ANALYTICAL EDITION
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CONCLUSIOWS The conclusions to be reached from digesting the results of these analyses are: 1. The A. P. H. A. and Winkler methods give reliable hydrate content in the absence of high silica and organic matter. I n the presence of organic matter the Winkler method appears to be the most reliable. 2, The determination of hydrate from the pH value gives erroneous results. 3. The carbonate determined by the A. p. H. A. method is accurate in the absence of organic matter. However, in boiler waters having the sodium carbonate below 50 p. p. m. this method is not reliable. 4. The determination Of carbonate using the short equilibrium method gave results which averaged about the same as those obtained by the A. P. H. A. method. 5. The carbonate determined by the carbon dioxide evolution method gave the most reliable results for carbonate.
Vol. 4, No. 3
LITERATURE CITED (1) American Public Health Association, “Standard Methods of Analyses,” New York, 1925. (2) Britton, H. T., “Hydrogen Ions,” pp. 142-7, Van Nostrand, 1929. (3) Hecht, M. H., and McKinney, D. S., Power Plant Eng., 35, 602-4, 649-52 (1931). ( 4 ) Johnston, J., J. Am. Chem. Soc., 38,939 (1916). (5) McKinney, D. h D . E X G . CHEM.,Anal. Ed., 3, 192-7 (1931). (6) Parr, S. W., and Straub, F. G., Eng. Expt. Sta. of Univ. of Ill., Bull. 216, 7 (1930). (7) Treadwell and Hall, “Analytical Chemistry,” Vol. 11, p. 592, Wiley, 1915. (8) Treadwell and Hall, Ibid., Vol. 11, p. 587.
s.,
RBCEIVED September 10, 1931. Presented before the Division of Water, Sewage, and Sanitation Chemistry at the 82nd Meeting of the American Chemical Society, Buffalo, N. Y., August 31 t o September 4, 1931. Part of the research being oonduoted in cooperation with the Utilities Research Commission, Inc., of Chicago, 111. Published by permission of Director Engineering Experiment Station, University of Illinois.
An Inexpensive Low-Temperature Thermostat FRANK 0. LUNDSTROM AND COLINW. WHITTAKER Fertilizer and Fixed Nitrogen Investigations, Bureau of Chemistry and Soils, Washington,
T
HE low-temperature thermostat described in this paper,
although inexpensive to construct and to operate, has been found to be accurate and reliable. Liquid ammonia is used as the cooling agent for temperatures between 7 ” and -25” C. This substance is cheap, readily available, and because of its exceptionally large heat of vaporization per unit weight, cools economically. Ice is used to cool the bath for temperatures between 7 ” and room temperature. The method of applying the cooling agent differs from the usual practice in that the heat is conducted away from the bath to the cooling agent by means of a &Der rod soldered through the wall of the bath, which is constructed of copper. The cooling of the bath liquid is thus accomplished by transfer of heat to the container wall, rendering the use of a cooling coil or other device in the bath itself unnecessary.
DESCRIPTION OF APPARATUS The bath and its associated apparatus are illustrated diagrammatically in Figure 1. The bath itself consists of a copper tank, a, made by soldering one end of a section of copper tubing 7.5 inches (19.05 em.) long and 3.5 inches (8.89 cm.) inside diameter into a groove turned in a copper disk 4.5 inches (11.43 em.) in diameter. The walls and bottom of the bath are 0.125 inch (0.32 cm.) thick. A 1-inch (2.54-em.) hole is drilled in the wall of t h e b a t h 2.5 i n c h e s (6.35 c m . ) f r o m t h e t o p , a n d t h r o u g h this hole is soldered a 1-inch (2.54-em.) copper rod, b, so that it extends 0.5 inch (1.27 cm.) into the bath. The rod is bent
