tube is connected to the mid-point of this voltage divider, its potential will change during the course of a titration in response to the change in light transmission of the solution. If there is a pronounced change in color at the end point of the titration, there will be a corresponding large change in the tube current. The vacuum tube serves as one arm of a Wheatstone bridge circuit, the other arms of which are R\, R2> and i? 3 . If the bridge is balanced at the outset of the titration, it will be markedly unbalanced at the end point, and the unbalance signal may be read, or employed to activate a relay and stop the addition of titrant. This circuit is not intended for precise measurements of the light transmission of the solution. The only requirement is that the characteristics remain fairly stable during the several minutes required for the titration to be completed. There is no problem about the input signal to the grid of the tube, since it has a moderate impedance, and is of large magnitude, and all the power needed is available from the 20-volt power supply. Hence, almost any commercial radio-type vacuum tube may be used, and no special circuit designs need be invoked. Single-Stage Electrometer. If it is necessary to obtain stable, reproducible measurements of the current flowing through a vacuum tube in order to sense the magnitude of the input signal impressed on the grid, this may be achieved by very careful selection and stabilization of the components of a simple circuit, or through a sophistication of design that eliminates the effect on the read-out of variations occurring in the characteristics of the components. The latter approach will be described shortly; the former is exemplified by the circuit shown in Figure 2 [Andreatch, A. J., Feinland, R.,
ANAL. CHEM. 32,
1021
(I960)]. The heart of this circuit is an electrometer-type tube, which is a triode vacuum tube that is especially designed to provide the maximum insulation between the electrode structures, so that the leakage current that flows between grid and
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Circle No. 126 on Readers' Service Card
VOL 34, NO. 2, FEBRUARY 1962
·
31 A
