerals at high temperatures it would be liberated as vapour, which would condense to liquid or solid white phosphorus, and would again be oxidised rapidly under most conditions. Sulphur, which resembles phosphorus in so many respects, does, however, occur as element in considerable quantities, because it is produced in many reactions which take place at comparatively low temperatures (such as that between H₂S and SO₂), and, when once produced, is far less easily oxidised than phosphorus. Phosphorus (0.142) is actually more abundant than sulphur (0.093), the numbers in brackets referring to the percentage of the earth's 10-mile crust, with atmosphere and hydrosphere, constituted by the element. Phosphorus is thus twelfth in the order of abundance, coming below chlorine but above carbon. In the 10-mile crust composed of the lithosphere only, phosphorus is present to the extent of 0.157 per cent., being now in the tenth place, since hydrogen and chlorine in the rocks occupy a position below phosphorus, although they are still above carbon. The average proportion in igneous rocks is 0.13 per cent., whilst the element is also common in sedimentary rocks, chiefly as phosphate of lime, and to a less extent as phosphates of iron and alumina. These secondary deposits are by far the most important sources of phosphates. They are derived probably from the widely but sparsely diffused ingredients of igneous rocks and from the well-crystallised but more stable minerals such as apatite (calcium fluo- or chloro-phosphate), which gradually disintegrate, pass into the soil, and are concentrated in plant tissues and in the bones of animals, which eventually yield deposits of phosphate of lime.

Phosphorus is present to the extent of about 0.1 per cent, in ordinary soils. From this source plants draw the supplies which are essential to their growth—the phosphorus is found mainly in the seeds, and is more concentrated in the germ of these. It is estimated that 100 lbs. of corn contain nearly ½ lb. of phosphorus. Phosphorus is also present in the bodies of animals, and although a minor constituent of the soft parts, is absolutely necessary to life. It is present as soluble phosphate in the blood, milk and other fluids, as a constituent of organic compounds in the brain, spinal cord, and other parts, and as calcium phosphate in the bones. The proportions vary considerably in different parts of the body. Ox brain has been found to contain nearly 2 per cent., human liver 1 per cent. In animal fats phosphorus occurs combined in the "lipoid" form—for example, as lecithin. Bones contain over 40 per cent, of calcium and magnesium phosphates. The human skeleton, weighing about 14 lbs., contains about 5 lbs. of calcium phosphate, or 1 lb. of phosphorus. Since the bones form about 10 per cent, of the total weight of the body, and the rest of the body, exclusive of the bones, contains of the order of 0.1 per cent, of phosphorus, it will be seen that there is slightly over 1 lb., or about 500 grams, of this element in the body. Analysis of certain parts of the body shows that about 90 per cent, of the phosphorus is present in the bones, 8 per cent, in the muscles, and a total of 2 per cent, in the brain, liver, lungs, and blood.

While plants use phosphorus very sparingly, animals require a greater proportion for their metabolism.

The continual secretion of phosphorus necessitates a constant supply. About 1 gram a day for the adult is required to maintain the body equilibrium. An adequate supply is also one of the factors which control growth. The milk of the cow contains about 0.2 per cent, of P₂O₅, that of the rabbit nearly 1 per cent. In other cases also there is a relation between the percentage of phosphorus in the milk and the reciprocal of the number of days required to double the weight at birth.

As just explained, phosphorus is concentrated in plant and animal products. These are not only more widely distributed than the rich phosphatic minerals, but they also form a more interesting and attractive object of experiment, and, what is perhaps more important, they contain their own reducing agents. It is not surprising, therefore, that the element was first discovered in its organic sources by the doctors and pharmacists who formed the majority of the experimentalists in the iatro-chemical and succeeding periods of chemical history. By distillation of the residue from the evaporation of urine (which contains phosphates and organic matter) the alchemist Brand in 1669 obtained a substance which glowed without any noticeable evolution of heat. Brand's process was described by Kirchmaier in 1676 and discovered independently by Kunkel in 1678, who designated the product " noctiluca " and "phosphorus mirabilis" respectively, by which names it was distinguished from Bolognese phosphorus ("lapis bononiensis ") and Baldwin's phosphorus ("phosphorus hermeticus "), which are phosphorescent sulphides of the alkaline earths.

Brand's secret was bought by Krafft, who exhibited " das kalte Feuer " at various Courts, and in 1677 at that of King Charles II., where it was seen by Boyle, who, being informed that it was prepared from an animal source, worked out the method of preparation and described this in a sealed paper which was deposited with the Royal Society in 1680 and published in 1693. The urine was evaporated down to a syrup, which was mixed with three times its weight of clean sand and distilled at the highest available temperature from a retort, the neck of which almost touched the surface of some water. White fumes given off at first were followed by a vapour which condensed and fell to the bottom of the water. At the beginning of the eighteenth century the " noctiluca" was made and sold by Boyle's assistant Hanckewitz, and consequently it became known as " Boyle's phosphorus or English phosphorus."

Phosphorus was detected also in mustard seed by Albinus in 1688. The existence of large supplies in minerals had been established by Gahn towards the end of the eighteenth century, who also recognised it in bone-ash (1769) and in the mineral pyromorphite (1779).

Phosphorus was prepared in various European countries during the eighteenth century, but it remained an expensive chemical curiosity until Gahn, about 1770, discovered in bone-ash a more suitable source of phosphorus, and Scheele developed a method of preparing the element which was described in several communications. According to this method the acid liberated from bone-ash by means of nitric acid and freed from lime by precipitation with sulphuric acid, was distilled with charcoal, phosphorus being collected under water. This method has the advantage of yielding the phosphorus at a comparatively low temperature, but the metaphosphoric acid liberated by the nitric acid is partly volatilised and so partly escapes reduction. The present method of preparation proceeds by the following stages:—

Ca₃(PO₄)₂ + 3H₂SO₄ = 3CaSO₄ + 2H₃PO₄ (1)

The calcium sulphate is filtered off and the phosphoric acid concentrated to a syrup and strongly heated in order to convert it into metaphosphoric acid—

H₃PO₄ = HPO₃ + H₂O (2)

This acid is mixed into a paste with charcoal or ground coke and distilled, finishing at a yellow heat—

4HPO₃ + 12C = P₄ + 2H₂ + 12CO (3)