DISCRIMINATION OF THE GASES. 7. Oxychloride of carbon 8. Sulphurous anhydride 9. Nitrous anhydride 10. Peroxide of chlorine 11. Chlorous anhydride 307 12. Hypochlorous anhydride 13. Chlorine 14. Carbonic anhydride 15. Chloride of cyanogen. Of these gases each of the first eleven reddens litmus-paper, when moistened and plunged into it. Hypochlorous anhydride and chlorine destroy its colour, and bleach it entirely. Carbonic anhydride is nearly without action, and chloride of cyanogen produces no effect upon its colour. The first six gases fume strongly when mixed with the air, owing to their action on the moisture which it contains: the solutions in water of hydrochloric, hydrobromic, and hydriodic acids are immediately distinguished by the usual tests for them. Each gas also presents certain peculiarities-viz., 1. A small quantity of chlorine produces no change in the hydrochloric gas; 2. In hydrobromic gas it occasions the separation of red fumes of bromine; and 3. In the hydriodic gas violet fumes of iodine appear. 4. Fluoride of silicon is recognised by the gelatinous deposit of silica which water produces when the gas is dissolved in this liquid. 5. The fluoride of boron produces a gelatinous precipitate in a solution of potash, but not in pure water. 6. Chloride of boron is decomposed by water into hydrochloric and boracic acids, which may be recognized in the solution by the appropriate tests. 7. Oxychloride of carbon has a peculiar, pungent odour, and is decomposed by water into hydrochloric acid and carbonic anhydride. 8. Sulphurous anhydride is immediately recognized by the suffocating odour of a burning sulphur match: it is absorbed by the peroxide of lead, and a white sulphate of lead is formed. Nitrous anhydride is sufficiently characterized by its colour and peculiar odour; and 10, the same may be remarked of peroxide of chlorine. II. Chlorous anhydride has a greener tinge than the peroxide of chlorine, and it yields a bright yellow solution when dissolved in water. 12. Hypochlorous anhydride has the odour of the bleaching compounds of chlorine with the alkalies and earths, and it rapidly destroys vegetable colours: these three oxides of chlorine detonate by the application of a temperature below that of boiling water. 13. Chlorine is distinguished by its green colour and remarkable odour, by its bleaching action on vegetable colours, and by its sparing solubility in water, which takes up about twice its bulk of the gas. 14. Carbonic anhydride extinguishes flame, renders lime-water turbid, and is soluble in about its own bulk of water, 9. 308 DISCRIMINATION OF THE GASES. 15. Chloride of cyanogen is recognized by its pungent odour, and its peculiar irritating effect on the eyes. (513) 2.-Gases absorbable by Potash and inflammable; these are only 4 in number :-viz., 1. Sulphuretted hydrogen 2. Seleniuretted hydrogen 3. Telluretted hydrogen 4. Cyanogen. These gases are recognized with great facility. 1. Sulphuretted hydrogen has a peculiar odour of putrid eggs; it burns with a blue flame, often attended with a deposit of sulphur: it blackens paper soaked in a solution of acetate of lead, and is decomposed by moist chlorine, with separation of sulphur; water dissolves about twice its bulk of the gas. 2. Seleniuretted hydrogen has an odour analogous to that of the preceding gas; its aqueous solution gradually deposits selenium in the form of a red amorphous precipitate: it precipitates salts of zinc of a flesh-red colour. 3. Telluretted hydrogen is also decomposed by chlorine, tellurium being set free, and subsiding as a brown powder. 4. Cyanogen burns with a roseedged purple flame; it has a penetrating characteristic odour. If mixed with an equal volume of oxygen, and a red-hot platinum wire be suspended in the mixture, red nitrous fumes are produced by the oxidation of the nitrogen contained in the gas. (514) 3.-Gases not absorbable by Potash, and not inflammable ; of these also there are four-viz., 1. Oxygen 2. Nitrous oxide 3. Nitrogen 1. Oxygen is at once distinguished from all other gases by its property of kindling a glowing match, by its power of producing red fumes when mixed with nitric oxide, and by its insolubility in water when agitated with it. It is absorbed by moistened phosphorus; by a solution of suboxide of copper in ammonia, rendering the colourless solution deep blue; and by a solution of pyrogallic acid in potash, the mixture becoming of an intense bistre colour. Solutions of the sulphides of the alkaline metals also absorb oxygen rapidly. 