Several Designs for
Rubin Banino
Illinois Institute of Technology Chicago, Illinois
Constructing Potentiometers "Student" potentiometers cost about $300 when all of the necessary attachments to complete them are included. (The attachments usually consist of a galvanometer, working battery, standard cell, tap keys, switch, and a variable resistor.) They are not compact and the circuitry is not readily apparent. The potentiometers described here are inexpensive ($40-$100 depending on the variations chosen), compact (everything is mounted on a Plexiglas sheet 10 X 14 inchesexcept for the thern~ocouplepotentiometer which uses a small external box galvanometer), and the circuitry is obvious since the Plexiglas is transparent. These potentiometers were developed for a National Science Foundation In-Service Institute Course (Modem Instrumental Analysis) for high school teachers which the author teaches. Since most of the participants never had built instruments before, it was necessary to design instruments for construction which were compact, simple, inexpensive, and rugged. Some of the potentiometers constructed in this course have been in use for over two years. Considerations concerning components will be discussed and then several ways of hooking them up will be shown.
The so-called "student" potentiometer which is commonly used in undergraduate laboratories is too costly in terms of its applications. This paper describes several designs for constructing inexpensive pot.entiometers which possess a practical degree of precision. Potentiometers are used in undergraduate courses primarily for three purposes: (1) to illustrate the principles of potentiometry (measuring emf's by utiliziug a balancing circuit and a nuU detector); (2) to measure the emf of electrochemical cells, and to determine the E" values for half-cells; and (3) to carry out potentiometric titrations. They can also be used as pH meters (with an appropriate conversion chart) and to measure the emf's of thermocouples. The "student" potentiometer is capable of a precision of about 0.05% or 0.0005 v on the 1-volt scale, but this precision is wasted on most experiments. I n actual student use important factors like temperature, pressure, purity of materials, construction and reproducibility of electrodes, proper stirring, junction potentials, concentration, and care in the use of the instrument (like restandardizing frequently and not abusing the standard cell) are uncontrolled or imprecisely known or both. These factors in combination lead to a practical precision of 1%or poorer in the student use of potentiometers. However, a precision of 1-2yo is adequate since the Eoof many half-cells is not reported to greater than two decimals (O.Olv), and the change in emf at the end point of many potentiometric titrations is of the order of 0.4 to 1.0 v (which readily permits the characterizationof boththetitrationcurveand the end point). Thus a potentiometer capable of a precision of 1-2% or 0.01 to 0.02 v would match the in-use requirements.
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Working Battery. The working battery provides current for the slide wire, and if the number of standardizations of the circuit are to be kept to a minimum, then the workine battery should have a long-term stability or be of the low-discharge type. IiIercury batteries are outstanding, since they give a rather constant potential over their lifetime and have a smaU temperature coefficient of emf. Two RMl2R batteries in series supply 2.7 v and have a capacity of 2400 mah. In the circuits described, the current drain is 1 ma and thus the batteries have a useful lifetime of over 2000 hours. The RIVI12R's are also small.
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Standard Cell. These may be purchased (about $17) or constructed1 for about $2.50, or a particularly stable mercury battery may be used. The RM42R has a capacity of 14000 mah. a voltage of 1 . 3 5 ~and . is stable to 1%for iong periods. ~ h e ' c o s tis $1.85. The Mallory No. 302540 is a bit more expensive ($3.50), has the same capacity, but is more stable (voltage is also 1.35~). Mercury batteries should be periodicauy checked against a Weston cell.
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Switches ond lap keys.
Volume 42, Number 4, April 1965
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Figure 2.
Variable resistor hook-ups.
