A Balanced Circuit for Electrornetric Titration EARL B. WORKING Kansas Agricultural Experiment Station, Ifanhattan, Kans.
(4)have included in their valuable study of solvents for the electrometric titration of petroleum oils a titrimeter circuit which has some points of advantage over the bridge-type circuits of Garman and Droz (2) and Willard and Hager ( 5 ) . However, this circuit is so expensive as t o discourage its use unless most of the parts required happen to be on hand. The circuit shown in Figure 1 was designed t o avoid unnecessary expense and t o simplify construction. The cost of parts for this circuit is less than one-third t h a t of Rescorla, Carnahan, and Fenske, and the space required is reduced in about the same proportion, yet the stability is as great, even though the sensitivity is increased by the use of a n output tube of higher amplification.
R
resistor with an adjustable clip may be used. After this adjustment, momentary fluctuations of line voltage should have no effect upon the meter tested with potentiometer ROset to give a reading of any magnitude on the meter scale. However, prolonged deviations of voltage will cause a drift of the reading due to change of cathode temperature. With modern control of commercial line voltage, it is unusual to find conditions where voltage will fluctuate sufficiently to int'erfere with even .the more sensitive electrometric titrations; but if voltage changes are large and prolonged, the bridge circuit of Garman and Droz (2) will probably be more satisfactory, unless a voltage control is used. The power transformer, T , may be one designed for midget radio use and, if it is skimped in iron to save weight and size, the increased magnetic saturation mill reduce voltage fluctuations in the output. Since there is only a difference of about 85 volts in potential between the cathodes of the two tubes, they may use the same filament winding. If two filament windings are present, they should be used, n-ith each cathode connected to the center tap of its own winding. The milliammeter connection shown allows about a half ndliampere plate current to flow at zero reading, P O that the entire scale of the instrument may be used. If the simpler connection as shown by Rescorla, Carnahan, and Fenske is desired, R, is omitted and R4 and R, are combined to form $1 single 5000-ohm resistor, but scale readings below about 0 , 1 milliampere will deviate considerably from linearity. The milliammeter chosen should have a fairly long scale, pref-
ESCORLA, Carnahan, and Fenske
The stability of the circuit against changes in line voltage is due in part to the use of two opposed tubes, but since the pentode has a much greater amplification than the triode, the circuit is balanced by drawing part of the grid bias of the pentode from the bleeder circuit. Therefore, Ri should be carefully adjusted until line-voltage changes produce a minimum effect on the meter reading. Adjustment will be most convenient if RI consists of a 150-ohm k e d resistor and a 100-ohm wire-wound volume cont,rol, but this is necessary so seldom that a simple nire-Found
FIGURE 1. CIRCUIT B. C. L. MA. .V. Rt.
Rz. R3. RI.
Rs. RQ.
Battery, 1.5 or 3 volts Electrolytic condenser, S mfd. Choke, 30 henries, not over 200 o h m direct current resistance .\Iilliammeter, 0 t o 1 milliampere Neon pilot bulb, 0.25-watt, candeiabra base 250-ohm adjustable resistor 5000 ohms, 5 watts 10,000 ohms, 1 0 watts 3500 ohms, 5 watts 1500 ohms, 5 watts 50,000 ohnis
R:. Ra.
Ro.
8, S . SI.
T.
TI. T2.
Ta.
397
100,000-ohm potentiometer or adjustable resistor 500,000 ohms 10,000-ohm wire-wound potentiometer Double-pole single-throw switch Single-pole single-throw switch Power transformer, delivering 350 volts each side of center t a p T y p e 6C6 or 97 t u b e , according t o filament voltage available T y p e i 6 or 3G tube, according t o filament voltage available T y p e 80 or similar rectifier t u b e
398
VOL. 10, KO.7
R7
1
+
343 YO/fS FIGCRE 2. CIRCUIT 9-4
Ri. R:. R3.
R4. Ra.
Rs. RF. R-. R9. Rio SI.
