A Continuous-Reading Electron-Tube Conductance Meter
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R. L. GARM.4N, Washington Square College, New York University, New York, N. Y.
nating current voltage from the bridge while adjusting RI and
to reduce the sensitivity, preventing the meter from reading offscale in the performance of titrations where large changes in
resistance are anticipated at the end point. Under these conditions of operation the meter will read the “off-balance” voltage of the bridge, the readings increasing as the bridge is progressively unbalanced.
ment A vacuum-tuba has been constructed instruwhich i n c o r p o r a t e s a n oscillator supplying audio frequency voltage to a selfc o n t a i n e d b r i d g e , a detector, and direct current meter which indicates the r e s i s t a n c e of a conducta n c e cell. These functions are performed with a single 6A6 tube whose cir-
As the resistance of the conductance cell increases, the shunt resistance between the plate side of choke L1 and cathode also increases. This has the effect of increasing the voltage across the bridge terminals and distorts the grid volts-plate current curve shown in Figure 2. This distortion is desirable, since the resulting plate current will be more nearly a linear function of the resistance of the conductance cell. The degree of linearity and sensitivity attained with this set-up is shown in Figure 4. Since the meter readings are linear with resistance over the range of plate current from 100 to 500 microamperes, the bridge must be unbalanced slightly before readings are taken. This setting may be performed after the titration has progressed to a certain extent, since the first readings are usually excluded in the final determination of the end point. This has a further advantage because almost full scale of the meter may be used in the region near the end point for greater accuracy.
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tion ofthe resistance of the conductance cell. The instrument derives all power from alternating or direct current mains.
Theory The output voltage of a bridge is a parabolic function of the resistance of any one of its arms. A typical example is shown in Figure 1. The plate current-grid volts curve of a vacuum tube is not a linear function in the region of high negative values of grid voltage. A typical curve for a high mu triode is shown in Figure 2. (Direct current voltages were used in these calculations for simplicity and are permissible since the curves are used as illustrations only.) If these curves can be completely superimposed, an instrument may be constructed in which the plate current of an electron tube is a linear function of the res i s t a n c e of one arm of a bridge. Complete superimposition is difficult to attain in practice, but may be a p p r o x i m a t e d sufficiently to satisfy the requirements of conductance titrations.
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FIGURE 3. DIAGRAM 7 mpnnhms. 1 watt RP. 1-megoFm-GZimi c o i t r o ~ Ra. 50 000-ohm volume control RI. Vdltage divider 1000 ohms 25 watts Ra. Voltage divider) 20 000 oh& 25 watts Re. Volume control: 15:OOO ohms R7. General radio potentiometer 1000 ohms RE. General radio potentiometer: 100 ohms Rs. 15 ohms 100 watts Rlo. 140 oh& 100 watts CI. Condenser 0.01 mfd., mica Cz,Cs,Ca. Condensers, paper, 0.5 mfd., 200 volts Cs. Electrolytic condenser, 8 mfd. Cn. Electrolvtic condenser. 4 mfd. TI. Low rad0 audio transformer Tz. Single-button carbon microphone transformer LI Choke 30 H. Lz: Choke: 30 H. 200 ohms pa, 0-500 microammeter Rt 5. tn ~.
Operation The upper section of the triode 6A6 (Figure 3) is 4.06 3.85 3.65 1.45 3.45 NEOATIVE ORTD VOLTS used as an oscillator and the alternating v o 1t a g e 2* Mu TRIODE CURVE developed across resistance HIGH Re is amlied to the bridge through-condensers C3 acd C,. The voltage from the unbalanced bridge is applied t o the primary of the step-up transformer Tz. The stepped-up voltage is applied to the grid of the lower section of the triode which is negatively biased t o zero plate current by adjustment of R3. A sensitivity control, RI, is used to attenuate the alter146
ANALYTICAL EDITION
MARCH 15, 1936
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AND SENSITIVITY ATT.4INED FIGURE4. LINEARITY
Plate current-resistance
of conductance cell
CC. OF nsommm ACETATE ADDED TO 10 ~0.010.1049 I SODIUM RlDlOXIDE
FIGURE5 , CONDUCTANCE TITRATION
Construction The oscillating circuit was designed to offer a maximum in stability and yet lend itself to easy construction. Transformer Tlmust be correctly poled to permit oscillations, and resistance R1should be as high as ossible yet allowing sufficient energy feed-back to sustain osciEations. A high value of this resistance assures smooth operation and good wave form. The setting of R4is not critical and may be adjusted for maximum oscillator output ( - 2 t o -3 volts are usually satisfactory). TI may be any low-ratio audio transformer, and since many of these units have a natural period of oscillation at approximately 1000 cycles per second, condenser C7may be omitted. The bridge may be standard equipment or may consist of large-size wire-wound potentiometers since no calibration is required. Since the ratio of resistance in the bridge arms determines the curvature of the curve shown in Figure 1, the values must be as stated t o attain the desired linearity. If, however, the apparatus is to be designed t o measure higher resistance values of the conductance cell, the resistance of the bridge arms as well as the impedance of transformer T z should be increased proportionately. Meter readings are relative and the proper setting of resistors R , and Rs may conveniently be determined for a particular range of resistance by substitution of a variable resistance box for the conductance cell. The settings of R, and Rg are varied until equal increments of resistance produce equal increments of microamperes. The inclusion of a rectifier and filter system permits satisfactory operation on both alternating and direct current mains. Figure 5 illustrates a typical titration curve. I n each case 10 cc. of 0.1049 N sodium hydroxide subsequently diluted to 200 cc. were titrated with magnesium acetate. The end point for curve 1, taken by the usual bridge methods. is 6.73 cc.; for curve 2 taken with the instrument it is 6.71 CC.
The error may be assumed to rest entirely with the inaccuracies encountered in the extrapolation. The circuit described here does not represent the only one suitable for the purposes outlined. It represents rather a simple scheme which may be translated into a compact instrument and the cost of the parts, including the meter and cabinet, need not exceed twenty-five dollars. A more complicated circuit extending the principles described here could certainly be made in which the conditions of linearity are more closely approximated. It is the ardent hope of the author that such circuits will be devised to further the technic of conductance titrations.
Summary A vacuum-tube conductance meter deriving power from alternating or direct current mains combining an audio oscillator-bridge and a detector-meter system has been described. The application to conductometric titrations has been illustrated and it has been shown that meter readings are linear with resistance over four-fifths of the meter range.
Literature Cited (1) Britton, “Conductometric Analysis,” London, Chapman and Hall, 1934. (2) Callan and Horrobin, J. Soc. Chem. Ind., 47, 329 (1928). (3) Jander, G., and Sohorstein, H., 2. angew. Chem., 45, 701 (1932). (4) Treadwell, Heh. Chim. Acta, 8 , 89 (1925). RECBIVED October 18, 1935. Presented before the Division of Physical and Inorganic Chemistry, Symposium on Recent Advances in Microchemical Analysis, under the title ”Use of Multi-Purpose Radio Tubes in Analytical Chemistry,” at the 89th Meeting of the American Chemical Society, New York, N. Y . , dpril 22 t o 26, 1935.