D. C.
downward 2.5 inches (6.35cm.) from the wall of the bath, passes through a cork, c, and extends down 8 inches (20.32 cm.) below the bend. The bath and that portion of the rod between i t and the cork are carefully lagged with 1.5 inches (3.81 cm.) of hair felt (not shown). The lagging is somewhat thinner between the rod and the bath to allow clearance for the vacuum bottle. Kerosene is used as the bath liquid. The bath is cooled continuously by inserting the copper rod into a 500-cc. silvered vacuum bottle, d, containing liquid ammonia, or into a beaker of ice and water. The cork, c, fits loosely into the neck of the bottle. Heat is sumlied intermittently by one or t; 21candle power automobile headlight bulbs, f and y, as conditions may require. The switches, o and o‘, are arranged to permit the use of one or both bulbs. The b u l b s , I I which are lighted by a standard 6-volt storage battery, are mounted with special clamps of brass strap, g and g’, which pass around the base of the bulb and are soldered to the 0.125-inch (0.32-cm.) copper rod, h, which passes through the cork, i, and forms one side of the lamp circuit. Copper wires, j and j ’ , are soldered to the other contact of the bulbs and pass through the cork to the insulating bracket, k . The lamp circuit is controlled by the 250-ohm relay, I, the primary circuit of which is operated by a conventional gas type thermoregulator which consists of a gas-filled 20-cc. bulb, m, connected by a small-bore capillary to a closed-end manometer of s l i g h t l y m o r e than barometric height, which was carefully boiled out after being filled with
July 15, 1.932
INDUSTRIAL AND ENGINEERING CHEMISTRY
pure mercury. Tungsten leads tipped with platinum are sealed through a t tl and tz. The circuit is made and broken by the rise and fall of the mercury in the capillary a t the tip of tl. The dead space above the mercury a t tl and in the connecting capillary is so small that variations in room temperature do not perceptibly affect the setting of the thermoregulator. The cross section at z-2, shown immediately below the bath in Figure 1, shows the arrangement of parts in the other plane. Most of the bath space is left free for other apparatus. Figure 2 is a photograph of the bath, vacuum bottle, and wooden stand used to support them. The bottle rests on a movable shelf which may be removed a t any time to permit lowering of the bottle. The picture shows the lagging and the method of securing it with metal straps ordinarily used for fastening lagging to steam pipes. The large cork, i, which closes the top of the bath and through which the lead wires, tubes, etc., pass, is bolted to a horizontal crosspiece (not shown) which forms part of the frame which supports the manometer and other equipment. The cork and other apparatus are thus supported independently of the bath which can be lowered to expose the apparatus by simply removing the stand. Because of the small size of the bath, no mechanical stirrer is used, but instead dried air is passed in through the tube p and allowed to bubble up through the bath liquid. This has been found quite satisfactory. Precooling of this air would make it possible to operate at somewhat lower temperatures with a given cooling agent. The operation of the apparatus is as follows: The bath is brought to the desired temperature by immersing the rod in liquid ammonia or ice while the cock, u, is open The height of the mercury in the manometer is then adjusted by means of the leveling bottle, s, until it makes contact a t t l , the cocks, u and w, are closed, and the heaters turned on. The bath will then hold the desired temperature. Additional ammonia or ice can be added at any time without interfering with the operation of the thermostat. I n cooling the bath initially to temperatures below 0" C., some time and liquid ammonia can be saved by precooling the bath liquid. This is done by immersing the kerosene in an ice bath before it is poured into the thermostat.
DISCUSSION The bath as described has been used only with liquid ammonia and ice, but there is no apparent reason why it could not be used equally well with other cooling media, for example, liquid air or carbon dioxide snow in acetone, and thus be made to operate at lower temperatures than are possible with liquid ammonia. The practical lower limit, using ammonia, is about -25" C., but the limit would not approach the temperatures of liquid air and carbon dioxide snow as closely as it does that of liquid ammonia, owing t o increased heat losses at lower temperatures, The bath has been operated at several temperatures between 0" and -25" C. using liquid ammonia, and a t 6.75", 9", and 19" C. using ice. The range between room temperature and -26" C. is thus readily covered with the two cooling agents, ice and liquid ammonia. S o change in the apparatus is required when cooling agents are changed. The highest practical operating temperature with any cooling medium is reached when the bath cools so rapidly that the heaters must operate almost continuously to maintain that temperature. With two bulbs in use, it is possible to operate a t higher temperatures with a given cooling agent, and by using one or two bulbs as necessary the heating and cooling periods can be made more nearly equal. The ammonia consumption is small, 400 cc. being sufficient
295
to cool the bath from 15' to -15" C. and maintain it a t the latter temperature for 5 hours. Starting with the bath already down to temperature, a single filling suffices for a day's run of about 7 hours. The apparatus is being used under a fume hood and the ammonia fumes have given no trouble. For operation in the open air the cork, c, should be made to fit tightly and a tube passed through it a t the side through which the ammonia f u m e s could be carried off as desired. Care must be exercised when placing the bottle of ammonia around the copper r o d , as t h e ammonia boils violently when it first touches the warm rod. The temperature change of the bath during its heating or cooling period was imperceptible on a pentane thermometer, and an attempt was made to follow it with a copper- c o n s t a n tan thermocouple. FIQURE 2. THERMOSTAT BATH AND STAND The c o u p l e was made of fine wire and was placed in direct contact with the bath liquid to reduce the lag as much as possible. The e. m. f. readings were made with a Leeds and Northrup Type K potentiometer and galvanometer to 1 microvolt. Although these readings could hardly have an absolute accuracy of 1 microvolt, if all errors affecting the instrument are assumed constant over the brief interval of a heating or cooling period, then the change in e. m. f. should be accurate to about 1 microvolt (0.027' C.). Of thirteen readings taken with the bath operating at -10" C., a readable difference was observed in only one case in which the e. m. f. changed 1.5 microvolts between the start and end of a heating period (67 seconds). Similar results were obtained with the bath operating at -19" C. Out of eleven determinations one readable difference of 5 microvolts was observed, all the others showing no perceptible variation between the start and end of a heating or cooling period. The two observations showing variation were probably due to some extraneous cause, especially since both showed a change of e. m. f. in the direction opposite to that which would be expected. The twenty-four determinations were made under a variety of conditions, with the vacuum bottle half filled with ammonia, with the bottle full, with one bulb in use, with both in use, with the thermocouple junction near the top, and with the junction near the bottom. The bath was explored with the thermocouple junction to determine whether adequate stirring was being obtained. Of five readings taken a t widely separated points in the bath, a variation of * 1 microvolt was observed, corresponding to a temperature variation of *0.03" C. Although this could perhaps be improved by mechanical stirring, it is entirely adequate for many purposes. I~ECEIVED February 24, 1932.
