A Simple Low-Cost Device for Temperature Control in Nonisothermic Chemical Kinetics in Solution F. Salvador, J. L. Gonzalez, and L. M. Tel Departamento de Quimica Fisica. Facultad de Quimica, Universidad de Salamanca, 37008 Salamanca, Spain There is considerable current interest in the kinetic study of rhemio~lreactions in solution by nonisorhermic methods.'-!' Thc advantages of using nonisorhermic procedures fur determining thekrrhenius equation parame& are evident and have been described elsewhere. Nonisothermic kinetics may even he feasible for practical classes in an undergraduate laboratory.2 The kinds of temperature variations with time proposed by us are linear3 and hyperhok2 To achieve these, one can purchase sophisticated pieces of apparatus which program the variations in temperature. We, however, propose the use of an operative procedure employing an electronic device that is cheap and easy to huild and that gives excellent result. The device is shown in Figure 1. A small drivine motor (M) with a verv low s ~ e e is d fixed ~ to the head of a cintact thermometer (CT) of a ihermostatted bath (T). In this way we ensure the heating rate of the water and that the temperatureltime relationship is linear. The water is pumped to the cell holder (CH) of the device for kinetic measurement (for example a spectrophotometer (S)) through well-insulated conduits (IC). A thermometer (Th) is fitted to the mouth of the cuvette containing the reaction in order to ensure a hermetic mixture with Teflon tape (TTB) seal and thus avoid loss by evaporation. The table shows the least-sauares fitting of the data collected for a linear heating. heg goodness offit appears in the corresponding parameters, and i t may be observed that the standard deviation is in agreement with the precision of the reading of the temperature (precision of thermometer = fO.IX0C). In order to vary the heating rate at will to obtain a given temperatureltime ratio (for example, of the hyperbolic kind) the electronic device (V) was included in the driving circuit of the motor (Fig. 2). This device, easy to huild and of low cost, is basically composed of an oscillating circuit with a Wein bridge, whose frequency may he changed at will by the potentiometers VR1 and VR2. It is thus possible to control at will the rotationspeed of the motor and hence the heating rate. We may therefore reproduce the desired temperatureltime curve by carrying out a previous calibration. A s i m ~ l emethod r which we have sometimes used consists in sirnul;rting the temperatureltirne hyperbolir rurvr with straight segments; we obtained highly satisl'artorily results since the temperatures recalculatrd nfter fitting the curve showed a de\.iation of less t h ~ O.0ZoC n [less than the ~rwision of the thermometer) with respect to the experimental results.
Figure 1. Diagram of the temperature controller in operation.
30 X'1
Figure 2. Circuit diagram for motor Comparison of the Measured and Calculated Temperature versus Time tor a Linear Heating
Acknowledgment The authors' thanks are due to Gutierrez Conde and Hernindez Hernindez for the supervision of the electronic device and to N. Skinner for revising the English manuscript.
' Brown M. E., and Philipotts, C.,J. Cum. E~uc.,55, 556 (1978).
Salvador. F., GonzAlez. J. L., and Tel, L. M., J. CHEM. EDUC. 61,921
(1984).
Gonzilez, J. L.. and Salvador, F., React. Kinef. Catal. Len.. 21, (1-2). 167 (1982). 1 rprn. 220 V. 60 cycle. 2.7 W, Type 117, PO4JAA07. made by Cramer Division, Old Saybrook, CT.
'
Slope = 028541 i 0.00033°C min-'. , '~ntercept= 18.963 i 0.012°C;carelstion coatllclent = 0.999993; standard devialion = i0.023'C.
Volume 62
Number 7 July 1985
613
