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., and Miller, 0. E., J . Opt. SOC.Amer., 43, 777-9
(2)
Anderson, D. H., and Woodall. N. B., ANAL.CHEM.,25,
(1953). 1906-9
(1953).
Schiedt, U., and Reinwein, H., Naturforschung.. 76, 270 (1952). (4) Stimson, &I. >I., J . Am. Chem. SOC.,74, 1805 (1952).
(3)
