EXERCISES

DENSITY BY EXPERIMENTAPPROXIMATE MOLECULAR WEIGHT (D. × 28.9)PERCENTAGE OF NITROGEN BY EXPERIMENTPART OF MOLECULAR WEIGHT DUE TO NITROGENNitrogen gas0.967127.95100.0027.95Nitrous oxide1.52744.1363.7027.11Nitric oxide1.038430.0046.7414.02Nitrogen peroxide1.58045.6630.4913.90Ammonia0.59117.0582.2814.03Nitric acid2.18063.0622.2714.03Hydrocyanic acid0.93026.8751.9013.94

Method of calculation.The densities of the various gases in the first column of this table are determined by experiment, and are fairly accurate but not entirely so. By multiplying these densities by 28.9 the molecular weights of the compounds as given in the second column are obtained. By chemical analysis it is possible to determine the percentage composition of these substances, and the percentages of nitrogen in them as determined by analysis are given in the third column. If each of these molecular weights is multiplied in turn by the percentage of nitrogen in the compound, the product will be the weight of the nitrogen in the molecular weight of the compound. This will be the sum of the weights of the nitrogen atoms in the molecule. These values are given in the fourth column in the table.

If a large number of compounds containing nitrogen are studied in this way, it is probable that there will be included in the list at least one substance whose molecule contains a single nitrogen atom. In this case the number in the fourth column will be the approximate atomic weight of nitrogen. On comparing the values for nitrogen in the table it will be seen that a number which is approximately 14 is the smallest, and that the others are multiples of this. These compounds of higher value, therefore, contain more than one nitrogen atom in the molecule.

Accurate determination of atomic weights.Molecular weights cannot be determined very accurately, and consequently the part in them due to nitrogen is a little uncertain, as will be seen in the table. All we can tell by this method is that the true weight is very near 14. The equivalent can however be determined very accurately, and we have seen that it is some multiple or submultipleof the true atomic weight. Since molecular-weight determinations have shown that in the case of nitrogen the atomic weight is near 14, and we have found the equivalent to be 7.02, it is evident that the true atomic weight is twice the equivalent, or 7.02 × 2 = 14.04.

Summary.These, then, are the steps necessary to establish the atomic weight of an element.

1. Determine the equivalent accurately by analysis.

2. Determine the molecular weight of a large number of compounds of the element, and by analysis the part of the molecular weight due to the element. The smallest number so obtained will be approximately the atomic weight.

3. Multiply the equivalent by the small whole number (usually 1, 2, or 3), which will make a number very close to the approximate atomic weight. The figure so obtained will be the true atomic weight.

Molecular weights of the elements.It will be noticed that the molecular weight of nitrogen obtained by multiplying its density by 28.9 is 28.08. Yet the atomic weight of nitrogen as deduced from a study of its gaseous compounds is 14.04. The simplest explanation that can be given for this is that the gaseous nitrogen is made up of molecules, each of which contains two atoms. In this respect it resembles oxygen; for we have seen that an entirely different line of reasoning leads us to believe that the molecule of oxygen contains two atoms. When we wish to indicate molecules of these gases the symbols N2and O2should be used. When we desire to merely show the weights taking part in a reaction this is not necessary.

The vapor densities of many of the elements show that, like oxygen and nitrogen, their molecules consist of two atoms. In other cases, particularly among the metals,the molecule and the atom are identical. Still other elements have four atoms in their molecules.

While oxygen contains two atoms in its molecules, a study of ozone has led to the conclusion that it has three. The formation of ozone from oxygen can therefore be represented by the equation

3O2= 2O3.

Other methods of determining molecular weights.It will be noticed that Avogadro's law gives us a method by which we can determine the relative weights of the molecules of two gases because it enables us to tell when we are dealing with an equal number of the two kinds of molecules. If by any other means we can get this information, we can make use of the knowledge so gained to determine the molecular weights of the two substances.

Raoult's laws.Two laws have been discovered which give us just such information. They are known as Raoult's laws, and can be stated as follows:

1.When weights of substances which are proportional to their molecular weights are dissolved in the same weight of solvent, the rise of the boiling point is the same in each case.

2.When weights of substances which are proportional to their molecular weights are dissolved in the same weight of solvent, the lowering of the freezing point is the same in each case.

By taking advantage of these laws it is possible to determine when two solutions contain the same number of molecules of two dissolved substances, and consequently the relative molecular weights of the two substances.

