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ANALYTICAL CHEMISTRY
glass disk, coarse porosity separates B from D. Compartment C contains saturated potashum chloride solution which is used to flush D from time to time. Compartment E, which contains the test solution, is separated from D by t~ Corning fineporosity sintered-glass disk. Resistance. The cell resistance was mesliured with an Industrial Instruments Model R G 1 6 conductivity bridge with a decade capacitor in parallel using 1000 cycles per second. The resistance was ea. 195 ohms when compartment E contained 0.1M potassium chloride and a diltinum foil electrode of 1 su. em. A second measurement, made through a dropping meriury electrode a t the instant of the drop fall, gave a value of ea. 265 o h m . The first value wm checked by a less sensitive oscillographic method with agreement of ea. 5%. Diffusion Experiment. Fifty milliliters of distilled water was allowed to stand in E for 21 hours with s slow stream of nitrogen passing through. At the end of this period, the chloride content of the water had increased to onlv 0.0064M. Therefore. over normal periods of time, the diffusioh of ions from D to E, dr vice versa, is negligible for most purposes. Moreover, C and D may he filled with an indifferent electrolyte in special cases where even traces of chloride ion must be excluded from the test solution.
container where electrical contacts might possibly touch, pieces of insulating plastic were cemented to the walls. The cost of t h e two basic items, the small heater and a control (rated a t 1 ampere and 115 volts) is from $10 to $17. For direct current operation, a
Electric Heater for Van Slyke-Folch Carbon Combustion Apparatus Figure 2.
1. V. Hankei, Medical Department, Brookhoven National Laboratory, Upton, 1. I., N. Y. N THE original Van
Separated P;arts of Heater Assembly
Box, cover. and plate to nhiob heatin unit ia attached am of mbest,os boar3
Slyke-Foloh manometric carbon combustion
I [Van Slyke, D. D., and Folch, J., J . B i d . C h a . , 136,5 0 9 4 1 (1940)l a micra gas burner is used a murce of heat. Air drafts as
sometimes ms?ne,the control of the heat difficult. An essily constructed electric heater; which provides more readily regulated heat and a temperature range adequate for the various steps of the process, simplifies the technique of the combustion. Figure 1 shows how the heater unit and power control appear when assembled an the Van Slyke machine for a carbon comhustion. The control illustrated is a variable auto-transformer (Variac Type No. 200-B, General R d i o Co., Cambridge 39, Mass.) of power rating suitable for the heater used. This control is mounted in a standard 4 X 5 X 6 inch aluminum box which has been reworked to receive it. At places on the inside of the
Figure 3. I.
Figure 1. Heater Assembly with Resistance Assembled with Van Slyke Combustion Equipment
Scale Drawings of Heater Unit Assembly
Vertical 8ectlon through middle of assembly 1. Hehter spacing plate (details in 11) ?. '/a-mph eirqular hales in tap and bottom
V O L U M E 2 7 , NO. 1, J A N U A R Y 1 9 5 5
167
rheostat must be used in place of the autotransformer. The use
of a 1-ampere rheostat of proper resistance for either alternating or direct current operation would reduce the cost of construction t o possibly $5.00 to $7.00. An autotransformer was preferred for the heater, as a transformer does not heat up as readily as a rheos t a t and, thus, provides a more constant volt-amperage supply t o the heater. Anexplodedview of the heater (ratedat 1lOrvattsand llsvolts, in Figure 2 shows how it may be diqassembled for replacement of burned out heating units (available from American Instrument Co., Silver Spring, Md., as replacement parts of an electric microKjeldahl digestion apparatus). When assembled, the small plate {containing side vents for air cooling) which has the heater attached to it, fits into the box and the lid fits tightly over the top. The details of the heater construction are shown in scale dranings in Figure 3. This box and support plate are made of Transite. The heater unit. A , is fastened to the support plate beneath it with four small spacing nuts. A similar set of spacing nuts fastens the electrical contacts to the unit: the nuts are easily removed to replace a heater unit As the heating unit never becomes estremely hot during an analysis, ordinary coated \$ire \vas found to be satisfactory for lead wire from the heater to the power control. Complete heater units will be available from ilrthur H. Thomas Co., Philadelphia, Pa. ACKSOWLEDGMEh-T
This rePearch was supported by the Atomic Energy Commission.
