t
ANALYTICAL CHEMISTRY
$62
LITERATURE CITED
The temperature of the sample has no detrimental effect upon
the measurement, provided it remains above the dew point of the sample and below the temperature of the cell wall. Long-time stability of calibration can be achieved. Calibration check can be made by using appropriate gas mixtures stored under pressure. No cooling system is required as an integral part of the measurement, in contrast to the condensation method of determining dew point. Maintenance of wick or spray devices is eliminated.
Daynes, H. A,, Proc. P h y s . SOC.London, 34, lxxxix (1921-22). Daynes, H. A., “Gas Analysis by Measurement of Thermal Conductivity,” p. 23, London, Cambridge Press. 1933. Gross, H., and Schmick, H., Wiss. Vera@. Siemens-Kontern, 7.
The extensive industrial and laboratory use of the thermal conductivity method of gas analysis for many years is generally indicative of its broad possibilities. However, relatively few applications have been reported for the determination of water vapor concentration and the material presented in this paper is intended to aid in the evaluation of this method in terms of any specific problem of humidity measurement.
Rosecrans, C. Z., IND. ENQ.CHEM.,ANAL.ED.,2, 129 (1930). Shakespear,G. A., Proc. Phys. Soc. London, 34, lxxxviii (1921-22) Walker, A. C., and Ernst, E. J., IND.EN^. CHEM., ANAL.ED.,2,
1, 202 (1928).
Keyes, F. G., and Smith, L. B., Refrig. Eng., 27, 129 (1934). Leone, 0. J.,Iron Steel Engr., 23, 112 (1946). Palmer, P. E., and Weaver, E. R., Natl. Bur. Standards, Tech Paper 249 (1924).
134 (1930). Weber, S..Ann. Physik, 54, 481 (1917). RECEIVED December 6, 1947. Presented before the Division of Anslytioal and Micro Chemistry a t the 112th hleeting of the BMERICAN CHEMICAL Piew York. N. Y. SOCIETY,
Gas Thermometer for Automatic Control of low Temperatures Application to Separation of Gases by Isothermal Distillation D. D. TUNNICLIFF, Shell Development Company, Emeryville, Calif#
A gas thermometer is described for the automatic control of temperatures down to -180’ C. as required in the separation of gases by isothermal distillation. The helium-filled reservoir of the thermometer is located in a copper block which surrounds the distilling tube. A constant heat loss from the copper block and the distilling tube to liquid air is offset by intermittent heating which is controlled through a relay by the position of the mercury meniscus in the manometer. The thermometer can be preset to control the temperature at any desired point by proper adjustment of the volume of mercury in the manometer. The temperature is controlled to approximately 10.3’ C. The apparatus is simple to construct and is easy to operate.
T
HE separation of gases a t low temperatures and pressures by isothermal distillation as described by Shepherd ( d ) , Ward (6),and hvelli, Seyfried, and Filbert (8) requires the use of distillation temperatures in the range from -75’ C. down to the boiling point of liquid air. Usually the distilling tube is placed inside a copper block which has a small electrical heater wound on the outside. The copper block is placed in a test tube immersed in liquid air to provide a small but constant heat loss. The temperature is maintained constant by manually adjusting the current through the heater so as just to offset the heat loss to the liquid air. The method described by Ward (6) and Savelli, Seyfried, and Filbert (8) uses a series of four distilling tubes maintained a t progressively lower temperatures. The temperature of each distilling tube is increased after each fraction has been removed. In applying this method, the initial adjustment and the maintenance of the temperatures a t the prescribed values with manual controls were found to be rather difficult. The gas thermometer described in this paper provides a simple means for both the adjustment and automatic control of these temperatures. APPARATUS
A schematic diagram of the temperature control unit developed for this purpose is shown in Figure l. Briefly, the operation of this control unit is as follows:
A gas reservoir which is filled with helium and is connected through a small tube to a manometer is maintained in close thermal contact with the distilling tube. Changes in temperature of
helium in the reservoir cause corresponding changes in the position of the mercury in the manometer. These changes actuate a relay through suitable electrical contact points sealed into the manometer. The relay cuts off and turns on the current through a small electrical heater which is also in thermal contact with the distilling tube. The temperature control is effected by balancing thiF intermittent heat input against a constant heat loss to liquid air. The temperature maintained by the unit can be adjusted by varying the volume of mercury in the manometer. The close thermal contact required between the distilling tube, the helium reservoir, and the electrical heater is secured by surrounding the distilling tube with a heavy copper block; the heater is wound around the outside of the block and a cavity in the walls of the block serves as the helium reservoir. The distilling tube fits into a hole inch (1.6 cm.) in diameter, drilled down the center of the block. A little carbon tetrachloride is added around the distilling tube to improve the thermal contact. The nine vertical holes 6/1a inch in diameter drilled into the walls of the block serve as the