Law of Dulong and Petit.In 1819 Dulong and Petit discovered a very interesting relation between the atomicweight of an element and its specific heat, which holds true for elements in the solid state. If equal weights of two solids, say, lead and silver, are heated through the same range of temperature, as from 10° to 20°, it is found that very different amounts of heat are required. The amount of heat required to change the temperature of a solid or a liquid by a definite amount compared with the amount required to change the temperature of an equal weight of water by the same amount is called its specific heat. Dulong and Petit discovered the following law:The specific heat of an element in the solid form multiplied by its atomic weight is approximately equal to the constant 6.25.That is,

at. wt. × sp. ht. = 6.25.

Consequently,

6.25at. wt.=————sp. ht.

This law is not very accurate, but it is often possible by means of it to decide upon what multiple of the equivalent is the real atomic weight. Thus the specific heat of iron is found by experiment to be 0.112, and its equivalent is 27.95. 6.25 ÷ 0.112 = 55.8. We see, therefore, that the atomic weight is twice the equivalent, or 55.9.

How formulas are determined.It will be well in connection with molecular weights to consider how the formula of a compound is decided upon, for the two subjects are very closely associated. Some examples will make clear the method followed.

The molecular weight of a substance containing hydrogen and chlorine was 36.4. By analysis 36.4 parts of the substance was found to contain 1 part of hydrogen and 35.4 parts of chlorine. As these are the simple atomicweights of the two elements, the formula of the compound must be HCl.

A substance consisting of oxygen and hydrogen was found to have a molecular weight of 34. Analysis showed that in 34 parts of the substance there were 2 parts of hydrogen and 32 parts of oxygen. Dividing these figures by the atomic weights of the two elements, we get 2 ÷ 1 = 2 for H; 32 ÷ 16 = 2 for O. The formula is therefore H2O2.

A substance containing 2.04% H, 32.6% S, and 65.3% O was found to have a molecular weight of 98. In these 98 parts of the substance there are 98 × 2.04% = 2 parts of H, 98 × 32.6% = 32 parts of S, and 98 × 65.3% = 64 parts of O. If the molecule weighs 98, the hydrogen atoms present must together weigh 2, the sulphur atoms 32, and the oxygen atoms 64. Dividing these figures by the respective atomic weights of the three elements, we have, for H, 2 ÷ 1 = 2 atoms; for S, 32 ÷ 32 = 1 atom; for O, 64 ÷ 16 = 4 atoms. Hence the formula is H2SO4.

We have, then, this general procedure: Find the percentage composition of the substance and also its molecular weight. Multiply the molecular weight successively by the percentage of each element present, to find the amount of the element in the molecular weight of the compound. The figures so obtained will be the respective parts of the molecular weight due to the several atoms. Divide by the atomic weights of the respective elements, and the quotient will be the number of atoms present.

Avogadro's hypothesis and chemical calculations.This law simplifies many chemical calculations.

1.Application to volume relations in gaseous reactions.Since equal volumes of gases contain an equal number ofmolecules, it follows that when an equal number of gaseous molecules of two or more gases take part in a reaction, the reaction will involve equal volumes of the gases. In the equation

C2H2O4= H2O + CO2+ CO,

since 1 molecule of each of the gases CO2and CO is set free from each molecule of oxalic acid, the two substances must always be set free in equal volumes.

Acetylene burns in accordance with the equation

2C2H2+ 5O2= 4CO2+ 2H2O.

Hence 2 volumes of acetylene will react with 5 volumes of oxygen to form 4 volumes of carbon dioxide and 2 volumes of steam. That the volume relations may be correct a gaseous element must be given its molecular formula. Thus oxygen must be written O2and not 2O.

2.Application to weights of gases.It will be recalled that the molecular weight of a gas is determined by ascertaining the weight of 22.4 l. of the gas. This weight in grams is called thegram-molecular weightof a gas. If the molecular weight of any gas is known, the weight of a liter of the gas under standard conditions may be determined by dividing its gram-molecular weight by 22.4. Thus the gram-molecular weight of a hydrochloric acid gas is 36.458. A liter of the gas will therefore weigh 36.458 ÷ 22.4 = 1.627 g.

