zood aereement with nublished work. and the students seem to enjoy this experimknt. Presented a t the 33rd Southwestl37th Southeast Reeional ACS Meeting, Memphis, TN, October 9, 1985.
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An Inexpensive Linear Thermistor Thermometer for Cryoscopic and Calorimeter Measurements and Lecture Demonstrations
provides a way to control the cooling rate of the sample. The modifications not only do not detract from the need for constant and careful stirring and the recording of the data for sufficient periods of time to obtain satisfactory cooling curves but em~hasizethese and other needs. A c o ~ of v the thermometer computer program and a report describing the modifications we have made to improve the quality of the results and the ease of performing this cryoscopic rneasurements may he obtained by writing to JWB.
..
James W. Beatty and Todd B. Colln Ripon College Ripon. WI 54971 Two years ago, inspired by papers in this Journal by Chan and Ng (19) and Srivastava and Meloan (201, we set out to build a precision computer thermistor thermometer for general use in the physical chemistry laboratory. In doing background work we discovered a commercially available component that is linear over a wider range, is even simpler to build, and for most operations does not require calibration. The use of a commercially available linear thermistor bridge network thermometer has general applications for those wishing a simple economical unbiased precision thermometer with 10-cm-high numerals for chemistry demonstrations and experiments. We have used this thermometer for lecture demonstrations and for calorimeter and cryoscopic measurements. The thermometer, a YSI8 44212 thermolinear component, consists of three thermistors in a composite and three precision resistors in a resistance bridge network that we incorporated in the thermometer leads. The linear component is the heart of expensive precision commercial thermistor thermometers. The component can be used in voltaee or resistance mode and produces a linear output. We used it in the resistance mode where i t has a sensitivity of 129 tl per degree Celsius. Temperature differences were measured with ease to 0.01 OC with our components and differences of 0.001 "C can be measured with the component. Our limitation was the stability of our resistance meter. The use of the network in the voltaae mode reauires a constant voltaee " sunnlv. .. . The reader interested in;sing the component in the voltage mode is referred to YSI literature and the cited papers (19,
POINT a
20).
A Keithleyg Model 179A 4 K digit multimeter with IEEE488 option was used to measure the resistance and convert i t to a digital signal. The digital signal was read by a BASIC program using a Commodore 8032 microcomputer. The data was converted to Celsius or Fahrenheit temperature displayed on the screen in 10-cm-high numerals suitable for lecture demonstrations. The temperatures and acquisition time were held in an array and printed out at the end of an experiment. The acquistion time was programmable. We have resisted the temptation to have the computer graph the temperaturetime data for we believe a t this stage the preparation of the graph is an important exercise for the student. In using the thermometer for freezing-point depression measurement we made modifications of the classic experiment (21) using a thermoelectric stirrer-eooler to remove the heat from a specially designed freezing-point depression cell. The thermoelectric cooler effectively and conveniently BYellow Springs Instrument Co., Inc., (YSI) type 44212 linear temperature transducer may be purchased from Newark Electronics for about $40. Our final temperature probe was this component sealed in a 25-cm-long. 3/1&diameter stainless tube obtained directly from YSI for around $100. The special order number was 03444020-8-RP-8-ST for the thermistor composite and 44312 for the resistor composite. A similar multimeter with an RS-232C interface is available from Omega Engineering, Inc. 500
I
Journal of Chemical Education
F w r e 7 Sample cwilng cLwe tor 35/65 w!w SnlPb mlxlde (a) Comp ete ID)Plot expanded in the regcon ot the break
cooling Carve
Om
025
050
M O V F R M K I N TIN
Figure 8. Sample tinllead phase diagram.
075