helium reservoir. The holes are all connected by a channel cut around the top of the block; the gas s ace is sealed off by soldering a ring into the top of this channef The volume of the resulting reservoir is approximately 53 ml. The gas connection to the reservoir is made through a length of hypodermic tubing 0.6 mm. in inside diameter soldered into a cop er plug which is in turn soldered into the top of the block, This ppug is used to provide an opening sufficiently large to permlt filling the reservoir with water for the purpose of determining the volume before final assembly. The manometer as shown in Figure 1 is constructed of glass tubing 6 mm. in inside diameter and has two tungsten contacts sealed into the right leg. The lower contact is located near the bottom of the manometer, where it is always immersed in the mercury. The upper contact is located approximately 30 em. above the bottom of the manometer. A check valve is located
V O L U M E 20, NO. 10, O C T O B E R 1 9 4 8
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just above this contact to prevent mercury from accidentally entering the copper block. The gas volume between the upper contact point and the hypodermic tubing to the reservoir is ke t as small as possible. I n the four thermometers constructel this space varied from 0.5 to 1.0 ml. The left leg of the manometer has an over-all length of approximately 110 cm. The top of this leg is connected to a vacuum pump or alternatively to a supply of helium. A 120' type, &way stopcock is placed in the left leg 2 to 5 em. above the bottom and is connected through a needle valve to a steel mercury reservoir. By means of vacuum or by air pressure above the mercury in the reservoir, i t is possible to change independently the position of the mercury meniscus in either leg of the manometer. When four distilling tubes are used! the manometers are all connected to the same mercury reservoir. The two tungsten contacts are connected through a source of 6 volts direct current to a relay which is arranged so as to turn off the current through the heater wound around the copper block when the electrical connection is broken at the upper tungsten contact, and vice versa. A small rheostat is placed in the heater circuit for the purpose of regulating the current. I t was found to be essential for proper operation to fill the manometer with a caustic solution and to pass a current between the two tungsten contacts from a 6-volt alternating current source sufficiently long to remove the oxide coating from the tungsten. The volume of the gas reservoir in the copper block and the volume of the gas space in the manometer must be determined before or during the assembly of the apparatus. The volume of the reservoir in the copper block was determined from the weight of water required to fill this space. The manometer volume was taken as the gas volume between the reservoir in tho copper blocb and the nicrcwiy meniscus in the right leg of the manoineter when the meniscus i. just touching the upper tungsten contact. The volume of the hypodermic tubing and the volume of the glass capillary on the manometer were calculated from their physical dimensions. The volume between the mercury meniscus and the capillarv was determined from the diameter of the left leg of the
nianometer and the lowering of the mercury in this leg required to move the nieniscus in the right leg from the tip of the tungsten contact to a pnsition up against the check valve a t the bottom of the capillary. Preparation of Apparatus. After assembly of the apparatus and determination of the two volumes as described above, the thermometer is ready for filling with helium. All mercury in the manometer is withdrawn first to permit evacuating the helium reservoir through the left leg of the manomet'er. Rlercury below the 3-wap stopcock which cannot be drained out to the mercury reservoir is removed by suction through a small tube inserted through the stopcock after removing the plug. After the entire apparatus has been evacuated, including the mwir in the copper block and the connect,ion between the 3. stopcock and the mercury reservoir, the reservoir in the copper block is filled through the manometer with pure helium to a prwsure of 600 to 650 mm. The helium is confined in the thermometer by fillinx the manometer with mercury through the 3-ivay stopcock. T h e upper part of the left leg of the manometer IS then evacuated. 'l?eniperatures are calculated from a slightly simplified form of the equation used by Southard and Milner (6). The siniplification consists in assuming that all the connecting tubing between the helium reservoir and the manometer is a t the temperature of the gas in the manomet.er. I n view of the very sinall volume of the gas in the connecting tubing, which is at a temperature intermediate between the temperature of the manometer and the temperature of the reservoir, this simplification does not introduce a significant error i n thk application. The simplified equation is as follows:
,
n hole
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Scaling Ring
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High Vacuum
i -Hypodermic Tubing,
u
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Copper elOCk
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u DETAIL A
Air
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OPER4TIOh O F THE CONTROL UNIT
Reservoir
Figure 1.