1.From the following data calculate the atomic weight of sulphur. The equivalent, as obtained by an analysis of sulphur dioxide, is 16.03. The densities and compositions of a number of compounds containing sulphur are as follows:

NAMEDENSITYCOMPOSITION BY PERCENTAGEHydrosulphuric acid1.1791S = 94.11H = 5.89Sulphur dioxide2.222S = 50.05O = 49.95Sulphur trioxide2.74S = 40.05O = 59.95Sulphur chloride4.70S = 47.48Cl = 52.52Sulphuryl chloride4.64S = 23.75Cl = 52.53O = 23.70Carbon disulphide2.68S = 84.24C = 15.76

2. Calculate the formulas for compounds of the following compositions:

MOLECULAR WEIGHT(1) S = 39.07%O = 58.49%H = 2.44%81.0(2) Ca = 29.40S = 23.56O = 47.04136.2(3) K = 38.67N = 13.88O = 47.45101.2

3.The molecular weight of ammonia is 17.06; of sulphur dioxide is 64.06; of chlorine is 70.9. From the molecular weight calculate the weight of 1 l. of each of these gases. Compare your results with the table on the back cover of the book.

4.From the molecular weight of the same gases calculate the density of each, referred to air as a standard.

5.A mixture of 50 cc. of carbon monoxide and 50 cc. of oxygen was exploded in a eudiometer, (a) What gases remained in the tube after the explosion? (b) What was the volume of each?

6.In what proportion must acetylene and oxygen be mixed to produce the greatest explosion?

7.Solve Problem 18, Chapter XVII, without using molecular weights. Compare your results.

8.Solve Problem 10, Chapter XVIII, without using molecular weights. Compare your results.

9.The specific heat of aluminium is 0.214; of lead is 0.031. From these specific heats calculate the atomic weights of each of the elements.

SYMBOLATOMIC WEIGHTDENSITYMELTING POINTPhosphorusP31.01.843.3°ArsenicAs75.05.73—AntimonySb120.26.7432°BismuthBi208.59.8270°

The family.The elements constituting this family belong in the same group with nitrogen and therefore resemble it in a general way. They exhibit a regular gradation of physical properties, as is shown in the above table. The same general gradation is also found in their chemical properties, phosphorus being an acid-forming element, while bismuth is essentially a metal. The other two elements are intermediate in properties.

Compounds.In general the elements of the family form compounds having similar composition, as is shown in the following table:

PH3PCl3PCl5P2O3P2O5AsH3AsCl3AsCl5As2O3As2O5SbH3SbCl3SbCl5Sb2O3Sb2O5BiCl3BiCl5Bi2O3Bi2O5

In the case of phosphorus, arsenic, and antimony the oxides are acid anhydrides. Salts of at least four acids of each of these three elements are known, the free acid insome instances being unstable. The relation of these acids to the corresponding anhydrides may be illustrated as follows, phosphorus being taken as an example:

P2O3+ 3H2O = 2H3PO3(phosphorous acid).P2O5+ 3H2O = 2H3PO4(phosphoric acid).P2O5+ 2H2O = H4P2O7(pyrophosphoric acid).P2O5+ H2O = 2HPO3(metaphosphoric acid).

P2O3+ 3H2O = 2H3PO3(phosphorous acid).

P2O5+ 3H2O = 2H3PO4(phosphoric acid).

P2O5+ 2H2O = H4P2O7(pyrophosphoric acid).

P2O5+ H2O = 2HPO3(metaphosphoric acid).

History.The element phosphorus was discovered by the alchemist Brand, of Hamburg, in 1669, while searching for the philosopher's stone. Owing to its peculiar properties and the secrecy which was maintained about its preparation, it remained a very rare and costly substance until the demand for it in the manufacture of matches brought about its production on a large scale.

Occurrence.Owing to its great chemical activity phosphorus never occurs free in nature. In the form of phosphates it is very abundant and widely distributed.Phosphoriteandsombreriteare mineral forms of calcium phosphate, whileapatiteconsists of calcium phosphate together with calcium fluoride or chloride. These minerals form very large deposits and are extensively mined for use as fertilizers. Calcium phosphate is a constituent of all fertile soil, having been supplied to the soil by the disintegration of rocks containing it. It is the chief mineral constituent of bones of animals, and bone ash is therefore nearly pure calcium phosphate.

Preparation.Phosphorus is now manufactured from bone ash or a pure mineral phosphate by heating the phosphate with sand and carbon in an electric furnace. The materialsare fed in atM(Fig. 70) by the feed screwF. The phosphorus vapor escapes atPand is condensed under water, while the calcium silicate is tapped off as a liquid atS. The phosphorus obtained in this way is quite impure, and is purified by distillation.