Microammeter, full-scale 200 microamperes 100-ohm resistor with adjustable t a p 1100 ohms, 10 watts 500 ohms, 10 watts 1100 ohms, 10 watts 1100 ohms, 10 watts 175C ohms, 20 watts 500,000 ohms 800,000 ohms 600,000 ohms 10,000-ohm wire-wound potentiometer Snitch
ertLbly over 3 inches, but high accuracy in calibration is not essential. Suit,able meters can be purchased for $4.50 and up. Sensitivity is adjusted by changing R7, which may be either a radio volume control or a resistor tapped at several points. A' fixed resistor may be used if it is large enough to protect the meter from damage and no change in sensitivity is desired. With RT at the full 100,000 ohms, about 0.6-volt change in voltage of the input is required to give a full-scale deflection of the meter, while if only 10,000 ohms are used, only about 0.1 volt is required. Higher sensitivit'y is available by using a microammeter instead of the milliammeter, but this is hardly feasible unless a good voltage control is used. Voltage control may be desired either because of excessive fluctuations in line voltage, or when high sensitivity and reproducibility of measurements are necessary. Constant-voltage transformers are available, and will give excellent results when connected to keep the primary of the power transformer, T,a t constant voltage. The regulation of an electronic voltage control is still more accurate, and this can be constructed at considerably less cash outlay for parts, though it may cost more if time of assembly is charged against it,. Description of such a control is to follow in another paper. However, the application of direct current from the electronic control to the circuit in Figure 1 is not a great improvement unless the tube filaments can also be supplied with constant voltage, as from a storage battery. To avoid this difficulty, the circuit shown in Figure 2 was designed to operate entirely from a direct current voltage regulator of reasonable capacity. The voltage across the bleeder is specified as 343 volts, and if this is exceeded the life of the tubes will be considerably shortened. If an accurate voltmeter of this range is not available, it will be equally satisfactory to adjust the voltage source until the voltage drop across each of the tube filaments is 2 volts, or t o a tot,al current of 60 milliamperes, if either of these can be measured with fairly high accuracy. This circuit as shown gives about the maximum sensitivity conveniently available. If less sensitivity is required, a 0- to 1-milliampere meter and a type 30 tube may be used in the output, with a saving of several dollars. In this case, R7 mag be 50,000 ohms and Rs, 100,000 ohms. Either arrangement is suitable for measurements of potential from hydrogen or quinhydrone electrodes, from thermocouples, and the like, and will retain its calibration over considerable periods. Preparation should be made for convenient checking of calibration, as with buffer solutions for pH measurements, or with a potentiometer for millivolt readings. For reading pH values between 3 and 8 with the quinhydrone electrode, it is often convenient to reverse the usual connections, connecting the saturated calomel electrode to the ground connection and the quinhydrone electrode to the negative or grid lead. The method of calibration is illustrated by the following example, in which the materials to be studied are all expected to lie between
J
4.0 and 6.0 pH. Buffer solutions of these maximum and minimum values are made up by the method of Clark (1). The quinhydrone electrode is placed in the 4.0 pH buffer and the bias potentiometer RID adjusted to give a zero meter reading. The final adjustment may be made with the maximum sensitivity or zero setting of Rs. Rs is then increased to lower the sensitivity, and adjusted t o give exactly a full-scale reading after the quinhydrone electrode has been changed to the 6.0 pH buffer.
The linearity of response is well within t h e accuracy t o be expected in the prepor lese a r a t i o n of s u c h buffer solutions, as is shown by the data recorded in Table I which were read to the nearest 2 microamperes using this circuit with a 0t o 200-microampere meter. The buffer solutions were checked t o the nearest 0.5 millivolt by means of a potentiometer. To test the stability of the instrument, the readings were repeated on the two following days with no change in the adjustment of the titrimeter or of the electronic voltage control. However, this stability is not t o be relied upon unduly, and a daily or even more frequent check of calibration is t o be recommended.
TABLE I.
READINGS O F TITRIMETER WITH QUINHYDROXE ELECTRODE IN STANDARD BUFFERSOLUTIONS
S o m i n a l pH E. m. f . , millivolts
4.0 218.0 Meter reading, microamperes: First d a y 0 Second d a y 0 Third day -2
4.8 171.5 78
80 76
5.4 134.5 140 142 138
6.0 99.0
200 202 198
It is possible t o use this instrument, or similar ones, with the glass electrode by checking against buffer solutions of the correct p H so as to balance out the voltage drop and polarization, b u t this is rather a slovenly makeshift. It is much better to introduce a condenser for ballistic discharge as in the method of Hemingway (S), and when this is done an alternating current amplifier as shown by him is preferable t o direct current amplification. KOgrid resistor is shown on the input tube in either Figure 1 or Figure 2. S o n e is necessary unless there is danger of considerable positive voltage being applied t o the grid accidentally, in which case 100,000 ohms may be introduced. With good shielding, resistance u p to 10 megohms may ordinarily be used, but there will be a noticeable decrease in sensitivity with values above 1 megohm.
Literature Cited (1) Clark, W. hl., "Determination of Hydrogen Ions," pp. 192-202,
Baltimore, Williams & Wilkins, 1928. (2) Garman, R. L., and Droz, M. E., IXD.ESG. CREM., Anal. Ed., 7, 341-2 (1935).
(3) Hemingway, Allan, Ibid., 7, 203-5 (1935). (4) Rescorla, A. R., Carnahan, F. L., and Fenske, 505-8 (1937).
M. R., Ibid., 9,
(5) Willard, H. H.,and Hager, 0. B., Jr., Ibid., 8, 144-5 (1936). RECEIVED April 8, 1938. Industry.
Contribution No. 55, Department of Milling
