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CHAPTER XIX

PHIOSPHORUS AND THE OTIIER ELEMENTS OF THE FIFTH GROUP

NITROGEN is the lightest and most widely distributed representative of the elements of the fifth group, which form a higher saline oxide of the form R,0;, and a hydrogen compound of the form RH3. Phosphorus, arsenic, bismuth, and antimony belong to the uneven series of this group. Phosphorus is the most widely distributed of these elements. There is hardly any mineral substance composing the mass of the earth's crust which does not contain some—it may be a small-amount of phosphorus compounds in the form of the salts of phosphoric acid. The soil and earthy substances in general usually contain from one to ten parts of phosphoric acid in 10,000 parts. This amount, which appears so small, has, however, a very important significance in nature. No plant can attain its natural growth if it be planted in an artificial soil completely free from phosphoric acid. Plants equally require the presence of potash, magnesia, lime, and ferric oxide, among basic, and of carbonic, sulphuric, nitric, and phosphoric anhydrides, among acid oxides. In order to increase the fertility of a more or less poor soil, the above-named nutritive elements are introduced into it by means of fertilisers. Direct experiment has proved that these substances are undoubtedly necessary to plants, but that they must be all present simultaneously and in small quantities, and that an excess, like an insufficiency, of one of these elements is necessarily followed by a bad harvest, or an imperfect growth, even if all the other conditions (light, heat, water, air) are normal. The phosphoric compounds of the soil accumulated by plants pass into the organism of animals, in which these substances are assimilated in many instances in large quantities. Thus the chief component part of bones is calcium phosphate, CazP,0g, and it is on this that their hardness depends.

1 Dry bones contain about one-third of gelatinous matter and about two-thirds of ash, chiefly calcium phosphate. The salts of phosphoric acid are also found in the mass of the earth as separate minerals; for example, the apatites contain this salt in a crystalline form, combined with calcium chloride or fluoride, CaR2,3Cag(PO4)2, where

Phosphorus was first extracted by Brand in 1669, by the ignition of evaporated urine. After the lapse of a century Scheele, who knew of the existence of a more abundant source of phosphorus in bones, pointed out the method which is now employed for the extraction of this element. Calcium phosphate in bones permeates a nitrogenous organic substance, which is called ossein, and forms a gelatin. When bones are treated exclusively for the extraction of phosphorus, neglecting the gelatin, they are burnt, in which case all the ossein is burnt away. When, however, it is desired to preserve the gelatin, the bones are immersed in cold dilute hydrochloric acid, which dissolves the calcium phosphate and leaves the gelatin untouched ; calcium chloride and acid calcium phosphate, CaH,(PO,)2, are then obtained in the solution. When the bones are directly burnt in an open fire their mineral components only are left as an ash, containing about 90 per cent. of calcium phosphate, Ca3(PO4)2, mixed with a small amount of calcium carbonate and other salts. This mass is treated with sulphuric acid, and then the same substance is obtained in the solution as was obtained from the unburnt bones immersed in hydrochloric acid-i.e. the acid calcium phosphate soluble in water, in which reaction naturally the chief part of the sulphuric acid is converted into calcium sulphate:

Caz(PO,)2 + 2H,S04 = 2Caso,+CaH,(PO)..

Caz(PO3)2 + 4HC1 = 2CaCl, + CaH,(PO,)». On evaporating the solution, crystallisable acid calcium phosphate is obtained. The extraction of the phosphorus from this salt consists in heating it with charcoal to a white heat. When heated, the acid phosphate, CaH,(PO,)2, first parts with water, and forms the metaphosphate, Ca(PO3)2, which for the sake of simplicity may be regarded, like the acid salt, as composed of pyrophosphate and phosphoric anhydride, 2Ca(PO3)2 = Ca.,P,0, +P,03. The latter, with charcoal, gives phosphorus and carbonic oxide, P,05 + 5C = P. + 5CO.

R--F or Cl, sometimes in a state of isomorphous mixture. This mineral often crystallises in fine hexagonal prisms; sp. gr. 3:17 to 3:22. Vivianite is a hydrated ferrous phosphate, Fes(PO)2,8H20. Phosphates of copper are frequently found in copper mines; for example, tagilite, Cu3(PO4),,Cu(OH),.,2H, O. Lead and aluminium form similar salts. They are nearly all insoluble in water. The turquoise, for instance, is hydrated phosphate of aluminn, (A1,03),,P2035H,O, coloured with a salt of copper. Sea and other waters always contain a small amount of phosphates. The ash of sea-plants, as well as of land-plants, always contains phosphates. Deposits of calcium phosphate are often met with ; they are termed phosphorites and osteolites, and are composed of the fossil remains of the bones of animals; they are used for manure. Of the same nature are the so-called guano deposits from Baker's Island, and entire strata in Spain, France, and in the Governments of Orloff and Kursk in Russia. It is evident that if a soil destined for cultivation contain very little phosphoric acid, the fertilisation by means of these minerals will be beneficial, but, naturally, only if the other elements necessary to plants be present in the soil.

