CHAPTER XXII METHODS INVOLVING EVOLUTION AND MEASUREMENT OF GASES DETERMINATION OF CARBON DIOXIDE IN LIMESTONE. DETERMINATION OF 349. General Remarks. There are several very important determinations which are based upon the evolution and measurement of gases, for instance, the determination of carbon dioxide in limestone and in baking powder, of carbon in steel, of nitrogen in nitrates and nitrites, of available oxygen in hydrogen peroxide, etc. In such methods the gas is either collected by absorption in some suitable medium and the amount which is so collected found from the gain in weight of the absorbing medium, or the gas is collected in a graduated burette and its weight calculated from its volume, after making the necessary corrections for the temperature of the gas, the barometric pressure under which it is enclosed, and the vapor pressure of the confining liquid. For our purposes we shall consider only the case where the gas is collected in an absorbing medium and its weight determined from the gain in weight of the absorbing medium; as examples coming under this case we shall take up the determination of carbon dioxide in limestone and of carbon in steel. 350. Exercise No. 34. Determination of Carbon Dioxide in a Limestone. The composition of limestones has already been given in § 206. The method of determining the carbon dioxide consists in adding acid to the limestone in a closed system and sweeping out the evolved carbon dioxide by means of air (free from carbon dioxide) into absorption tubes that are filled with soda lime. 351. Apparatus and Assembly. These are illustrated in Fig. 48. A is a guard tube 10 cm. long by 1.8 cm. diameter, filled with soda lime. B is a dropping-funnel of about 100 c.c. capacity. C is a short-neck, wide-mouth, flat-bottom flask of about 150 c.c. capacity. D is a condenser of the Hopkins type. E, F, G, and H are glass-stoppered U-tubes with limbs about 12 cm. long by 2 cm. outside diameter, and equipped with glass braces to give them strength. E contains glass beads and just enough saturated silver sulphate solution in nine molar sulphuric acid to fill the bend in the tube. F contains anhydrous calcium chloride, while G and H are filled about three-quarters with soda lime and one-quarter with anhydrous calcium chloride. J is a guard tube similar to A, but filled with anhydrous calcium chloride. K is a bubble counter or gas bubble indicator containing 18 molar sulphuric acid to the depth of about 3 cm.; the inlet tube of K, which should have an inside diameter of 0.5-0.7 cm., should be immersed in the sulphuric acid to the depth of 1.6 cm. L is a side-neck suction-flask which is connected to a water pump and which serves as a water trap should the pump flow back. 352. Filling the Various Members of the Gas Train and Assembling the Apparatus. The guard tube A which is to remove carbon dioxide from the air drawn through the apparatus is filled as follows: hold the tube with its wide mouth on top and place upon the bottom of the bulb a small loose wad of cotton in order to keep any soda lime from falling through the narrow outlet tube and then place on top of the cotton sufficient soda lime (12 mesh) containing about 15% moisture to within about 2 cm. of the top. Fill the remaining space with cotton and hold the cotton in place by using a two-hole rubber stopper, 0.5 cm. thick. Tube J is filled in the same manner with anhydrous calcium chloride (8 mesh) which has been treated with carbon dioxide as described below, except that the wide end of the tube is fitted with a onehole rubber stopper containing a short glass tube. The tube J is employed between H and K because if H and K were directly connected to each other, water in the form of vapor would be abstracted from the anhydrous calcium chloride in H by the sulphuric acid in K because 18 molar sulphuric acid has a much lower aqueous vapor tension than anhydrous calcium chloride. It is always safer to protect an absorption train by the same drying agent as that used in the train. To the tube E add enough glass beads to fill the bend in the tube and add sufficient saturated silver sulphate solution in nine molar sulphuric acid to cover the beads. This solution is conveniently introduced by means of a small funnel. In the hollow part of the exit stopper place a wad of glass wool or of cotton. This serves to catch any spray which may form. Now add a good grade of stop-cock lubricant to the glass stoppers, sufficient to coat the ground surfaces