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Switches and T a p Keys. Toggle switches can be used for on-off and for selecting circuits. The author finds the DPDT configuration useful since it can be hooked up in several ways. Figure 1(a) and l(b) show two ways that a DPDT can be hooked up for selecting one of t,wo circuits or as an on-off. I n l(a) the contacts are gauged so as to minimize (roughly halve) the contact resistance. I n l(b) the arrangement is such as to minimize thermal emf's a t the contacts. The contacts A, B, and C can be of three different materials and the thermal emf's will cancel. The simple mercury switch operates in essence as in l(b) with the rnaterialA (or C) being mercury. Also the us* of mercury switches greatly reduces contact resistance. Figure l(c) shows the arrangement for reversing polarity. Figure l(d) shows a way of hooking up a mercury switch as an on-off device. Mercury switches are inexpensive (ca. 50R, especially when compared to good tap keys. The tap key arrangement in l(f) was devised by Tabbutt,' and that in l(e), which utilizes a spring to hold two tap keys, by Mr. Clayton Fawkes (Glenbad East High School, Glenbard, Ill.). Galvanometers. The important characteristics of a galvanometer are its coil resistance, period, critical damping resistance (C.D.R.X.), and sensitivity; these should be matched to the problem a t hand. A 0-20 ,a meter, Model no. TM 9014 (coil resistance, 2000 ohm) is available from Herbach and Rademau (1204 Arch St., Philadelphia, Pa.) for about $6.00; this has proved adequate for the full volt potentiometers. It is also small enough to be panel mounted. The Cenco model no. 82186 (No. 2 pointer galvanometer) costing about $35 has proved adequate in the thermocouple range (characteristics are 700 ohm C.D.R.X., 100 ohm coil resistance, and 0.2 ,a/mm). Galvanometer sensitivity is changed by varying a resistance in series or
parallel with it. The series arrangement is simpler This can be done with two tap keys, one connecting in a resistance which is 15-30 times the coil resistance for a low sensitivity position and the second connecting in no resistance for maximum sensitivity. One other arrangement' uses one tap key and a variable resistance whose magnitude is about 100 times the coil resistance. Slide Wire. A useful and convenient form of slide wire is a lo-turn potentiometer with a 10-turn dial. With a current of 1 ma and a resistance of 1000 ohms full scale is 1 volt and the dial can thus be read to the nearest millivolt although the linearity is not that good. To double the range, a 1000 ohm 1% precision resistor is added in series to the 10-turn potentiometer. To extend the range to 1.5 volts, either a 500 ohm 1% precision resistor or a 500 ohm 5-turn potentiometer may be added in series to the 1000 ohm variable. Variable Resistors. Variable resistors may be hooked up as in Figure 2. The circuits in 2(h) and (c) are preferable to that in (a) since if the sliding contact breaks contact the resistance will still be in the circuit. Turning the center tap clockwise increases the resistance in 2(b) and decreases it in 2(a) and (c). A 25-turn trimmer potentiometer is a compact way of attaining a highly adjustable variable resistor. Binding Posts. These should be of the very best quality available, as they take a great deal of abuse. Figure 3 shows the basic potentiometer for the 0-2 volt range. Figure 4 shows several variations which can be used for hooking up the standard cell such that the standardization can be checked without disturbing the slidewire setting. R7 is a 50 ohm 25-turn trimmer. If a mercury battery is used as a standard cell, then a
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Figure 5.
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Bark potentiometer.
= 2K = 200 = 20 = 1K 1 % = 1K 10-turn = 30K
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S1 = DPST S2SS = DPDT 53.54 = Hg Switches BAT = 2 RMl2R in series = 2 0 pa. 2 0 0 0 coil resistance S.C. = Mallory No. 3 0 2 5 4 0
Journal o f Chemical Education
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Standard cell hook-ups.
0-2" and 0-2 mv potentiometer.
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RS R7 R9 = 2 0 5 0 or R4 RiO = 203K or 9 9 x (R4 f R5 R l l = 2K or 20K variable 5 7 = DPDT center off S8 = DPDT or two switches G 2 = Cenco 8 2 1 8 6 No. 2
324 or 330 ohm resistor (R8) should be added in series to R7. The subcircuit shown in 4(c) can be used for accommodating both Weston and mercury standard cells. The arrangement in 4(a) requires a standardizing procedure which is of interest, especially if the calculations as to currents, voltages, and resistances in the circuits are made. First the S.C. is hooked up as the EMF, 52 is turned to the 1 volt range, and then the current is adjusted (Rl, R2, and R3) so that the galvanometer shows no deflection (when the high sensitivity tap key is used). Then the S.C. is hooked up to the S.C. terminals and R7 is adjusted until the galvanometer shows no deflection. The procedure is re-
peated until no further adjustment of R7 is necessary. The circuits shown in 4(b) and (c) would require this procedure being followed only once. Figure 5 shows the full circuit for a double range instrument (0-2 v, 0-20 mv) which can be used for measuring the emf's of thermocouples as well as larger emf's. The controlling factor here is that R9 = R4 R5 R7 and that R10 = 99 X R9. The author gratefully acknowledges the insights gained through many helpful discussions with Dr. T. J. Neubert. Also acknowledged is Dr. Tabbutt's work. The reader is referred to footnote 1 for a fuller bibliography on the subject and Dr. Tabbutt's contributions.
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