JOURNAL O F CHEMICAL EDUCATION
0
SOME USES OF RADIO SERVICE KITS IN TEACHING QUANTITATIVE ANALYSIS JOHN P. PHILLIPS University of Louisville, Louiwille, Kentucky
THEuse
of instrumental analysis in small colleges is restricted by the cost of commercial instruments which usually seem to be designed with much higher precision than is absolutely necessary for instructional purposes, though not for research. Although it is possible to relieve the cost difficulty by constructing titrimeters and colorimeters from directions in Analytical Chemistry and Tms JOURNAL, not all teachers have the skill and experience in electronics, especially when using directions often scantily detailed, to produce attractive constructions. No matter how well the instrument functions, students are not likely to consider it a professional tool unless it has a professional appearance. The problem of obtaining inexpensive and practical instruments to supplement conventional equipment specifically designed for chemical purposes has been partially solved in our quantitative analysis course by the purchase of radio sewice instrument kits, which can be put together without previous training in electronics simply by following the detailed assembly instructions. Such kits are available from several manufacturers a t low cost. For less than $125, for example, a battery eliminator, a vacuum-tube voltmeter, a 400-volt power supply, and an oscilloscope can be obtained.' The battery-elimimator kit was easily put together in one hour. This unit supplies voltages varying from 0-15 volts and is convenient for electrodeposition experiments or to charge storage batteries to be used for this purpose. Although the voltage may not be sufficiently well filtered for close electrolytic separations, it is more than adequate for the usual electrodeposition of copper in brass when used with platinum electrodes and a stirrer. An a.-c., d.-c. vacuum-tube voltmeter, with a full scale deflection of 1.5 volts on its minimum range, is the next easiest of the four instruments to construct. This instrument is not completely satisfactory for use as a titrimeter because it is not very sensitive to the small voltage changes of most potentiometric titrations, but it is suitable for titrations with large potential breaks,
for example the reaction of ferrous ion with eeric salts. A power-supply kit giving up to 400 volts of direct current is useful as the supply voltage for vacuum phototubes in homemade colorimeter designs. It is practical to connect the phototube in series with a high resistance (1-20 megohms), the power supply, and a galvanometer or microammeter. The electrical circuit of a one-cell colorirneter thus obtained is as useful as the more common barrier-layer cell colorirneters and is more versatile, since different types of phototubes (such as the RCA 917, 926, and 929) functioning in different portions of the spectrum may be used. An oscilloscope is the most complex of these instruments. I t is used in our laboratory as balance detector in an a-c. Wheatstone bridge circuit for conductometric titrations. Although some may question the replacement of a $2 pair of ear phones with a $45 oscilloscope, the advantage of visual over aural operation is a real one. The well-known uses of oscilloscopes for other purposesZmake this a very desirable instrument. In addition to these four instruments, inexpensive decade resistance, decade condenser, and resistance substitution kits have proved useful as components in student Wheatstone bridge, potentiometer, and other circuits. The accuracy of these kits is not high (of the order of 1 per cent) by research standards but is sufficient for student experiments. Potentialuses in chemistry for such other kits as an audio oscillator, grid dip meter, end impedance bridge are easily seen but will not be discussed here. These radio service kits are excellent as an introduction to the construction of more difficult circuits described in the chemical literature. The variety introduced into a course in quantitative analysis by the use of these instruments is very helpful in dispelling the frequent impression that analysis consists of the monotonous manipulation of balance and buret. In modem chemistry analysis actually offers a greater variety of methods than any other branch of chemistry, and it is good to convince the student of this as early as possible.
Most of the kits here described were purchased from the Heath Company of Benton Harbor, Michigan.
' Rrn~En, J. H., "Modern Oscilloscopes and Their Uses," Murray Rill Books, New York, 1949.