2. Nitrous oxide, though it rekindles a glowing match, is dissolved when agitated with water. 3. Nitrogen extinguishes the flame of burning bodies; it is insoluble in water, and does not render lime-water turbid. 2. Nitric oxide is instantly recognized by the red fumes which it occasions when mixed with air or free oxygen; it is immediately absorbed by a solution of ferrous sulphate, giving the liquid a deep brown colour when oxygen is present. DISCRIMINATION OF THE GASES. 309 (515) 4-Gases not absorbable by Potash, which are inflammable; these gases are 7 in number:—viz., 1. Hydrogen 2. Light carburetted hydrogen 3. Olefiant gas 4. Oil-gas 5. Phosphuretted hydrogen 6. Arseniuretted hydrogen 7. Carbonic oxide. 1. Hydrogen is inodorous, if pure; it burns with a feebly luminous flame, and if mixed with half its volume of oxygen produces water either by the transmission of an electric spark, or by the action of a ball of spongy platinum. 2. Light carburetted hydrogen burns with a yellowish flame; it is not acted upon if mixed with chlorine over water and screened from light, and is not dissolved by fuming sulphuric acid. 3. Olefiant gas, when mixed with an equal volume of chlorine, even in the dark becomes condensed to an oily liquid which is insoluble in water; it is also absorbed by perchloride of antimony, and by the Nordhausen sulphuric acid it burns with a brilliant smoky flame. 4. Oil-gas is soluble in oil of vitriol, and in alcohol; it burns with a brilliant smoky flame. When the last two gases are mixed together, there is considerable difficulty in identifying the existence of each in such a mixture, and a still greater difficulty occurs when methyl, ethyl, trityl, or their hydrides, are present. 5. Phosphuretted hydrogen is distinguished by its peculiar alliaceous odour: it burns with a luminous flame, producing white fumes of phosphoric anhydride; solutions of the salts of copper, silver, and mercury dissolve it and form brown precipitates. 6. Arseniuretted hydrogen is decomposed if passed through glass tubes heated nearly to redness, a ring of metallic arsenic being deposited: it burns with a peculiar white flame, and deposits a brown stain of metallic arsenic on cold bodies introduced into the burning jet. It is extremely poisonous, and has a peculiar odour of garlic. It may be distinguished from antimoniuretted hydrogen by methods to be described hereafter (846). 7. Carbonic oxide burns with a pale blue flame, producing carbonic anhydride; it is insoluble in water, and is dissolved by a solution of cupreous chloride in hydrochloric acid. (516) General Principles of the Analysis of a Mixture of Gases. -In a mixture of gases a qualitative examination must be made as a preliminary step, in order to ascertain what gases are present. It is of course needless to search for those which mutually condense or decompose each other. Thus ammonia would not be found in a mixture which contained hydrochloric, hydriodic, hydro 310 ANALYSIS OF A MIXTURE OF GASES. bromic acid gases, nor in the presence of sulphurous or nitrous anhydrides. Oxygen would not occur in a mixture in which nitric oxide was present. Neither could free chlorine or its oxides co-exist with hydriodic or hydrobromic acid, or with olefiant gas, or with the compounds of hydrogen with sulphur, selenium, tellurium, phosphorus, or arsenic : chlorine and its oxides are equally incompatible with ammonia. The complete analysis of a mixture of different gases is one of the most delicate and difficult branches of chemical analysis, and it is not intended on the present occasion to attempt to give more than an idea of the principles on which such an operation is conducted, and of the apparatus by which it is effected. As an illustration of the method of proceeding we may take a case of frequent occurrence: viz., the determination of the composition of a sample of coal-gas. In this gas, the ingredients which may be