1 , = abwlutr temperature of helium reservoir, i o = volume of reservoir in millili'lers as determined a t temperature il;, P = corrected pressure of helium in millimeters of mercury, C = constant determined by calibration a t any known temperature, a = cubical coefficient of expansion of copper, T,i = volume of gas space in manometer in milliliters (including volume in hypodermic tubing), and Tm = absolute temperature of T.,i The value of a, the cubical coefficient of expansion of copper, was taken as 0.000028; this value \\as calculated from the data of Buffington and Latinier (1). Constant C was calculated from , the pressure observed when the copper block was cooled to 0" C. In measuring this pressure it is essential that the volume of the niercury in the ma- , n o m e w be adjusted so that the meniscus in the right leg just touches the upper tungsten contact. , Each of the four thermometers constructed was , checked at the oxyqen point against an oxygen vapor preswre thekmometer ( 3 ) . Sone of the thermometers was in error by more than 0.5" C. Khen in iisc the thernionieters are checked periodically a t the oxygt'n point as a test for unssihle leaks.
Temperature Control Unit for Isothermal Distillation Apparatus
The temperature of the distilling tube is adjusted to any desired value by first calculating the corresponding pressure from Equation 1. The position of the mercury meniscus in the right leg is then adjusted just to touch the upper contact point; the nieniscus in the left leg is set a t a height above the meniscus in the right leg corresponding to the calculated pressure corrected to room temperature. The two legs of the manometer are then connected
964
ANALYTICAL CHEMISTRY
together through the 3-way stopcock. If the copper block is a t room temperature, the meniscus will fall in the right leg- and rise in the leftleg. The copper block is then cooled by direct immersion in liquid air until the meniscus in the right leg is back to a Dosition near the upper tungsten contact. Theliquild air is remdved, a glass test tube slipped over the copper block, and the test tube immersed in liquid air. The position of the mercury in the manometer then turns the heater current off and on, so as to maintain the meniscus near the contact point. The rheostat in the heater circuit and the depth of the liquid air around the test tube are adjusted so that the off and on periods for the heater are approximately equal in duration. Whe: roperly adjusted the temperature remains constant to *0.3 When it is desired to increase the temperature of the distil1in.g tube to the next value, the height of the mercury in the left leg is reset to the proper position with the mercury in the right leg still a t the tungsten contact. Without further attention the tpmperature quickly rises to the desired value and then is automatically maintained constant at this point.
For use as both a method of temperature measurement and temperature control the 120" type 3-way stopcock in the man. ometer should probably be replaced with a T-type stopcock. This type of stopcock has an advantage when measuring temperatures, as it permits adding mercury to both legs of the manometer simultaneously until the meniscus in the right leg just touches thP contact point. However, when the main application is the presetting of the control point a t some particular temperature and the control of the temperature a t this point, the 120" type s t o p cock is believed to be preferable. Although the location of the stopcock above the bottom of the manometer makes somewhat inconvenient the removal of all the mercury as required when filling the thermometer with helium, it has the advantage of permitting relubrication of this stopcock without loss of the helium from the reservoir.
DISCUSSION
(1) Buffington, R. M., and Latimer, TV. M., J . Am. Chem. SOC.,48, 2305 (1926). (2) Savelli, J. J., Seyfried, W. D., and Filbert, B. M.,IND.ENQ CHEM.,A N ~ LED., . 13, 868 (1941). (3) Scott, R. B., J . Research .VatZ. BUT. Standards, 25, 459 (1940). (4) Shepherd,M., Ibid., 2, 1145 (1929); 26, 227 (1941). (5) Southard, J. C., and Miluer, R. T., J . Am. Chem. SOC.,55, 4384 (1933). (6) Ward, E. C., IND.ENG.CEIEM.,ANAL.ED.,10, 169 (1938).
'
8
This control unit has been found to be a very satisfactory means of both adjusting and controlling the temperature required for
isothermal distillations a t low temperatures. The operation of the thermometer is very simple and it requires a minimum of attention from the operator. The apparatus should also be applicable to the control of low temperatures as required in other problems.
LITER4TURE CITED
RECEIVEDMaroh 1, 1948.
Quantitative Colorimetric Microdetermination of Methanol with Chromotropic Acid Reagent R. N. BOOS, Merck & Co., Znc., Rahway, N. J . .4 rapid, accurate, and specific method for the quantitative determination of
methanol is described in which the methanol is oxidized to formaldehyde and the latter measured colorimetrically with chromotropic acid. The method permits the determination of methanol with a relative error of
