V O L U M E 2 6 , NO. 10, O C T O B E R 1 9 5 4 Improved Trigger Circuit for Automatic Titrations. W. Ii. Carson, Jr., Hanford Atomic Products Operation, General Electrir Co., Richland, Wash. LTHOUCH
the trigger circuit described previously [h.u
A CHEM.,25,226 (1953)] has proved satisfactory for automatic coulometric titrations, several changes to improve its perform-
ance have been found desirable: replacement of the dry cell in the comparison circuit with an electronic equivalent, and replacement of the output relay with a delay output relay circuit. The first change was made to eliminate the freqLent adjustment of the triggering point which was caused by the slow drop of the dry cell voltage under continuous drain. The electronic circuit furnishes a constant voltage to the comparison circuit slide-wire and thus stabilizes the setting of the trigger. The eecond change eliminates a tendency for an autotitrator to shoiz oscillation or chatter a t end points; this oscillation arises hecause the turning on or off of the electrolysis current give9 a small voltage kick to the indicator system, which in turn give< a voltage kick to the trigger. If this kick reverses the trigger avtion, oscillation can occur. By adding a delay to the action of the trigger, this oscillation is prevented. In addition to these changes, it as found desirable to piovide for thp use of the Leeds and Northrup Model A7664A-1 pII meter in place of the modified Beckman Model H-2 pH meter deycribed previously. The Leeds and Korthrup meter provides a recoider output which is suitable for the input to the trigger. Both meters are satisfactory in so far as trigger action is concerned,
1-4
1673 the Leeds and Sorthrup meter can be used with grounded solutions and, after warm-up, exhibits good stability. The basic trigger circuit is given in Figure 1. Figures 2 and 3 give the comparison circuits for use with either the modified Beckman meter or the Leeds and Sorthrup meter. These are connected to the basic trigger circuit a t the points shown. In actual construction the basic trigger circuit plus one or the other of the comparison circuits is wired as one unit. I n the basic circuit the elements enclosed by the dotted lines are the components furnished in the Brown amplifier. The previous trigger circuit used the S o . 351921-1 amplifier; this amplifier has since been discontinued, and the manufacturer furnishes the No. 356358-1 amplifier. The main difference is in the tubes used. Since, as before, only the voltage amplifying stages are used, it saves considerable space to remove the components and remount them on the chassis with the rest of the circuit. This part of the circuit is unchanged from the manufacturer’s design. The output of the third amplification stage is coupled via eondenser C-3 to a thyratron relay circuit identical to that previously described. The plate relay, K-1, operates the two thyratron delay circuits which control K-2 and K-3. The contacts of these relays are connected so that K-4 cannot close until K-2 closes, and after it closes, it cannot open until K-3 closes. Relay K-4 is the output relay and operates the controller relays of the autotitrator as previously described. Relay K-2 does not close until a short time elapses after K-1 closes, and K-3 does not close until a short time after K-1 opens. The delay prevents oscillation of the addition of titrating current, since if K-1 oper-
-.-.-.-.-.-.-.
110 vhc
Figure 1. Basic Trigger Circuit Aini,l~fier. Brown 356358-1. All parts enclosed by dotted lin es are f u rnished with amplifier except tubes and the converter. TI. Rectifier, 6 x 5 T2. Voltage regulator VR-105 Ta. Voltage regulator 5651 (90 volts) Tc. Ta, Ts. Thyratron 2080 Tr-1. Transformer, filament 110 to 6.3 volts Tr-2. Transformer, power, Pacific JOB 361 C-1. Condenser, 1 pf., 600 volts C - 2 . C-6, C-7. Condenser, 2 pf., 600 volts Condenser, 0.05 p f . c-3. C-4. C-8. Condenser, 1 pf., 150 volts C-8. Condenser, 20-20 pf., 450 volts
R-1. R-2. R-3. R-4, R-5. R-6. R-7, R-8. R-9. R-10.
K-I.
K-2, K-3. K-4.
Resistor, 3000 ohms, 1 watt Resistor, 270,000 ohms, 1 watt Resistor, 1-megohm, 2 watt Potentiometer, 1-megohm Resistor, 100,000-ohm, 0.5 watt innn-ohm 2 =.%tt
1674
ANALYTICAL CHEMISTRY
ates because of a voltage kick from the indicator circuit, it must remain either open or closed for a short time before any change in the mode (open or closed) of K-4 occurs. By setting the delay long enough to revent K-4 from responding to transients, the oscillation causei by the turning off and on of titration current is prevented. This time is on the order of 0.02 minute. The manner of operation of the thyratron delay circuits has been described. The electronic circuit used to replace the dry cell uses the high voltage winding of the amplifier transformer normally used for the power stages. This circuit is a conventional cascaded voltage regulator tube circuit and furnishes a constant voltage across terminals C and D to the comparison circuit. Resistors R-9 and R-10 must be adjusted so that both T-2 and 2'-3 fire. The circuit adds little to the cost of the trigger and eliminates drifting of the triggering point.
