Electronic Controlled-Potential Coulometric Titrator SIR: Circuit changes have been made to improve the performance of the electronic controlled-potential coulometric titrator [ANAL. CHEW 31, 488 (1959)l. CIRCUIT
1. Replace the 1000-ohm 0.5-watt resistor connected to the base of the transistor by a 4700-ohm 2-watt resistor. 2. Insert an M-500 (1N 1084) rectifier a t the reduce-oxidize switch in the lead from the reduce-oxidize switch that goes to the 2.5-, 25-, and 250-ohm resistors. Connect the anode (arrow) terminal of the rectifier to the reduceoxidize switch. The rectifier is thus added in series with the current metering circuit. 3. Return the ground connection from the common connection of the two 10,000-ohm resistors in the integrator bias network by a separate wire directly soldered to pin E of the power supply connector. 4. Return the ground for the integrating amplifier (pin 4 of connector 26-4200-16s B) to only the common connection of the two 10,000-ohm resistors in the integrator bias network. 5. Replace the 470,000-ohm resistor that is connected to the limit light by a 560,000-ohm resistor. 6. Connect a 0.1-pfd. Vit. Q 400-volt capacitor across the two terminals of the “titrate-off-reset” switch that are wired to the relay coils. 7 . There should be a total of four wires attached directly to pin E of the power supply connector. 8. The resistor connected to pin B of the cell connector should be a 400,000-ohm 0.5% wire-wound resistor. CONSTRUCTION
Note 9 (Table I) should read: “The integrator bias should be adjusted for essentially zero drift of the integral readout voltage a t zero volt output of the integrator.” The integrator bias may be adjusted and the titrator checked for drift by the following procedure : 1. Turn on the a.c. power switch on the GAP/R R-100A power supply. 2. After a delay of 1 minute, turn on the d.c. power switch on the GAP/R R-100A power supply. 3. Warm the instrument up for 15 minutes or longer. 4. Connect a readout device to the instrument. 5. Disconnect the cell leads. 6. Throw the titrate switch to the reset position. 7. Turn the reduce-oxidize switch to reduce. 956
ANALYTICAL CHEMISTRY
8. Turn the readout switch to X10 position. 9. The readout device should indicate less than 0.0001 volt. 10. Throw the titrate switch to the titrate position. 11. After 5 minutes, measure the readout voltage. If it has changed more than 0.0001 volt, readjust the integrator bias potentiometer and repeat steps 6 through 11. 12. Repeat the drift measurement and integrator bias adjustment until the readout changes less than 0.0001 volt after 15 minutes in the titrate position. 13. Turn the reduce-oxidize switch to the oxidize position. 14. Throw the titrate switch to the reset position. 15. The readout device should indicate less than 0.0001 volt. 16. Throw the titrate switch to the titrate position. 17. After 15 minutes in the titrate position, the readout voltage should change less than 0.0001 volt. 18. If it is not possible to adjust the integrator bias potentiometer to meet these drift specifications, a leakage current exists a t the input of the integrator. The leakage current must be eliminated. The most common sources of leakage currents are ground loops. ABSOLUTE CALIBRATION PROCEDURE
Connect a 4-pfd. capacitor of good quality across the calibrated resistor. IN-SERVICE OSCILLATION CHECK
Under some titration conditions the output of the control amplifier includes oscillations a t about 60/kc. Oscillation of the control amplifier adversely affects the titration results. This can be eliminated by reducing the gain of the control amplifier loop. A simple and effective method is to increase the value of the resistor connected to the transistor base from 1000 to 4700 ohms. The titrator should be checked for the absence of oscillations when it is put into service. Connect a cell containing suitable electrolyte and a reversible redox sample to the titrator. Attach an oscilloscope between chassis ground and the working (isolated) electrode. Perform an oxidation and a reduction titration. Ascertain that the control amplifier does not oscillate during these titrations. A small amount of 60 c/s ripple is normal during thghcurrent periods. INITIAL-CURRENT LIMITING
The maximum initial current that
may be allowed to pass through a specific titration cell without adverse effect on the analytical results seems to depend on cell and electrode geometry, stirring efficiency, and current density and varies with the ion being titrated. As a result, the 160-ohm current limiting resistor is included in the original circuit. If the amount of material titrated causes the initial current to be higher than the current that can be handled by the cell, the size of the current limiting resistor must be increased, the cell design changed, or the control potential initially set a t a lower value until some of the sample has reacted and the titration completed a t the full value of the control potential. FRACTION OF CELL CURRENT INTEGRATED
I n the original circuit 1/40,000 of the cell current is integrated. This is suitable for most coulometric titrations. It would be advantageous to increase this fraction to titrate high equivalent weight substances such as plutonium. This may be done by substituting a wire-wound resistor of lower value for the 400,000-ohm resistor connected to pin 1 of the integrating amplifier. As the 10-ohm resistor must handle high current, it is a wire-mound power resistor, not a precision resistor. However, the value of these resistors is stable over long periods. TRANSISTOR
The transistor type used in the seven instruments constructed to date is a Motorola 2x375. If similar transistors having higher frequency response become available, it mould be worthwhile to test their performance in this circuit. It would also be possible and desirable to simplify the circuit, if an KPN transistor could be switched in for reduction titrations in a circuit similar to that used for oxidation with the P N P transistor. At this time no S P N commercially available transistor has sufficient gain, current rating, and voltage ratings. CHECKOUT PROCEDURE
A checkout and test procedure has been written for the electronic controlled-potential coulometric titrator (available from the authors upon request). AI. T. KELLEY H. C. JONES D. J. FISHER Oak Ridge National Laboratory Oak Ridge, Tenn.
