AUGUST, 1952
chart. The pen, pointer, and holder are attached t o a light metal carriage which slides across a '/&-in. guidebar. The carriage is pulled along the track by a stainless steel wire attached to the grooved wheel to which the potentiometer is bolted.
switch in the mercury pool circuit is set t o the position marked Normal, the last range being obtained only if this switch is in the position marked Polar. The current selector consists of a two-pole, eight@sition switch which inserts one of the resistors in series with the electrode system. The current ranges POLAROGRAPH SWITCHING UNIT are 1,2, 5, 10,20, 50, 100, and 250 microamperes. One Figure 1shows the wiring diagram of this unit which microampere through the 5000-ohm resistor will result may be considered t o have four integrated parts: in a drop of potential of 5 mv. across it, as also will the power supply with its voltage selector a t the right two microamperes through the 2500-ohm resistor. of the diagram, the current selector in the lower two The two lower gangs of the Ox-Red switch maintain the thirds of the central portion, the amplifier power and proper polarity of this voltage. Damping of the oscilbias portion at the left, with the standardizing or lations of the recorder pen with the formation and calibrating unit in the top third of the middle portion. growth of each fresh drop of mercury is accomplished by Potentiometers Ra and RB, mounted in the recorder insertion of capacitors across the jacks labeled Cond. in unit, are drum-type potentiometers. All fixed resis- Figure 1. The polarity to be observed with electrolytic tors are precision type (*0.2 per cent), and all switches capacitors is marked on the diagram. have either two or four contacts, parallel-connected to The amplifier power and biasing portion is a switchavoid contact trouble. ing arrangement by means of which the before-menThe power circuit for the electrode system makes tioned 5-mv. drop of potential across the continuousprovision for controlling the range and polarity of the balance potentiometer is maintained. As can be seen potentials applied to the electrodes. Polarity is from the diagram, no bias is applied to the recorder de~ermined by the position of the four-pole, two- when switch Sa is in the position indicated and switch posit,ion switch labeled Ox-Red in the disgram. The S3 set to the right, and a potential drop of from zero four-pole, five-position switch (83 allows selection of to 5 mv. may be recorded. Should Spbe set t o the ranges 0 to -1, 0 to -2, -1 t o -3, 0 to -4, and +2 next position, S8being still to the right, a 2'/,millivolt t o -2 volts, when the former switch is in the Reduc- bias will be applied to RB, and the recorder may now tion position. To obtain the first four ranges, the register potential differences of between 2.5 and 7.5 mv.;
JOURNAL OF CHEMICAL EDUCATION
A RECORDER FOR POLAROGRAPHY E. B. THOMAS and R. J. NOOK John Carroll University, Cleveland, Ohio that the resistance of RB should be 20 ohms. Subsequent discovery that a potentiometer of 25 ohms resistance was available required that a 100-ohm shunt be placed across it, as all the other resistors of the biasing and standardizing network for the amplifier and recorder had been assembled by this time. This RECORDER UNIT proved fortunate as it allowed inclusion of the 100-ohm The strip-chart recorder was originally a war-surplus potentiometer and switch Saof Figure 1, by means of recorder of the multiple printing type, which recorded which the recorder pen may be "zeroed" at any posiinformation from seven thermocouples at what appeared tion on the chart. This is of considerable advantage to be quarter-minute intervals, the chart moving one in the recording of several polarograms on the same division per hour. The paper drive mechanism was chart, and in the recording of potential changes in modified to move the paper a t either one and one-half, titrations where the polavity of electrodes changes as or three divisions per minute, and turn a potenti- the titration proceeds. Smooth, positive contact of ometer (Ra of Figure 1) a t the rate of 25 degrees per the slider with the potentiometer wire is an essential minute. This potentiometer, (shown as 1 in Figure 2), to proper operation. Any oxidized coating over that by means of which a varying potential is applied to portion of the wire where contact should be made may the polarograph cell, is mounted on the revolving, worm- be removed with a very line sandpaper, the wire cleaned geardriven table from an A-5 Auto Pilot Gyro (Part carefully and coated with white petroleum jelly. iVo. 656029 M-1). The balance and print mechanism Poor contacts and had capillaries account for most of of the original recorder was removed and replaced by a the troubles experienced in polarographic work. Antwo-phase (Minneapolis-Honeywell) motor which drives other appendage on the recorder unit, though not used the continuous balance potentiometer (RB of Figure 1) in polarography, is a B e n d i Synchro generator. and the recorder pen. Two motors have been tried, (C 78248; S of Figure 2), geared 1 : 1 to the main drive 54 and 162 r. p. m., the latter being preferred, as full shaft of the chart drive mechanism which turns at the scale travel of the chart requires only 3.2 seconds with rate of 25 r. p. m. This generator may be connected this motor. This motor is controlled by a Brown electrically to a synchro motor, and thus serve as a Electronik Amplifier. Five millivolts drop of potential remote drive for other apparatus external to the reacross RB was arbitrarily adopted mheu it was decided corder yet synchronized with the movement of the T m s switching arrangement and potential recorder was planned primarily for polarographic work, but may he adapted to many other uses which employ small changes of potential or current to measure or control chemical processes.
