A. M. Ferguron ond L. F. P h i l l i ~ r University of Canterbury Christchurch, New Zealand
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b. Thermistor Bridge "sing Light-Emitting Diodes An undergraduate thermochemistry experiment
A precision calorimetry experiment, in which the molar enthalpies of neutralization of HN03, HCI, HzSOa, and phenol are measured and compared, has evolved in our elementary physical chemistry laboratory over a period of years. The calorimeter consists of a 250-ml unsilvered Dewar into which are inserted a motor-driven stirrer, a small (15 W) heating coil, and an accurate mercury-inglass thermometer (15-21°C in % o 0 ) . The temperature rise (typically 3') produced by neutralizing 100 ml of N/4 acid 2 ml of concentrated NaOH solution (added from with a plastic syringe) is compared with the rise produced in the same volume of liquid by adding a known amount of electrical energy via the heating coil. Normal cooling corrections are applied, and the heat of dilution of the strong NaOH solution (typically 0.5"C rise in 102 ml of liquid) is also determined. The experiment normally yields values accurate to within about 6 kJ for the enthalpies of ionization of phenol and hisulfate ion. The most expensive and also the most fragile component of the calorimeter system is the thermometer, and although each set of apparatus is kept permanently assembled on a stand, many breakages have occurred. The heat capacity of the calorimeter plus the solution is determined by direct comparison with electrical energy from the heater, so the actual temperature readings do not enter into the final results. Consequently we have now replaced the thermometers by thermistors, whose relative temperatures are determined using the circuit shown in Figure 1. The hridge-balance detector of Figure 1 consists of an
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LM301A operational amplifier driving light emitting diodes LEDl and LED2 in such a way that if the input to the LM301A is positive LEDl is turned on, and if the input is negative LED2 is turned on. At the balance point both diodes are almost extinguished. The two diodes are mounted side by side on the bridge case, under a small lip which serves as a light shield and adjacent to the ten-turn potentiometer P1. P1 is used to compensate for changes in the resistance of the glass-encapsulated thermistor T I . At balance the dnodial which indicates the reading of P1 can he set reproducihly to within one division; this corresponds to a temperature resolution of 0.0l0C. The preset potentiometer P2 is used to fix the range of temperature spanned by the duodial readings. The whole circuit may he mains or battery operated; the maximum current drain is approximately 20 mA. Approximate costs of components in Figure 1 are P1 with duodial, $16; thermistor, $2; LM301A, < $2; light-emitting diodes, 506 each; and printed circuit hoard, $3. The basic circuit, comprising the operational amplifier and light emitting diodes, has many potential applications as a null-detector. A typical graph of duodial reading versus temperature is given in Figure 2. Although the deviation from linearity is significant, over a range of 3-4" the linearity is adequate for calorimetry measurements such as we have described. If greater precision were required, a calibration curve, such as that of Figure 2, could he used.
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Figure 1. Thermistor bridge circuit. Thermistor T1, glass encapsulated far thermometry, nominal resistance 10 K, rating 50 mW. R = AeB'T where B = 3800 K in our apparatus. Light-emitting diodes LEDl and LED2. Litranix type Red-Lit 2 or equivalent.
684
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Journal of Chemical Education
20 24 T -2 Figure 2. Typical variation with temperature of the duodial reading at baiance point. 16
