GASOMETRIC ANALYSIS. In • chemis try, the art of separating and of estimating, quantitatively, the several constituents of a gaseous mixture. The methods employed may he divided into three general classes: (1) Those based on diffusion; (2) those based upon the absorption of certain constituents by sub stances over or through which the mixed gases are passed; and (3) those in which the given mixture is oxidized and its original composition inferred from an examination of the products of the oxidation. In the application of diffusion methods, the mixture is caused to pass through a porous septum of graphite, gypsum or baked clay. The lighter constituents pass through faster than the heavier ones, so that a partial separation is effected. By causing the mixture to pass through a succession of such porous partitions, the concentration effects may be cor respondingly increased. This method has been employed in many chemical researches, espe cially for effecting the concentration of gaseous substances that are present in a mixture in very small quantity. Its value as an experi mental method was well demonstrated in con nection with the study of the rare gases of the atmosphere, in effecting the separation of helium from argon. In general, the rates of diffusion of two gases are proportional to the square roots of their densities; and the den sity of argon being 10 times that of helium, it follows that helium will diffuse through a por ous septum about 3.2 times as fast as argon.
In the analysis of gases by the absorption of certain of their constituents by means of chem ical substances, use is made of the following facts (among others) : Water absorbs HCl, HBr, and HI, very readily; solid caustic potash, when moist, absorbs all acid gases, such as CO,, SO,, H,S, HCI, etc.; dilute sulphuric acid absorbs all alkaline gases, such as am monia; concentrated sulphuric acid also absorbs water, alcohol, ether, methyl oxide, and (more slowly) propylene and its homologues; alkaline solutions of pyrogallic acid absorb oxygen very readily; cuprous chloride in solution in hydro chloric acid absorbs oxygen and carbon mon oxide; and solutions of Cra and of KMna absorb HiS and S02. In the study of special problems, certain unusual absorbents also sug gest themselves. Thus in the isolation of the rare gases of the atmosphere, great use was made of the fact that red-hot metallic magne sium absorbs nitrogen gas, while it is without effect upon argon, helium, and the other gases of that group.
The combustion methods are particularly ap plicable to those cases in which the mixture to be analyzed is capable of being burned so that the final products are water, carbon dioxide, and free nitrogen, together with excess of such gas as may have been added in order to effect the combustion. The combustion is effected in an instrument called a "eudiometer," which com monly consists of a graduated glass tube that is closed at the upper end, and which is provided with a pair of platinum electrodes fused through the glass near the closed end. A sample of the gas to be analyzed is introduced into the tube (the lower end of which dips into a mercury bath), and its volume is determined by reference to the graduation marks; readings being simultaneously taken of the thermometer and barometer, so that the observed volume of the gas can be reduced, in the subsequent cal culations, to standard conditions of tempera ture and pressure. A known quantity of such gas as may be required to effect the combustion is next added; pure oxygen being used if the gas under examination is rich in carbon and hydrogen, and pure hydrogen being used if it is highly oxygenated or chlorinated. It is usual, also, to add a known quantity of which is prepared by the electrolysis of water and consists of pure oxygen and hydro gen in the proportion in which they combine to form water. The mixture is then exploded by passing an electric spark between the electrodes that are sealed into the eudiometer near its closed end, and after the heat developed by the explosive combustion has been lost by radiation, the volume of the mixture is again determined. The several constituents that remain in the eudiometer tube are then removed, one by one, by the temporary introduction, into the tube, of suitable absorbent substances. The volume of the gaseous contents of the eudiometer tube are observed after each partial absorption, and from the data so • obtained the quantities of carbon, oxygen, hydrogen and nitrogen that were present in the original sample may be calculated. Consult Hempel, 'Methods of Gas Analysis> (trans. New York 1912) ; Sutton, 'A Systematic Handbook of Volumetric Analysis' (10th ed., London 1911). See also, in this encyclopmdia, CHEMICAL. ANALYSIS; SPEc-rao SCOPE.