Self-Balancing System for Continuous Control of Current or Voltage Frank J. Dunn, Joseph B. Mann, and John R. Modey University of California, Lor Alamos Scientific laboratory, 1 0 s Alamos, N. Mex.
GALVANOMETER
AMPLIFIER
I
I
I
,
n
w
T W I N PHOTOTUBE ( T Y P E 9201
2-STAGE CLASS B AMPLIF,ER
MANUAL
ADJUST
TRANSICOIL
MOTOR
Figure 2.
METROh
GEAR
BOY
IO-TURN HELIPOT
FPiCTlON COUPLING
Block Diagram of Galvanometer Servo
In general, the thermal conductivity method consists of passing a constant current through a \Tire surrounded by the gas to be analyzed, and measuring the wire’s re-istance. The experience of the authors has been that the greatest single difficulty with the method is in the precise control of this heating current. Since gases vary in their abilitie? to conduct heat, the temperature of the filament and hence its resistance will be a function of the gas and of its pressure. Empirical calibration curves may be constructed by using known gases or known mixtures of gases. CURRENT CONTROL
HE thermal conductivity metliod has long been used for the T a n s l y s i s of binary gas mixture?, including mixtures of isotopes (2-6). The accuracy usually claimed for this method is approximately 0.05 to O . l % , and the gas analyzed may be recovered unaltered and undiluted. Theye facts led to the adoption of this method of analysk for hvdrogen-deuteriuiii and hydrogen-
I
tritium mixtures during the course of an extended series of separations of these isotopes, employing Hertz-type diffusion pumps ( 1 ) . Because these separations proceeded almost continuously, it was advantageous to achieve as nearly automatic operation of the analysis apparatus as was feasible. Reproducibility to 0.02% has been achieved with the aid of the servo-system described below.
The bridge circuit employed is outlined in Figure 1. Upon operation of the thermal conductivity cells as received from the manufacturer (Leeds and Xorthrup), it became apparent that the limiting factor in the sensitivity was instability of the heating current, even though three thermostated 120 amp.-hr., 12-volt storage cells were employed in parallel to supply this current. Therefore, a &ohm resistor in series with the bridge and a 500ohm, 10-turn Helipot and a 200-ohm fixed resistor in parallel with this resistor were added. The galvanometer employed had a sensitivity of 0.02 pv. per mm. This combination served to allow more precise current control, and with the servo-driven Helipot, allowed this control to be accomplished automatically. The servo is of the cloqed-loop type (Figure 2); its operation depends on the differential signal received by a tn-in phototube (Type 920), masked except for a narrow vertical slit a t the front to prevent interference from room lighting, and also to increaPe the slope of the signal cs. beam deflection function. It is now possible to control the 0.5-ampere current to 1 part in 100,000, which allows reproduction of analyses to i.0.027& For cmtinuous monitoring of the e.m.f. developed across the bridge, a Rubicon Type B potentiometer, a Leeds and Xorthrup microvolt direct current amplifier, and a Brown 2.5-mv. Electronik potentiometer were employed. The potentiometer is used to oppose all but a few microvolts of the e.m.f. to be measured, and the remainder is fed to the direct current amplifier, where its magnitude is increased to a value suitable for recording on the Brown instrument. Thus, the output e.m.f. is recorded, making possible continuous and automatic observation of such phenomena as the self-equilibration of Hz Tz = 2HT.
+
VOLTAGE CONTROL
I
THERMOSTATED CONWCTlVlTY BRIDGE
I
Figure 1. Schematic Diagram of Current Control for Gas Analysis Apparatus
Recently the servo system has been adapted to assist in the continuous and accurate control of the temperature of an electric furnace, as shown in Figure 3. The Brown recorder is fed
