AIDS F O R THE ANALYST addition of two stirring paddles immediately above the electrode itself. At 400 to 700 r.p.m., the stirring is sufficiently efficient so that current readings may be made within a few seconds after each addition of reagent yet meter needle fluctuations are negligible. The glass cell, designed for easy draining and rinsing, is 60 mm. in diameter and 120 mm. long (dimensions are not critical) with a 2-mm. bore stopcock a t the bottom. The rotating electrode is driven by a suitable adjustable speed motor shown as R. In order to select the potential applied to the electrodes, the conventional method (8) is used with the exception that the rotating platinum cathode and the sheet platinum anode are used in place of the dropping mercury electrode and saturated calomel electrode when recording the polarogram.
Modified Amperometric Titration Apparatus Which Eliminates Agar Bridge. W. J. Seagers and H. A. Frediani, Chemical Control Division, Merck & Co., Inc., Rahway, N. J. METHOD
for eliminating the agar bridge in the conventional
A amperometric titration apparatus ( 3 ) without the use of a mercury pool has been developed in this laboratory. The reference electrode in the modified apparatus is a platinum electrode of large area whose potential relative to that of the rotating platinum electrode is maintained by an external potential source.
Table I. Parts List 0-100 pa. microammeter, Welsh Scientific Co., Model 451 SI, S3. Single-pole single-throw toggle swltches Sz. Three-position two-circuit rotary switch RI, B2. 1.5-volt 4FH Burgess dry cells RI. 500-ohm wire-wound potentiometer Rz. 300-ohm wire-wound potentiometer R3. 100-ohm wire-wound potentiometer Ra. 1500-ohm 0.5-watt carbon resistor Ra. 5000-ohm wire wound potentiometer 0-3 volts d.o. voltmeter, Welsh Scientific Co., Model 451 V.
.Ti‘.
R, A
.
C
For continuous routine use on a single type of titration-Le., chloride vith silver nitrate-the cost of the apparatus may be decreased by eliminating the voltmeter and substituting a 5000ohm adjustable power resistor for potentiometer R3. The potential applied to the electrodes is adjusted by moving the contact of the power resistor until a voltmeter inserted across the electrodes indicates the proper potential (for chloride with silver nitrate under the usual conditions, 4, this potential is 1.1 volts). The batteries may be eliminated and the instrument made lineoperated by the use of a suitable rectifying system if desired.
Figure 1. Circuit Diagram of Modified Amperometric Titration Apparatus
T h e present apparatus has two advantages over previously described setups. The platinum anode eliminates the difficulties usually enountered in the use of an agar bridge as the means of electrical contact between the anode and cathode compartments. This makes maintenance simple. The potential source is such that changes from one set of titration conditions to another require only a simple adjustment. This makes the instrument versatile and eminently suitable for investigational use. APPARATUS
This routine instrument has given excellent results for the determination of chloride in such varied products as corn distiller’s dried grains, saline suspensions of cortisone, and procaine penicillin. The instrument built according to the circuit diagram has proved to be an excellent tool for research and versatile enough for performing many varied amperometric titrations.
n
LITERATURE CITED
The circuit diagram is shown in Figure 1. All parts are available from radio supply houses (Table I). The tolerances of all parts are not critical, so that reasonable substitutions can be made. The zero-shunt circuit ( 1 ) powered by battery B1 is necessary for the utilization of the full sensitivity of the microammeter, since a relatively large initial current may flow through the circuit unlike most applications of the conventional apparatus. The potentiometers, R1 and Rt, are adjusted , so that the microammeter may be shunted and its range extended to 0 to 200 and 0 to 300 Ma. in addition to the 0- to 100-pa. range. A diagram of the cell used is shown in Figure 2. The anode, A . is a 0.05-mm. platinum sheet 15 X 50 mm. curved to fit the inside of the cell. The rotating platinum electrode, C, is of conventional construction using No. 18 platinum wire but with the
(1) Groves, L. G.. and King, J., J . SOC.Chem. Ind., 65, 320 (1946). (2) Kolthoff, I. hI., and Lingane, J. J., “Polarography,” 2nd ed., Vol. 11, p. 888, New York, Interscience Publishers, 1952. (3) Ibid., p. 901. (4) Mader, W. J., and Frediani, H. A , , J . Am. Pharm. Assoc., Sci. Ed.,
40, 24 (1951).
Inexpensive Conductivity-Determining Equipment. Robert M. Creamer and D. Harry Chambers, U. S. Bureau of Mines, College Park, hid. URING
experiments with nonaqueous electrolytes it became
D necessary to measure the conductivity of many solutions.
Figure 2. Diagram of Cell and Electrodes A . Sheet platinum anode B. Cone of stirring motor C. Rotating platinum electrode
Experiments conducted with several signal sources and detecting devices, when used in conventional bridge circuits, indicated that a vacuum-tube oscillator source and a sensitive alternating current vacuum-tube voltmeter gave excellent results. These instruments have been recommended before for conductivity determinations but their cost has been high. Recently instruments in an unassembled form have been made available a t a very reasonable cost (Heath Co., Benton Harbor, Mich., Electronic Instrument Co., Brooklyn 11, N. Y., and others). These instruments provide good precision, ease of operation, and excellent flexibility in determining conductivities. The vacuum-tube voltmeter has the following advantages: insensitivity to room noise, wide frequency sensitivity, sensitivity
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