and to make the joints gas tight. Wipe away any excess lubricant. This tube is designed to remove any hydrogen chloride which escapes the condenser. Tube F is filled with calcium chloride (8 mesh) to catch the water vapor which is carried over from E. Since many samples of anhydrous calcium chloride are alkaline, due to the presence of some calcium oxide, the calcium chloride must first be treated with a dry stream of carbon dioxide and the excess displaced by using dry air. Sufficient of the treated calcium chloride is introduced into the tube by means of a short wide-stem funnel to fill the bend. This layer is now covered on each side by a loose wad of cotton about 0.5 cm. thick and some more calcium chloride is added to within 1 cm. of the bottom of the glass stopper. This intervening space is filled with a loose wad of cotton. The hollow exit stopper is charged with cotton to retain any small particles which might be carried out of the tube by the moving gas into its neighbor. The cotton wads are used in the calcium chloride in order to prevent any channeling by the gas. The ground surfaces are now given a thin coating of stop-cock lubricant sufficient to seal the joints and any excess removed by a cloth. The tubes G and H must be carefully prepared since they are to absorb the carbon dioxide liberated in the reaction. These tubes are weighed separately in each determination, as well as in each blank, tube H serving as a guard tube which indicates by an increase in its weight when tube G has absorbed its maximum amount of carbon dioxide. Both tubes contain the same reagents and are filled in the same manner. The bend of each tube is filled with soda lime (12 mesh) containing 15% moisture. A wad of cotton 0.5 cm. thick is now placed in each limb over the soda lime; then in the entry limb of the tube, soda lime is placed upon the cotton until the soda lime is within 1 cm. of the bottom of the glass stopper, whereupon the remaining space is filled with cotton; while in the exit end of the tube, soda lime is placed upon the cotton to a depth of about 2.5 cm., then a wad of cotton is placed on the soda lime, and above the cotton is placed the anhydrous calcium chloride (8 mesh) to within 1 cm. of the bottom of the glass stopper, whereupon the remaining space is filled with cotton. The hollow exit stopper is also filled with cotton. The ground surfaces are coated with a thin film of stop-cock lubricant and any excess removed. Be sure that the exposed top of the stoppers are wiped free of it, otherwise it may be wiped away prior to weighing at the end of the determination. The calcium chloride is placed in the upper part of the limbs of the tubes because the soda lime contains moisture to the extent of 15%. Anhydrous soda lime cannot be used because it absorbs carbon dioxide slowly and incompletely.! The bubble counter K contains 18 molar sulphuric acid to the depth of about 3 cm., the inlet tube of K being immersed to the depth of 1.6 cm. All rubber stoppers that are used should be of the very best grade of rubber, while the rubber tubing used for connecting the tubes should be of the heavy-walled pressure kind. Tubing 1.5 cm. outside diameter with 0.3 cm. bore is | satisfactory. The connecting pieces should be about 4 cms. long. 353. Procedure. Connect the apparatus as illustrated in Fig. 48, and test it for any leaks as follows: close the stop-cock, which of course is lubricated, on the dropping funnel and open all the other stop-cocks so that they are in the "running" position. Turn on the water pump and draw a slight vacuum on the system, then close the stop-cock s while the pump is still running. After s is closed shut off the pump. The sulphuric acid in the bubble counter K will be depressed in the inner tube and elevated in the outer. If at the end of three minutes the meniscus in the inner tube has remained at the same position, the apparatus may be considered air-tight. If the levels should become equal, then the apparatus is leaking and each joint should be wired and coated with collodion. Do not draw too great a vacuum because the tubes and flasks are not designed to withstand great pressures and might shatter if care is not taken in this respect. 354. The Blank. When the apparatus is found to be airtight, turn all the stop-cocks in a "running position" and turn on the water pump so that the bubbles passing through the counter 1 See Lamb, Wilson and Chaney, "Gas Masks Absorbents," J. Ind. & Eng. Ch. 11, 437 (1919). |