24 METHOD OF RELATIVE VOLUMES. brought to redness in the usual way by means of a charcoal fire, the decomposition being caused to take place less rapidly than usual. When the part of the retort containing the matter for analysis is red-hot through its entire extent, heat is gradually applied to the hydro-sodic carbonate, and the last portions of gas furnished by the combustion are driven into the receiver by the carbonic anhydride disengaged from the carbonate. The products of the combustion are only water, carbonic anhydride, and nitrogen; the former two are retained by the solution of potash, whilst the nitrogen alone presents itself for measurement. When the apparatus by standing for an hour or two has reached the temperature of the atmosphere, the height of the barometer and thermometer must be carefully noticed; and since the gas will be saturated with moisture, its volume must be corrected by the known methods for the three points of temperature, pressure, and moisture: then, since a litre of nitrogen at o° C. and 760 mm. barometric pressure weighs 1256 grm., or 100 cubic inches of nitrogen at 60° F. and under a pressure of 30 inches of mercury weigh 30.15 grains, it is easy to calculate the weight of the nitrogen that is contained in a given quantity of the matter analysed. In this process, as in every case where the proportion of nitrogen, alone forms the object of the experiment, after the weight of the material for analysis has been once accurately ascertained, it is evident that there is nothing to fear from a slight absorption of moisture. (1049) Method of Relative Volumes.-When the quantity of nitrogen present is not less than one-fourth of the weight of the carbon contained in the compound, its proportion may be advantageously determined by making the combustion just as though we were going to ascertain the proportion of carbon and hydrogen; but, instead of condensing the carbonic anhydride and weighing it, the whole of the gases produced are collected over mercury. A bent gas-delivering tube, g, fig. 381, is substituted for the usual apparatus for the absorption of water and carbonic anhydride. In this case it is best to begin at the closed extremity of the tube, and having expelled the atmospheric air by a portion of gas generated from the substance, to collect the rest of the gaseous products in a graduated jar; by agitating the gas with a solution of potash, the proportion of nitrogen to the carbon is at once determined. since equal volumes of carbonic anhydride and nitrogen gases represent single atoms of carbon and nitrogen. It is not necessary in this case to weigh accurately the quantity of material acted upon. DETERMINATION OF SULPHUR, PHOSPHORUS, AND ARSENIC. 25 Experience has shown that in the preceding process for organic analysis, the quantity of hydrogen deduced from it is always slightly in excess, usually about o'2 parts in 100, whilst, unless chromate of lead or potassic chlorate be employed, the carbon is sometimes deficient to the same extent. A deficiency of carbon also occurs if the ash contain carbonates of the metals of the alkalies or of the earths. (1050) Determination of Sulphur, Phosphorus, and Arsenic.— One of the methods employed for ascertaining the amount of unoxidized sulphur in an organic compound, consists in mixing I part of the substance for analysis with 10 parts of nitre, 2 of dried sodic carbonate, and 30 of pure sodic chloride, and heating the mass to redness in a tube of hard glass. The sulphur is thus converted into sulphuric acid, which forms a salt with a portion of the alkali. The object of adding the chloride of sodium is simply to moderate the violence of the deflagration. The residue is to be dissolved in water rendered slightly acid with hydrochloric acid, and the sulphuric acid precipitated by the addition of baric chloride. If phosphorus or arsenicum be present, it will remain in the acid liquor in the form of phosphoric or arsenic acid. Its amount may be determined by adding sulphuric acid to throw down the excess of barium salt, filtering, supersaturating with ammonia, and adding an ammoniacal solution of magnesic sulphate; the phosphoric or arsenic acid is precipitated as the ammonio-magnesic phosphate or arseniate, and is to be collected in the usual manner. Another method, and one less liable to error, consists in sealing up the substance in a tube with strong nitric acid (of sp. gr. 152) and heating it for twenty-four hours to about 392° (200° C.), then neutralizing with soda, and after evaporating to dryness, fusing the residue in a platinum dish; after which the amount of sulphate, phosphate, or arseniate is determined in the usual way. When a large quantity of sulphur is present, an error might easily occur in the estimation of the amount of carbon, since a portion of the sulphur becomes converted during the process of combustion with cupric oxide into sulphurous anhydride, and this would be condensed by the potash along with the carbonic anhydride which would thus be estimated in excess. This source of error may be avoided by interposing a short tube filled with peroxide of lead between the tube of chloride of calcium and the potash bulbs; the sulphurous anhydride is then arrested, and is 26 DETERMINATION OF CHLORINE, BROMINE, AND IODINE. retained as sulphate of lead, the anhydride passing into a higher state of oxidation, PbᎾ, + ᎦᎾ, becoming PbᎦᎾ 2 (1051) Determination of Chlorine, Bromine, and Iodine.