Confining Liquids for Gas Analysis Solubility of Carbon Dioxide in Salt Solutions KENNETHA. KOBEAND JOHN S. WILLIAMS Department of Chemical Engineering, University of Washington, Seattle, Wash.
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HE use of mercury as a confining liquid in the buret for gas analysis is recognized as best practice for accurate work, as all common gases analyzed in the usual apparatus are insoluble in and do not react with mercury. However, the cost and great density of mercury are two disadvantages that make it common practice to substitute an aqueous solution for mercury in routine work.
allowance for any water of crystallization. The per cent salt or acid content is weight per cent, except where volume per cent is noted for acids. The solution was heated t o boiling to remove dissolved gases, care being taken t o condense and return all water vapor evolved, and the flask was stoppered tightly and cooled to 25" C.
The two most commonly used confining solutions are saturated sodium chloride with 2 volume per cent hydrochloric acid, and 20 per cent sodium sulfate with 5 volume per cent sulfuric acid, the solutions being colored with methyl orange (7). Hoffmann ( 1 ) investigated 19 solutions by determining the solubility of a one to one carbon dioxide-air mixture. A number of the salts used by him give alkaline solutions and several concentrations of Nome salts were used, so that his conclusion that a 22 per cent sodium chloride solution is best is based on a study of very few practical solutions. Hoflmann further states (8) that there is no advantage in using acid in the confining liquid but, on the contrary, its use is likely to cause error, since he found an increase in the solubility of his mixed gas when acid was added to a saturated solution of sodium chloride. Tropsch (6) points out that the addition of a little sulfuric acid is of advantage to prevent the liquid from becoming alkaline. Passauer (6) determined the solubility 01 pure carbon dioxide a t 20" C. in 17 different saturated salt solutions. He found that sodium dichromate dissolved the least gas, but it had the disadvantage of being a very viscous solution. Silver nitrate solution was the next best and gave a solution but slightly VISCOUS. Some of these data were checked by other workers (4). Wolf and Krause (8) determined the solubility of carbon dioxide in water and in solutions of sodium chloride and calcium chloride. They found that acidulation with sulfuric or hydrochloric acid was useless or harmful, except for alkaline waters. They recommend a nearly saturated sodium chloride solution. It was believed that an investigation of solubility of carbon dioxide in the usual confining solutions, in solutions of inexpensive salts, and in acid solutions would give data to enable the selection of the most practical c o n h i n g solution.
FIGURE1. DIAGRAM OF APPARATUS Pure carbon dioxide was selected as the gas t o be used, as it has the greatest solubility of the gases commonly encmntered in ordinary work. It is believed that the data secured in this way have more value than when gaseous mixtures are used. The carbon dioxide was first saturated with water vapor by passing it through a Friedrichs type gas-washing bottle a t 25" C. containingsolutions being tested. The gas was introduced to A a t C, 100 ml. of the saturated gas being taken into A . Fifty milliliters of the solution were pipetted into bulb F , over mercury. By lowering the mercury reservoir, K , the solution was drawn into bulb HI 0.46 ml. of solution remaining in the stem which connected F with H. The carbon dioxide in A was then introduced into H and the latter shaken for 15 minutes. This period of shaking was found to be sufficient to obtain equilibrium between the solution and the carbon dioxide. The pressure in H was adjusted by E with the barometric pressure, so that the total pressure in H was 783.5 mm. Thus, for pure water at 25" C., the partial pressure of the carbon dioxide is 760 mm. Solutions were assumed to have the same vapor pressure as pure water, as the vapor pressure lowering by the salts is negligible for this work. The pressure in H during the shaking was adjusted by raising K so that the correct reading on E was maintained. After the shaking period the carbon dioxide was flushed back to A and measured.