JOURNAL OF CHEMICAL EDUCATION
I.
Potentiometer RA;
2. blinn~a~,olis-Honey\%~ell 2-phase ntotnr: Bendix synohro eenerator: 4. worm gear drive for R.4.
3.
in other words, switch Sa allows the recorder pen to be "stepped-hack" in half-scale steps for each position that it is advanced from the posibion indicated in the diagram. By this means ooe can raise the voltage level a t which the recorder operates, so that currents five t,imes as great as those indicated by the current selector switch may he recorded and yet maintain the same high sensitivity of response of the amplifier. When employing this bias arrangement in polarographic work, it is necessary to stop the progressof the chart (and Ra), set S4 to the next postion, allow several seconds for the recorder to travel back to its new position, and then resume recording by reactivating the chart drive mechanism. Instead of a two-section switch (Sn) as shown in the diagram, we now employ a met,er insertion switch (Mallory 1400 L) with a single decade of 10-ohm resistors. This switch will open any one of t,he eleven positions and insert RB (and the 100ohm potentiometer shunting it) a t that point. When switch SI is thrown to the left and S4is in the position shown in the diagram, the 100-ohm potentiometer shunting RE a l l o ~ st,he recorder pen to be "zeroed" a t any desired point on the chart. This is especially convenient when it is desired to repeat a polarogram on the same chart, as shown in Fignre 3, or in the recording of titration curves with other apparatus operating through this switching unit and recorder. The calibrating circuit, shown in the top, central portion of Figure 1, consists of a Weston standard cell, galvanometer, and a switch for selecting either the power circuit or the amplifier circuit for standardizing. In the power circuit two resistors have been chosen with values such that when 4.000 volts is impressed across them, the drop of potential across one of them is that of the standard cell, namely 1.0186 volts. To obtain a 5-mv. drop of potential across RE (with the 100-ohm potentiometer shunting it) a current of 250 microam-
peres is required. This current through a 4074.4-ohm resistor produces a drop of potential of 1.0186 volts, and accordingly a resistor of this value is incorporated in the circuit. With the calibrating switch thrown to the left, as shown in Figure 1, t,he drop of potential across this resistor opposes the voltage of the standard cell through a 10,000-ohm galvanometer-protecting resistor. Balanre is att,ained by adjustment of the rheostat marked 5000 ohms in the diagram. Actually a 3000-ohm fixed resistor and a 2000-ohm rheostat (General Radio 314-A) are used a t this point. For fine adjust,ment the protective resistance is short-circuited by depressing the button of a normally open microswitrh shunted across this resistor. The power circuit is standardized in the same manner, but. with the calibrating sv.itch thronm to the right,. When both cirruits have been standardized, t,he calibrating switch is set to t,he central or neutral position. I t should be mentioned t,hat t,he power circuit requires readjustment whenever the range of potent.ial applied to t,he polarographic cell is changed, inasmuch as the load on the hattery is changed. A somewhat more complex switching system mould avoid this minor inconvenience. Figure 3 shows four polarograms run on the same solution of copper, cadmium, and zinc chlorides in a supporting electrolyte of potassium chloride using the mercury pool as the anode. Curves A and B were run under idendical conditions, but, displaced from each other by means of switch S1 and the 100-ohm potentiometer paralleling RE. A 3000-microfarad capacitor was used in running A and B, 2000 microfarads with C
(at half the original sensitivity), and no damping with
D, which was run a t one quarter of the original sensitivity. The pronounced irregularities in curve A were caused by attempts to realign t h e chart on the driveroll while the polarogram was being recorded. The chart is reset manually and requires careful alignment of the perforations of the paper on the sprocket.