present are numerous. These are 1, hydrogen; 2, olefiant gas and other heavy hydrocarbons; 3, light carburetted hydrogen; 4, carbonic oxide; 5, carbonic anhydride; 6, sulphuretted hydrogen; 7, ammonia; 8, oxygen; and 9, nitrogen; the last two derived from the atmosphere. A qualitative examination is made thus:-the proportion of ammonia and of sulphuretted hydrogen is usually very minute, and in most cases these gases must be sought for by placing the tests for their presence for some time in a current of the coal-gas. In searching for ammonia, a piece of moistened litmus-paper, feebly reddened, is placed for a minute in a jet of the issuing gas: if the blue colour be restored, ammonia is present. Paper soaked in a solution of acetate of lead may be subjected to a similar trial: if it turn brown, sulphuretted hydrogen is present. The presence of oxygen is detected by admitting a bubble of the nitric oxide into a tube filled with the gas under trial, and looking through the tube obliquely upon a sheet of white paper; very small traces of oxygen may thus be detected by the red tinge produced, owing to the formation of peroxide of nitrogen. Carbonic anhydride may be detected by the turbidity which it produces in lime-water or in a solution of basic acetate of lead, if thrown up into the gas, whilst standing in a tube over mercury. The existence of the other gases may be assumed, for they are certain to be present, in greater or less quantity. The sulphuretted hydrogen and ammonia are too small in amount to be quantitatively determined; but supposing that oxygen and carbonic anhydride are found to be present, the proportion of seven different gases will remain to ANALYSIS OF COAL-GAS. 311 be ascertained. The following method may be adopted for their quantitative determination :— 1. Carbonic Anhydride.-A volume of the gas is confined over mercury, and its bulk is measured, with due attention to temperature and pressure. A piece of caustic potash which has been melted upon the end of a long platinum wire, to serve as a handle, is introduced from below, through the mercury into the tube. After two or three hours the potash is withdrawn; the amount of the absorption indicates the proportion of carbonic anhydride which was present. 2. Olefiant Gas and Heavy Hydrocarbons.-These gases are absorbed by introducing another ball, consisting of porous coke moistened with fuming sulphuric acid. It is necessary, however, before reading off the volume of the gas, to introduce a ball of potash a second time, in order to withdraw the vapour of sulphuric anhydride, which possesses sufficient volatility to introduce a serious error by dilating the bulk of the gas unless it be completely removed. The total amount of absorption will indicate the proportion of olefiant gas, together with the vapours of condensible hydrocarbons. 3. Oxygen. This gas is determined in a similar manner, by employing a ball of moist phosphorus, which must be left in the gas for twenty-four hours; the fresh diminution in bulk, shows the proportion of oxygen.* 4. Carbonic Oxide.-The accurate separation of carbonic oxide from the other gases is not easily effected. The gas may be divided into two portions, one of which is to be carefully measured as it stands over mercury, in the jar, h, fig. 285, p. 61; a small quantity of a solution of subchloride of copper in hydrochloric acid is next added by means of the syringe, i, and the mixture is briskly agitated; the gas is then withdrawn by means of the gas pipette, also shown in fig. 285, and transferred by its means to a second graduated tube, also standing over mercury; into this tube a ball of hydrate of potash on the end of a platinum wire is introduced, for the purpose of absorbing the vapours of *A ball of coke moistened with a concentrated solution of potash and pyrogallic acid may be employed for the same purpose: the absorption in this case is much more rapid. The use of pellets of appropriate materials may be extended to other gases: for example-Sulphurous anhydride may be absorbed by using a ball of moistened peroxide of manganese, or of peroxide of lead; and hydrochloric acid is rapidly absorbed by a ball consisting of crystallized rhombic phosphate of sodium. |