Fig. 70Fig. 70

Explanation of the reaction.To understand the reaction which occurs, it must be remembered that a volatile acid anhydride is expelled from its salts when heated with an anhydride which is not volatile. Thus, when sodium carbonate and silicon dioxide are heated together the following reaction takes place:Na2CO3+ SiO2= Na2SiO3+ CO2.Silicon dioxide is a less volatile anhydride than phosphoric anhydride (P2O5), and when strongly heated with a phosphate the phosphoric anhydride is driven out, thus:Ca3(PO4)2+ 3SiO2= 3CaSiO3+ P2O5.If carbon is added before the heat is applied, the P2O5is reduced to phosphorus at the same time, according to the equationP2O5+ 5C = 2P + 5CO.

Na2CO3+ SiO2= Na2SiO3+ CO2.

Silicon dioxide is a less volatile anhydride than phosphoric anhydride (P2O5), and when strongly heated with a phosphate the phosphoric anhydride is driven out, thus:

Ca3(PO4)2+ 3SiO2= 3CaSiO3+ P2O5.

If carbon is added before the heat is applied, the P2O5is reduced to phosphorus at the same time, according to the equation

P2O5+ 5C = 2P + 5CO.

Physical properties.The purified phosphorus is a pale yellowish, translucent, waxy solid which melts at 43.3° and boils at 269°. It can therefore be cast into any convenient form under warm water, and is usually sold in the market in the form of sticks. It is quite soft and can be easily cut with a knife, but this must always be done while the element is covered with water, since it is extremely inflammable, and the friction of the knife blade is almostsure to set it on fire if cut in the air. It is not soluble in water, but is freely soluble in some other liquids, notably in carbon disulphide. Its density is 1.8.

Chemical properties.Exposed to the air phosphorus slowly combines with oxygen, and in so doing emits a pale light, or phosphorescence, which can be seen only in a dark place. The heat of the room may easily raise the temperature to the kindling point of phosphorus, when it burns with a sputtering flame, giving off dense fumes of oxide of phosphorus. It burns with dazzling brilliancy in oxygen, and combines directly with many other elements, especially with sulphur and the halogens. On account of its great affinity for oxygen it is always preserved under water.

Phosphorus is very poisonous, from 0.2 to 0.3 gram being a fatal dose. Ground up with flour and water or similar substances, it is often used as a poison for rats and other vermin.

Precaution.The heat of the body is sufficient to raise phosphorus above its kindling temperature, and for this reason it should always be handled with forceps and never with the bare fingers. Burns occasioned by it are very painful and slow in healing.

Red phosphorus.On standing, yellow phosphorus gradually undergoes a remarkable change, being converted into a dark red powder which has a density of 2.1. It no longer takes fire easily, neither does it dissolve in carbon disulphide. It is not poisonous and, in fact, seems to be an entirely different substance. The velocity of this change increases with rise in temperature, and the red phosphorus is therefore prepared by heating the yellow just below the boiling point (250°-300°). When distilled and quickly condensed the red form changes back to the yellow. This is in accordance with the general rule that when a substance capableof existing in several allotropic forms is condensed from a gas or crystallized from the liquid state, the more unstable variety forms first, and this then passes into the more stable forms.

Matches.The chief use of phosphorus is in the manufacture of matches. Common matches are made by first dipping the match sticks into some inflammable substance, such as melted paraffin, and afterward into a paste consisting of (1) phosphorus, (2) some oxidizing substance, such as manganese dioxide or potassium chlorate, and (3) a binding material, usually some kind of glue. On friction the phosphorus is ignited, the combustion being sustained by the oxidizing agent and communicated to the wood by the burning paraffin. In sulphur matches the paraffin is replaced by sulphur.In safety matchesredphosphorus, an oxidizing agent, and some gritty material such as emery is placed on the side of the box, while the match tip is provided as before with an oxidizing agent and an easily oxidized substance, usually antimony sulphide. The match cannot be ignited easily by friction, save on the prepared surface.

In safety matchesredphosphorus, an oxidizing agent, and some gritty material such as emery is placed on the side of the box, while the match tip is provided as before with an oxidizing agent and an easily oxidized substance, usually antimony sulphide. The match cannot be ignited easily by friction, save on the prepared surface.

