1. N. Klatt.' D. M. Coleman, R. J. Domalakes, and R. E. Van AttaZ
Southern Illinois University Carbondale, Illinois 62901
I
An Inexpensive Resirtometer for Student Laboratory Use
T h e relatively simple resistometer described in this article may be constructed from inexpensive components by students with little previous knowledge of electrical instrumentation. The device (Fig. 1) has sufficient sensitivity and accuracy for reasonably sophisticated experiments in conductometry, yet is rugged enough for freshman experiments such as those described by C a r l t ~ n . ~
where R, is the value of the precision resistor in the circuit a t the final RANGE control setting employed. For solution conductance measurements, a variable capacitor C may be incorporated, if desired, to compensate for the capacitance effects of the conductance cell. Performance
Experimental measurements of resistances picked from a Heathkit Decade Resistance Model DR-1 showed agreement well wit,hin the tolerance of the precision resistors em~lovedin both oieces of eouinment. Figure 3 illu&tes the perfirmance of' tGe Ports List for Resistometer -
1
I\licroammeter, 0-50 rA dc ahminnm, sloping panel, 4 X 4 X 7 in. Potentiometer, 10-turn 1K ohma, +0.5%, , 1)ial. 10-turn. for oaterkiometer. 1000-div~slon
1 Cabinet,
1 1 1 Figure 1.
Assembled resistometer.
The circuit diagram for the instrument is given in Figure 2 and a detailed parts list appears in the table. Total cost of components is about $50 for the device as shown in Figure 1; breadboard construction with less expensive components can make the cost considerably less. Although the circuit as shown iuvolves three ranges for resistance measurement, more or fewer ranges may be incorporated a t the option of the builder. The series incorporation of a silicon diode permits the use of a relatively sensitive dc microammeter as the null detector, while allowing ac operation in order to eliminate the polarization effects encountered with simpler dc bridges. I n operation, the unknown resistance or conductance t o he measured is connected across the R, terminals, the RANGE control set t o its highest value, and the TEST push-button switch depressed; imbalance, if any, is then offset by rotating the 10-turn potentiometer control until meter null is observed. Final balance is obtained with the lowest possible RANGE control setting, and the dial reading O is read. The unknown resistance R, is calculated via the relationship 1 Present address: Department of Chemist,ry, University of Georgia, Athens, Georgia 30601. 2 Present address: Department of Chemistry, Ball State University, Muncie, Indisna47306. T. S., J. CHIIM. EDUC., 44,769 (1967). CARLTON,
1
1 1 1 1
-
Switch, ~.otary Switch, laggle, SPST Swit,ch,p!wll-button, SPST-NO Ihde, silicon Resistor, 100K, 20% Itesistors, precision, varions valoes, ly0 l\liacellmeow (jxks, plugs, line cord, pilot lieht. knobs, hook-ut, vire, etc.)
RANGE
Figvre 2.
Circuit diagram of resittometer.
Volume 47, Number 5, Moy 1970
/
403
instrumeut as an end-point detector in the titration of 2 4 3 ml of 0.0316 N sulfamic acid with 0.399 N NaOH; the equivalence points were found to be 1.97 ml (a) by conventional pH measurement with a Fisher Model 210 Accumet pH meter and 1.96 ml (b) via conductometric measurement with the instrument herein shown. The conductometric cell consisted of a 50-ml beaker and bright platinum electrodes about 1 cm square rigidly mounted about 4 cm apart; magnetic stirring was used. Absolute conductance measurements may be made, if desired, through use of platinized electrodes and appropriate cell constant determination.
32 28
2 4.f 0
-.,
20;
o
'6
12 ay D
a
Acknowledgmenl
The authors are grateful to the National Science Foundation, whose grants GW-595 (1967) and GW-2 969 (1968) made possible the participation of DMC and RJD.
404
/
Journal of Chemical Education
-.
2
o
1
~
3
4
NOOH. ml Figure
3.
A comparative patentiometric-cond~ctomrntrictitration
5
~
