SIMPLE DEMONSTRATION POLAROGRAPH CHARLES L. RULFS Purdue University, Lafayette, Indiana
THE RELATIVE virtues of the "bread board" versus the more compact "closed box" type of construction have been too thoroughly discussed in the past to justify their consideration here. With respect to polarographs, an excellent example of the former type of construction is the circuit of Miiller described in THIS JOURNAL.^ On the other hand, a compact, self-contained, "boxed" instrument is preferable in many respects when actual student exercises in quantitative polarographic analysis are to be performed. The apparatus to be described fulfills the need for a simple, rugged, and portable instrument. It necessarily involves compromises in certain respects. Thus, the employment of a microammeter in place of a galvanometer leads to a decrease in sensitivity which limits the usable range of the instrument to the relatively higher concentrations of ions. Similarly, the use of a small circular voltage divider will obviously not permit the same precision of voltage settings as is obtainable with a divider of meter-stick length. On the other hand, the instrument has a hished, professional appearance and is readily transportable to the bench top of the individual user. It is sufficiently rugged for student use, and replacement parts are relatively inexpensive. The essential elements of the circuit will be evident from the diagram of Figure 1. The choice of the current-measuring meter will obviously be determinative as regards the usable range of the h a 1 instrument,. The largest size microammeter which is readily available and sufficiently rugged (functions in vertical mounting) for the present purpose is an @/*-inch met,er of 50 microampere range. On the other hand, the available 4'/%-inch meters in 20 or 25 microampere ranges give the same sensitivity. The 4'/,inch, 50 microampere instrument specified on the circuit diagram is about the coarsest usable meter. The range and internal resistance of the meter will, of course, determine the values of the shunt resistors. With the meter and shunt shown on the diagram three current ranges of 50, 100, and 200 microamperes are available. Condenser-damping (for the oscillations due to drop formation) of the meter is a considerable convenience in reading.2 The feature of counter-current compensation extends the usefulness of the instrument but may be omitted for simplicity. This feature permits the determination' ' MihLEn, 0. H., J. CHEM..EDUC., 18, 1 1 1 5 (1941).
LINGANE, J. J., AND H. KEELINGEE,Ind. Eng. Chem., Anal. Ed., 12, 750-2 (1940).
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Circuit DiaQTsrn.
BI, 2-Burgees 4FH cells; 88, 1-Burgeas 4FH mil: R1. 100-ohm rheostat; RB, 10-ohm rheostat; RS,20ohm potentiometer; Rm, 1500-ohm meter resiatsnoe: R4, 1500-ohm precision resistor; R6. 500-ohm preoision resistor; R8. 10.000-ohm rheostat; KT. 100-ohm potentiometer; V. 0-3 or 0-5 voltmeter; U.4, 4V2inoh, 0-50 microammeter: C, 1000-microfarad, 6working volt, dry eleotrolytia oondenser; Sw 1, single-pole, single-throw switoh; Slo d , double-pole, double-throw switch; Slo 8, 8-position, aingle-pole rotary switoh; Sw 4, double-pole, single-throw switch; J P , jack and plug lea& for electrode oonneotions.
of small amounts of one ion a t a proper sensitivity setting of the shunt, despite the presence, in relatively larger amounts, of another, more easily reducible ion.% The polarity-reverse switch permits the running of an anodic oxidation followed by a cathodic reduction without the necessity for reversing the external electrode leads. This feature, also, is obviously expendable if one desires the maximum in simplicity of construction. The "on-off" switch for the main circuit may be conveniently incorporated with the 100-ohm resistor as a
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APRIL. 1948
A flange-mounted rod for holding the mercury reservoir is mounted atop the cabinet. An adjustable clamp mounted on one side of the cabinet is convenient for holding the polarographic cell. The mounting and insulation of all components is greatly facilitated if the steel panel at the front of the cabinet is replaced with a Masonite panel. The appearance of the finished instrument is shown in Figure 2. The total cost of all components when purchased from a large radio supply house is approximately $25. On the other hand, the microammeter alone will cost about 830 if purchased a t the manufacturer's list price. The usable concentration range for an instrument having a sensitivity such as to make curve inflections of 1 microampere readable and with shunts to give a meter range up to 500 microamperes is approximately 5 x M to 1 x M. Excellent curves for the cathodic reduction of Cu, Pb, T1, Cd, Zn, Cr, and nitrate ions have been obtained using this instrument. Anodic oxidation curves for chloride ion and quinone have been obtained. Figure 3 gives the calibration curve obtained for Cd ion showing the actual experimental points. For student exercises designed to yield the maximum results in a minimum of time, "one-point" calibration curves may be used; i. e., the diffusion current for a single; relatively high, concentration of an ion is determined and plotted using the point, zero-current at zero-concentration and assuming a straight-line relationship. Thus, for example, the student may run standard Cd and Zn solutions of 5 X M concen-
Figure 2.
General View of Instrument.
,,cover-platete"type
of switch. The 10-ohm "fineadjustment" resistor is a worth-while convenience in obtaining exact voltage settings but could be omitted. The voltmeter may be of a relatively inexpensive make hut should not be of the very cheap vane-repulsion variety. This type of meter will be found absolutely useless in the presence of the strong magnet of the microammeter. The voltage divider slide wire may consist of any 15or 20-ohm, linear taper, radio potentiometer whose effective electrical rotation matches the scale divisions of an available dial plate. The dial plate is most conveniently graduated in 0 to 100 divisions, and should be at least 3 inches in diameter. Such an assembly will sufficefor most practical purposes, but leaves much to be desired in the way of accuracy. It is a relatively simple matter, however, to hand-wind a potentiometer wheel of any requisite accuracy.
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LOG. CONC. Cd++, M. Fi.uro 3
tration and then proceed a t once to an unknown mixture containing Zn and Cd ions in concentrations of approximately 2 X 10Wa M. Results of plus or minus 5 per cent accuracy are obtainable by such a simple expedient,.
