A Convenient Absorption and Titration Flask for Carbon Dioxide Determination ROBERT GARDNER, Colorado Agricultural College Experiment Station, Fort Collins, Colo.
A
PRECISE method of determining carbon dioxide in air has been described by Thomas (2). A modified form of
the absorption unit described, which has been found very useful for a variety of carbon dioxide determinations, consists of a 250-cc. Erlenmeyer flask with a side arm drawn down at the end for tube connection and a fritted glass disk sealed in the tube at the point where it begins t o taper. The flask was made by blowing a hole in the end of a test tube, sealing on a small piece of tubing, and fusing a porous disk at A (Figure 1). The tube was then sealed into the side of the flask. The disk was made of 100- to 200-mesh glass and was prepared by the method described by Bruce and,Bent (1). To operate, 25 to 50 cc. of the absorbing solution with a few drops of normal butyl alcohol are placed in the flask, the side arm is attached by a rubber connection to the source of gas to be analyzed, and suction applied through a tube in the stopper. As soon as suction is applied, the flask is rotated so that the side arm is nearly vertical. Before the suction is removed and the pressure equalized, the side arm is brought to a horizontal position to prevent liquid passing through the disk. The contents are titrated through a tube in the stopper. For precise work, carbon dioxide is removed before starting a determination by filling with water and then replacing the water with carbon dioxide-free air through the side arm. Usually, a correction for the initial carbon dioxide in the flask, made from a blank determination, is sufficiently accurate. Tenth normal sodium hydroxide is satisfactory as an absorbing solution. When absorption is complete, an excess of 10 per cent barium chloride solution is added through a tube in the stopper and the excess base titrated against 0.1 N hydrochloric acid. It is necessary to clean the porous disk occasionally with hydrochloric acid to prevent clogging with barium Carbonate. The contents of the flask may be protected f r o m c o n t a c t with outside air by i n t r o d u c i n g a l l solutionk through a rubber tube in the sto per, which is kept closed by a clamp except wiile the solutions are being introduced. Barium hydroxide
FIGURE
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FIGURE 3. ASSEMBLYFOR DETERMINING TOTAL CARBON is not suitable for an absorbent if much carbL.1 dioxide is present, because t h e b a r i u m carbonate clogs the pores in the disk, but barium chloride may be added before titration. The equipment as assembled in Figure 1 has been used to determine carbon dioxide in the soil air and to measure the concentration of carbon dioxide in desiccators over a number of materials, including soil and cereals. When measuring carbon dioxide in the soil air, the side arm was connected directly to a tube inserted into the soil. The device has been found very convenient in determining the carbonate and total carbon content of soils, particularly where the carbon content is low. Figure 2 shows the carbonate apparatus in operation,
The sample to be analyzed for carbonate is weighed into a wide-mouthed Erlenmeyer flask which is connected to a source of carbon dioxide-free air and through a condenser to the side arm of the absorber. Suction is applied by an aspirator pump. Acid is added to the soil through a large buret and the rate of gas flow regulated by the pump. Total carbon may be determined conveniently by connecting the flask as shown in Figure 3, instead of the usual absorption train, to a combustion tube containing the sample. Ti2 . APPARATUS IN trating the contents of the flask takes no more OPERATION time than weighing the absorption bulbs and is more accurate for small amounts of carbon. When determining total carbon in soil the sample is mixed intimately with manganese dioxide in a boat and placed in a combustiontubeinthefurnace. Thegasesfrom the tube pass throu h an absorber containing granular zinc to remove acids from t8e oxidation of sulfur and nitrogen compounds, through a sulfuric acid bulb (not shown in the figure, usually may be dispensed with) to remove an ammonia, and then to the carbon dioxide absorption flask. stream of carbon dioxide-free oxygen is passed through the system during the determination and the furnace is run a t about 900' C. About 10 minutes are required for a determination.
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FIGURE 1. ASSEMBLED EQUIPMENT
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INDUSTRIAL AND ENGINEERING CHEMISTRY
If the method is to be used for determining carbon in a variety of compounds, the chain preceding the carbon dioxide absorber would depend, of course, upon the composition of the material being studied. The zinc absorber does not remove nitric acid completely, but the error resulting is usually small with soils and, if necessary, can be corrected by a nitrate determination of the residue in the flask after titrating.
The extremely small bubbles produced by the porous glass disk when the surface tension of the absorbing liquid has been lowered with a higher alcohol make absorption very rapid. The author's results confirm the data presented by Thomas
VOL. 7, NO. 6
(2) which show that several hundred nlilliliters of air per Of absorber and get very high absorption.
minute can be passed through this type
Literature Cited (1) Bruce, W. F.,and Bent, H. E., J. Am. Chem. SOC.,53, 990 (1931). (2) Thomas, M.D., ISD. Esc. CHEM.,Anal. Ed., 5,193 (1933). RECEIVED June 26, 1935.