So that in reality a somewhat complicated process takes place here, yielding ultimately products according to the following equation :

2CaH,(PO4)2 + 5C = 44,0 + Ca,P,0, +P,+ 5CO. After the steam has come over, phosphorus and carbonic oxide distil over from the retort and calcium pyrophosphate remains behind, 1 bis

As phosphorus melts at about 40°, it condenses at the bottom of the receiver in a molten liquid mass, which is cast under water in tubes, and is sold in the form of sticks. This is common or yellow phosphorus. It is a transparent, yellowish, waxy substance, which is not brittle, almost insoluble in water, and easily undergoes change in its external appearance and properties under the action of light, beat, and of various substances. It crystallises (by sublimation or from its solution in carbon bisulphide) in the regular system, and ? (in contradistinction to the other varieties) is easily soluble in carbon bisulphide, and also partially in other oily liquids. In this it recalls common

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1 bis By subjecting the pyrophosphate to the action of sulphuric or hydrochloric acid it is possible to obtain a fresh quantity of the acid salt from the residue, and in this inanner to extract all the phosphorus. It is usual to take burnt bones, but mineral phosphorites, osteolites, and apatites may also be employed as materials for the extraction of phosphorus. Its extraction for the manufacture of matches is everywhere extending, and in Russia, in the Urals, in the Government of Perm, it has attained such proportions that the district is able to supply other countries with phosphorus. A great many methods have been proposed for facilitating the extraction of phosphorus, but none of them differ essentially from the usual one, because the problem is dependent on the liberation of phosphoric acid by the action of acids, and on its ultimate reduction by charcoal. Thus the calcium phosphate may be mixed directly with charcoal and sand, and phosphorus will be liberated on heating the mixture, because the silica displaces the phos

Pig. 83. -Preparation of phosphorus, phoric anhydride, which gives carbonic oxide and The mixture is calcinell in the retort phosphorus with the charcoal. It has also been c. The vapours of phosphorus pass

through a into water without coming proposed to pass hydrochloric acid over an incan

into contact with air. The phosdescent mixture of calcium phosphate and charcoal ; phorus condenses in the water, and

the gases accompanying it escape the acid then acts just as the silica does, liberating

through i. phosphoric anhydride, which is reduced by the charcoal. It is necessary to prevent the access of air in the condensation of the vapours of phosphorus, because they take fire very easily; hence they are condensed under water by causing the gaseous products to pass through a vessel full of water. For this purpose the condenser shown in fig. 83 is usually employed.

? Vernon (1891) observed that ordinary (yellow) phosphorus is dimorphous. If it be melted and by careful cooling be brought in a liquid form to as low a temperature as possible, it gives a variety which melts at 45%-3 (the ordinary variety fuses at 44°3), sp. gr. 1.827 (that of the ordinary variety is 1'818) at 13°, crystallises in rhombic prisms (instead of in forms belonging to the cubical system). This is similar to the relation between octahedral and prismatic sulphur (Chapter XX.).

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sulphur. Its specitic gravity is 1:84. It fuses at 44°, and passes into vapour at 290°; it is easily inflammable, and must therefore be handled with great caution ; careless rubbing is enough to cause phosphorus to ignite. Its application in the manufacture of matches is based on this.2 bis It emits light in the air owing to its slow 3 oxidation, and is therefore kept under water (such water is phosphorescent in the dark, like phosphorus itself). It is also very easily oxidised by various oxidising agents and takes up the oxygen from many substances.3 bis Phosphorus enters into direct coinbination with many metals and with sulphur, chlorine, &c., with development of a considerable amount of heat. It is very poisonous although not soluble in water.

Besides this, there is a red variety of phosphorus, which differs considerably from the above. Red phosphorus (sometimes wrongly called amorphous phosphorus) is partially formed when ordinary phosphorus

2 bis According to Herr Irinyi (an Hungarian student), the first phosphorus matches were made in Austria at Roemer's works in 1835.

3 The absorption of the oxygen of the atmosphere at a constant ordinary temperature by a large surface of phosphorus proceeds so uniformly, regularly, and rapidly, that it may serve, as Ikeda (Tokio, 1893) has shown, for demonstrating the law of the velocity (rate) of reaction, which is considered in theoretical chemistry, and shows that the rate of reaction is proportional to the active mass of a substance-.e. dr dt =k(A-2) where t is the time, A the initial mass of the reacting substance-in this case oxygen-> the amount of it which has entered into reaction, and k the coefficient of proportionality. Ikeda took a test-tube (diameter about 10 mm.), and covered its outer surface with a coating of phosphorus (by melting it in a test-tube of large diameter, inserting the smaller test-tube, and, when the phosphorus had solidified, breaking away the outer testtube), and introduced it into a definite volume of air, contained in a Woulfe's bottle (immersed in a water bath to maintain a constant temperature), one of whose orifices was connected with a mercury manometer showing the fall of pressure, x. Knowing that the initial pressure of the oxygen (in air nearly 750 x '0209) was about 155 mm.- A, the coefficient of the rate of reaction k is given, by the law of the variation of the rate of reaction with the mass of the reacting substance, by the equation : k = log A , where

8A-x'" t is the time, counting from the commencement, of the experiment in minutes. When the surface of the phosphorus was about 11 sq. cm., the following results were actually obtained. t=10

30

50

60 minutes
r=105 215

31:1
10:7
19:1

57.3 mm
10,000 k= 32

32
82
33

33

20

10

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The constancy of k is well shown in this case. The determination takes a comparatively short time, so that it may serve as a lecture experiment, and demonstrates one of the most important laws of chemical mechanics.