Die for Preparation of Potassium Bromide Disks under Vacuum. Don H. Anderson and Richard G. Smith, Industrial Laboratory, Eastman Kodak Co., Rochester 4, PI'. Y. N C R E A ~ I K Gapplication
1 densing system
of the silver chloride lens beam con-
(1, %)in this laboratory has encouraged the de-
velopment of dies to form transparent potassium bromide pressed plates. An inexpensive, simply constructed, and easily manipulated die has been devised for the preparation of 20- to 50-mg. potassium bromide disks under vacuum. The advantages outlined by Stimson ( 4 ) and Schiedt (S) of disks so prepared have been obtained under conditions less severe than are required for the larger plates they made. In addition, the disks prepared a8 described here are firmly mounted in a metal ring and handling and mounting are considerably facilitated. CONSTRUCTION AND OPERATION
The construction of the die is shown in Figure 1. A plastic or steel shell, 2, is drilled to accept steel sleeves 7, 8, and 9, which are inserted from top to bottom in the order shown. A steel plunger passes through these sleeves and presses the powdered potassium bromide inside ring 8 against the fixed steel peg, 3. Rubber gasket 6 and rubber stopper 5 serve to seal the chamber above and below the potassium bromide on the fixed peg. Outlet 10 provides for attachment of a line to a vacuum pump during the evacuation.
@ m
EASIC TRlogR C W T
T
-" Figure 2.
P
Steel sleeves 8
,'E
Beckman Comparison Circuit
The comparison circuits for the Beckman pH meter (Figure 2) are complicated by the fact that separate pH and millivolt connections are made to the meter. The circuits differ enough in the zero and span adjustments required to make it convenient to have separate adjustments. An ordinary telephone switch is satisfactory for the multiple switch.
I
1
Figure 1. Die for Preparing Potassium. Bromide Pellets under Vacuum 1. Steel base plate 2. Plastic or metal die shell 3. Hardened steel fixed peg 4. Movable hardened steel plunger 5. Rubber stopper 6. Rubber gasket 7. Upper steel sleeve 8. Steel tube in which completed pellet is held 9. Lower steel sleeve 10. Vacuum connection
TRIGGER CRCUIT
Figure 3.
L & N Comparison Circuit
The comparison circuit for the Leeds and Northrup meter (Figuie 3) is complicated by the fact that the output of the pH meter reverses sign in going from one end of the scale to the other. This requires an offset voltage in series with the lead to the comparison circuit slide-mire (the 1000-ohm Helipot). This is furnished in the circuit by placing the end of the slide-wire about 50 mv. above zero by inserting a 50-ohm potentiometer, R-1. The input from the meter is connected to the slider of this potentiometer. This permits setting the amount of offset voltage used. These changes and modifications of the trigger circuit have 1 e-ulted in improved titrations and operation.
In operation, sleeves 9 and 8 are mounted on the peg fixed in the base plate and steel barrel 2 is fitted over the assembly snugly against rubber gasket 6 . After the insertion of upper sleeve 7, the powdered potassium bromide is added and lightly tamped down, and the vent hole of sleeve 7 is aligned with evacuation outlet 10. After the plunger-stopper unit is added so as not to block the air vent of 7, the entire die is positioned in a hydraulic press but not subjected to pressure. Air is removed from the die for half a minute by an oil pump attached to evacuation outlet 10. Then a pressure equivalent to 7500 pounds dead weight on a pellet inch in diameter is applied for 1 minute, and lowered gradually and the die is disassembled. The transparent disk is thus fixed in the ring and is easily mounted in a plastic holder as described (21, with no maskingof the sample being required. LITERATURE CITED (1) Anderson, D. H., a n d Miller, 0. E., J . Opt. SOC.Amer., 43, 777-9 (1953). (2) Anderson, D. H., a n d Woodall. N. B., ANAL.CHEM.,25, 1906-9 (1953). (3) Schiedt, U., a n d Reinwein, H., Naturforschung.. 76, 270 (1952). (4) Stimson, &I. >I., J . Am. Chem. SOC.,74, 1805 (1952).
