Page:Encyclopædia Britannica, Ninth Edition, v. 5.djvu/557

Rh ORGANIC.] CHEMISTKY 545 drawn off in the blowpipe flame so as to leave a tailed extremity of the form shown in figure 1. The open end of the tube is then held in the flame of the blowpipe till the sharp edge of the glass is rounded off. This tube, known as the combustion tube, is from 500 to 800 millimetres in length, according as the substance contains little or much carbon. The substance generally employed to effect the combustion of carbon compounds is cupric oxide (CuO), and as this oxide is of a somewhat hygroscopic nature, it is necessary to dry it thoroughly before use, and then exclude it carefully from the air. Should these precautions not be observed, the oxide absorbs water from the air, and when heated in the combustion tube this water distils over and is weighed with that produced by the combustion of the substance. It is therefore found convenient in practice to have another glass tube sealed at one end and closely corked at the other for the reception of the cupric oxide. This tube is of sufficient capacity to contain enough cupric oxide to fill the combustion tube, and its diameter is such as to allow it to be introduced into the open end of the combus tion tube. The substance for analysis, after being finely powdered, is dried in a water oven at 100 C. till it ceases to lose weight, or, if decomposed by heat, in a vacuum over some water- absorbing substance, such as strong sulphuric acid or calcium chloride. When dry it is transferred to a small stoppered bottle or corked tube, sealed at one end, and of about 40 or 50 millimetres in length. The apparatus for absorbing the carbon dioxide is a series of light glass bulbs (figs. 2 and 3) containing a solution of caustic potash (KHO) of sp. gr. 1-27, through which the gases evolved during the combustion are obliged to pass. It is customary to attach to one end of the potash bulbs a small glass tube drawn to a point at its free end, and containing a small piece of solid potash enclosed between plugs of cotton wool. This appendage is for the purpose of retaining any trace of carbon dioxide or mois ture carried over from the bulbs by the escaping gases. FIG. 2. Liebig s Potash Bulbs. FIG. 3. Geissler s Potash Bulbs. The water formed by the combustion is collected in a tube (fig. 4) filled with fragments^ of dried calcium chloride or pumice stone wetted with strong sulphuric acid. The combustion tube when charged is heated in a furnace which consists essentially of a row of gas burners con structed so as to burn a mixture of coal gas and air, and thus obtain a non- luminous but very hot flame. Two forms of furnace are in common use in laboratories. In Hofmann s furnace a clay cylinder perforated by a large number of very fine holes is fitted over the end of each gas burner, thus dividing the escaping gas into a number of fine streams, and so ensuring its perfect combustion. In Griffin s furnace the burners are constructed on Bunsen s principle, that is, each burner is perforated by holes at its lower end so that a mixture of gas and air escapes from its upper extremity. The com bustion tube is supported on a thin iron trough lined with asbestos, the whole being enclosed by unglazed tiles. FIG. 4. Calcium Chloride Tube. The cupric oxide intended for use is first heated to red ness in a crucible, and transferred while still hot to its tube, wherein it is allowed to cool. Whan sufficiently cool the cork is withdrawn, and enough cupric oxide poured into the combustion tube to occupy about ^th of its length. The tube or bottle containing the dried pulverized sub stance having been weighed, about half a gramme is then introduced into the combustion tube, and a small quantity of cupric oxide is poured down after it. The bottle being again weighed gives the exact weight of the substance employed. The substance in the combustion tube is thoroughly mixed with cupric oxide by stirring it about with a long wire with corkscrew point. More cupric oxide is now added till the tube is filled nearly to its open end, and a plug of freshly ignited asbestos is finally introduced. The tube is then gently tapped while in a horizontal position so as to cause a slight subsidence of the contents, thus allowing a free passage for gas throughout its whole length. The calcium chloride tube is fitted into the open end of the combustion tube by means of a tightly fitting cork or caoutchouc stopper, and the potash apparatus is attached to the calcium chloride tube by means of a short piece of caoutchouc tubing bound tightly round with thin copper wire. Both the potash bulbs and calcium chloride tube are carefully weighed before being attached. The whole apparatus as arranged for the combustion is shown in fig. 5 : FIG. 5. Apparatus arranged for a Combustion. The front part of the tube is first heated to redness, and the gas burners are then gradually turned on so as to cause the slow combustion of the substance. When no more bubbles pass into the potash solution the gas burners are turned out, the point of the tail of the combustion tube is broken off, and dry air passed through the whole system to sweep out the last traces of carbon dioxide and water vapour lingering in the tube. In cases where great accuracy is required it is desirable to sweep out the com bustion tube first with a current of pure oxygen and then with air. This ensures the complete oxidation of any trace of carbon that may have escaped the first combustion. When the operation is completed the potash bulbs and calcium chloride tube are allowed to cool down to the atmospheric temperature and then again weighed. The increase in weight gives the respective amounts of carbon dioxide and water produced. Since carbon dioxide contains in 1 1 parts by weight 3 of carbon (CO 2 =12 + 2x16 = 44), ^Y of the weight of the C0 2 obtained is due to carbon. Similarly -| of the weight of the water found is due to hydrogen (H 2 = 2x1 + 16 = 18). It is customary to ex press the results in parts per cent. In practice the hydro gen usually comes out a little too high, and the carbon a little too low. Cupric oxide is sometimes replaced by lead chromate in cases where the substance is difficultly combustible, and more particularly when the compound contains chlorine, bromine, or sulphur. In these latter cases were cupric oxide employed, cupric chloride or bromide would be formed, and might volatilize over into the calcium chloride tube, thus unduly increasing its weight. Sulphur iu V. 69