3 bin Not only do oxidising agents like nitric, chromic, and similar acids act upon phosphorus, but even the alkalis are attacked-that is, phosphorus acts as a reducing agent. In fact it reduces many substances, for instance, copper from its salts. When phosphorus is heated with sodium carbonate, the latter is partially reduced to carbon. If phosphorus be placed under water slightly warmed, and a stream of oxygen be passed over it, it will burn under the water.

remains exposed to the action of light for a long time. It is also formed in many reactions ; for example, when ordinary phosphorus combines with chlorine, bromine, iodine, or oxygen, a portion of it is converted into red phosphorus. Schrötter, in Vienna, investigated this variety of phosphorus, and pointed out by what methods it may be produced in considerable quantities. Red phosphorus is a powdery redbrown opaque substance of specific gravity 2:14. It does not combine so energetically with oxygen and other substances as yellow phosphorus, and evolves less heat in combining with then). Common phosphorus easily oxidises in the air ; red phosphorus does not oxidise at all at the ordinary temperature; hence it does not phosphoresce in the air, and may be very conveniently kept in the form of powder. It does not, like yellow phosphorus, fuse at 44°. After being converted into vapour at

+ The thermochemical determinations for phosphorus and its compounds date from the last century, when Lavoisier and Laplace burnt phosphorus in oxygen in an ice calorimeter. Andrews, Despretz, Favre, and others have studied the same subject. The most accurate and complete data are due to Thomsen. To determine the heat of combustion of yellow phosphorus, Thomsen oxidised it in a calorimeter with iodic acid in the presence of water, and a mixture of phosphorous and phosphoric acids was thus formed (was not any hypophosphoric acid forined ?-Salzer), and the iodic acid converted into hydriodic acid. It was first necessary to introduce two corrections into the caloriinetric result obtained, one for the oxidation of the phosphorous into phosphoric acid, knowing their relative amounts by analysis, and the other for the deoxidation of the jodic acid. The result then obtained expresses the conversion of phosphorous into hydrated phosphoric acid. This must be corrected for the heat of solution of the hydrate in water, and for the heat of combination of the anhydride with water, before we can obtain the heat evolved in the reaction of P, with Og in the proportion for the formation of P.O. It is natural that with so complex a method there is a possibility of many small errors, and the resultant figures will only present a certain degree of accuracy after repeated corrections by various methods. Of such a kind are the following figures determined by Thomsen, which we express in thousands of calories:-P,+05 - 370; P2+O3 + 3H.,O= 400; P2 + 0g + a mass of water = 405. Hence we see that P,03 + 3H2O = 30; 2PH30, + an excess of water - 5. Experiment further showed that crystallised PH:04, in dissolving in water, evolves 2:7 thousand calories, and that fused (399) PH:04 evolves 5-2 thousand calories, whence the heat of fusion of H3PO4--2-5 thousand calories. For phosphorous acid, H3PO3, Thomsen obtained P., + 0; + 3H2O= 250, and the solution of crystallised H;P03 in water = -0:13, and of fused H3PO3 = + 2:9. For hypophosphorous acid, H3P02, the heats of solution are nearly the same ( - 0:17 and + 2:1), and the heat of formation P2+0+ 3H.,O= 75; hence its conversion into 2H3PO3 evolves 175 thousand calories, and the conversion of 2H;P03 into 2H3PO4= 150 thousand calories. For the sake of comparison we will take the combination of chlorine with phosphorus, also according to Thomsen, per 2 atoms of phosphorus, P2+3C1,= 151, P2 + 5Cl2 = 210 thousand calories. In their reaction on an excess of water (with the formation of a solution), 2PC);= 130, 2PC), = 247, and 2POClz=142 thousand calories.

Besides which we will cite the following data given by various observers : heat of fusion for P (that is, for 31 parts of phosphorus by weight) -0:15 thousand calories; the conversion of yellow into red phosphorus for P, from + 19 to +27 thousand calories; P+H3= 4:3, HI + PH;= 24, PH;+ HBr=22 thousand calories.

At the ordinary temperature (20° C.) phosphorus is not oxidised by pure oxygen; oxidation only takes place with a slight rise of temperature, or the dilution of the oxygen with other gases (especially nitrogen or hydrogen), or a decrease of pressure.

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