EXPERIMENTAL The apparatus used for this work is shown in Figure 1, A buret of the mine-air type, A , is used, as this gives greater accuracy in the range 75 to 100 ml. Mercury is used as the confining liquid in A and in absorption bulb HI both being waterjacketed and maintained at 25' C. by a jet of warm air. Buret A and pressure compensator B, of standard type (7), are connected to absorption bulb H , which is a 200-cc. balloon flask with a bottom tube attached to a leveling bulb, K , containing mercury. Passing through the rubber stopper, G, in the neck of bulb H a r e three capillary tubes. The first is from the dropping funnel, F , through which the solution is introduced into H; the second is the capillary line, D,from the buret. and the third is from a small mercury manometer, E, by which a known pressure can be maintained above the solution in H. The top of the gas bulb is sealed with a cement to prevent leakage around the tubes or rubber stopper G. Since the liquids used have little if any effect on rubber, use of rubber is not objectionable. The neck of H is clam ed into a shaking mechanism which tilts the bulb about 20" ea& side of vertical, while the bulb is in the constant-temperature bath I at all times. The shaker is operated by a motor, reducing gears, and an eccentric (not shown) which turns at 100 r. p. m. The capillary line connecting the buret to the absorption bulb contains a short piece of capillary rubber tubing, D, which allows the bulb to be shaken. The solutions were made u p by dissolving the accurately weighed amount of analytical grade salt, calculated as anhydrous salt, into the weighed amount of water, making
Results are shown as milliliters of carbon dioxide at 25" C. and 760 mm. dissolved in 1 ml. of solution a t 25" C. The Bunsen absorption coefficient, a,which is the volume of carbon dioxide calculated to 0' C. and 760 mm. dissolved by 1 ml. of solution at 25" C., has also been calculated (3). The experimental data and results are shown in Table I. The data given in Table I are the average of two determinations, accurate to 0.005 in the values of the solubility. The experimental value, a = 0.754, for distilled water at 25" C. agrem very well with the International Critical Tables (3)value of Q: = 0.756. 37
INDUSTRIAL AND ENGINEERING CHEMISTRY
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TABLEI. SOLUBILITY OF CARBON DIOXIDE IN SALT SOLUTION SA~T
(Temperature 25O C., total pressure 783.5 mm.) CONVOLUMEOF CARBON DIOXIDE BUNSEN
USED
OlNTRATION
WeQht % None Has04 HB0r NaCl NaCl NaCl His04 NaCl -*m. NrtCl nu NaaSOc NaaSOc Ha604 NaaPOd HsPO4 CaClz MgCL ZnCh AlCla A h (804)I 0 Milliliters of 25' C.
*.
5 10 10 20 20 5
SOLVTION
DIBSOLVED
M1.
M1.
24.54 24.54 24.54 24.54 49.54 49.54
20.20 18.30 17.60 13.52 16.54 16.29
COEFFICIENT
Ml./ml.a soh. 0.823 0.746 0.717 0.551 0.334 0.328
25 49.54 12.75 49.54 17.48 20 1 20 49.54 12.9 20 13.07 49.54 5 (vol.) 13.89 10 49.54 7 49.54 7.82 40 30 49.54 8.12 19.12 50 49.54 25 49.54 12.00 20 49.54 10.96 COS at 25" C.,760 mm., dissolved
0.754 0.683 0.657 0.505 0.306 0.302
0.257 0.352
0.236 0.323
0.260 0.263
0.237 0.242
0.280
0.256
0.158 0.164 0.386 0.243 0.221 per ml. of
0.144 0.150 0.354 0.222 0.203 solution at
DISCUSSIONOF RESULTS From Table I it is seen that the concentrated solution of calcium chloride dissolves the least carbon dioxide. However, this has three disadvantages as a confining liquid: the solution is quite viscous, it seems to become easily contaminated with impurities and precipitate, and when spilled it does not evaporate and leave crystals but remains as a sticky fluid. The solution may be satisfactory for a confining liquid in gas storage containers, but does not seem suitable for use in the gas analysis apparatus. The magnesium chloride solution is much less viscous than the calcium chloride solution. The solubility of carbon dioxide is almost equal in the calcium and magnesium chloride solutions, but is much lower than for the sodium chloride or sodium sulfate soIutions. Its use in the gas analysis apparatus has the disadvantage that whenever the concentrated