Compounds of phosphorus with hydrogen.Phosphorus forms several compounds with hydrogen, the best known of which is phosphine (PH3) analogous to ammonia (NH3).

Preparation of phosphine.Phosphine is usually made by heating phosphorus with a strong solution of potassium hydroxide, the reaction being a complicated one.

Fig. 71Fig. 71

The experiment can be conveniently made in the apparatus shown in Fig. 71. A strong solution of potassium hydroxide together with several small bits of phosphorus are placed in the flaskA, and a current of coal gas is passed into the flask through the tubeBuntilall the air has been displaced. The gas is then turned off and the flask is heated. Phosphine is formed in small quantities and escapes through the delivery tube, the exit of which is just covered by the water in the vesselC. Each bubble of the gas as it escapes into the air takes fire, and the product of combustion (P2O5) forms beautiful small rings, which float unbroken for a considerable time in quiet air. The pure phosphine does not take fire spontaneously. When prepared as directed above, impurities are present which impart this property.

Properties.Phosphine is a gas of unpleasant odor and is exceedingly poisonous. Like ammonia it forms salts with the halogen acids. Thus we have phosphonium chloride (PH4Cl) analogous to ammonium chloride (NH4Cl). The phosphonium salts are of but little importance.

Oxides of phosphorus.Phosphorus forms two well-known oxides,—the trioxide (P2O3) and the pentoxide (P2O5), sometimes called phosphoric anhydride. When phosphorus burns in an insufficient supply of air the product is partially the trioxide; in oxygen or an excess of air the pentoxide is formed. The pentoxide is much the better known of the two. It is a snow-white, voluminous powder whose most marked property is its great attraction for water. It has no chemical action upon most gases, so that they can be very thoroughly dried by allowing them to pass through properly arranged vessels containing phosphorus pentoxide.

Acids of phosphorus.The important acids of phosphorus are the following:

H3PO3phosphorous acid.H3PO4phosphoric acid.H4P2O7pyrophosphoric acid.HPO3metaphosphoric acid.

These may be regarded as combinations of the oxides of phosphorus with water according to the equations given in the discussion of the characteristics of the family.

1.Phosphorous acid(H3PO3). Neither the acid nor its salts are at all frequently met with in chemical operations. It can be easily obtained, however, in the form of transparent crystals when phosphorus trichloride is treated with water and the resulting solution is evaporated:

PCl3+ 3H2O = H3PO3+ 3HCl.

Its most interesting property is its tendency to take up oxygen and pass over into phosphoric acid.

2.Orthophosphoric acid (phosphoric acid)(H3PO4). This acid can be obtained by dissolving phosphorus pentoxide in boiling water, as represented in the equation

P2O5+ 3H2O = 2H3PO4.

It is usually made by treating calcium phosphate with concentrated sulphuric acid. The calcium sulphate produced in the reaction is nearly insoluble, and can be filtered off, leaving the phosphoric acid in solution. Very pure acid is made by oxidizing phosphorus with nitric acid. It forms large colorless crystals which are exceedingly soluble in water. Being a tribasic acid, it forms acid as well as normal salts. Thus the following compounds of sodium are known:

NaH2PO4monosodium hydrogen phosphate.Na2HPO4disodium hydrogen phosphate.Na3PO4normal sodium phosphate.

These salts are sometimes called respectively primary, secondary, and tertiary phosphates. They may be prepared by bringing together phosphoric acid and appropriate quantities of sodium hydroxide. Phosphoric acid also forms mixed salts, that is, salts containing two different metals. The most familiar compound of this kind is microcosmicsalt, which has the formula Na(NH4)HPO4.

Orthophosphates.The orthophosphates form an important class of salts. The normal salts are nearly all insoluble and many of them occur in nature. The secondary phosphates are as a rule insoluble, while most of the primary salts are soluble.

3.Pyrophosphoric acid(H4P2O7). On heating orthophosphoric acid to about 225° pyrophosphoric acid is formed in accordance with the following equation:

2H3PO4= H4P2O7+ H2O.

It is a white crystalline solid. Its salts can be prepared by heating a secondary phosphate:

2Na2HPO4= Na4P2O7+ H2O.

4.Metaphosphoric acid (glacial phosphoric acid)(HPO3). This acid is formed when orthophosphoric acid is heated above 400°:

H3PO4= HPO3+ H2O.

