Stabilized Magic Eye Indicator for Karl Fischer Titrations RICHARD KIESELBACH Bakelite Corporation, Bound Brook, N . J . SIMPLE three-tube titration
instrument is described
A which is unaffected by ordinary line voltage fluctuations.
The shadow angle of the magic eye changes by about 5’ for a 10% line voltage change. Component tolerances are 10% 01 greater. The instrument is usable with any platinum-platinum titration cell, the effective resistance of which, a t the end point, is between 100 ohms and 1 megohm. DISCUSSION
The use of the electronietric “dead-stop” end-point technique ( 1 ) is generally accepted as the most satisfactory method of
carrying out Karl Fischer moisture titrations. A number of instruments have been applied to the determination of this end point. For the sake of simplicity, the straightforward galvanometer circuit is preferred by some ( 7 ) . This circuit, and its modifications perform sat,isfactorily. However, as has been shown by Pott,s ((i), optimum sensitivity requires the use of a special low-resistance titration cell, matching the internal resistance of the galvanometer. A cheaper and more rugged cell, of high resistance, may be used, if an electronic amplifier is substituted for the galvanometer. An electronic amplifier, in itself, may also be somewhat more durable than a galvanometer. Several electronic “magic eye” circuits suitable for the present purpose have been described ( 2 , 5,6 , 6 ) , and that described by the writer has been in use in this laboratory for several years. All these circuits, however, are somewhat unstable to line voltage fluctuations, and several, including the writer’s, are sensitive as to certain component tolerances. I n particular, difficulty has been encountered in duplicates of the writer’s model, because of the wide variations in target current between individual magic eye tubes. I n order to eliminate the nuisance of line voltage drift and adjustments necessitated by aging of components, the circuit described here was developed. I n addition to improved stability and noncritical components, i t is very flexible as to the resistance of the cell with which i t is used, and is very simple in construction and operation. The circuit diagram of the instrument is given in Figure 1. &Isopposed to the galvanometer circuit, in which a constant voltage is applied to the cell, i t is convenient with an electronic amplifier to pass a constant current through the cell, and to measure the change in potential across the electrodes as the t h a t i o n proceeds. A satisfactory approximation may be obtained by connecting the cell, through a relat,ively high resistance, to a fixed voltage source. Inasmuch as the effective resistance change of the cell at the end point of a Karl Fischer titration is of the order of tenfold, extreme sensitivity in the amplifier circuit is not required. Using a high-mu triode driving a GE5 (magic eye) t’ube, full shadow deflection is obtained from a 0.1-volt change in electrode potential, a value that is easily obtained by supplying the cell and series resistor with about 0.5 volt. I n the present circuit, this series resistor is made variable, so as to permit adjustment of the cell current in accordance with the end-point resistance of the particular cell used. One side of the input is grounded, and a shielded cable is used to connect the electrodes, to minimize stray pickup. One section of a GSLT twin triode is used as the amplifier in this circuit, direct-coupled to a conventionally operating 6E5 magic eye tube. I n order to minimize grid current, the tube is operated a t a very low plate current, and gain is maintained by use of a high value of plate load. Any alternating current pickup is by-passed by the grid-to-cathode capacitor a t the 6E5 tube. This part of the circuit is conventional. The effects of line voltage fluctuations are neutralized by use of the second triode of the 6SL7 as a cathode follower, which is cathode1578
coupled to the amplifier triode. The low effective plate resistance of the cathode follower minimizes degeneration of amplifier gain by the high value of cathode resistor. By means of this circuit, any change in supply voltage affecting the grid voltage of the amplifier affects its cathode voltage to almost the same extent, thereby maintaining a substantially constant bias. Drift may also be introduced by changes in amplifier heater voltage. I n this circuit such an effect is neutralized by the equal effect on the cathode of the cathode follower, and is minimized by the reduction of heater voltage to 5.7 volts, by means of a series resistor. The canceling of heater voltage variations by means of the cathode follower depends, for its effectiveness, upon close matching between the cathodes of both triodes. Although twin triodes will generally match fairly closely in this respect, an occasional tube will be found unsatisfactory. Examples of variations between 6SL7 tubes are shown in Table I. Where the additional expense is justified, almost perfect performance may be obtained by use of the Type GSU7 tube, which is directly interchangeable with the 6SL7. Using this more care-
Table 1 .