Recent Developments in Platinum
Thermocouples BERT BRENNER, Sigmund Cohn, 44 Gold St., X e w York, N. Y.
I T H the use of the National Bureau of Standards 1933 reference tables (3)for platinum to platinum-rhodium thermocouples, the deviation curves obtained for individual couples have no points of inflection and are mostly linear. The improvement in these tables over those previously used is found principally a t the higher temperatures, and particularlyabove 1200" C. (2200" F.), The accuracy of temperature measurements obtainable with such couples is now limited by the quality of the thermocouple wires and (unless corrections based upon an individual calibration of the actual couple used are applied to each reading) upon the limits within which the manufacturer can keep the deviations of the e. m. f. of his couples from the standard values. This situation seemed to make closer tolerances desirable. I n the past few years definite improvements have been made in the manufacture of platinum-rhodium thermocouples, increasing their effective life, their resistance to deterioration a t high temperature, their homogeneity, and their reproducibility. The following data are presented to indicate what tolerances it is possible for a manufacturer of rare metal thermocouple wire to meet. I n 1934, this company adopted shop specifications with tolerances much closer than had ever been used for platinum and platinum-rhodium, and has been able to adhere rigidly to these specifications. While they may not represent the ultimate tolerances which might be met in special cases, they are significant in that they indicate the quality (as demonstrated by actual experience) which a manufacturer may be reasonably asked to maintain in the general run of his product.
General Considerations Since the first effect in service is, in general, a lowering of the e. m. f. of a platinum-rhodium thermocouple, it is obvious that a couple which originally yields an e. m. f. lower than the standard will, in service, gradually depart further. It is, therefore, important to allow only positive deviations from the standard (N. B. S.) reference tables. Tests of high-grade thermocouples a t the National Bureau of Standards by Neville in 1923 (2) have shown that during a service test of 25 hours a t 1500" to 1600"C., the e. ni. f. (measured a t 1200°C.) of the platinum element decreased 5.5 microvolts while that of the alloy element decreased 14.5 microvolts. The e. m. f . of the couple consequently decreased 9 microvolts a t 1200" C. Neville also found that the corresponding decrease in the e. m. f. of thermocouples then made commercially was from 4 to 12
times as great (36 to 120 microvolts). The author has observed an average drop in the e. m. f . in this company's thermocouples a t 1200" C. of less than 10 microvolts (less than 1" C.) after 36 hours a t 1500" C. (uniform electrical heating in air). The ability of the best thermocouples to retain their original characteristics when subject to high temperatures furnishes a basis for deciding how far it is worth while to go in tightening up on specifications. The limit is indicated as about * 1" C. Upon this basis and keeping in mind the fact that originally negative deviations from the standard values should not be permitted, a tolerance of +2" to 0" C. a t 1200" C. as the maximum deviations from the 1933 reference tables will insure the utmost that can be obtained from platinumrhodium thermocouples. These tolerances, therefore, form the basis for the essential requirements and shop specifications adopted. The fact t h a t they are rigidly adhered to demonstrates that manufacturers may be reasonably expected to meet such close specifications.
Essential Requirements PURITY OF COMPONENT METALS. The purity of the elements is of the utmost importance. The platinum, drawn into wire and electrically annealed for 3 minutes a t 600" C., must have an e. m. f. of less than 10 microvolts against Pt 27 (the N. B.S. pIatinum standard) a t 1200" C., and a temperature Rioo - Ro coefficient for the fundamental interval 0" to 100" C., of 0.00392 or higher. (Table 11.) This corresponds ( I ) to a purity of between 99.999 to 99.9999 per cent. Rhodium must have a temperature coefficient of resistance between 0" and 100" C. of 0.00434 or higher. The temperature coefficient of electrical resistance is the most reliable test for purity. PREPARATION OF WIRE. The platinum and the platinumrhodium are melted in selected pure lime, following the usual procedure, using an oxy-hydrogen blow torch (with platinum tip), with strongly oxidizing flame. They are cast into ingots, forged, rolled, swaged, and finally drawn through diamond dies to size. The utmost care must be taken throughout the entire procedure to keep the material absolutely clean if contamination is to be avoided. The finished wire must be sound, uniform, smooth, and round. The maximum tolerance in size a t finish is *0.0002 inch. INTERCHANGEABILITY AND REPRODUCIBILITY. Platinum can be made to meet closer specifications for interchangea-