It is also formed when phosphorus pentoxide is treated with cold water:

P2O5+ H2O = 2HPO3.

It is a white crystalline solid, and is so stable towards heat that it can be fused and even volatilized without decomposition. On cooling from the fused state it forms a glassy solid, and on this account is often called glacial phosphoric acid. It possesses the property of dissolving small quantities of metallic oxides, with the formation of compounds which, in the case of certain metals, have characteristic colors. It is therefore used in the detection of these metals.

While the secondary phosphates, on heating, give salts of pyrophosphoric acid, the primary phosphates yield salts of metaphosphoric acid. The equations representing these reactions are as follows:

2Na2HPO4= Na4P3O7+ H2O,NaH2PO4= NaPO3+ H2O.

2Na2HPO4= Na4P3O7+ H2O,

NaH2PO4= NaPO3+ H2O.

Fertilizers.When crops are produced year after year on the same field certain constituents of the soil essential to plant growth are removed, and the soil becomes impoverished and unproductive. To make the land once morefertile these constituents must be replaced. The calcium phosphate of the mineral deposits or of bone ash serves well as a material for restoring phosphorus to soils exhausted of that essential element; but a more soluble substance, which the plants can more readily assimilate, is desirable. It is better, therefore, to convert the insoluble calcium phosphate into the soluble primary phosphate before it is applied as fertilizer. It will be seen by reference to the formulas for the orthophosphates (see page 244) that in a primary phosphate only one hydrogen atom of phosphoric acid is replaced by a metal. Since the calcium atom always replaces two hydrogen atoms, it might be thought that there could be no primary calcium phosphate; but if the calcium atom replaces one hydrogen atom from each of two molecules of phosphoric acid, the salt Ca(H2PO4)2will result, and this is a primary phosphate. It can be made by treatment of the normal phosphate with the necessary amount of sulphuric acid, calcium sulphate being formed at the same time, thus:

Ca3(PO4)2+ 2H2SO4= Ca(H2PO4)2+ 2CaSO4.

The resulting mixture is a powder, which is sold as a fertilizer under the name of "superphosphate of lime."

Occurrence.Arsenic occurs in considerable quantities in nature as the native element, as the sulphides realgar (As2S2) and orpiment (As2S3), as oxide (As2O3), and as a constituent of many metallic sulphides, such as arsenopyrite (FeAsS).

Preparation.The element is prepared by purifying the native arsenic, or by heating the arsenopyrite in iron tubes,out of contact with air, when the reaction expressed by the following equation occurs:

FeAsS = FeS + As.

The arsenic, being volatile, condenses in chambers connected with the heated tubes. It is also made from the oxide by reduction with carbon:

2As2O3+ 3C = 4As + 3CO2.

Properties.Arsenic is a steel-gray, metallic-looking substance of density 5.73. Though resembling metals in appearance, it is quite brittle, being easily powdered in a mortar. When strongly heated it sublimes, that is, it passes into a vapor without melting, and condenses again to a crystalline solid when the vapor is cooled. Like phosphorus it can be obtained in several allotropic forms. It alloys readily with some of the metals, and finds its chief use as an alloy with lead, which is used for making shot, the alloy being harder than pure lead. When heated on charcoal with the blowpipe it is converted into an oxide which volatilizes, leaving the charcoal unstained by any oxide coating. It burns readily in chlorine gas, forming arsenic trichloride,—

As + 3Cl = AsCl3.

Unlike most of its compounds, the element itself is not poisonous.

Arsine(AsH3). When any compound containing arsenic is brought into the presence of nascent hydrogen, arsine (AsH3), corresponding to phosphine and ammonia, is formed. The reaction when oxide of arsenic is so treated is

As2O3+ 12H = 2AsH3+ 3H2O.

Arsine is a gas with a peculiar garlic-like odor, and is intensely poisonous. A single bubble of pure gas has been known to prove fatal. It is an unstable compound, decomposing into its elements when heated to a moderate temperature. It is combustible, burning with a pale bluish-white flame to form arsenic trioxide and water when air is in excess:

2AsH3+ 6O = As2O3+ 3H2O.

When the supply of air is deficient water and metallic arsenic are formed:

2AsH3+ 3O = 3H2O + 2As.

These reactions make the detection of even minute quantities of arsenic a very easy problem.