Effect of Line Voltage on RIagic Eye Shadow Angle
6SL7 Make
RCA
RC.4
Line voltage
Shadow Angle, Degrees Tung-Sol Lafayette
RCA
120
0
110 100 90 80
5 10
..
0 5
45
10
15 30
..
0
..
..
0 5 10 15 20
Unmarked 0
0
..
26 .. 30
45 90
I O 5 V.
- 2 -6SL7-GT
5600 2 w
I
,
7;:0.5 V
-100 v
Figure 1.
Schematic Diagram
A l l resistors 0.5 watt except where noted.
M. Megohm
V O L U M E 21, NO. 1 2 , D E C E M B E R 1 9 4 9
1579
fully matched tube, line voltage variations as high as 40% have negligible effect on the magic eye shadow angle. The cathode follower triode serves the additional function of a zero adjustment, for setting the eye shadow angle a t the start of a titration. This is accomplished by the potentiometer i n its grid circuit, by means of which the grid is returned to the same potential as that of the amplifier grid, as indicated by the shadow angle. This is desirable, in any case, in order to achieve the best cancellation of line voltage changes. Because of the differences betweell individual 6SL7 tubes, it is necessary to provide a compensating control: the potentiometer in the 6SL7 cathode circuit. It ordinarily need be adjusted only 011 changing tubes. To make this adjustment, the electrodes are disconnected from the amplifier, the zero-adjusting potentiometer is rotated to the stop, in the direction which opens the eye, and the compensating potentiometer is then adjusted until the eye just closes. (When the back-titration procedure is followed, the shadow angle is adjusted to just under the maximum.) R i t h the exception of the 6SL7 tube, none of the component values in the circuit is critical, and 10% tolerances are adequate. 111 most cases, the choice of the 6SL7 may also be considered unimportant, so long as it is made by a reliable manufacturer. Only where extreme line voltage fluctuations are the rule will it be necessary to select BSL7's or to use the more carefully matched tube type mentioned. Operating procedure is the same as with circuits previously described, except that a polarizing current control replaces the sensitivity control. For any given cell, a position of mavimum sensitivity will be found for this control. Generally, something
less than maximum sensitivity will be adequate, and this control normally need not be changed after initial adjustment to the cell used. In actual performance tests, the instrument has been entirely satisfactory. When compared with the unit formeily used, the lack of flutter and drift in the eye position is immediately app a i t ~ t . In addition, the instrument is ready for use only 30 seronds after being turned on, because warm-up drift is effectively cancelled. Titrations were performed with cells of various effective resistances, with equal success. Satisfactory results were obtained with an electrode system consisting of a pair of 18-gage platinum wires, sealed through g l w tubes, and then ground off flush with the glass. A spacing of about 1 em. between electrodes was used. This cell has the obvious advantage of eliminating the problem frequently encountered in other cells, in which the wires break off close to the glass seal. Cleaning of the electrodes is also facilitated. LITERATURE CITED
(1)
E'oulk. C. W., and Bawden, A . T., J . Am. Chem. SOC.,48, 2046 (1926).
Kieselbaoh, R., IND. ENG.CHmi., - ~ X A I , .ED., 18, 726 (1946). ( 3 McKinney, C. D., Jr., and Hall. It. L.. I b i d . , 15, 460 (1943). (4) Potts, J. E., private communications. ( 5 ) Serfass, E. J., ISD.ENG.C H E M A . , N A LED., . 12, 536 (1940). (6) Smith, G. F., and Sullivan,V. I