-When the quantity of chlorine, or of any other halogen, is to be estimated, the substance for analysis is to be mingled with about 10 times its weight of lime, and introduced into a tube of Bohemian glass 10 or 12 inches long, and sealed at one end. The tube is to be filled up with fragments of pure lime, which is gradually brought to a red heat, commencing at the open extremity. When the combustion is complete, the tube is corked, its outer surface cleared from ashes, and whilst hot it is plunged into a beaker of cold water. It is thus cracked, and its contents are then treated with nitric acid, and the chlorine, iodine, or bromine is precipitated from the filtered liquid by means of nitrate of silver. (1052) Calculation of the Combining Proportion of an Organic Body. We will suppose the labour of analysis thus brought to a successful issue. It is evident that the information derived from this source alone is but scanty, for it furnishes no idea either of the number of atoms of each element entering into the molecule of the organic body, or of the relations of the body to the substances concerned in its production or obtainable from it by its decomposition. Whenever it is possible, the combining proportion of the compound must be determined. This is effected by preparing a compound of the body with some substance the combining proportion of which is well known, and proceeding to analyse the new product. If the organic substance be soluble in water and capable of forming a compound with silver, this compound is for many reasons to be preferred. Silver forms with many organic bodies compounds insoluble or but sparingly soluble in water; and they may generally be obtained by double decomposition, by adding a solution of the nitrate or some soluble salt of silver to an aqueous or alcoholic solution of the substance under examination; the precipitate must then be washed from all adhering impurities. Supposing a silver compound to have been prepared in a state of purity, a gramme or more of it is to be accurately weighed in a counterpoised porcelain crucible. It is then to be carefully incinerated till pure silver alone remains. On again weighing, the loss will give that of the body combined with the silver. The residual silver should be soluble without remainder in nitric acid. From the weight of the metallic silver, the combining number of the organic body that had combined with it may be readily calculated. An example will best explain the method of proceeding : CALCULATION OF THE EQUIVALENT OF AN ORGANIC BODY. 27 4873 grms. of silver acetate left 3'149 grms. of metallic silver. 1724 will therefore express the loss, due to the weight of acetion (the acetic acid radicle) combined with silver; then, Sat. wt. silver. 3'149:1 { 108 :: 1724 x (=59); and : 59 = the combining number of acetion. Another example will show the method of calculating the number of atoms of each element in a molecule of the compound:By combustion with oxide of copper it is found that I gramme of silver acetate yields 05277 grm. of carbonic anhydride And (from previous expt.). The deficiency Grm. = 01439 carbon = 01919 oxygen. From this the formula of glacial acetic acid is easily deduced, as it contains one atom of hydrogen in the place of the atom of silver in the acetate of silver; acetate of silver=Age, H12, and glacial acetic acid=HЄ,H,2. 3 3 In some cases the required compound with silver cannot be obtained: a salt of lead is then, if practicable, substituted for it. The residue after incineration in this case does not consist entirely of metallic lead, neither is it all oxide of lead. In order to determine the proportion of each, the residue is carefully weighed, and treated with acetic acid in the crucible itself; the oxide of lead is thus dissolved and washed away. When the contents of the crucible have been carefully dried, a second weighing gives the quantity of metallic lead, whilst the loss furnishes that of the oxide. From the metal the quantity of oxide to which it is equiva 28 COMBINING PROPORTION DISTINCT FROM MOLECULAR weight. lent may be calculated; this, added to the portion dissolved by acetic acid, furnishes the whole quantity of oxide contained in the compound a calculation, similar to that employed for the silver salt, then supplies the equivalent number of the body analysed. The method is not quite so accurate as the preceding one; it involves more manipulation, and the compounds of lead are apt at a high temperature to undergo loss by volatilization. Salts of barium or of potassium may be substituted when needful for those of silver or lead. Just as the combining number of an acid can be ascertained by determining the amount of any metallic monad with which it forms a salt, so may the combining number of a body possessed of marked basic properties-morphia or aniline, for example-be discovered by causing it to enter into combination with a welldefined acid such as the hydrochloric, and ascertaining the amount of such acid with which a given weight of the base will unite. The hydrochlorates of many bases form crystallizable double salts with platinic chloride; when this is the case, the double salt is frequently employed in the determination of the combining proportion of the organic base which it contains. (1053) Distinction between the Combining Proportion and the Molecular Weight.-It is comparatively easy to determine the combining proportion, or chemical equivalent of compounds possessing decided acid or basic characters when referred for comparison to some well-known acid, such as the hydrochloric or the nitric, or to some base, such as potash; and up to this point the chemist is strictly within the domain of facts. These facts do not decide the question of the molecular weight of a compound, which is necessarily based on theoretical considerations. If, for instance, we analyse the normal and neutral potassic tartrate after it has been dried sufficiently to expel the water of crystallization, we shall find that the composition of a quantity of the salt equivalent to KCl, (containing, that is to say, a quantity of potassium equal to that in the amount of chloride represented by the formula KCl), may be expressed by the formula K¤‚Í‚Ð ̧. A second salt of potassium with tartaric acid, the sparingly soluble acid tartrate, may however be obtained; and this, when subjected to analysis is found to contain, combined with the same amount of potassium as is present in KCl, other elements, in proportions which must be indicated by the formula KHЄH. Now if we imagine the potassium of these two salts to be displaced by hydrogen, we obtain two different formulæ, either of which may be the molecular formula for tartaric acid. These formulæ are--1. HЄ2H ̧ ̧ 4 4 |