Gas Analysis

Gas Analysis. The first operation in gas analysis is the collection of the sample, and this frequently offers a great many difficulties. In ordinary cases the gas is collected in glass vessels over water or mercury, the former being most convenient, but only applicable when the gas to be analysed contains no soluble constituents. The vessels are filled with the liquid, and then by means of a tube attached to their upper end are placed in communication with the chamber, etc., containing the gas. The mercury or water is then run out from below, the gas being thus aspirated into the vessel which, when full, is securely closed. The sample being thus collected, the method usually adopted for its analysis is the absorption in turn of each of its various constituents, by suitable substances, the decrease in volume after each absorption being observed. For this purpose the gas is transferred to a graduated measuring tube, and from this it is forced into the bulb containing the particular absorbent, being again forced back into the measuring tube. Owing to the alteration of the volume of a gas with variations in the temperature and barometric pressure [Gas], these data must be also noticed at each observation. The principal substances employed as absorbents are the following: - Caustic jiotash, either solid or in solution, which absorbs acids, or their anhydrides, being most frequently used for the determination of carbonic acid C02. Pyroyallol or pyrogallic acid, which is used to absorb oxygen, for which purpose also phosphorus may be employed. Cuprous chloride, either in acid or in ammoniacal solution, used chiefly for determination of carbon monoxide, acetylene, or oxygen. Sulphuric acid (1) dilute, by which basic gases, as ammonia, etc., are absorbed; (2) concentrated, by which defiant gas and allied compounds can be estimated. Bromine can also be employed for these latter, while nitric acid is used to absorb vapours of benzene. Besides these, alcohol, lead acetate, and other substances are used in special cases. Many gases, however, containing carbon, oxygen, hydrogen, and nitrogen, cannot be estimated by any of the preceding, and these are then determined by exploding with a known and sufficiently large volume of oxygen in a graduated tube known as an eudiometer. From the volume of gas before and after explosion, and also the quantity of carbonic acid, C02 (determined as above), and of aqueous vapour (found by heating the tube to 100° C.) formed, the quantities of all four constituents in the original gas can be determined. For the determination of a single constituent of gases, special methods are frequently employed - e.g. as by absorption in some liquid, and afterwards analysing the solution obtained.