Fig. 72Fig. 72

Marsh's test for arsenic.The method devised by Marsh for detecting arsenic is most frequently used, the apparatus being shown in Fig. 72. Hydrogen is generated in the flaskAby the action of dilute sulphuric acid on zinc, is dried by passing over calcium chloride in the tubeB, and after passing through the hard-glass tubeCis ignited at the jetD. If a substance containing arsenic is now introduced into the generatorA, the arsenic is converted into arsine by the action of the nascent hydrogen, andpasses to the jet along with the hydrogen. If the tubeCis strongly heated at some point near the middle, the arsine is decomposed while passing this point and the arsenic is deposited just beyond the heated point in the form of a shining, brownish-black mirror. If the tube is not heated, the arsine burns along with the hydrogen at the jet. Under these conditions a small porcelain dish crowded down into the flame is blackened by a spot of metallic arsenic, for the arsine is decomposed by the heat of the flame, and the arsenic, cooled below its kindling temperature by the cold porcelain, deposits upon it as a black spot. Antimony conducts itself in the same way as arsenic, but the antimony deposit is more sooty in appearance. The two can also be distinguished by the fact that sodium hypochlorite (NaClO) dissolves the arsenic deposit, but not that formed by antimony.

Oxides of arsenic.Arsenic forms two oxides, As2O3and As2O5, corresponding to those of phosphorus. Of these arsenious oxide, or arsenic trioxide (As2O3), is much better known, and is the substance usually called white arsenic, or merely arsenic. It is found as a mineral, but is usually obtained as a by-product in burning pyrite in the sulphuric-acid industry. The pyrite has a small amount of arsenopyrite in it, and when this is burned arsenious oxide is formed as a vapor together with sulphur dioxide:

2FeAsS + 10O = Fe2O3+ As2O3+ 2SO2.

The arsenious oxide is condensed in appropriate chambers. It is a rather heavy substance, obtained either as a crystalline powder or as large, vitreous lumps, resembling lumps of porcelain in appearance. It is very poisonous, from 0.2 to 0.3 g. being a fatal dose. It is frequently given as a poison, since it is nearly tasteless and does not act very rapidly. This slow action is due to the fact that it is not very soluble, and hence is absorbed slowly by the system. Arsenious oxide is also used as a chemical reagent in glass making and in the dye industry.

Acids of arsenic.Like the corresponding oxides of phosphorus, the oxides of arsenic are acid anhydrides. In solution they combine with bases to form salts, corresponding to the salts of the acids of phosphorus. Thus we have salts of the following acids:

H3AsO3arsenious acid.H3AsO4orthoarsenic acid.H4As2O3pyroarsenic acid.HAsO3metarsenic acid.

Several other acids of arsenic are also known. Not all of these can be obtained as free acids, since they tend to lose water and form the oxides. Thus, instead of obtaining arsenious acid (H3AsO3), the oxide As2O3is obtained:

2H3AsO3= As2O3+ 3H2O.

Salts of all the acids are known, however, and some of them have commercial value. Most of them are insoluble, and some of the copper salts, which are green, are used as pigments. Paris green, which has a complicated formula, is a well-known insecticide.

Antidote for arsenical poisoning.The most efficient antidote for arsenic poisoning is ferric hydroxide. It is prepared as needed, according to the equation

Fe2(SO4)3+ 3Mg(OH)2= 2Fe(OH)3+ 3MgSO4.

Sulphides of arsenic.When hydrogen sulphide is passed into an acidified solution containing an arsenic compound the arsenic is precipitated as a bright yellow sulphide, thus:

2H3AsO3+ 3H2S = As2S3+ 6H2O,2H3AsO4+ 5H2S = As2S5+ 8H2O.

2H3AsO3+ 3H2S = As2S3+ 6H2O,

2H3AsO4+ 5H2S = As2S5+ 8H2O.

In this respect arsenic resembles the metallic elements, many of which produce sulphides under similar conditions. The sulphides of arsenic, both those produced artificially and those found in nature, are used as yellow pigments.

Occurrence.Antimony occurs in nature chiefly as the sulphide (Sb2S3), called stibnite, though it is also found as oxide and as a constituent of many complex minerals.

Preparation.Antimony is prepared from the sulphide in a very simple manner. The sulphide is melted with scrap iron in a furnace, when the iron combines with the sulphur to form a slag, or liquid layer of melted iron sulphide, while the heavier liquid, antimony, settles to the bottom and is drawn off from time to time. The reaction involved is represented by the equation

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