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hydroxide absorbents come into contact with the magnesium chloride, even though acidified, a precipitate of magnesium hydroxide is formed, which plugs up the capillary tube. The use of magnesium chloride is recommended when this contact with alkalies is not possible. The solubilities in 20 per cent sodium sulfate and 25 per cent sodium chloride are nearly equal; however, the 25 per cent sodium chloride is more nearly saturated than the 20 per cent sodium sulfate, so the latter is recommended. The addition of 5 per cent by weight of sulfuric acid to 20 per cent sodium chloride and 5 per cent by volume of sulfuric acid to 20 per cent sodium sulfate has been shown to produce a negligible change in the solubility of carbon dioxide in these solutions. As a confining solution must never be alkaline, the addition of acid is necessary, and no error is introduced as suggested by Hoffmann (2). CONCLUSIONS
A 20 per cent by weight sodium sulfate and 5 per cent by volume sulfuric acid solution is recommended as the most practical confining liquid for use in technical gas analysis equipment. The addition of acid to sodium sulfate or sodium chloride solutions causes no appreciable increase in the solubility of carbon dioxide, and is necessary to prevent the solution from becoming alkaline a t any time. LITERATURE CITED (1) Hoffmann, F. G., Feuermngstech., 14,98(1926). (2) Hoffmann, F.G . ,Z . angew. Chem., 39,23(1926).
(3) International Critical Tables, Vol. 3,pp. 260,279. (4) Nahocsy, A.,Bdnydsz. KoMsz. Lapok, 66,332(1933). (5) Passauer, H., Feuermngstech., 19, 142 (1931). (6) Tropsch, H., 2. angew. Chem., 39,401 (1926). (7) U. S. Steel Corp., "Methods for Sampling and Analysis of Gases," Pittsburgh, Carnegie Steel Co., 1927. (8) Wolf, O.,and Krause, Arch. Wtirmewirt., 8, 216 (1927). RECEIVED November 5, 1934.
Determination of Chloride A Modification of the Volhard Method JOHN R. CALDWELL AND HARVEY V. MOYER,Ohio State University, Columbus, Ohio HE Volhard method for the determination of chloride nate. Kolthoff (3) suggests the subtraction of 0.7 per cent of has been subjected to many modifications. Kolthoff the percentage of chloride found in order to correct for adsorp(8) in a review of the subject points out the fundamental tion of silver nitrate on the silver chloride and on the silver errors in several of the proposed methods and concludes that thiocyanate. In this paper a modification is proposed which only one, the method of Schoorl (4,gives accurate results. reduces the errors mentioned above to negligible values and This procedure is somewhat tedious, and since it requires spe- considerably shortens the time required for a determination. cial precautions it is limited in application. The method in Repeated trials in the hands of several operators have given general use at the present time requires the removal of the accurate results with errors no greater than the probable erprecipitated silver chloride by filtration before back-titrating rors in reading the volumetric apparatus. In view of the successful use by Caldwell of the addition with potassium thiocyanate. A modification is desirable which eliminates this filtration. Recently Stschigol (6) has of an organic substance to improve the end point in the iodoproposed covering the chloride solution with a layer of toluene metric determination of copper (I), this principIe was applied or benzene. These immiscible liquids cause the silver chloride to the Volhard titration. It was found that nitrobenzene had to be drawn to the interface and thus remove it from the the desired properties. The experimental evidence seems to aqueous solution. This principle was suggested earlier by indicate that in the presence of nitrobenzene no appreciable Rothmund and Burgstaller (4, but the method has certain amount of silver nitrate is carried down and that the nitrodisadvantages as shown by Kolthoff (5). Furthermore, the benzene forms an insoluble layer over the precipitate, so that authors have observed that the end point is partially ob- the rate of solution of the silver chloride is reduced to such an scured because the precipitated silver chloride turns dark extent that it does not interfere in the thiocyanate titration. more rapidly in the presence of these organic liquids. EXPERIMENTAL The principal objection to the Volhard method is the fading end point when the silver chloride is not removed, due to the SILVERNITRATE. A 0.1 N solution of silver nitrate was fact that silver chloride is more soluble than silver thiocya- prepared by dissolving the appropriate weight of reagent silver
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